Characteristics of Oil-in-Oil Emulsions under AC Electric Fields

Emulsions have been applied in a
number of industries such as pharmaceutics,
cosmetics, and food, which are also of great scientific interest.
Although aqueous emulsions are commonly used in our daily life, oil-in-oil
(o/o) emulsions also play an irreplaceable role in view of their unique
physics and complementary applications. In this paper, we investigate
typical behaviors of organic droplets surrounded by organic medium
(o/o emulsions) with different functional groups controlled by the
AC electric field. Droplet behaviors can be catalogued into five types:
namely, “no effect”, “movement”, “deformation”,
“interface rupture”, and “disorder”. We
identify the key dimensionless number Wee·Ca, combined with the channel
geometry, for characterizing the typical behaviors in silicon oil/1,6-hexanediol
diacrylate and mineral oil/1,6-hexanediol diacrylate emulsions. Unlike
aqueous emulsion, the Maxwell–Wagner relaxation inhibits the
electric effect and leads to an effective frequency, ranging from
0.5 to 3 kHz. The increasing viscosity of the droplet facilitates
the escalation by promoting the shearing effect under the same flow
conditions. Ethylene glycol droplets primarily show the efficient
coalescence even at a low Wee·Ca, which is attributed to the attraction
of free charges induced by the increasing conductivity. In 1,6-hexanediol
diacrylate/silicon oil emulsion, the droplet tends to form a liquid
film that expands into the entire channel due to the affinity of the
droplet to the channel wall. A variety of elongated columns are observed
to oscillate between the electrodes at high voltages. These findings
can contribute to understanding the electrohydrodynamic physics in
o/o emulsion and controlling droplet behaviors in a fast response,
programmable, and high-throughput way. We expect that this droplet
manipulation technology can be widely adopted in a broad range of
chemical synthesis and biological and material science.