Calendar of MRSEC Events

Abstract: Optical techniques can enable non-invasive, in situ measurements of the properties of biologically or industrially relevant colloidal particles. One such property is the fractal dimension of fractal-like aggregates, which can be formed by some proteins when heated. Here, we consider recent experiments that used holographic microscopy, a technique based on optical interference, and an optical scattering model based on an effective-medium theory to determine the fractal dimension of colloidal aggregates. We numerically generate fractal aggregates with a known structure, simulate the images that would be experimentally obtained from those aggregates, and analyze the simulated images using the proposed scattering model. Our results show that holographic microscopy accurately determines the fractal dimension (to within ~10%) when multiple scattering within the aggregates is negligible. They also suggest that holographic microscopy is useful because it probes the extinction cross section of the aggregates.

José Andrés (@chefjoseandres), Think Food Group, minibar, Jaleo
Harold McGee, (@Harold_McGee), author of "On Food and Cooking", "Curious Cook"

Joan Roca (@CanRocaCeller), El Celler de Can Roca, Girona, Spain, best restaurant in the world 2013 and 2015
Heloise Vilaseca, (@heloislois), director of R&D, El Celler de Can Roca, Girona, Spain
Ahmoy Panagiotis, El Celler de Can Roca, Girona, Spain

Animals fly, run, swim, and crawl by self-deforming, changing the shape of their bodies using internally generated forces, to interact with their surroundings. While traditionally viewed as obstacles to locomotion, limbless animals like snakes must self-deform to take advantage of environmental heterogeneities and overcome drag on the elongate body. While progress has been made in understanding undulatory locomotion in fluids, little is known about how snakes ably traverse terrestrial terrains ranging from desert to forest. In this talk I will present our studies of the motion of live snakes in laboratory models of terrestrial terrain. I will discuss how a permanently deformable substrate, granular matter, was found to help or hinder snakes depending on how the waveform remodeled the material. Surprisingly, locomotion was non-inertial despite the surface slithering speed, and resistive force theory accurately predicted performance of a desert specialist snake moving in a frictional-fluid-like regime. We next studied the neuromechanical control strategy of this desert specialist by observing it traversing a simple multi-component terrain—a single row of posts in a uniform substrate. Combining trajectories from many trials revealed a pattern of trajectory reorientations reminiscent of the diffraction of, e.g., light. Using this mechanical diffraction phenomenon, we show that the desert specialist could rapidly transit the sparse obstacles without additional neural input by relying on passive body buckling. Lastly, I will discuss the role of control specialization by comparing the waveform of the desert specialist with that of a habitat generalist snake as they move through a 2D array of rigid posts embedded in a slippery whiteboard. We used persistent homology to search for periodic cycles in the kinematics and found that the sand snake, adapted to use the omnipresent GM in its natural habitat, targeted desired kinematics while those of the generalist, which encounters a range of materials and obstacle densities, were aperiodic. Force measurements in a simplified terrain, a single post in the whiteboard, suggested that the generalist is instead targeting an advantageous force pattern. Studying the interplay between animal and environment furthered our understanding of the terrain itself as well as the animals' goals and their strategies for achieving them.

In this talk I will show several examples of an interesting and surprising competition between buoyancy and Marangoni forces.

First, I will introduce the audience to the jumping oil droplet—and its sudden death—in a density stratified liquid consisting of water in the bottom and ethanol in the top : After sinking for about a minute, before reaching the equilibrium the droplet suddenly jumps up thanks to the Marangoni forces. This phenomenon repeats about 30-50 times, before the droplet falls dead all the sudden. We explain this phenomenon and explore the phase space where it occurs.

Next, I will focus on the evaporation of multicomponent droplets, for which the richness of phenomena keeps surprising us. I will show and explain several of such phenomena, namely evaporation-triggered segregation thanks to either weak solutal Marangoni flow or thanks to gravitational effects. The dominance of the latter implies that sessile droplets and pending droplets show very different evaporation behavior, even for Bond number << 1. I will also explain the full phase diagram in the Marangoni number vs Rayleigh number phase space, and show where Rayleigh convections rolls prevail, where Marangoni convection rolls prevail, and where they compete.

The research work shown in this talks combines experiments, numerical simulations, and theory. It has been done by and in collaboration with Yanshen Li, Yaxing Li, and Christian Diddens, and many others.

Enric Rovira (@enrovira), Master Chocolatier, Barcelona, Spain
Ramon Morató, (@ramonmorato_), Master Chocolatier, author of "Chocolate" and "Four in One"

How do we move when walking alone or inside dense crowds? Is it possible to develop mathematical models capable of accurately describing the statistical properties of pedestrian dynamics? These challenging and fascinating scientific questions can also directly impact on our daily comfort, and even safety, when visiting crowded urban areas.

In recent years we^[1] have been conducting a number of real-life experiments aimed at providing some answers. We employ 3d depth-sensor cameras to observe the dynamics of millions of pedestrians in a variety of real-life settings: from a small University corridor, to a museum entrance, city festivals, crowded train stations, etc. Our 3d depth-map cameras have allowed us to record pedestrian trajectories with high space and time accuracies while preserving the privacy of single individuals. Thanks to the large acquired datasets we could study fluctuations and not just average behaviours.

To describe our observations, we borrowed and extended mathematical tools ordinarily used in physics. In this talk we will present some of them and we will discuss how the dynamics of single individuals can be modeled in terms of path integrals or stochastic differential equations. We will discuss how to model collisions between individuals, in low density crowds, and how to learn generic space-time patterns. We will also show how machine learning algorithms can be used to accurately extract orientational data from depth-map images and, finally, we will discuss experiments that we conducted in order to investigate the possibility to nudge human crowds via light stimuli.

Alessandro Corbetta, Jasper Meeusen, Chung-min Lee, Roberto Benzi, Federico Toschi
Physics-based modeling and data representation of pairwise interactions among pedestrians Journal Article
Physical Review E, 98 , pp. 062310, 2018.

Alessandro Corbetta; Chung-min Lee; Roberto Benzi; Adrian Muntean; Federico Toschi
Fluctuations around mean walking behaviours in diluted pedestrian flows
Physical Review E, 95 , pp. 032316, 2017.

