Research On The Effect Of Surface Microtopography Onboundary Slip And Flow Resistance

The popularization and application of the microfluidic devices in the field of biology, chemistry and medicine have greatly promoted the development of microfluidic science, meanwhile it also put forward new problems and challenges, among which, the channel in micro/nano scale, due to its high surface-area/volume ratio, has larger resistance than channels in macro scale when fluid flow in them. How to reduce the flow resistance effectively in micro/nano scale channel has become a research topic to many researchers.Boundary slip has been considered to be able to reduce the flow resistance effectively since it was first proposed, and has attracted much attention of many researchers. Among the factors affecting the boundary slip, surface microtopography can significantly change the contact state at the solid-liquid interface and influence the fluid flow, at the same time, studies has found that the surface microtopography has an important influence in the wettability of the surface and the viscous force between solid and liquid. Therefore, further research in the effect of surface microtopography on boundary slip and flow resistance is feasible and meaningful.In this dissertation, we first studied the influence of surface microtopography in surface wettability by measuring the contact angles of deionized water on hydrophilic silicon samples and hydrophobic octadecyltrichlorosilane(OTS) self-assembled molecular membrane samples with different surface microtopography. Results show that both surface micro morphology and roughness have impact on the wettability of hydrophilic silicon surface: intensive sharp bumps can enhance hydrophobicity, while smooth gentle bumps enhance hydrophilicity, and roughness can slightly increase high hydrophobicity; as to hydrophobic OTS surface, micro morphology nearly has no effect on the wettability while roughness can enhance the hydrophobicity significantly.In the next chapter, we measured the boundary slip length of deionized water on hydrophilic silicon surfaces and hydrophobic OTS surfaces with different surf ace microtopography through atomic force microscopy(AFM) colloid probe technique, in order to find how the surface microtopography, as well as wettability, by combing conclusions obtained in the former chapter, influences the boundary slip. Results show that micro morphology and roughness have no effect on the boundary slip, but wettability has: there is nearly no slip on hydrophilic silicon surfaces and wettability has no obvious effect on boundary slip on these surfaces, while there are tens of to one hundred nanometers slip on hydrophobic OTS surfaces and wettability can influence the slip length a lot.Finally, we studied how the surface microtopography, ration of gas-liquid area, height of the channel and driving pressure difference influence the effec tive boundary slip and flow resistance of pressure-driven thin film flow past Cassie state surfaces through finite element simulation. Results show that all the four factors have influence in the flow resistance, and surface microtopography and ratio of gas-liquid area can affect the effective boundary slip significantly, while height of the channel and driving pressure difference have no effect on boundary slip. The increase of ratio of gas-liquid area can reduce the flow resistance and increase the boundary slip. Comparing the five Cassie state models, flow resistance decrease and slip increase more than others when fluid flow past grooved surface in its longitudinal direction.Above all, we found that the surface microtopography influence the boundary slip indirectly through influencing the wettability rather than directly and the essence of the effect of mocrotopography on boundary slip and flow resistance is the ratio of gas-liquid area. These conclusions can provide new references to the development and application of resistance reduction through slipping.