Diffiusiophoretic Self-Propulsion of a Colloid Near a Solid Boundary

Active colloids are micron and submicron size swimmers which are designed to move along an envisioned path in small scales to ascertain new applications in nanotechnology including cargo shuttling in microfluidics, directed drug delivery to targeted cells, and transporters for bottom-up materials assembly. The deriving force for their motion originates from chemo-mechanical transduction mechanisms which utilize solutes in the continuous surrounding environment of the colloid as a fuel to actuate the movement. In diffusiophoresis, gradients in the solute concentration across the colloid create an imbalance in the interactions of the solute with the particle which brings about colloid motion. Solute-colloid interactions can be emerged from van der Waals attractive forces, steric repulsion or electrostatic forces for charged colloids and solutes. These forces can also be integrated into a self-propulsion by choosing a reactant as a solute which undergoes a surface reaction only on one face of a colloid (eg. a Janus colloid) to generate the required concentration gradient for the motion. In this way reactant and product solutal gradients are created.

Most of the theoretical studies of the hydrodynamics of diffusiophoretically self-propelled reactive Janus colloids have been principally studied in the context of an infinite medium, but most applications envision motion in the vicinity of boundaries. The effect of the boundaries is not purely to retard the hydrodynamic motion, because the boundaries alter the solutal gradient. In this presentation, we consider two examples of boundary interactions: The first is the axisymmetric locomotion perpendicular to an infinite plane wall, driven by the colloid’s reacting side oriented either directly in front of, or facing away from the wall. The second is locomotion along the wall , driven by an orientation in which the reacting side points along the lateral direction of the wall. Continuum calculations are undertaken in which the repulsive interactions of the solute product with the colloid energizes the motion, and the reactive side produces a constant flux of product. Colloid velocites at Stokes flow regime are obtained using Reynolds Reciprocal Theorem (RRT) based on boundary layer approach in which the net interaction creates a slip-velocity at the surface which actuates the motion.

Our analysis indicates when the reacting surface faces the wall boundary, and the repulsive interactions thereby repel the colloid from the surface, the accumulation of product in the region between the colloid and the wall enhances the self-propulsion and the locomotion away from boundary becomes larger than the value in an infinite medium. When the reacting surface faces away from the wall, the concentration gradient is not changed significantly, and the locomotion toward the wall is slower than in an infinite medium due to the hydrodynamic interaction. When the reacting side faces along the lateral direction of the wall, the colloid locomotes along the surface, but also rotates causing the reacting side to redirect its orientation relative to the wall. This rotation creates a component of the propulsion velocity perpendicular to the wall, causing the swimmer to move either away or towards the wall.