(5d) Active Liquid Droplet/Janus Colloid Core Composites Self-Propelled By Marangoni Forces

Synthetic colloidal motors propel themselves through a continuous liquid phase by reaction with solutes in their fluid environment. In the usual design, the colloids are engineered solid particles with a catalytic side (Janus swimmers); upon consumption of fuel, an asymmetric distribution of the reactant and product solutes is created across the motor. Phoretic motion results from the resultant asymmetric interaction of the reactants and products with the colloid. Several interactions have been examined, including van der Waals (self-diffusiophoresis) and electrostatic (self-electrophoresis) forces. Immiscible liquid droplets in a suspending liquid phase, rather than particles, can also be formulated for autonomous motion: Surfactants adsorbed onto the droplet interface react with solutes in the environment to produce products; symmetry breaking reaction instabilities create concentration gradients on the droplet surface which drive tension gradients (Marangoni tractions) that propel the droplet.
In this presentation, we study a colloidal motor design in which a Janus particle with a catalytic side is first coated with an liquid wetting film and then suspended in a continuous liquid phase immiscible with the film. Thus the motor is a composite droplet with a reactive Janus core. Solute fuel in the film reacts on the catalytic side of the Janus particle to produce a surface active product which is released into the film and adsorbs to the fluid interface in proximity to the active side of the colloid. Local adsorption creates Marangoni tensions which drive the motion of the composite droplet through the continuous phase. This design has the advantages that it is powered by Marangoni tractions that can create larger velocities than self-diffusiophoretic- or self-electrophoretic forces, and relies on the steady catalysis of the active side of the Janus colloid, rather than symmetry breaking to drive the Marangoni motion. Furthermore, the proposed motor configuration can be driven by thermocapillary forces as well. In this case, the Janus core is used to generate an asymmetric temperature field that can generate the Marangoni traction forces that propel the composite particle.

Assuming low Reynolds number flow, solutions are obtained using bispherical coordinates for the propulsion velocity of the composite droplet as a function of the ratio of the film thickness to the colloid radius, the rate of reaction at the colloid surface, and the surface activity of the product at the fluid interface. The Janus colloid at the center moves relative to the composite droplet, occupying eccentric positions along the center axis of the droplet. The eccentric movement is tracked, and optimum velocities are obtained as a function of the wetting layer thickness.