Electroosmotic flow (EOF) in microfluidic systems is frequently subjected to thermal effect because of temperature-dependent material properties. Boltzmann equation is usually used to describe the ion distribution in EOF. This study will compare the ion distribution under the thermal effect with the Boltzmann distribution. Moreover, for thin electrical double layer (EDL), constant potential model always be used to simplify the calculation of EOF at constant charge. In this study, the thermal effects on EOF at both constant potential and constant charge are analyzed. In addition, as the surface charge density increases largely with higher temperature, in this study efforts are also made to address the thermal effect on EOF induced by the temperature-dependent charge density.
In particular, a numerical model is presented for investigating the steady EOF under the thermal effect. The proposed model involves several coupled governing equations including the Nernst-Planck equations, the Poisson equation, the modified Navier-Stokes equations, and the energy equation. The simulation results show that the Boltzmann equation cannot fully describe the ionic concentration distributions under the large thermal effect when EDL overlap. Moreover, for thin EDL, the electroosmotic velocity under the thermal effect at constant potential is lower than that at constant charge, due to the negative electrothermal force at constant potential. Furthermore, it is revealed that the temperature-dependence of surface charge can significantly modify the characteristics of EOF.