Surface Tension Driven Motion

Plants and fungi have evolved ingenious and reliable ways to store and release energy using mechanisms driven by surface tension. At millimeter scales, these forces are considerable and can serve as an effective source of energy for driving motion. Leptosporangiate ferns produce their spores in spherical capsules lined on one side by a long row of specialized cells, the annulus (see picture above). These cells are filled with water but soon after spore maturation they become exposed to the ambient air. This initiates a process of evaporation forcing the cells to contract as their water content decreases. The contraction forces the capsule to open thus exposing the spores. The tension in the annulus causes the cells to cavitate, leading to a fast closure of the capsule and release of the spores. Dumais has determined the critical tension for cavitation. He is using this knowledge to design microdevices, which, like the capsule of ferns, can use water to do work!

Jacques Dumais
Harvard MRSEC (DMR-0820484)