Fig. 1. Swelling of a hydrogel confined in a 3D granular medium.

(A) Image of apparatus for testing hydrogel swelling in confinement. A hydrogel (orange) is embedded within a granular medium composed of glass beads (hazy transparent circles) packed within a transparent acrylic chamber with an overlying loaded piston. (B) Image of a hydrogel swollen within a medium in the absence of an applied load, showing that it swells freely and retains its spherical shape. (C) Image of a hydrogel swollen within a medium under a strong applied load, corresponding to a confining stress of 22 kPa, showing that it deforms strongly and exhibits hindered swelling. The white dotted circle shows an inscribed circle, representing the size of the region of the hydrogel in contact with surrounding beads, for which swelling is hindered. The black dotted circle shows the outline of the hydrogel under no applied load from (B). As shown by the white space in between the black dashed outline and the projection of the hydrogel, its projected area is smaller under strong applied load. Photo credit: Jean-François Louf, Princeton University.