(474a) A Molecular Dynamics Simulation Study On Active Diffusiophoresis of Swimming Colloids
Microbots and nanobots are locomoting objects which are designed to transverse liquids in a programmed way along small scale landscapes in highly imaginative applications such as the targeted delivery of drugs to individual cells, roving sensors for chemical detection or deployed agents for capturing targeted molecules, engines for micropumping, shuttles for moving cargo through microfluidic cells, and transporters for directly assembling supramolecular structures from molecular building parts. Phoretic mechanism has long been considered as a top-down approach to accomplish colloid locomotion, however the attendant miniaturizations in the force fields through the small scale venues envisioned for these locomotors are challenging and therefore the idea of self-propulsion has been thrived as an alternative approach for autonomous motion of colloids. We study theoretically one potential transduction mechanism, diffusiophoresis, which derives from unbalanced van der Waals forces which are exerted on the molecules of the colloid by low molecular weight solute molecules (less than one nanometer in diameter) distributed asymmetrically around the colloid. The van der Waals interaction of a solute molecule in solution near a colloid with all the molecules comprising the colloid is confined to an interaction zone around the particle with thickness $L$ of the order of $1-10$ nm (a few to several molecular diameters). Within this zone, the concentration gradient required for the colloid motion can be generated and sustained by an irriversible surface chemical reaction of solutes only on one face of the colloid. We utilized the molecular dynamics (MD) simulation to investigate the colloid self-propulsion due to a surface chemical reaction. We focus on a simple system in which only three species (solvent, solute and molecules belong to colloid) exist in our simulation and the pair interactions were depicted via modulated Lennard-Jones interaction potential. We furthermore assigned a greater strength of attractive interaction for the particular pair (solute vs. colloid molecules) to actuate the motion which result in an inward flux of solute into the colloid and ultimately a specifically adsorbed layer of solutes due to a balance of attractive vs repulsive forces. Within this layer, we adopt the reaction on the active cites simply by converting solute to solvent. The colloid trajectories clearly demonstrated the enhanced motility of the swimming colloid compared to the controlled ones (colloid in the solvent bath undergoes only brownian motion) and with the aid of MD simulation results, we quantified the force as well as the directed velocity of the swimming colloid at nanoscale and lastly compared them with the continuum description.