Topological Guidance of Self-Propelled Janus Locomotors Along Solid Boundaries

Cuny City College, New York NY

Investigators

Abstract

This research studies self-propelling microbots. These are ultra-miniaturized locomoting vehicles which are microns in size - the dimensions of a red blood cell or a grain of pollen. Microbots travel independently in a liquid environment such as water by consuming fuel present in the environment, and converting the chemical energy of the fuel into mechanical motion. Due to their small size and their ability to motor around by themselves as active matter, self-propelled microbots are at the center of innovative applications on the technological horizon. Their medical applications are the most promising: Microbots are envisioned to be able to dock with sperm and assist their movement towards an egg for fertilization, navigate blood vessels to unclog arteries, and ferry drugs to targeted sites in the human body such as tumors or diseased organs. The ability to steer microbots along prescribed pathways is essential. While on board guidance systems which respond to external instructions can be used for navigation, this research focuses on passive guidance in which the microbot responds to cues in its environment to guide itself. In particular, this research studies how a microbot can passively follow along a planar wall at a fixed height above the wall, or steer around a post. The passive guidance originates from how the boundary changes the conversion of the chemical energy into mechanical motion, and how the microbot interacts fluid dynamically with the boundary. Educationally, the grant will fund two graduate students, one on theory and one on experiments; their synergistic effort will broaden each of their academic horizons. An outreach effort to two local high schools, one in New York and one in New Jersey, is in place, and this research will be integrated into the research curriculum experience of these high schools. This research studies a model locomotor consisting of a spherical particle in which one face of the particle is coated with a platinum catalyst to form a two-faced Janus particle. The platinum reacts with hydrogen peroxide, as a fuel, in the environment to produce oxygen and water. The reaction of the peroxide on the active face creates gradients in the ionic disassociation products of the peroxide across the particle which in turn generates electrokinetic propulsive forces. While this mechanism explains the locomotion of these Janus engines in free space, this study examines the electrokinetic propulsion in the vicinity of boundaries and aims to determine theoretically and experimentally if boundary guidance is possible with electrokinetic propulsion. Two examples will be investigated, the ability of Janus locomotors to be guided along a wall, and their ability to circumnavigate a post attached (perpendicular) to a wall. Theoretically, the electrohydrodynamic equations will be solved to obtain deterministic trajectories, and Brownian dynamics simulations will be undertaken to understand the stability of guided states to thermal fluctuations. Experiments will also be undertaken on guidance on these two surface topologies. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.