The swelling of cellulose foams due to liquid transport

Cellulose-based foams are available commercially in a dry pre-compressed state, but can swell upon infiltration by a suitable liquid. A series of experiments reveal that pre-compressed foams can swell in one direction by an order of magnitude when infiltrated by water or glycerol; in contrast, no swelling accompanies infiltration by ethanol. The kinetics (and underlying mechanisms) of infiltration and of swelling are quantified by a series of critical experiments on both pre-compressed foam and pre-expanded foam, using water, glycerol and ethanol. Infiltration is driven by capillarity, cell wall diffusion and by opening of capillaries during swelling of the foam. Ethanol-infiltration of pre-expanded foam or of pre-compressed foam occurs by a combination of capillary action and diffusion without swelling of the foam. Water induces swelling of the pre-compressed foam on a time scale of 1 s, whereas glycerol swells the foam progressively over 10(5) s, after an initial incubation period of 10(3) s. When the foam is in the pre-expanded state, water and glycerol seep horizontally into the foam by capillary action: in contrast, water-rise and glycerol-rise in the vertical direction is initially by capillary action and then, once the Jurin height has been attained, diffusion leads to a much slower rate of seepage. Confirmation of the existence of a Jurin height for the vertical rise of water into pre-compressed foam or pre-expanded foam is obtained by X-ray computer tomography. Infiltration into pre-compressed foam by water or by glycerol involves the propagation of a swelling front, with liquid feeding first by capillary flow and then by diffusion within the cell walls. An understanding of liquid infiltration and the resultant swelling of foams is the first step in the design of actuating multi-scale lattices made from pre-compressed foam. (C) 2019 Elsevier Ltd. All rights reserved.