Role of Solution Conductivity in Self-Electrophoresis

Bimetallic particles, typically cylindrical and 2 microns in length, move autonomously in hydrogen peroxide solutions by a novel electrochemical/electrokinetic mechanism commonly known as self-electrophoresis (Figure 1). These particles are but one example of catalytic nanomotors, colloids that can “swim” by converting chemical energy from their surrounding environment into mechanical energy of motion, typically via catalytic reactions on their surfaces. Catalytic nanomotors show promise for a variety of potential applications in nanotechnology and nanomedicine, such as targeted drug delivery in the human body. In general, these applications will require the motors to propel themselves through electrically conductive media. However, in the case of bimetallic rods, it is well-known that the swimming speed decreases sharply when electrolyte is added even at sub-millimolar concentrations, as initially reported by Paxton et al. [J. Am. Chem. Soc. 2006, 128, 14881-14888]. It is thus crucial to understand whether this trend is indicative of a fundamental limitation on the rods’ capabilities, or if it may be circumvented.

In this talk, I will first give a general overview of the bipolar electrochemical/electrokinetic propulsion mechanism for the self-propulsion of bimetallic rods. I will then present numerical simulations and scaling analyses that elucidate the physical reasons for the electrolyte-induced speed decrease. The numerical results show strong agreement with the scaling arguments and previously published analytical results. Finally, I will discuss the implications of these results for future applications of these particles.