Untangling structure-function relationships on the nanoscale is critical to understanding biological molecular machinery and to engineering human-made nano-devices. In this talk I will give an overview of my lab’s approaches to probing and controlling sub-nanometer atomic arrangements in biological and synthetic systems. First, I will introduce our single molecule conductance measurements which show that molecular conductance signatures can serve as Angstrom-scale rulers and demonstrate how we apply these techniques to probe the conductance of DNA building blocks. Next I will discuss our approaches to single molecule absorption spectroscopy using an optical tweezer and outline some future research directions.

Spontaneous collective motion or flocking of a large collection of self-propelled entities, is seen on many scales in nature, ranging from bird flocks to the coherent migration of cells during development, along with several synthetic realizations in granular and colloidal systems. Understanding the distinct nonequilibrium phenomena in such active fluids remains crucial to developing insight into the organizational principles of living matter, while still posing a serious challenge to conventional statistical mechanics. On the other hand, condensed matter now uses a wider range of tools, particularly topology, that is not rooted in equilibrium physics. In this talk I will present two examples of topological phenomena that acquire unique realizations in polar active fluids. In the first half, I will show how broken time-reversal symmetry due to spontaneous motion allows polar flocks on a curved surface to support topologically protected sound modes, analogous to edge states in a quantum Hall system. Later I will switch to discuss flocks flowing through a disordered environment, where the ordered flock undergoes a two-step melting transition through an intriguing intermediate state - a vortex glass with frozen topological defects. I will end with a brief outlook on the broader role of topology in active matter.

Carles Tejedor (@CarlesTejedor), Oilmotion and Sofia Be So Restaurant, Manresa, Spain
Pere Planagumà, ROM and Mas de Torrent restaurants, Spain

The invasion of one fluid into another of higher viscosity is unstable in a quasi-two dimensional geometry. In isotropic systems, this viscous-fingering instability typically produces complex patterns that are characterized by repeated branching of the evolving structure, which leads to the common morphologies of fractal or dense-branching growth. In anisotropic systems, by contrast, the growth morphology changes to a highly ordered dendritic growth characterized by stable needle-like protrusions decorated with regular side-branches. We investigate such morphology transitions between dendritic growth and dense-branching growth in an intrinsically anisotropic liquid; a lyotropic chromonic liquid crystal in the nematic phase. We show that the transition is remarkably sensitive to the interface velocity and the viscosity ratio between the less-viscous inner fluid and the more-viscous outer liquid crystal. We discuss the importance of a stable shear alignment of the liquid crystal in governing the morphology transition to dendritic growth.

Expanding our work to more complex fluids, dense suspensions, that exhibit both shear-thickening and shear-jamming behavior as a response to an applied stress allows us to probe transitions from flow instabilities to fracture instabilities. Displacing a cornstarch suspension by a pressure-controlled injection of air, we observe a variety of patterns: smooth fingering in the fluid regime and different modes of fractures, ranging from slow branched cracks to single fast fractures. We discuss strategies to predict and control these different failure modes in dense suspensions.

Marsia Taha (@marsia_taha_gustu), Gustu, La Paz, Bolivia
Natalie Del Carpio, Gustu, La Paz, Bolivia

Plant matter is being used increasingly in construction and in various other applications thanks to its remarkable porous and mechanical properties, but water transfers play a critical role on these properties and their possible alteration. Water in plants may be either in a “free” liquid state in capillaries, or in a “bound” state after adsorption in cell-walls, associated with significant deformation of the structure. Here we show that the coexistence of these two effects strongly affect the transport properties.

We demonstrate this from Synchrotron and MRI observation in hardwoods, which exhibit a relatively simple hydraulic structure. Capillary imbibition dynamics appears to be dramatically damped (velocity decreased by several orders of magnitude), but the liquid can still climb over significant heights (in contradiction with its dynamics) as soon as sufficient bound water has been adsorbed. This contradiction is confirmed by 3D Synchrotron images of the internal structure obtained during imbibition, which show that the liquid-air interfaces in the capillary vessels remain planar, which implies negligible Laplace pressure, but significantly advance along the vessels, again unexpectedly.

From MRI measurements allowing to distinguish bound and free water, but also direct measurements of the induced macroscopic deformation distribution in time, we then show that this contradiction is explained by the adsorption of a slight amount of bound water in the capillary walls. This adsorption governs the process: it momentarily damps wetting and then allows further advance later when the walls are saturated with bound water. The generality of the process for hygroscopic systems is demonstrated with a model material, i.e. hydrogel, from which both the position and shape evolution of liquid-air interface and the adsorption and propagation of bound water may be directly observed (see below). This suggests the development of bio-inspired porous materials able to absorb liquid with a tunable timing, for pharmaceutical or chemical engineering applications.

We finally discuss the opposite process, i.e. liquid transfers in hardwood structures during drying, as observed from MRI and Synchrotron imaging, and in particular show the essential role of bound water.

Affinity maturation (AM) is the process through which the immune system evolves antibodies (Abs) which efficiently bind to antigens (Ags), e.g. to spikes on the surface of a virus. This process involves competition between B-cells: those that ingest more Ags receive signals (from T helper cells) to replicate and mutate for another round of competition. Modeling this process, we find that the affinity of the resulting Abs is a non-monotonic function of the target (e.g. viral spike) density, with the strongest binding at an intermediate density (set by the two-arm structure of the antibody). We argue that, to evade the immune system, most viruses evolve high spike densities (SDs). This is indeed the case, except for HIV whose SD is two orders of magnitude lower than other viruses. However, HIV also interferes with AM by depleting T helper cells, a key component of Ab evolution. We find that T helper cell depletion results in high affinity antibodies when SD is high, but not if SD is low. This special feature of HIV infection may have led to the evolution of a low SD to avoid potent immune responses early on in infection.

