Reducing viscous losses in microchannels using modified surfaces.

摘要

With current MEMS expanding, microfluidics was developed in terms of control and distribution of fluids in the micro-devices. Microchannels are distributed between those systems to transport or contain fluids. With the devices shrunk, the nanochannels are employed. The efficiency of reducing the friction losses in nano- and microchannels using air gaps was examined. Theoretical predictions were conducted first based on Navier-Stokes equations for three simplified 1-dimensional models: slip-wall model, air-gap model, and depleted-water model. The results showed that all models are of great benefit to increasing liquid flow rate with the same pressure gradient.; Numerical simulations were conducted for the two-phase flow in microchannel with air gaps by activating both Flow and VOF modules in CFD-ACE(U)(TM). The results showed that the air gaps in the microchannels are able to increase water flow rate and/or reduce the friction losses. The parameter studies showed that the small block widths and larger periodic lengths are preferred. The optimum values of the air gap heights exist, but are much less than the theoretical result. The studies of flow directions showed that parallel flow is more effective than the transverse flow. With a slip velocity on solid surface, the flow rate can be increased much more than just having air gaps. Similar finds were concluded with the simulations that used to verify the experimental work at UCLA, which confirmed the effectiveness of the air gaps in reducing friction losses.; The particle coating methods were used to form micro-/nano-textured surfaces inside the microchannels, which are the premise of producing super-hydrophobic microchannels. With the dip coating method, only a small part at the inlet had particles coated on its surface because of the small geometry size of the channel and the large viscosity and surface tension of the slurry. With pumping or pressure driving slurry flow, the flow coating method can coat the entire surfaces. The coating showed arbitrary results with two major problems: low particle density and poor particle distribution. Three factors affect the coating results were considered: the slurry properties, the geometry and surface properties of the microchannels, and the flow in microchannels.