Motion of chemically active micro-objects in confined geometries

• Applicants: Professor Dr. Samuel Sanchez Ordonez; Dr. Mykola Tasinkevych
• Subject Area: Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
• Term: from 2013 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 230022700

Micrometer sized objects can move by various mechanisms, such as self-diffusiophoresis or bubble-induced pumping, when parts of their surface catalyze a chemical reaction in a surrounding liquid. In future, such chemically active micro-objects (we will call them chemical "self-propellers") may serve as autonomous carriers working within microfluidic devices. The limited understanding of their dynamics near walls or within micro-channels hinders, however, the development of applications in, e.g., micro-mechanics or targeted drug-delivery. In the present project we propose a systematic study of carefully thought-out model systems. This study synergistically combines theoretical, numerical, and experimental approaches towards the aim of achieving a clear understanding of the influence of boundaries on the motion of chemical self-propellers. The specific objectives of this project are summarized as follows: i) to elucidate the effects of confinement on the motion of various types of chemical self-propellers; ii) to apply the knowledge gained in i) towards developing novel methods to control such active motion, for example via patterned smart surfaces when the motion stems from self-diffusiophoresis, as well as to optimize cargo-transport by self-propellers. Two paradigmatic cases of chemical self-propellers will be studied: spherical Janus colloids covered by a catalyst over cap-like regions, and cylindrical tubes with a small gradient in the inner radius, and covered by a catalyst on the inner side. Our choice is motivated by the fact that these cases are at the extremities of the spectrum of chemical self-propellers, the former employing self-diffusiophoresis and the latter, bubble-induced pumping as the propulsion mechanisms, respectively. We will consider the following confining geometries which are relevant for motion within microfluidic chips: a single chemically homogeneous or patterned wall; two parallel walls or two walls forming a wedge; rectangular or cylindrical channels. The relation between the self-propulsion and the boundaries will be characterized through the dependence of the linear and the angular velocities of the chemical self-propeller on the various parameters, such as the distance from walls or the specific boundary conditions for the hydrodynamic flow and reaction product(s) concentration fields. By taking advantage of the fact that in general the motion takes place near the bottom or top wall we will attempt to achieve persistent directionality of the motion, e.g., by using chemical stripes on the bounding walls.

• DFG Programme: Research Grants
• International Connection: Australia
• Participating Person: Dr. Mihail Popescu