Effect of hydrophobicity and surface roughness on two-phase flow in rectangular microchannels

"Two-phase flows in microchannels have received significant attention recently, and have become the cornerstone of numerous microfluidic devices. Microscale devices used for bioengineering applications, oil recovery, and chemical and catalytic microreactor applications involve the transport of bubbles in confined fluidic networks in channels of micrometer length scale. These types of two-phase flows result in pressure variations, leading to an overall increase in pressure drop. Among various flow parameters, pressure drop is extensively used in design of microfluidic devices. There are several parameters that affect the pressure drop across two-phase flow in microchannels. In the present study, the goal is to be able to predict the pressure drop of two-phase flow in rectangular microchannels as a function of hydrophobicity, surface roughness, and bubble size. The SU-8 channels are fabricated using photolithography to ensure a perfectly smooth surface, which eliminates the effect of surface roughness. The fabricated channels are treated to alter the contact angle of water on SU-8, isolating the effects of hydrophobicity. Pressure drop data of air-water two-phase flow across the channels was collected, and compared to a previously published model, which predicts the pressure drop across a smooth hydrophilic rectangular microchannel with an air bubble flowing through it. Deviations of the experimental pressure drop from the predicted values were observed as a function of hydrophobicity and bubble size; this information was used to introduce a term, accounting for the effects of hydrophobicity and bubble size, into the existing model. A method of fabricating rough SU-8 channels was proposed to isolate the effects of surface roughness. The model was validated using channels of varying aspect ratios. It was found that the proposed model was independent of the aspect ratio"--Abstract, page iii.