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 mm 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 mm), 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.

Thermophoresis refers to the motion of particles under a temperature gradient and it is one of the particle manipulation techniques. Regarding the thermophoresis of particles in liquid media, however, many open questions still remain, especially the role of the interfacial effect. This work reports on a systematic experimental investigation of surfactant effects, especially the induced interfacial effect, on the thermophoresis of colloids in aqueous solutions via a microfluidic approach. Two kinds of commonly used surfactants, sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), are selected and the results show that from relatively large concentrations, the two surfactants can greatly enhance the thermophilic mobilities. Specifically, it is found that the colloid-water interfaces modified with more polar end groups can potentially lead to a stronger thermophilic tendency. Due to the complex effects of surfactants, further theoretical model development is needed to quantitatively describe the dependence of thermophoresis on the interface characteristics.

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