Interfacially-Adsorbed Particles Enhance the Self-Propulsion of Oil Droplets in Aqueous Surfactant

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.