(296c) The Use of Fluorescence Imaging to Determine Orientation and Directionality In the Self-DIffusiophoretic Motion of Particles Driven by An Asymmetric Surface Catalytic Reaction

Phoretic transport is the process by which colloidal particles migrate due to a gradient of a thermodynamic variable such as electric potential, temperature or concentration which is applied across the particle. Schemes which utilize phoretic transport to allow particles to propel themselves without externally applying a gradient are of great interest as “motors” to direct particles to a targeted location. Alternatively, autonomously powered particles can be used as ferries to convey cargo attached to the particles to a delivery point. In autonomous motion, particles use a fuel present in the surrounding media to provide the energy source for self propulsion. The phoretic transport provides the transduction mechanisms to convert the energy stored in the fuel into motion. A great deal of recent interest has centered on the use of diffusiophoresis for self-propulsion as this phoretic mechanism allows a direct chemo-mechanical transduction. Particles are coated on one side with a catalyst which converts a reactant present in the liquid phase around the particle to a product. Limitation of a reaction to one side of an asymmetric particle alters the local concentration of reactant, product and solvent molecules in the fluid immediately surrounding the particle. This leads to an imbalance in the van der Waals forces exerted by these molecules on the particle which cause there to be a pressure effect which drives the particle into motion.

Experimentally, recent studies have utilized the catalytic decomposition of hydrogen peroxide to produce oxygen and water to implement a prototype for a self-diffusiophoretic propulsion scheme. These studies have incorporated a platinum catalyst onto one side of a particle, creating a Janus particle. The platinum consumes hydrogen peroxide as a fuel from solution and produces oxygen and water on only one side of the particle. The composition difference in water, hydrogen peroxide, and oxygen across the particle produces an imbalance in van der Waals forces which drives the particle in self-diffusiophoretic motion. In these recent studies, the diffusiophoretic velocity of platinum coated spheres in hydrogen peroxide solutions has been investigated in systems with various concentrations of hydrogen peroxide by video tracking the particles. While these studies have measured the velocity, the direction of the motion relative to the platinum catalyst cap is unclear because the video tracking did not distinguish the platinum half and the inert sides of the particle. In principle the direction of motion is determined by the net interaction of the fluid mixture with the particle, which is not known a priori since the Hamaker constants for this mixture are unknown. A very recent report on the use of the platinum/hydrogen peroxide system for self-diffusiophoresis (Schowalter et al., J. Phys. Chem. A , 114, 5462) takes advantage of an asymmetry in the transmitted light through the particle caused by the platinum cap to determine the particle orientation, demonstrated that the platinum coated region is opposite to the direction of motion. This indicated a net repulsive interaction between the mixture on the platinum side relative to the inert side.

In this study, measurements are made of the directionality of diffusiophoretically self-propelled particles driven by the catalytic surface conversion of hydrogen peroxide by video tracking fluorescent beads which are half surface coated with platinum. By coating half of the surface of the fluorescent bead with a platinum catalyst cap, the particle, observed through its fluorescent light, shows spatially distinct bright (uncoated) and dark (Pt coated) regions due to the cap obscuring the underlying fluorescence of the bead. This enables the particle orientation to be determined. To date, such techniques have been utilized to assess Brownian rotational motion proximal to surfaces. Confinement of the particles to chambers in a microfluidic device limits the range of motion to two dimensions and effectively allows for clear imaging of the directionality of the particles and associated rotational Brownian behavior.