Advances in optical stepper lithography have opened the door to high-throughput on-chip parallel fabrication and assembly of addressable complex nanoscale structures, including processors and memory, that have amazingly low defects. By combining these advances with an enabling particle fabrication-release protocol, which involves roughness controlled depletion attractions, we make idealized two-dimensional monolayers of differently shaped mobile colloidal tiles that can be pre-assembled at high densities in interesting new ways without the need for patchy programmable interactions. The size and volume fraction of the depletion agent are tuned so that the face of each tile remains near the flat smooth substrate, yet the edges of the tiles have nearly hard interactions. As a first demonstration of lithographically pre-assembled monolayers (litho-PAMs), we produce a Brownian Penrose P2 quasi-crystal (QC) of dense microscale kite and dart tiles with low defects over large areas, and we study this fluctuating five-fold P2 QC using time-lapse optical video microscopy. Beyond measuring the equilibrium fluctuations and structure of this multi-scale form of soft matter, we examine motif dynamics of sets of tiles and calculate motif superstructural pair correlation functions. Moreover, by removing a confining wall, we melt this P2 QC and deduce the tile area fraction associated with its melting. Since a wide range of shape-designed tiles can be fabricated and pre-configured lithographically, litho-PAMs represents a major advance over prior work that merely fabricated different colloidal shapes using lithography, and it opens the door to a wide range of new experimental soft-matter systems. The related paper is: Nature 561: 94–99 (2018)

An odor is not composed of a single molecule but a mixture of various molecules in most cases. These molecules are so-called ‘odorous molecules’, and it is known that the number of odorous molecules amounts to approximately 400,000 or more. As hundreds of, or sometimes thousands of the odorous molecules mix together to form an odor, there are a vast number of odors existing in the world. Such versatility is one of the reasons why odor analysis takes time and requires special analytical instruments which are usually bulky and very expensive. On the other hand, the complex and versatile nature of odors indicates that they obviously contain detailed information on their origin which we have not been fully understood and exploited so far. In this talk, I will introduce our olfactory sensor project and share with you some of the recent results including fundamental physical/chemical studies and several examples of applications. By using a newly developed sensing platform—Membrane-type Surface stress Sensor (MSS)—with high sensitivity and tunable selectivity, we are trying to overcome various barriers to a practically useful mobile olfactory sensing system which has never been achieved.

David Zilber (@David_Zilber), Director of Fermentation at Noma in Copenhagen, Co-author with René Redzepi of "The Noma Guide to Fermentation"
Jason White

The best understood crystal ordering transition is that of 2D freezing, which proceeds by the rapid eradication of lattice defects as the temperature is lowered below a critical threshold. This behavior is described by the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) model, which assumes that the system in question is confined to an infinite flat plane. However, many natural and man-made 2D systems possess finite curvature and/or non-trivial topology. Crystals that assemble on such surfaces may be required by topology to have a minimum number of lattice defects, called disclinations, that act as conserved topological charges—consider the 12 pentagons on a soccer ball or the 12 pentamers on a viral capsid. Moreover, crystals assembled on curved surfaces can spontaneously develop additional lattice defects to alleviate the stress imposed by the curvature. A priori, it is unclear how—or even if—crystallization can proceed in such a system. In this talk, I will discuss the results of experiments and simulations that address the phase behavior of repulsive particles on the surface of a sphere, the simplest surface on which it is impossible to eliminate lattice defects. Our work shows that freezing on a sphere proceeds by the formation of a single, encompassing crystalline 'continent', which sequesters remaining defects into 12 isolated 'seas' with the same icosahedral symmetry as soccer balls and viruses. By aligning the vertices of an icosahedron with the defect seas and unfolding the faces onto a plane, we can use this broken symmetry to construct a new order parameter that reveals the underlying long-range orientational order of the lattice.

Joanne Chang '91 (@jbchang), Flour Bakery and Café, Myers + Chang, author of "Flour", "Flour Too", "Myers + Chang at Home", and "Baking With Less Sugar"

Dave Arnold (@CookingIssues), Booker and Dax, author of "Liquid Intelligence", host of "Cooking Issues", founder of the Museum of Food and Drink
Harold McGee (@Harold_McGee), author of "On Food and Cooking", "Curious Cook"

Many cellular and developmental processes rely on self-organization of protein patterns in space and time for proper functioning. These proteins can couple to molecular force generators and pattern mechanical stresses, leading to cell shape deformation. But how does cell shape information get transmitted to these upstream regulatory proteins? In this talk, I will present a robust mechanism that couples cell shape to biochemical regulators in starfish oocytes. When the oocytes are confined in different geometries, we find that the meiotic Rho pulse adapts to cell shape. Through mathematical modeling, we show that the excitable Rho pulse is tightly regulated by a bistable front of the Rho regulator Ect2. In turn, the Ect2 front decodes the cell shape information encoded in the Cdk1 (cell cycle regulator) cytosolic gradient, allowing cell shape to provide feedback on biochemical processes. We posit that the hierarchical coupling between a biochemical gradient and self-organization is a robust mechanism for cell shape sensing and mechanochemical feedback.

Many cellular and developmental processes rely on self-organization of protein patterns in space and time for proper functioning. These proteins can couple to molecular force generators and pattern mechanical stresses, leading to cell shape deformation. But how does cell shape information get transmitted to these upstream regulatory proteins? In this talk, I will present a robust mechanism that couples cell shape to biochemical regulators in starfish oocytes. When the oocytes are confined in different geometries, we find that the meiotic Rho pulse adapts to cell shape. Through mathematical modeling, we show that the excitable Rho pulse is tightly regulated by a bistable front of the Rho regulator Ect2. In turn, the Ect2 front decodes the cell shape information encoded in the Cdk1 (cell cycle regulator) cytosolic gradient, allowing cell shape to provide feedback on biochemical processes. We posit that the hierarchical coupling between a biochemical gradient and self-organization is a robust mechanism for cell shape sensing and mechanochemical feedback.

