Elastohydrodynamics of hygroexpansive porous media: Swelling and coiling

Abstract

We study fundamental mechanism and mechanics of hygroscopic deformation of soft porous materials. We first explore wicking in hygroscopic swelling porous media, in particular a cellulose sponge with heterogeneous porosity. We theoretically and experimentally investigate the capillary imbibition of various aqueous solutions in the sponge that isotropically swells at the same time. We find that the rate of liquid rise against gravity deviates from the classical Lucas-Washburn rule beyond a certain threshold height. The observation using the environmental scanning electron microscopy (ESEM) reveals the coalescence of microscale wall pores with wetting, which allows us to develop a mathematical model for pore size evolution and the consequent wicking dynamics. We rationalize the power law of the rise height versus time by combining Darcy's law with a hygroscopic swelling equation. The scaling law constructed through this work agrees well with the experimental results, which can build a theoretical framework to understand the physics of water absorption in hygroscopic responsive soft materials. <br/> Secondly, we study a coiling movement of hygroexpansive an-isotropic materials. As a botanical inspiration, we consider a \textit{Pelargonium} seed awn that can generate helical motions in response to humidity change. The mechanism of its helical morphing is due to the anisotropic expansion in the multilayer structures. To predict the entire helix shape of the awn, we develop and corroborate a theoretical model. We further fabricate hygroresponsive helical actuators by electrospinning to mimic the botanical motion. We anticipate our work will shed light on kinematics of helical motion of the laminated plates and provide an essential design parameter of tunable helical motion actuator.