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Molecular dynamics simulations of solid-liquid interfaces

Authors Chen, Shuyu
Issue Date 2011
Summary In this thesis, I discuss our recent studies on two types of interfacial phenomena that were explored using molecular dynamics (MD) simulations. The first of these is the formation of molecular dipolar filaments across gaps of nanocontacts between two polarizable substrates. This phenomenon provides a microscopic mechanism for the recently discovered giant electrorheological (GER) effect. It has been shown that the theoretical upper bound of the traditional electrorheological (ER) effect is no longer applicable to this new type of ER fluids. A phenomenological model of the GER mechanism, based on the saturation polarization of urea molecular dipoles in the contact regions of the nanoparticles, yields an adequate account of the observed effect but without a microscopic picture on how this can occur. By using MD to simulate a urea-silicone oil mixture confined between two bounding surfaces of a nanocontact, we demonstrate that the urea molecules, which have a molecular dipole moment of 4.6 Debye, can form aligned dipolar filaments that penetrate the oil film and bridge the two substrates, with an attendant lowering of the aligning field for the urea dipoles. This phenomenon is explainable on the basis of a 3D to 1D crossover in the urea molecules’ microgeometry, which is realized through the confinement effect provided by the oil chains. The resulting electrical energy density gives an excellent account of the observed yield stress variation as a function of the electric field. Further analysis showed that the hydrogen-bonding interaction is critical for enhancing the molecular polarizability of urea and lowering the energy barrier for filament formation. Every distinct feature of the GER effect was revealed in our simulation. The remainder of this thesis concerns the slip of fluid flow at the solid-liquid interface. MD simulations of the 2D Poiseuille flow of a simple fluid between two parallel planar solid walls were performed, and the wall-fluid atomic interactions were modeled according to the fluid-fluid atomic interactions in three ways. Slip coefficient was measured with a changing fluid viscosity, which was controlled by adjusting the fluid-fluid atomic interaction energy, for each model. Relationship between fluid viscosity and slip coefficient shows model-dependence. Unlike fluid viscosity that is only associated with fluid-fluid atomic interactions, the slip coefficient depends on both types of microscopic interactions. This result implies that in addition to wall-fluid interactions, interfacial slip may also depend on a fluid’s bulk viscosity.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2011
Language English
Format Thesis
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