(290b) The Diffusiophoretic Self-Propulsion of Patchy Particles Driven by a Catalytic Reaction On the Particle Surface

The ability to propel micron or nanosized solid particles or liquid droplets through a liquid and along prescribed trajectories is essential to many envisioned applications, particularly in biological contexts. For example: On the micron scale, ?labs on a chip? are networks of microfluidic channels designed to undertake the chemical steps ? mixing, bioconjugation and separation - necessary to complete a clinical diagnostic assay. Digital systems that use droplets or particles to implement these steps require their directed motion to arrange for the steps and their sequencing. On the nanoscale, drug-laden nanoparticles released into the cell cytoplasm, can be used to delivery the drug to targeted cellular sites by direct propulsion.

Propulsion mechanism can generally be divided into two classes. The first are phoretic mechanisms where either external fields are imposed, and the particle reacts to and follows the field, e.g. the electrical fields of electrophoresis, the concentration gradients of diffusophoresis and chemotaxis, the magnetic fields of magnetophoresis and the temperature gradients of thermophoresis. The second are self-phoretic or autonomous mechanisms where the particle itself, operating as a motor, imposes the directing field. Autonomous motions are of greater interest because the self-swimming particle is untethered to an external input of energy, and hence avoids the problems of focusing fields or arranging concentration gradients. For nanofluidic applications in which the fluids are confined to dimensions less than a micron, self-swimming is the only alternative because of the difficulty of imposing an external field along these length scales.

Experimentally, the autonomous motion generated by using a catalytic reaction on part of the surface of a particle to generate a concentration gradient across the particle and an attendent diffusophoretic propulsion has received the most attention. This motor consists of a metallic cap (platinum) deposited on a micron sized solid particle (polystyrene), and immersed in a hydrogen peroxide solution. The catalytic decomposition of the hydrogen peroxide on the metal into oxygen and water at the surface of the patch particle creates an asymmetric distribution of the peroxide, oxygen and water species on either side of the particle surface. Intermolecular interactions between the particle and these species near the surface are asymmetric as a result of the concentration distributions, and this drives a diffusophoretic motion. Theoretical studies of this propulsion mechanism have examined the coupled mass transfer and hydrodynamic motion in the limit in which the forces render a slip of the fluid on the surface of the cap.

In this presentation, we provide a more complete theoretical description of the effect of the intermolecular forces on the hydrodynamic propulsion. The analysis is undertaken in the limit in which the fluxes generated by diffusion and the intermolecular forces are larger than the convective flux (small Peclet number), the flow is inertialess (low Reynolds number), the catalytic reaction rate is infinite and concentration and velocity fields are azimuthally symmetric about the cap. Finite element calculations are used to determine the concentration fields as a function of the size of the cap, and different forms for the intermolecular interaction (short range repulsion, van der Waals attraction). The velocity field and propulsion velocity are computed analytically in the low Reynolds number limit in terms of the first moment of the concentration field, and the finite element calculations of the flow are then used to compute the propulsion velocity. This velocity is analyzed as a function of the intermolecular interactions, and the results compared with our own recent experimental measurements of this motor.