  Microcapsules allow for the controlled containment, transport, and release of
cargoes ranging from pharmaceuticals to fragrances. Given the interest from a
variety of industries in microcapsules and other core-shell structures, a
multitude of fabrication strategies exist. Here, we report on a method relying
on a mixture of temperature-responsive microgel particles,
poly(N-isopropylacrylamide) (pNIPAM), and a polymer which undergo fluid-fluid
phase separation. At room temperature this mixture separates into colloid-rich
(liquid) and colloid-poor (gas) fluids. By heating the sample above a critical
temperature where the microgel particles shrink dramatically and develop a more
deeply attractive interparticle potential, the droplets of the colloid-rich
phase become gel-like. As the temperature is lowered back to room temperature,
these droplets of gelled colloidal particles reliquefy and phase separation
within the droplet occurs. This phase separation leads to colloid-poor droplets
within the colloid-rich droplets surrounded by a continuous colloid-poor phase.
The gas/liquid/gas all-aqueous double emulsion lasts only a few minutes before
a majority of the inner droplets escape. However, the colloid-rich shell of the
core-shell droplets can solidify with the addition of salt. That this method
creates a core-shell structures with a shell composed of stimuli-sensitive
microgel colloidal particles using only aqueous components makes it attractive
for encapsulating biological materials and making capsules that respond to
changes in, for example, temperature, salt concentration, or pH.

Soft Condensed Matter