Molecular Dynamics Simulation of Diffusiophoretic Motion of Nanoparticles

Diffusiophoresis refers to the spontaneous motion of a particle or a polymer chain in a solution induced by a concentration gradient of a different type of solutes. Diffusiophoresis is suggested to be responsible for the counterintuitive stratification phenomena recently found in colloidal and polymer solutions undergoing rapid drying. To understand the underlying physics, large scale molecular dynamics simulations are performed to investigate the diffusiophoretic motion of particles in a colloidal suspension with the solvent modeled explicitly as a Lennard-Jones liquid. A concentration gradient of the suspended nanoparticles is induced by using a molecular pump. After the concentration gradient is stabilized, a large nanoparticle is added to the system and its motion along and normal to the gradient direction is recorded. The results indicate that diffusiophoretic motion indeed emerges when the magnitude of the concentration gradient and the size contrast between the solute and diffusiophoretic particles are above certain thresholds. The simulation results are used to test the existing models and theories of diffusiophoresis. By elucidating the physics of diffusiophoresis, the results will help reveal the fundamental mechanism underlying the stratification phenomena in suspensions of colloidal particles with polydisperse size distributions.