Thermophoresis of charged colloidal particles in aqueous media - Effect of particle size

We investigated the particle size effect on the thermophoresis of colloidal particles in an aqueous medium via a microfluidic approach. A microfluidic device was used to directly observe and quantitatively characterize the thermophoretic behaviors of charged hydrophobic polystyrene particles within a wide diameter range from 100 nm to 5 mu m dispersed in deionized water. Our experimental findings showed an interesting particle size effect - a sign switch of the thermodiffusion coefficient from positive to negative with increasing particle size from submicron to micron. Moreover, a linear size-dependence of the thermodiffusion coefficient for micron-sized particles was obtained. Through comparing our experimental results with Duhr-Dhont's analytical model that includes effects of the electric double layer ionic shielding and the hydration entropy, we found that both the sign switch and the particle size linear dependence can be explained by this model. In addition, a separation of submicron particles from micron-sized particles was demonstrated experimentally via the particle size-dependent sign switch of thermophoresis. (C) 2016 Elsevier Ltd. All rights reserved.