We study the deformations of pH-responsive spherical microcapsules -- micrometer-scale liquid drops surrounded by thin, solid shells -- under the influence of electrostatic forces. When exposed to a large concentration of NaOH, the microcapsules beco
me highly charged, and expand isotropically. We find that the extent of this expansion can be understood by coupling electrostatics with shell theory; moreover, the expansion dynamics is well described by Darcys law for fluid flow through the microcapsule shell. Unexpectedly, however, below a threshold NaOH concentration, the microcapsules begin to disintegrate, and eventually rupture; they then expand non-uniformly, ultimately forming large, jellyfish-like structures. Our results highlight the fascinating range of behaviors exhibited by pH-responsive microcapsules, driven by the interplay between electrostatic and mechanical forces.
Colloidal capsules can sustain an external osmotic pressure; however, for a sufficiently large pressure, they will ultimately buckle. This process can be strongly influenced by structural inhomogeneities in the capsule shells. We explore how the time
delay before the onset of buckling decreases as the shells are made more inhomogeneous; this behavior can be quantitatively understood by coupling shell theory with Darcys law. In addition, we show that the shell inhomogeneity can dramatically change the folding pathway taken by a capsule after it buckles.
