Study On The Effect Of Surface Chargeat Solid-liquid Interface On The Fluid Drag On Micro/Nano Scale

Micro/nano fluidic systems are widely used in mechanical, physical, chemical and biomedical fields. Because of the decreasing dimensions and increasing surface-to-volume ratio of micro/nanofluidic devices, the fluid drag at the solid-liquid interfaces is of scientific interest, and how to reduce the fluid drag has inspired widely scientific attentions. Theoretical and experimental studies show that surface charge and boundary condition at the solid-liquid interfaces are believed to affect the fluid drag. But there is few research to investigate the effect of surface charge on the boundary condition and no study considers the effect of coupling between the surface charge and boundary condition on the fluid drag. Considering the effect of surface charge on the boundary condition, the present study comprehensively analyzes the effect of surface charge on the fluid drag on the micro/nano scale.This study firstly develops a method to measure the surface charge density at the solid-liquid interface based on the atomic force microscopy(AFM) operated in contact mode. The theoretical model between the surface charge densities of AFM colloidal probe and the sample surface immersed in the liquid with the electrostatic force applied on the AFM probe is firstly established. Then the theoretical model is used to analyze the electrostatic force applied on the AFM probe to obtain the surface charge density. Based on this method, the surface charge density of borosilicate glass and silica surfaces immersed in DI water and Na Cl solution are measured, and the mechanism of origin of surface charge are analyzed. In addition, the effect of p H and ionic concentration of the solutions on the surface charge density at these solid-liquid interface are studied.Then, the samples with different degrees of surface wetting are prepared using nanoparticle-binder system by changing the roughness of these samples. The surface wetting of the droplet of DI water, ethylene glycol and hexadecane on these sample are studied. Then AFM operated in contact mode is used to measure the slip length of these samples immersed in the DI water, ethylene glycol and hexadecane. The study investigates the boundary slip of different solid-liquid interfaces, and find the changing mechanisms of boundary slip.Based on the AFM operated in contact mode, the effect of p H value of solutions and applied electric field on the surface charge density and slip length of a smooth hydrophobic self-assembled monolayer of octadecyltrichlorosilane(OTS) are studied. Then the effect of surface charge at the solid-liquid interface on the boundary slip condition is analyzed. The result shows that a larger magnitude of surface charge density leads to a smaller slip length. The analysis of the effect of surface charge on the boundary slip make it possible to accurately study the effect of surface charge on the fluid drag on the micro/nano scale.Finally, a physical model of one-dimensional fully developed pressure-driven incompressible laminar flow confined by two infinitely large parallel plates is established. Considering the effect of surface charge on the boundary slip condition, a theoretical model based on the modified Navier-Stokes equation is developed to study the effect of surface charge-induced electric double layer(EDL) on the fluid drag. Based on the theoretical model, the effect of both the no overlapped EDL and overlapped EDL under symmetric surface charge density on the drag is studied. The results show that the existence of surface charge will reduce the velocity of the fluid and increase the fluid drag. In addition, the boundary slip will further promote the effect of surface charge on the drag. The effect of no overlapped EDL with asymmetric surface charge density on the fluid drag is also studied using the theoretical model, and it is found that when the surface charge at upper and lower walls has the same magnitude but opposite sign, the effect of EDL on the fluid drag is minimum because of the cancellation of electrical force caused by surface charge. This is a feasible method to reduce the fluid drag.In summary, the role of surface charge at the solid-liquid interface on the boundary slip condition is studied, and the effect of surface charge density on the fluid drag on the micro/nano scale is analyzed. The theoretical medol of reducing the effect of surface charge on the fluid drag is proposed.