Pt-SiO₂ Janus Particles and the Water/Oil Interface: A Competition between Motility and Thermodynamics

Abstract

Various aspects of the behavior of Janus particles near liquid/liquid interfaces have been studied through different experimental and theoretical realizations, but the effect of motility on the behavior of Janus particles near liquid/liquid interfaces has not been investigated, yet. Here, we demonstrate the ability to engineer the behavior of highly interfacial active Janus particles near a water/oil interface by introducing motility to the system. Passive, i.e., nonmotile, platinum-capped 8 μm silica (Pt-SiO 2 ) Janus particles exhibit a strong tendency to attach to water/oil interfaces with the Pt-cap facing the oil and the SiO 2 side facing the water phase. In contrast, we show that active, i.e., motile, 8 μm Pt-SiO 2 Janus particles approach the interface, orient in a sideways fashion with the Janus boundary perpendicular to the interface, and then swim in the vicinity of the interface similar to observations reported near solid/liquid interfaces. Active Pt-SiO 2 Janus particles near the water/oil interface show motility as a result of adding H 2 O 2 to the particle solution. The decomposition of H 2 O 2 into O 2 and H 2 O creates a nonuniform gradient of O 2 around the particle that hydrodynamically interacts with the water/decalin boundary. The interaction enables rotation of the particle within the swimming plane that is parallel to the interface but restricts rotation in and out of the swimming plane, thereby preventing adsorption to the liquid/liquid interface.

Study of Active Janus Particles in the Presence of an Engineered Oil–Water Interface

“…Immediately after merging the droplets, the particle activity can be observed by a change in orientation of the colloids and swimming behavior sets on at a speed of 4−5 μm/s, which correlates well with normal Pt@SiO 2 behavior. Just as we have observed on solid interfaces and has been found for Pt@SiO 2 by the Kretzschmar group 21 the orientation of the directional vector of the particles is of about 0°to the bottom flat interface (Figure 2c). 9 The pinned oil droplets that we refer to as dimples (indicated by red lines in Figure 3c−e), at first seemed to be unfortunate.…”

Investigating Time‐Dependent Active Motion of Janus Micromotors using Dynamic Light Scattering

“…These values of k 1 and k 2 are in reasonable agreement with prior reports for the decomposition of H 2 O 2 on Pt coated JMs. [ 34,42,43,57 ]…”

Phoretic colloids close to and trapped at fluid interfaces

“…Such predictions can, possibly, be verified using setups similar to that discussed in Refs. [80,81].…”