Principles and Engineering of Self-Propelling Particles

We report here the development and characterization of novel techniques for powering and actuating the self-propulsion of small particles, and potentially future microdevices, in liquids. This is a challenging research area because the interplay of forces acting on a small particle swimming in a liquid is very different from that acting on a macroscopic swimmer. The inertial forces, dominant at the macroscale, are overpowered by other effects such as Brownian collisions and viscous drag at microscales. Over the last decade, several techniques have been explored to propel different kinds of objects (a few millimeters or smaller in size) in liquids and to control or tune their motion. Self-propelling particles could find applications in performing a broad range of functions such as drug/vaccine delivery and medical diagnostics in the biomedical field, shuttling cargo and pumping/mixing fluid in lab-on-chip devices, and others. Recently, proof-of concept of their ability to perform some of these practical tasks has been demonstrated by various groups. We developed two new categories of self-propelling particles driven by a periodic cycle of Marangoni effect flows and catalytic activity of live cells, and proposed a novel technique based on AC field modulation for remotely controlling the direction of motion of the diode-based self-propelling particles. We also demonstrated a novel simple application of such autonomously propelling particles floating on water surface for rapid oil spill cleanup. Our Marangoni effect driven self-propelling particles, comprising of an ethanol infused hydrogel, exhibit a unique pulsating motion in water over long duration. Their pulsation results from the emergence of a self-sustained periodic cycle of surface tension gradient driven flows. We developed scaling relations for the pulse interval and the distance propelled by these particles. On the basis of the quantification of this mass-transfer driven motion, we constructed floaters of different designs programmed to move in complex trajectories over the water surface. Such programmable "dancing" swimmers serve as early prototypes of functional autonomously propelling devices capable of performing practical tasks, as demonstrated by us in a follow-up project where we use them for efficient oil collection. To achieve functionality, we incorporated an oil absorbent "payload" into the Marangoni effect driven particles, which already contain an "engine" component for selfpropulsion. Such "engine-payload" particles self-propel over the oil-covered water surface while simultaneously collecting and dispersing the oil film. Oil gets collected into the absorbent end and the release of the surface active material from the hydrogel end causes oil film dispersion. We found that such mobile absorbents are more efficient (due to convective transport of oil) compared to immobile absorbents that depend on diffusion or natural drifting for gathering oil, which makes the former a viable alternative for tackling oil spills. The overall approach of enhancing the rate of mass-transfer by self-propelling an otherwise stationary particle may be also used to dramatically increase the efficiency of other processes such as catalysis. A drawback prevalent with chemical Marangoni effect driven self-propulsion is that the motion is limited by the amount of "fuel" on-board the particle. With the objective of overcoming this drawback, we developed biocatalytic self-propelling particles that use yeast cells as catalyst to power their propulsion by fermenting glucose or decomposing hydrogen peroxide (H2O2) present in the surrounding solution. Catalytically driven propulsion in H2O2 has already been accomplished using synthetic catalysts or isolated enzymes. Our work is the first demonstration of employing live cells directly as catalysts for this process, and will stimulate further exploration of novel catalyst-fuel combinations. Finally, we devised a novel technique for controlling the direction of motion of diodebased self-propelling prototypes of electronic microdevices on water. The diodes are remotely powered by an external uniform AC electric field, a technique reported earlier by Velev group. We found that by modifying the wave symmetry of the AC signal, the selfpropelling diodes could be rotated due to their orientation-dependent polarizability and made to shuttle back and forth on water. Analogous to the dipole-dipole interactions, diodes prefer to orient such that the DC field across them is anti-parallel with respect to the external field. We believe that this new principle of AC field modulation driven control of the direction of motion of the self-propelling microcircuit elements is a first step towards the development of "intelligent" particles that can perform complex functions in the fields of MEMs and microrobotics.