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 in this work we study a bottom-up approach based on a chemo-mechanical transduction mechanism, diffusiophoresis. In this case, motion results from unbalanced molecular 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 various parameters controlling colloids motion. We have undertaken both continuum and molecular dynamics (MD) simulations in order to obtain insight into these sub-micron colloid propulsion schemes. From continuum framework, in self-diffusiophoretic motion of a colloid in an infinite medium, we have first examined the effects of reaction rate, solute advection and the particle size on the colloid swimming velocity. Furthermore, we extended our analysis for the colloid self-propulsion in presence of geometrical constraints and we particularly investigated the solid plane wall effect and also pair active colloids interactions. Although the continuum approach provides a complete solution to the governing equations of motion at micron and larger length scales, but the correct incorporation of molecular interactions requires an unspecified molecular cutoff. The MD approach provides a self consistent account of all interacting atomic species at nano-scales. We show that these MD simulations establish a cutoff 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.
展开▼
