Fluid velocity slip and temperature jump at a solid surface

by Melvin Ji Teo

Institution: Nanyang Technological University
Year: 2015
Keywords: DRNTU::Engineering::Mechanical engineering::Fluid mechanics; DRNTU::Science::Physics::Heat and thermodynamics
Record ID: 1375777
Full text PDF: http://hdl.handle.net/10356/61997


This thesis focuses on the discontinuous transport profiles across fluid-solid interfaces that are commonly encountered in micro and nano fluid systems. Despite the long history of the jump boundary conditions, most of the existing theoretical models for gas- solid and liquid-solid interfaces fail to provide satisfactory predictions of experimental findings. We first develop an adsorption model for fluid-solid interactions that is applicable to both gases and liquids. The various adsorption processes that take place simultaneously depend on factors such as the molecular energies, surface chemistry and surface fraction of vacant adsorption sites. Fluid molecules in each of these adsorption states emerge from the surface with different momenta and energies. The net velocity and temperature of the near-wall molecules, equivalent to the slip velocity and temperature jump, can be evaluated by considering the relative rates of adsorption. Our first theoretical model focuses on the subject of fluid slip over solid surfaces, where the corresponding velocities of various adsorbed fluid molecules are analysed based on the dynamics of the adsorption processes. The slip velocity expression is obtained through the overall velocity distribution of the near-wall molecules. Predictions from the new general model are compared with experimental results from the literature for gas and liquid systems. The motion by which mobile adsorbed fluid molecules traverse across a solid substrate has been suggested to occur through hops between adsorption sites. However, the slip velocity from such a mechanism has been shown to be significantly lower than that II observed experimentally. Surface diffusion of adsorbed molecules may develop in several ways other than surface hopping. We propose two surface diffusion mechanisms by which molecular slip may take place. These alternative mechanisms are capable of producing elevated molecular slip velocities that are much closer to measured quantities. Using the proposed adsorption framework of fluid-solid interactions, we derive an interfacial temperature jump expression for gas-solid and liquid-solid interfaces. In this model, the temperature jump is evaluated by considering the energies of fluid molecules that correspond to their adsorption states. Experimental data from the literature is used as corroboration for the new model, which addresses the inadequacies of current temperature jump theory in the prediction of observed temperature jump behaviour.