Like charges stabilize emulsions, whereas opposite charges
break emulsions. This is the fundamental principle for many industrial and
practical processes. Using micrometer-sized pH-sensitive polymeric hydrogel
particles as emulsion stabilizers, we prepare emulsions that consist of
oppositely charged droplets, which do not coalesce. We observe noncoalescence
of oppositely charged droplets in bulk emulsification as well as in
microfluidic devices, where oppositely charged droplets are forced to collide
within channel junctions. The results demonstrate that electrostatic
interactions between droplets do not determine their stability and reveal the
unique pH-dependent properties of emulsions stabilized by soft microgel
particles. The noncoalescence can be switched to coalescence by neutralizing
the microgels, and the emulsion can be broken on demand. This unusual feature
of the microgel-stabilized emulsions offers fascinating opportunities for
future applications of these systems.
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