Diffusiophoresis of Colloids at Nano-scales
Benjamin Levich Institute for Physico-Chemical Hydrodynamics
City University of New York
Microbots and nanobots are self-locomoting objects intended to move through liquids along a programmed path in small-scale landscapes to facilitate novel applications such as targeted drug delivery to individual cells and shuttles for moving cargo through microfluidic domains. Phoretic mechanisms have long been constructed using a top-down approach to colloid locomotion, however here we study a bottom-up approach based on a chemo-mechanical transduction mechanism, diffusiophoresis. In this case, motion results from unbalanced van der Waals forces exerted on a colloid particle by solute molecules distributed asymmetrically around it. In (passive) rectified diffusiophoresis, the concentration gradient is applied externally whereas in (active) self-diffusiophoresis, the concentration gradient is sustained by a surface reaction with a solute on one face of colloid. A key issue in applications as technology turns to nano-scales is to understand the dependence of the propulsion velocity on the colloid size.
We have undertaken both continuum and molecular dynamics (MD) simulations in order to obtain insight into these sub-micron colloid propulsion schemes. The continuum framework provides a complete solution to the governing equations of motion at micron and larger length scales, but the correct incorporation of van der Waals interactions requires an unspecified molecular cut-off. The MD approach provides a selfconsistent account of all interacting atomic species at nano-scales. We show that these MD simulations establish a cut-off for the interaction potential in the continuum theory allowing the continuum and MD to be in full agreement. We are thus able to predict the motion of colloids from nanometers to microns.