Influence of nanoscale surface roughness on flow behaviour in fluidic microchannels

Gao, A. (2010) Influence of nanoscale surface roughness on flow behaviour in fluidic microchannels.

Abstract:Microsieve is largely used in the area of filtration and selective gas permeation. Such applications can benefit from a microsieve with a low flow resistance. The flow resistance is usually achieved by reducing the thickness of the microsieve, however, at the mean time microsieve membrane strength becomes low. To solve this dilemma, a microsieve has to be thick enough to withstand high pressure or forces while at the same keep its resistance remaining the same as a very thin membrane. The conductance difference for thick and thin microsieve membranes is caused by the rough surface of microsieve channel. Therefore, it is postulated that a microsieve with extremely smooth channel surface would prevent the influence on total resistance from the channel wall roughness. A silicon microsieve with smooth channel surface is designed and fabricated to examine the influence of surface roughness on flow conductance. DRIE is applied to drill holes through the wafer, which will create microscale roughness. The circular holes will be getting a post processing with KOH wet etching: a rhombus shaped channel is fabricated with smooth surface. By measuring the flux through the microsieve with nanoscale roughness and comparing this with the microsieve with micro roughness, the influence of surface roughness on flow flux or flow conductance is determined. The experiment results show the flow conductance of the microsieve with smooth channel surface is almost the same as that of the microsieve with rough channel surface, which means most gas molecules are reflected diffusively by the channel walls. The silicon surface property is analyzed and dry nitrogen and HF treatment is applied to prevent the influence from water molecules. Larger gas molecules SF6 are also tried. Both of these two experiment results show the same diffusive reflection behaviour. Therefore, it is concluded that the diffusive reflection is not only caused by large scale surface roughness, but also highly depended on the surface solid atoms arrangement as well as gas molecule size and shape. Specular reflection can occur on the condition that gas molecule can be treated as sphere and its size is larger than the distance between solid atoms, while at the same time the solid atoms at surface have to be regularly patterned and closely packed. Such kinds of surface can be achieved by graphane or boron nitride (BN) coating.
Item Type:Essay (Master)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Subject:33 physics
Programme:Nanotechnology MSc (60028)
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