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Fluid Dynamics Seminar

Monday, Nov. 7, 2011, 4:00 PM
Cullimore, Room 611
New Jersey Institute of Technology

The Self-Propulsion of Colloidal Particles

Charles Moldarelli

Department of Chemical Engineering and Levich Institute, City College of City University of New York

Abstract

The self-­‐propulsion or swimming of colloidal particles in a liquid has attracted attention as a means of moving colloids to desired locations without the application of external fields. Attachment of cargo to these swimming particles or “motors” enables many technological possibilities, as for example the ferrying of nanomaterial building blocks to an evolving structure in “bottom-­‐up” materials fabrication, or the fine control targeting of therapeutic molecules to a cellular landscape in drug delivery. In autonomous motion, the colloid particle motor “consumes” fuel in the surrounding media, and “converts” this fuel into motion via a chemo-­‐mechanical engine. One paradigm for this engine is to utilize a particle that is asymmetrically functionalized (a Janus particle). One side of the particle is inert, while the other side either reacts with or binds solute present in the continuous phase surrounding the particle. The binding or reaction generates a gradient in solute concentration across the particle. This gradient in turn create an imbalance in the attractive intermolecular (van der Waals) forces exerted on the particle by the solute which provides the engine for phoretic locomotion (diffusiophoresis). In this presentation, continuum and molecular dynamics calculations of the diffusiophoretic colloidal motion of Janus particles are presented, and experiments verifying the motoring concept are detailed. The continuum calculations demonstrate that the propulsion velocity is found to be independent of the particle radius for micron-­‐sized particles whose diameters are larger than the lengthscale of the van der Waals attractive interaction (typically 10-­‐100 nm). For submicron particles, the velocity decreases, and the MD calculations verify this dependence and provide a molecular level view of the propulsion for nanometer-­‐sized particles. Experiments are also reported on the asymmetric binding of a solute to one side of a functionalized, one micron, Janus particles. Fluorescence and optical microscopy measurements of the movement of these particles shows a directed velocity much larger than the chaotic, background motion of the particle driven by Brownian forces.