Infection with Human Immunodeficiency Virus (HIV) is incurable, despite the existence of antiretroviral therapy (ART) that effectively targets the viral life cycle. The lack of a cure for HIV infection has been linked to an extremely rare population of replication-competent HIV proviruses that persists during ART even when viremia is undetectable. To study the latent population requires a system that can 1) process millions of cells at ultra-high throughput, 2) detect one infected cell within a background of a thousand uninfected cells, and 3) single cell sort and sequence the rare cell population. This talk will discuss the development of a microfluidic workflow that can single cell RNA-sequence infected cells containing a single HIV proviral sequence. Although the system was design to understand the genomic mechanisms that control HIV latency in vivo, it can be applied to find and sequence any cell based on the presence of DNA or RNA biomarkers.

Living systems are out of equilibrium and nearly all cellular processes require a continual supply of energy. The realization of the importance of non-equilibrium processes for subcellular organization helped to inspire the field of active matter, which seeks to understand the behaviors of system in which energy is input at the microscale. Despite intensive work on the dynamics and mechanics of active matter, there is very little known about the thermodynamic flows in these systems which are ultimately responsible for their non-equilibrium nature. Such thermodynamic flows are also extremely important (and extremely poorly understood) in biological systems, because the breakdown of metabolic processes responsible for energy transduction has diverse cellular and developmental consequences, and underlies a variety of diseases. In this talk, I will describe quantitative experiments on: 1) the non-equilibrium thermodynamics of mixtures of microtubules and molecular motors; 2) the metabolism of mammalian oocytes.

From power grids to neurons, oscillator networks represent a broad class of human-made and naturally occurring dynamical systems. Behaviors of multicellular organisms are organized by specialized neural tissues, underscoring the rich spatiotemporal patterns such networks exhibit. Towards developing synthetic analogs, we build and study a rudimentary model oscillator network system composed of Belousov-Zhabotinsky chemical oscillators housed in microfluidic wells and diffusively-coupled through PDMS membranes. In this talk, I will highlight recent work on two different network topologies with inhibitory coupling: stars or "hub and spoke" networks and a ring of three oscillators. For the former, I will discuss how the phase relationship between the hub and arm nodes is altered by the number of arms. For the latter, I will show how optimal control theory can be used to generate spatiotemporal patterns of photochemical perturbations that drive the network between its two attractors: wave states that propagate around the network with opposite sense. Such switching can leverage the system's inherent bistability to robustly generate different gaits at will or store information.

Colloidal suspensions are ubiquitous in our daily life. Micrometric particles dispersed in a solvent are indeed present in common liquids such as paints, inks or even food products. We will discuss the properties of those colloidal suspensions from their liquid phase to special properties of their solid deposits after drying. First, colloidal suspensions exhibit a wide range of rheological behaviors from shear-thinning to yield stress fluids. We will focus on the shear-thickening transition when dense suspensions experience a dramatic increase in viscosity above a critical shear-stress. By changing the physico-chemistry of the particles, we can tune this rheological transition and thus understand the interactions involved in this behavior. Increasing concentration can also be noticed during drying when solvent evaporates: particles finally form a solid deposit. After drying, a drop of a colloidal suspension leads to a variety of patterns from coffee-stain to more homogeneous coatings in paintings. We will discuss the effect of the initial concentration of particles on the drying pattern and on the subsequent mechanical instabilities. Finally, after the whole drying of the colloidal suspension, coatings are achieved. Depending of the nature of the particles, we can tune the wettability of the substrate up to superhydrophobic solid. We will briefly discuss how such a water-repellent substrate can allow levitation of liquids.

Metabolism is fundamental to all living systems. It drives cell growth, fuels ion pumps, powers cell migration, and much more. In the absence of cell metabolism, active processes that define life would cease to exist. Despite great leaps in understanding metabolism's role in maintaining cell homeostasis and contributing to pathological disorders, connections between cell mechanical events and energy metabolism remain poorly understood. In this talk, I present work focused on measuring the metabolic state of cells during collective cell migration achieved through unjamming of the confluent epithelial layer.

Professor David (Dave) Weitz is the Mallinckrodt Professor of Physics and Applied Physics at Harvard University He is the director of the NSF Materials Research Science Engineering Center at Harvard, as well as the co director of the BASF Advanced Research Initiative His research group at Harvard investigates the behavior of soft materials, studies a variety of biological systems from a soft matter perspective, and is well known for pioneering work in the area of microfluidics, including the physics of microfluidics, the engineering of microfluidic devices, and the application of microfluidics to biology, materials science, and two phase flow in porous media Weitz and his research group have extensive interactions with industry, with some of their work motivated by science that addresses technologically important problems Research in the group has led to several start up companies In addition to his research, Professor Weitz teaches courses in innovation and entrepreneurship, and is one of the founding faculty of the highly popular Harvard and EdX course Science Cooking From Haute Cuisine to Soft Matter Science.

Professor Weitz will talk about the relationship between scientific innovation and commercialization, and will describe different routes to entrepreneurship based on his experience in creating and advising biotech and materials startup companies He will also answer student questions on when and how to create or work for a start up company.

Professor Weitz received his B Sc In Physics from the University of Waterloo and his PhD from Harvard University He worked as research physicist for 18 years at Exxon, leading the Interfaces and Inhomogeneous Materials group and Complex Fluids area Prior to joining Harvard, he was Professor of Physics at University of Pennsylvania.

Microswimmers, and among them aspirant microrobots, generally have to cope with flows where viscous forces are dominant, characterized by a low Reynolds number (Re). This implies constraints on the possible sequences of body motion, which have to be nonreciprocal. Furthermore, the presence of a strong drag limits the range of resulting velocities. In this presentation, I will propose a swimming mechanism which uses the buckling instability triggered by pressure waves to propel a spherical, hollow shell. With a macroscopic experimental model, I'll show that a net displacement is produced at all Re regimes. An optimal displacement caused by nontrivial history effects is reached at intermediate Re. I'll show that, due to the fast activation induced by the instability, this regime is reachable by microscopic shells. The rapid dynamics would also allow high-frequency excitation with standard traveling ultrasonic waves. Scale considerations predict a swimming velocity of order 1  cm/s for a remote-controlled microrobot, a suitable value for biological applications such as drug delivery.

