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Bioinspired shape shifting of liquid-infused ribbed sheets
At small scales, capillary forces can deform flexible structures. The aggregation of wet hair into bundles is a daily example. With the miniaturization of technologies, these capillary forces have become important in engineering since they can lead to the catastrophic collapse of structures obtained by lithography but can also appear as an ingenious self-assembly solution at scales where conventional techniques fail. Until now, studies have mainly focused on elastocapillary deformation of either thin free membranes in contact with a drop (Bico et al) or networks of slender structures fixed on a rigid substrate. Here, we are interested in the case of flexible textured sheets consisting in an array of grooves and ribs attached to a thin membrane.
When these initially wet sheets dry out, we observe three scenarios depending on their geometry and their elasticity : absence of deformation, curling of the membrane or aggregation of textures. We develop an analytical model combining sheet geometry, material stiffness, and capillary forces to rationalize the onset of such deformations and predict the transition between the different observed regimes. We further show that, in the curling regime, the final local curvature depends uniquely on the geometry of the textures. This allows us to develop a simple geometric inverse model and thus to inverse program target shapes requiring fine control over the local curvature. We finally demonstrate the potential irreversibility of the transformation by UV-curing a photosensitive evaporating solution. This programming technique, therefore, appears to be an efficient method for the fabrication of small-scale three-dimensional structures.
When the same structured sheets are partially immersed in a liquid bath, the liquid rises along the grooves of the structure due to capillarity. Here again, the three aforementioned scenarios can occur depending on the texture geometry. If no deformation occurs, the liquid rises in the open channels until it reaches an equilibrium height. If capillary forces are strong enough to deform the ribs, they collapse and the equilibrium liquid height increases due to the reduction of the channel section. Finally, if curling occurs, two concomitant dynamics are at play : the thin membranes between adjacent ribs deforms until the ribs get into contact leading (i) to a reduction of the channel section and an increase of the capillary rise and (ii) to the deformation of the initially flat sheet to a cylinder in which a second capillary rise occurs, increasing significantly the liquid capture.