(57d) Motion of Self-Propelled Janus Particles Inside Double Emulsion Drops

There is growing interest in the self-propelled motion of particles with two distinct hemispheres, also known as Janus particles. Previous work has shown that motion in Janus particles can be accomplished by having a catalytic material on one side of the particle and leaving the other half-intact. If a chemical reaction occurs at one hemisphere of the object, it will generate a gradient in concentration causing the particle to move by a process called self-diffusiophoresis. Recent experiments performed on an aqueous system composed of polystyrene microspheres half-covered with platinum, and using hydrogen peroxide as a reactant, showed that the particle velocity depends strongly on reactant concentration. It has been theorized that directed particle motion could be achieved in the presence of a concentration gradient and that confinement within a small volume could lead to the assembly of particles at an interface. The present study aims to corroborate these theories by characterizing the motion of polystyrene particles half-coated with platinum inside double emulsion drops in the presence of a hydrogen peroxide concentration gradient. A microcapillary microfluidic device was used to generate water/oil/water double emulsion drops composed of a polydimethilsiloxane (PDMS) oil drops containing a diluted aqueous suspension of Janus particles in the internal droplet. The PDMS oil drop acts as a semipermeable membrane that regulates the concentration of hydrogen peroxide inside the inner droplet. A baseline test was performed to track the motion of free-flowing Janus particles in the presence of different concentrations of hydrogen peroxide. In a second experiment, double emulsion drops containing the Janus particles in the inner droplet were placed in different solutions of hydrogen peroxide and the particle motion was tracked as a function of time. Particle velocities showed a strong dependence with hydrogen peroxide concentration both outside and inside the double emulsion drops. The results in the form of velocities as function of concentration showed indeed that this is correct and it is expected that having a gradient of concentration within the double emulsion will generate autonomous motion of the Janus Particle.