2023 AIChE Annual Meeting

Effects of Geometry on the Locomotion of Active Particles Driven By Self-Diffusiophoresis and Induced-Charge Electrophoresis

The discovery of self-propelled particles has spurred interest in creating synthetic microswimmers that mimic biological entities for autonomous propulsion with medical and environmental applications. Phoretic phenomena demonstrate how colloidal particles can be driven by external fields generated by a variety of gradients such as concentration (diffusiophoresis), temperature (thermophoresis), or electrostatic potentials (electrophoresis). This work examines the geometric effects of patch shape on the motion of patchy spherical colloids for two mechanisms: self-diffusiophoresis and induced-charge electrophoresis (ICEP).

Our work on self-diffusiophoretic particles focuses on achieving tunable propulsion by modifying phoretic interaction between the catalytic particle surface and the solute gradient. We determine the resultant solute concentration by numerically decomposing the patch effects into a combination of spherical harmonic modes through an optimization scheme. To calculate the propulsion velocity, we obtain a continuum-scale slip velocity from the tangential gradient of the surface solute concentration and employ the Lorentz reciprocal theorem. We validate our methodology using well-known results for model problems, such as: (i) a uniformly active particle (no motion); (ii) a Janus particle (translates). Our results demonstrate how catalytic patch and the phoretic interaction can be fine-tuned to control both translational and rotational motion.

Additionally, we have examined how patch orientation affects the motion of particles when they are propelled via ICEP. To theoretically describe this range of ICEP propulsion, we first solve for the electric potential in the fluid bulk arising out of an imposed electric field and a given surface potential profile. From a given polarizability asymmetry, we can obtain an expression for the induced electrophoretic slip via the Einstein-Smoluchowski equation. Finally, using the obtained slip velocity, we derive expressions for the translation and rotation of the particle. Through this theoretical analysis, we found qualitative agreement between the predicted and experimentally observed cluster trajectories[1].

Finally, we have begun investigating geometric effects of patch shape for particles driven via ICEP. By fabricating arbitrary patches on spherical particles using a two-photon lithography technique, our preliminary results indicate that the particle trajectory depends on patch geometry and can result in linear, helical and trochoidal trajectories. In the future, we plan to conduct further computations and experiments to fully describe the autonomous motion of these particles.

Relevant Publications:

  1. Lee, Jin Gyun, et al. “Magnetically locked Janus particle clusters with orientation-dependent motion in AC electric fields.” Nanoscale, 2023, p. 9.