Understanding the chemo-mechanical mechanisms that direct the motion of self-propulsive colloids is important for the development of active materials and exploration of dynamic, collective phenomena. Here, we demonstrate that the...
Stabilization
of oil–oil interfaces is important for nonaqueous
emulsions as well as for multiphase oil-in-water emulsions, with relevance
to a variety of fields ranging from emulsion polymerization to sensors
and optics. Here, we focus on examining the ability of functionalized
silica particles to stabilize interfaces between fluorinated oils
and other immiscible oils (such as hydrocarbons and silicones) in
nonaqueous emulsions and also on the particles’ ability to
affect the morphology and reconfigurability of complex, biphasic oil-in-water
emulsions. We compare the effectiveness of fluorophilic, lipophilic,
and bifunctional fluorophilic-lipophilic coated nanoparticles to stabilize
these oil–oil interfaces. Sequential bulk emulsification steps
by vortex mixing, or emulsification by microfluidics, can be used
to create complex droplets in which particles stabilize the oil–oil
interfaces and surfactants stabilize the oil–water interfaces.
We examine the influence of particles adsorbed at the internal oil–oil
interface in complex droplets to hinder the reconfiguration of these
complex emulsions upon addition of aqueous surfactants, creating “metastable”
droplets that resist changes in morphology. Such metastable droplets
can be triggered to reconfigure when heated above their upper critical
solution temperature. Thus, not only do these bifunctional silica
particles enable the stabilization of a broad array of oil-fluorocarbon
nonaqueous emulsions, but the ability to address the oil–oil
interface within complex O/O/W droplets expands the diversity of oil
chemical choices available and the accessibility of droplet morphologies
and sensitivity.
We have demonstrated that adsorption of silica nanoparticles at the
interface of a solubilizing oil droplet in surfactant solution can
significantly accelerate the droplets’ self-propulsion speed. Using fluorescent
particle visualization, we correlated the degree of particle surface coverage
on bromodecane droplets to the droplet speed in TX surfactant. Slowest speeds
were found at the lowest and highest surface coverages and the fastest speeds
were achieved at intermediate surface coverages of about 40%. The particle-assisted propulsion acceleration
was further demonstrated in nonionic, anionic, and cationic surfactants and a
range of oils with varying solubilization rates. We propose that particles at the droplet interface hinder solubilization by
displacing oil-water interfacial area, providing asymmetry in the distribution
of oil-filled micelles along the droplet surface and accelerating Marangoni
flow. We describe a fluid-mechanical model to rationalize the effect of the
particles by considering the effect of a non-symmetrical distribution of
solubilized oil at the droplet surface. Approaches by which to modulate the
distribution of solubilization across droplet interfaces may provide a facile
route to tuning active colloid speeds and dynamics.
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