Droplet-based microfluidics, a method to produce emulsion drops in a controlled way and at high-throughputs, enables the miniaturization and automation of biological and chemical experiments. Each emulsion drop is used as a closed reaction vessel, enabling the performance of thousands of experiments per second, at throughputs traditional technologies cannot meet. To prevent emulsion drops from coalescing, they can be stabilized with surfactants. They adsorb at the liquid-liquid interface, lower the interfacial tension, and add steric stability. For most drop-based biological assays, aqueous drops are dispersed in fluorinated oils. These drops are stabilized with fluorinated surfactants, composed of two perfluoropolyether blocks linked to a polyethylene glycol (PEG) block. We studied how the composition of fluorinated surfactants influences the stability of emulsion drops and how the surfactant can contribute to the transport of encapsulants between drops leading to cross-contamination. We show that the mechanical and thermal stability of single emulsion drops strongly depends on the composition of the surfactants. We studied the leakage from double emulsions, resulting in cross-contamination, and show that it is mainly caused by tiny emulsion drops with sizes around 100 nm that spontaneously form at the water-oil interface. We show that the leakage can be reduced by an order of magnitude if surfactants that only moderately lower the interfacial tension or double emulsions with shell thickness similar to the diameter of the spontaneously forming drops are used. Lastly, I will present novel types of surfactants containing a catechol group in the hydrophilic block, that can be ionically cross-linked at the interface of a drop, creating a viscoelastic shell. We demonstrate that by cross-linking the surfactant at the drop interface, we create capsules that show high mechanical stability, and are impermeable to encapsulants. Leveraging their stickiness, we demonstrate that these capsules, if densely packed, can be printed into 3D structures.

Surface acoustic wave (SAW) technology integration into microfluidics has gained significant attention over the past years and has great potential to support and advance flow cytometry, cell sorting, and sample handling. SAW-based microfluidics provides many advantages such as versatile fluid handling, biocompatibility, easy integration into various channel designs, rapid prototyping, and is compact while maintaining all the benefits of using a microfluidic device. One of the more notable advantages that has been shown are the abilities to actively sort or provide spatial control of particles. We discuss this technology and its various applications with an emphasis on cell sorting. I report a fully enclosed microfluidic fluorescence activated cell-sorting (µFACS) device that employs a single transducer to generate traveling surface acoustic waves (TSAW) to sort cells upon fluorescence detection at rates comparable to conventional jet-in-air FACS machines, with sort purity and cell viability in excess of 90%. The device operates in continuous flow without encapsulating cells in droplets or labeling them with responsive beads prior to sorting. By studying SAW’s fast fluidic actuation and forces in microchannels, we can engineer a variety of biocompatible SAW-based microfluidic devices for the field of flow cytometry and cell sorting.

Cartographers have early realized that it is impossible to draw a flat map of the Earth without deforming continents. Gauss later generalized this geometrical constrain in his Theorema Egregium. Can we invert the problem and obtain a 3D shape by changing the local distances in an initially flat plate? This strategy in widely used in Nature: leaves or petals may develop into very complex shapes by differential growth. From an engineering point of view, similar shape changes can be obtained when a network of channels embedded in a flat patch of elastomer is inflated. Can we program the final inflated shape?

Understanding the forces on small bodies at a fluid interface has significant relevance to a range of natural and artificial systems. In this talk, I will discuss two recent investigations in my lab where we’ve developed custom experiments to explore different forces on “capillary disks”: centimeter-scale hydrophobic disks trapped at an air-water interface.

In the first part, we investigate the friction experienced by a capillary disk sliding along the interface. We demonstrate that the motion is dominated by skin friction due to the viscous boundary layer that forms in the fluid beneath the moving body. We develop a simple model that considers the boundary layer as quasi-steady, and that is able to capture the experimental behavior for a range of disk radii, masses, and fluid viscosities.

In the second part, we present direct measurements of the attractive force between capillary disks. It is well known that objects at a fluid interface may interact due to the mutual deformation they induce on the free surface, however very few direct measurements of such forces have been reported. In the present work, we characterize how the attraction force depends on the disk radius, mass, and relative spacing. The magnitudes of the measured forces are rationalized with a simple scaling argument and compared directly to numerical predictions.

Future directions in this area will also be discussed, in particular, we are beginning to investigate the motion and interactions of "active" capillary disks at the interface.

Controlled and programmable fabrication of functional materials at the nano to micro scales under mild aqueous conditions is an unmet challenge. Our approach to addressing these challenges is biofabrication, that is to understand and utilize the programmable functionalities of biologically derived materials and interactions. Viral assemblies have attracted substantial attention as templates for materials synthesis due to their precisely controlled dimensions, chemical functionalities and the ability to impart additional modalities through genetic modification. We harness several unique properties of tobacco mosaic virus (TMV) for facile synthesis of catalytically active palladium (Pd) nanoparticles. Unique advantages of TMV templates include robust nanotubular structure providing large surface area, high stability in a wide range of conditions, easy mass production and biological safety. We have demonstrated size-controlled synthesis of small palladium nanoparticles under mild aqueous conditions via spontaneous reduction of precursors. High catalytic activity and stability of TMV-templated Pd nanoparticles are examined via dichromate reduction reaction. The results show that the TMV-templated Pd nanoparticle synthesis offers attractive routes to highly active, controlled and stable catalyst systems.

Polymeric hydrogels offer attractive platforms for a large range of applications including bioassays, separation and catalysis. We exploit simple photo-induced radical polymerization of poly(ethylene glycol) diacrylate and related materials to capture the as-prepared viral-nanoparticle complexes for facile catalytic reaction applications via replica molding and interfacially initiated hydrogel layer synthesis. This simple, robust and readily tunable scheme allows the large nanocomplexes to be captured and utilized without aggregation or leakage in a stable fashion while small molecule reactants and products can access the catalytic sites with minimal mass transfer limitation. Combined, our facile synthesis-capture strategy integrates potent viral nanotemplates, high catalytic activity and stability of the small nanocatalysts, and robust polymerization schemes. We thus believe that our strategy can be readily extended to programmable manufacturing of a large array of multifunctional materials.

