  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 become 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 Darcy's 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.
