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Second Thermal and Fluids Engineering  Conference

ISSN: 2379-1748
ISBN: 978-1-56700-430-4

Capillary rise and swelling in cellulose sponges




Abstract

A cellulose sponge, which can absorb and hold a significant amount of liquid, is a mundane porous material that exhibits hygroscopic swelling. Materials capable of hygroscopic swelling have practical importance in many fields such as biomimetic engineering, soft robotics, and medicine. When a sponge is dipped in water, water permeates through the sponge, driven by capillary pressure, and leads to swelling exhibiting hygroexpansive characters. Here we show that the rate of water rise against resistance caused by gravity and viscous forces deviates from Washburn's rule, where the rise height increases like t^(1/2) with t being time, beyond a certain threshold height. We found that numerous micropores coalesce to form large pores when cellulose fibers absorb water, significantly decreasing the rise speed. Furthermore, we rationalize the novel power law of the rise height versus time by combining Darcy's law with the hygroscopic swelling equation and predict the threshold height. We anticipate our work to shed light on the physics of capillary flow in deforming porous media, which provides compelling implications on our daily lives including hygiene items and foods.

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