The Design and Control of Catalytic Motors: Manipulating Colloids and Fluids with Self-Generated Forces
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The Design and Control of Catalytic Motors: Manipulating Colloids and Fluids with SelfGenerated Forces Timothy R. Kline1, Jodi Iwata1, Paul Lammert1, Darrell Velegol2,3, Thomas Mallouk1, and Ayusman Sen1 1 Pennsylvania State University, University Park, PA, 16802 2 Department of Chemical Engineering, Pennsylvania State University, 111 Fenske Building, University Park, PA, 16802 3 Materials Research Institute, Pennsylvania State University, University Park, PA, 16802 ABSTRACT
Microfabrication was employed to pattern silver (Ag) on a gold (Au) surface. The two metals served as bimetallic heterogeneous catalysts for the heterogeneous decomposition of H2O2. Silver was the cathode, carrying out H2O2 reduction (to water) and gold the anode carrying out H2O2 oxidation (to oxygen). Both protons and electrons are created at the anode (as a part of the reaction) and migrate to the cathode (migration of ions is a current) where they are consumed. An electric field is thereby established (migration of ions obeys Ohm’s law), which passively pumps fluids through electroosmosis. Electrophoresis is also present as either an additive component to the electroosmotic flow or results in pattern formation (occurs at point where electroosmosis is equal and opposite to that of electrophoresis). Herein, we describe the testing of the electrokinetic model, chemical methods to tune the tracer behavior (convection to pattern formation) and design of asymmetric patterns through microfabrication. INTRODUCTION
Generating controlled fluid flow at the micron-scale remains an interesting challenge in nanotechnology; in the past flows have been driven by externally applied pressure, electric field, thermal or concentration gradients.[1-4] Heterogeneous catalysis offers an unusual method to move fluids, deriving its energy from a “fuel” converted locally at the catalyst surface, possibly eliminating external pumps or power sources. We previously reported the autonomous movement of platinum-gold nanorods and micro-gears in dilute solutions of H2O2.[5-8] Herein, we describe pattern formation and convective fluid flow at the micron-scale induced by the bipolar electrocatalytic decomposition of H2O2.[9, 10] Control of fluid flow and pattern formation have been reported with externally applied electric fields.[11-16] Electrophoresis (1) is a well-established method to move colloids[4] and has been theoretically described by Dukhin, Derjaguin[17] and later O’Brien.[18] In general, for non-conducting colloids the movement is attributed to the polarization of the electric double layer surrounding the particle (i.e. electric double layer potential is described by the zeta potential) in an external electric field. The resultant particle movement was first predicted theoretically by the Smoluchowsky equation for classical electronkinetics. Electroosmosis (2) is the movement of the fluid through ion-drag, governed by the zeta potential of the surface over which the fluid is moving (rather than zeta potential of the particle as in electrophoresis)
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