Collective effects on thermophoresis of colloids: A microfluidic study within the framework of DLVO theory

Collective effects on thermophoresis of aqueous particle suspensions are studied experimentally and theoretically. A microfluidic device is used to characterize thermophoretic transport of 100 nm, 500 nm and 1 μm particles of various concentrations in deionized (DI) water. Our experimental findings show two interesting collective effects on the Soret coefficient of colloids: (i) for smaller particles (e.g., 100 nm and 500 nm), a sign change of the Soret coefficient is observed when increasing the particle concentration; (ii) for larger particles (e.g., 1 μm), a negative Soret coefficient is always seen. A model is derived to account for the collective effect on the thermophoresis of colloids using the well-known Derjaguin-Landau-Verwey-Overbeek (DLVO) theory that combines the van der Waals (VDW) attraction and the electric double layer (EDL) repulsion. Such DLVO interactions in an inhomogeneous particle suspension can exert an additional force on particles and thus modify the mass transport of particles under both temperature and concentration gradients and also alter the corresponding Soret coefficient. It is found that the proposed theoretical model can favorably explain our experimental observations.