In this presentation, our recent progress on the fabrication of multifunctional membranes via interfacially initiated radical polymerization offering controlled macroporous structures for size-selective protein purification as well as catalytic remediation of toxic compounds will be highlighted.

Early in embryonic development the blastocyst consists of a mix of two different cell populations, one primed towards the epiblast which will later develop into the fetus, and one primed for the endoderm, which will develop into the amniotic sac. Initially, those two populations are mixed and later they segregate into the fetus and amniotic sac. Using embryonic stem cells as a relevant model system, we use optical tweezers to quantify the physical properties of those two populations and demonstrate that their viscoelastic properties are significantly different. By modeling, we also show that this relatively small but significant difference in viscoelasticity in those two cell populations is enough to cause a segregation of the two populations, hence, the biomechanical properties alone can drive the process.

Mechanical forces and biophysical properties of cells are also vital for the morphogenesis of organs and embryos. However, how mechanical force and biophysical properties specifically contribute to tissue formation is poorly understood, predominantly due to a lack of tools to measure and quantify biomechanical parameters deep within living developing organisms without causing severe physiological damage. Using an adapted version of optical tweezers we quantify cellular viscoelasticity as deep as 100-150 µm within living embryos and demonstrate that liver and foregut morphogenesis in zebrafish entails progenitor populations with varying mechanical properties. Gut progenitors exhibit are more elastic compared to the more viscous neighboring cell populations, indicating that viscoelastic properties influence specific morphogenetic behaviors. The higher elasticity of gut progenitors correlates with an increased cellular concentration of microtubules and may be decisive for organ positioning. This approach opens new possibilities for quantitative in vivo investigation of cell mechanics in biological systems with complex 3D organization, such as embryos, explants or organoids.

Controlled and programmable fabrication of functional materials at the nano to micro scales under mild aqueous conditions is an unmet challenge. Our approach to addressing these challenges is biofabrication, that is to understand and utilize the programmable functionalities of biologically derived materials and interactions. Viral assemblies have attracted substantial attention as templates for materials synthesis due to their precisely controlled dimensions, chemical functionalities and the ability to impart additional modalities through genetic modification. We harness several unique properties of tobacco mosaic virus (TMV) for facile synthesis of catalytically active palladium (Pd) nanoparticles. Unique advantages of TMV templates include robust nanotubular structure providing large surface area, high stability in a wide range of conditions, easy mass production and biological safety. We have demonstrated size-controlled synthesis of small palladium nanoparticles under mild aqueous conditions via spontaneous reduction of precursors. High catalytic activity and stability of TMV-templated Pd nanoparticles are examined via dichromate reduction reaction. The results show that the TMV-templated Pd nanoparticle synthesis offers attractive routes to highly active, controlled and stable catalyst systems.

Polymeric hydrogels offer attractive platforms for a large range of applications including bioassays, separation and catalysis. We exploit simple photo-induced radical polymerization of poly(ethylene glycol) diacrylate and related materials to capture the as-prepared viral-nanoparticle complexes for facile catalytic reaction applications via replica molding and interfacially initiated hydrogel layer synthesis. This simple, robust and readily tunable scheme allows the large nanocomplexes to be captured and utilized without aggregation or leakage in a stable fashion while small molecule reactants and products can access the catalytic sites with minimal mass transfer limitation. Combined, our facile synthesis-capture strategy integrates potent viral nanotemplates, high catalytic activity and stability of the small nanocatalysts, and robust polymerization schemes. We thus believe that our strategy can be readily extended to programmable manufacturing of a large array of multifunctional materials.

In this presentation, our recent progress on the fabrication of multifunctional membranes via interfacially initiated radical polymerization offering controlled macroporous structures for size-selective protein purification as well as catalytic remediation of toxic compounds will be highlighted.

Bubbles are commonly found in the world around us, from industrial processes to carbonated beverages. In this talk I will present two studies about bubbles, from an applied use to a fundamental phenomenon. First, I will discuss the use of bubbles to prevent the growth of marine biofouling. Bubbles rising along a submerged surface have been shown to inhibit biofouling growth, but little work has been done to determine the primary mechanisms responsible for their antifouling behavior. Here I will present a combination of field and laboratory experiments used to gain insight into the mechanisms at play, thus laying a foundation for optimization of this antifouling technique.

For the second half of the talk, I will discuss the fundamental stability of bubbles in volatile liquids. When a bubble arrives at a free surface, we typically expect the film of the bubble cap to thin over some period of time until it ruptures. Traditionally, the drainage of this film has been considered inevitable with evaporation only hastening the film rupture. Here I will present air bubbles at the free surface of liquids which appear to defy traditional drainage rules and can avoid rupture, persisting for hours until dissolution. Using pure, volatile liquids free of any surfactants, we highlight and model a thermocapillary phenomenon in which liquid surrounding the bubble is continuously drawn into the bubble cap, effectively overpowering the drainage effects.

The symposium will feature interactive discussion sessions with faculty and is catered toward early-career researchers.

Hagfish slime is a unique predator defense material containing a network of long fibrous threads each 10-15 cm in length. Hagfish releases the threads in a condensed coiled state known as skeins (~100 μm), which must unravel within a fraction of a second and form a soft hydrogel to thwart a predator attack. The mechanisms of how the hagfish controls the unraveling rates, and the properties of the resulting gel are not well understood. I will present results from our recent experiments and modeling that will address these questions. First, the hypothesis that the viscous hydrodynamics may be responsible for the rapid unravelling rates is considered, and the scenario of a single skein unspooling as the fiber peels away due to viscous drag is discussed. As a result, I will show that under reasonable physiological conditions viscous-drag-induced unravelling can occur within a few hundred milliseconds, comparable with the physiological time scales. Subsequently, I will show that key rheological and structural features of hagfish slime are insensitive to its concentration, in spite of the uncontrolled gelation process, and this peculiar characteristic may be vital for its physiological use.

Asatekin lab focuses on new polymeric materials designed to self-assemble to impart improved and/or new functionality to separation membranes by controlling nano-scale morphology and surface functionality. Our work aims to develop new membranes for generating fresh water, treating wastewater, and process separations. We focus on preventing membrane fouling and controlling membrane selectivity while maintaining high flux and simple, scalable manufacturing methods.

