Cracks invade a drying film of silica nanoparticles (left). Drying stresses, up to 1000 times atmospheric pressures, can fracture this fluid-solid composite material (right).
Drying is a crucial step in the processing of a variety of materials, from the common coat of paint to emerging photonic band-gap structures. As the suspending fluid evaporates, fracture, delamination, and buckling can ruin the material. Profs. Weitz, Hutchinson, and Xie have been exploring drying nanoparticle suspensions, which are particularly prone to fracture. In the nanoparticle regime, drying stresses, originating from the affinity of the fluid for suspended particles, can exceed 1000 times atmospheric pressure. Furthermore, the fluid immersing nanoparticles can behave strangely. Since the gaps between tightly packed nanoparticles are only a few molecules across, fluid molecules can be arrested by strong short-range interactions with the nanoparticles. Recently MRSEC REU students have demonstrated the importance of these molecular-scale interactions on the macroscopic dynamics of drying nanoparticle suspensions (to appear in Physical Review Letters).
David A. Weitz (Physics & Applied Physics)
Harvard MRSEC (DMR-0820484)