Colloidal thermodiffusion in aqueous solutions and ion thermodiffusion effect on electrokinetic flow
Date of Issue2015
School of Mechanical and Aerospace Engineering
The thermodiffusive motion of colloidal particles suspended in aqueous media due to a thermal gradient is named as thermophoresis, which has found its applications in microfluidic macromolecule separation/trapping, colloidal crystals formation, and protein functionality. Numerous experimental and theoretical studies show that the thermophoresis of colloids is related to physicochemical parameters, such as particle size, particle and salt concentrations, particle and salt types, and background temperature. Despite many years of studies, there still exist open questions for the thermophoresis of colloids in aqueous media, such as its dependence on particle and liquid thermal conductivities, and on the thermoelectricity for arbitrary electrical double layer (EDL) thickness. Moreover, the existing theoretical and experimental investigations of particle size effect on the thermophoresis are inconsistent and inconclusive. In addition, the underlying mechanisms of effects of the electrolyte concentration and the particle concentration on thermophoresis of concentrated but non-interacting particles are still unclear. On the other hand, thermophoretic manipulation in microfluidics can involve electroosmotic flow (EOF), which is widely used in micro- and nano-fluidic systems. However, the thermal effect on EOF with the consideration of ion thermodiffusion has not been studied before. The objective of this thesis is to address some of above-mentioned problems on thermophoresis in liquid solutions and ion thermodiffusion effect on EOF. The thermal conductivity effect on the thermophoresis of a colloidal particle in aqueous solutions was numerically analysed, with the thermal conductivity ratio of particle to liquid ranging from 0.1 to 100. The numerical simulations reveal that the unequal thermal conductivities of particle and liquid cause a non-linear temperature distribution around the particle surface. When such non-linear temperature region is thicker than the EDL region, the thermal conductivity effect on thermophoresis becomes significant. The thermophoretic coefficient decreases with increasing thermal conductivity ratio of particle to liquid. The thermal conductivity effect on the thermoelectricity of a charged particle in aqueous solutions was also theoretically analysed. An analytical model for thermoelectricity of a “normal” particle (with an equal thermal conductivity as that of liquid) was firstly formulated and solved based on the linear response theory. The analytical results show that the thermoelectric effect induced thermophoretic coefficient of a “normal” particle is independent of the ratio of particle radius to EDL thickness. On the other hand, a numerical model of the thermoelectric effect induced thermophoresis of a charged particle was developed. When the non-linear temperature region around the particle surface is thicker than the EDL region, the thermoelectric effect induced thermophoretic coefficient was found to strongly depend on the ratio of particle radius to EDL thickness. Furthermore, experiments were carried out to investigate the particle size effect on the thermophoresis of dilute particles in aqueous solutions by using a microfluidic approach. The experimental findings show that the sign of thermophoretic coefficient switches from positive to negative with increasing particle size from submicron to micron. Moreover, a linear particle size-dependence of the thermophoretic coefficient for micron-sized particles was obtained. Such experimental results can be explained by a modified Duhr and Braun's analytical model, with consideration of the hydrophobic hydration entropic effect induced by the breakdown of hydrogen bond network. In addition, the electrolyte and particle concentration effects on the thermophoresis of non-interacting charged hydrophobic particles in aqueous solutions were studied experimentally and theoretically. The signs of thermophoretic coefficient were found to be switched from positive to negative with decreasing electrolyte concentrations or increasing particle concentrations. Duhr and Braun's analytical model was further modified with consideration of electrolyte concentration-dependent hydrophobic hydration entropic effect. Such model can well explain the experimentally observed dependence of thermophoresis on both particle and electrolyte concentrations and in particular, the sign change of thermophoretic coefficient. It was found that the strong hydrophobic hydration entropic effect gives rise to the thermophilic behaviours of non-interacting charged hydrophobic particles in dilute aqueous solutions. Finally, for potential thermophoresis-based microfluidic particle manipulations, an analysis of the thermal effect on EOF under an imposed temperature difference was reported, with consideration of ion thermodiffusion. In particular, an analytical model was formulated for the thermal effect on a steady, fully developed EOF in a slit microchannel, and the model was solved using the regular perturbation method. The parametric studies show that compared to the isothermal cases, the presence of imposed transverse temperature difference/gradient results in a faster EOF. The thermodiffusion-induced free charge density was also identified to play a key role in the thermodiffusion-induced electroosmotic velocity.