In one research direction, we aim to understand how zwitterion-containing copolymers self-assemble, and utilize their behavior to develop membranes with improved capabilities. Zwitterions, functional groups with equal numbers of positive and negative charges, strongly resist fouling, defined as performance loss due to the adsorption and adhesion of feed components onto the membrane. They also easily self-assemble due to strong intermolecular interactions. We have developed high flux, fouling resistant, size-selective membranes utilizing the self-assembly of random copolymers of zwitterionic and hydrophobic monomers. The effective membrane pore size or ~1 nm closely matches the size of self-assembled zwitterionic nanodomains. These membranes are exceptionally fouling resistant, showing little to no flux decline during the filtration of a wide range foulants and complete flux recovery with a water rinse.

We also aim to develop membranes that can separate small molecules of similar size based on their chemical properties. For this purpose, we prepared membranes by depositing micelles formed by random copolymers of a highly hydrophobic fluorinated monomer with methacrylic acid on a porous support. The gaps between the micelles act as 1-5 nm nanochannels functionalized with carboxylic acid groups. These membranes show charge-based selectivity between organic molecules. Furthermore, the carboxyl groups can be functionalized to alter the selectivity of the membrane. We used this method to prepare membranes that exhibit aromaticity-based selectivity. We believe these approaches will eventually lead to novel membranes that are capable of new separations and can replace more energy intensive methods such as distillation or extraction.

taste of science is an annual festival in cities across the United States. This year, the physics event focuses on soft and active matter. Aditi Chakrabarti (postdoc in Mahadevan's group) and John Berezney (postdoc from Brandeis on active nematics) will be speaking at Sissy K's.

Cells must move through tissues in many important biological processes, including embryonic development, cancer metastasis, and wound healing. Often these tissues are dense and a cell's motion is strongly constrained by its neighbors, leading to glassy dynamics. Although there is a density-driven glass transition in particle-based models for active matter, these cannot explain liquid-to-solid transitions in confluent tissues, where there are no gaps between cells and the packing fraction remains fixed and equal to unity. I will demonstrate the existence of a new type of rigidity transition that occurs in confluent tissue monolayers at constant density. The onset of rigidity is governed by a model parameter that encodes single-cell properties such as cell-cell adhesion and cortical tension. I will also introduce a new model that simultaneously captures polarized cell motility and multicellular interactions in a confluent tissue and identify a glassy transition line that originates at the critical point of the rigidity transition. This work suggests an experimentally accessible structural order parameter that specifies the entire transition surface separating fluid tissues and solid tissues. Finally, I will discuss recent work using a culture of human lung epithelial tissue to compare a newly discovered mode of fluidization of jammed cells—the unjamming transition (UJT)—with the canonical epithelial-to-mesenchymal transition (EMT).

The extracellular matrix (ECM) is a complex assembly of structural proteins that provides physical support and biochemical signaling to cells in tissues. Over the last two decades, studies have revealed the important role that ECM elasticity plays in regulating a variety of biological processes in cells, including stem cell differentiation and cancer progression. However, tissues and ECM are often viscoelastic, displaying stress relaxation over time in response to a deformation, and can exhibit mechanical plasticity. My group has been focused on elucidating the impact of ECM viscoelasticity and plasticity on cells. Our approach involves the use engineered biomaterials for 3D culture, in which the mechanical properties can be independently modulated. In this talk, I will discuss our recent findings on how cells sense ECM viscoelasticity through stretch activated ion channels, how cancer cells generate extracellular force in order to divide in confining ECMs, and on how matrix mechanical plasticity regulates cancer cell migration.

Liquid crystalline materials are pervasive, enabling devices in our homes, purses, and pockets.

It has been long-known that liquid crystallinity in polymers enables exceptional characteristics in high performance applications such as transparent armor or bulletproof vests.

This talk will generally focus on a specific class of liquid crystalline polymeric materials: liquid crystalline elastomers. These materials were predicted by de Gennes to have exceptional promise as artificial muscles, owing to the unique assimilation of anisotropy and elasticity.

Subsequent experimental studies have confirmed the salient features of these materials, with respect to other forms of stimuli-responsive soft matter, are large stroke actuation up to 400% as well "soft elasticity" (stretch at minimal stress).

This presentation will survey our efforts in directing the self-assembly of these materials to realize distinctive functional behavior with implications to soft robotics, flexible electronics, and biology. Most notably, enabled by the chemistries and processing methods developed in my laboratories, we have prepared liquid crystal elastomers with distinctive actuation and mechanical properties realizing nearly 20 J/kg work capacities in homogenous material compositions.

Local control of orientation dictates nonuniformity in the elastic properties, which we recently have shown could be a powerful means of ruggedizing flexible electronic devices. Facile preparation of optical films, prepared with the cholesteric phase, capable of concurrent shape and color change will also be discussed.

In addition, there will be an Industrial Forum Luncheon hosted by Sigma-Aldrich in Pierce 209 at noon, sandwiches and refreshments from Flour Bakery will be provided. This lecture will be a wonderful opportunity to hear more about industrial career opportunities.

Representatives from Sigma-Aldrich will include:

Beth Rosenberg is the Manager of Research Technology Specialist (RTS)-North America. She leads a team of Chemistry, Materials Science and Life Science RTS whose mission is deepen scientific relationship.

Na Li is the Global Product Manager for Electronic Materials at MilliporeSigma.

Tyler Gravelle is the Harvard MilliporeSigma representative and his been working with campus researchers for the past two years.

Until very recently, most cancer biologists operated with the assumption that the most common route to metastasis involved cells of the primary tumor transforming to a motile single-cell phenotype via complete EMT (the epithelial-mesenchymal transition). This change allowed them to migrate individually to distant organs, eventually leading to clonal growths in other locations. But, a new more nuanced picture has been emerging, based on advanced measurements, and on computational systems biology approaches. It has now been realized that cells can readily adopt states with hybrid properties, use these properties to move collectively and cooperatively, and reach distant niches as highly metastatic clusters.

