Collapse and cavitation during the drying of water-saturated PDMS sponges with closed porosity.

In this paper, we study the drying of water-saturated porous polydimethylsiloxane (PDMS) elastomers with closed porosity in which the evaporation of water is possible only via the diffusion across PDMS. Starting from water/PDMS emulsions, we fabricate soft macroporous samples with different pore diameter distributions and average diameters ranging from 10 to 300 μm. In these materials, the drying may lead to either a collapsed state with low porosity or the cavitation and reopening of a fraction of the pores. Using optical microscopy and porosity measurements, we showed the influence of the pore diameters and interactions on the result of drying. At pore diameters lower than 30 μm, the majority of pores remain collapsed. We attribute the permanence of the collapse of most small pores to a low probability of cavitation and to the adhesion of the pore walls. Pores with diameters larger than 100 μm reopen via cavitation of the water they contain. The behavior of pores with diameters ranging from 30 to 100 μm depends on the porosity and drying temperature. We also visualize collective cavitation upon the drying of sponges initially saturated with sodium chloride solution. In this case, the cavitation in the largest pores leads to the reopening of small pores in a neighboring zone of the sample. To our knowledge, our results present the first experimental proof of the pore-size-dependent and cooperative nature of the response of soft sponges with closed porosity to drying.