  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 the PDMS.
Starting from water/PDMS emulsions, we fabricate soft macroporous samples with
different pore diameter distributions and average diameters ranging from 10 to
300 $\mu$m. In these materials, the drying may lead either to a collapsed state
with low porosity or to the cavitation and reopening of a fraction of the
pores. Using optical microscopy and porosity measurements, we showed the
importance of the pore diameters and interactions on the result of drying. At
pore diameters lower than 30 $\mu$m, 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 $\mu$m tend to reopen after the water they contain
cavitates. The behavior of pores with diameters ranging from 30 to 100 $\mu$m
depends on the porosity and drying temperature. We also visualize collective
cavitation upon drying of sponges initially saturated with a sodium chloride
solution. In this case, the cavitation in the largest pores leads to 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.