This talk will focus on the accumulating evidence for this revised perspective, the role of biological physics theory in instigating this whole line of investigation, and on open questions currently under investigation.

Abstract: Smart fluids - or a fluid with suspended particles in which an applied field produces a substantial change in properties – are a fascinating class of soft materials that are widely used in applications ranging from automotive suspension to high performance speakers. However, predicting their performance from first principles remains challenging. Our underlying hypothesis is that a deeper understanding can be gained by studying highly controlled model particles and connecting these to emergent behavior of the system. Here, we explore two model systems, (1) the electric-field directed assembly of nanoparticles and (2) the mechanical properties of particle-laden interfaces. In the first section, we introduce a novel method to measure the polarizability of nanoparticles based upon modest trapping fields and fluorescence microscopy. By using this assay, we determine that nanoparticles can exhibit polarizabilities that are >30 times larger than would be expected based upon simple models, corroborating early theoretical work that includes contributions from space charge in the Debye layer. In the second section, we discuss the mechanical properties of liquid marbles, or drops of liquid coated with a layer of hydrophobic particles that render the entire structure non-wetting. By developing a novel method to study the deformation of liquid marbles, we find that while the elastic mechanics of liquid marbles are invariant of the particle coating, the failure properties of marbles depend strongly on the particle identity, suggesting that more weakly interacting particles constitute tougher marbles. These observations are further explored using a series of macroscopic experiments studying the break-up of particle rafts based on a funnel method. While there remain open questions about the behavior of smart fluids, these lessons illustrate that important insights can be gained by combining novel multi-scale characterization strategies and well-characterized particles.

Abstract: In many biological phenomena, cells migrate through confining environments. To study such confined migration, we place migrating cells in two-state micropatterns, in which the cells stochastically migrate back and forth between two square adhesion sites connected by a thin bridge. We adopt a data-driven approach where we learn an equation of motion directly from the experimentally determined short time-scale dynamics, decomposing the migration into deterministic and stochastic contributions. This equation captures the dynamics of the confined cell and accurately predicts the transition rates between the sites. We thereby derive the emergent non-linear dynamics that governs the migration directly from experimental data. In particular, we find that the deterministic dynamics is poised near a bifurcation between a limit cycle and bistable behaviour. As a result, we find that cells are deterministically driven into the thin constriction; a process that is sped up by noise. Our approach yields a conceptual framework that may be extended to describe cell migration in more complex confining environments.

Abstract: Like mixtures of oil and water, artificial lipid membranes undergo liquid-liquid phase separation. Unraveling the physical mechanisms behind the organization of these liquid phases in membranes is a central goal in biophysics, while the ability to reproduce them in synthetic structures holds great potential for applications in self-assembly and drug delivery. Previous studies on vesicles and supported lipid bilayers have unveiled a fundamental interplay between the membrane geometry and position of different lipid domains. However, the detailed mechanisms behind this coupling remain incompletely understood, because of the difficulty of independently controlling the membrane geometry and composition. In this talk, I will show how we overcome this limitation by fabricating multicomponent lipid bilayers supported by colloidal scaffolds of prescribed shape. Thanks to a combination of experiments and theoretical modeling, we demonstrate that the substrate local curvature and the global chemical composition of the bilayer determine both the spatial arrangement and the amount of mixing of the lipid domains.

Abstract: Packing problems occur in numerous applications, such as granular media under confinement, Pickering emulsions, viral structure, etc. When identical particles are packed occurs on a curved geometry, crystalline order is necessarily disrupted by the curvature, necessitating introduction of defects: a paradigmatic example is the “scars” or grain boundaries found in the packing of spherical particles on a spherical shell. Conversely, for very polydispersed or heterogeneous systems, packings are amorphous. We connect these two regimes by investigating packings as a parameter of particle shape, finding that the spherical crystallography and amorphous regimes are linked by growth and percolation of the scar network. Prospects for experimental realizations of such systems will also be presented.

Abstract: A mixture of particles suspended in a fluid is ubiquitous around us, in nature and applications. When a suspension is sheared, it exhibits a complex, rich and varied behavior such as shear thinning, shear thickening, shrinking or dilating, and so on. Despite extensive available data on mixture reactions to shearing, determination of macroscopic properties of a suspension has remained a challenge, as such mixture properties are dependent on particle systems and sizes, shearing magnitude, experimental duration and experimental methods, even when hydro-clustering is avoided, and the suspension is composed of uniform rotationally-neutral monodispersed spherical particles. At the same time, microstructural ordered states of sheared suspensions have been detected and reported for numerous spherical mixtures. In this presentation, relationships between dynamic and effective ordered states and two components of stress tensor will be explored, the first linear viscosity coefficient and the second normal stress. A proper relationship between these two components will also be developed.

We consider solid spherical particles that are interacting both locally and globally, and form a single effective ordered or unordered structure in the mixture when viscosity is the dominant dissipation mechanism. It is demonstrated that this assumption is sufficient to predicting stress components of the mixture response to an applied shear stress under different conditions, for both colloidal and non-colloidal suspensions, up to but not including the jamming limit. We’ll show that our analysis is consistent with the light-scattering observations of Ackerson group in the past decades. For non-colloidal mixtures, both viscosity and the 2nd-particle pressure data confirm the first Ackerson transition at particle volume fractions 45-50%. For colloidal suspensions, the observed non-Newtonian effects such as shear thinning at higher volume fractions or at higher shear-rates coincide with emergence of the second Ackerson transition to a higher-order crystalline state by the shear flow.

Through the REU program, we provide a coordinated, educational and dynamic research community to inspire and encourage young scientists to continue on to graduate school. We emphasize professional development workshops and seminars for a career in science and engineering. Weekly faculty seminars highlight research and community activities are integrated into the program. Undergraduate students research in world-class laboratories, and focusing on an in-depth research project while exploring multidisciplinary research topics to hone your science communication skills. We are seeking undergraduates from chemistry, physics, biology, computer science, mathematics (applied and pure), statistics, and engineering. Students without prior research experience, including freshman and sophomore students, are especially encouraged to apply.