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A stacked microchannel heat sink was developed to provide efficient cooling for microelectronics devices at a relatively low pressure drop while maintaining chip temperature uniformity. Microfabrication techniques were employed to fabricate the stacked microchannel structure, and experiments were conducted to study its thermal performance. A total thermal resistance of less than 0.1 K/W was demonstrated for both counter flow and parallel flow configurations. The effects of flow direction and interlayer flow rate ratio were investigated. It was found that for the low flow rate range the parallel flow arrangement results in a better overall thermal performance than the counter flow arrangement; whereas, for the large flow rate range, the total thermal resistances for both the counter flow and parallel flow configurations are indistinguishable. On the other hand, the counter flow arrangement provides better temperature uniformity for the entire flow rate range tested. The effects of localized heating on the overall thermal performance were examined by selectively applying electrical power to the heaters. Numerical simulations were conducted to study the conjugate heat transfer inside the stacked microchannels. Negative heat flux conditions were found near the outlets of the microchannels for the counter flow arrangement. This is particularly evident for small flow rates. The numerical results clearly explain why the total thermal resistance for counter flow arrangement is larger than that for the parallel flow at low flow rates. In addition, laminar flow inside the microchannels were characterized using Micro-PIV techniques. Microchannels of different width were fabricated in silicon, the smallest channel measuring 34 mm in width. Measurements were conducted at various channel depths. Measured velocity profiles at these depths were found to be in reasonable agreement with laminar flow theory. Micro-PIV measurement found that the maximum velocity is shifted significantly towards the top of the microchannels due to the sidewall slope, a common issue faced with DRIE etching. Numerical simulations were conducted to investigate the effects of the sidewall slope on the flow and heat transfer. The results show that the effects of large sidewall slope on heat transfer are significant; whereas, the effects on pressure drop are not as pronounced.

The condition of no relative velocity of fluid past solid is termed as ‘no-slip boundary condition’. This condition is a general observation in fluid mechanics. However, several research groups have recently reported slip of water for surfaces with water repelling chemistry (referred to as hydrophobic surfaces). The effect has been attributed to disruption of H-bonding network of water molecules at such surfaces and resulting nucleation of dissolved gases and even reduced water density locally in absence of dissolved air. Slip of water on hydrophobic surfaces has been demonstrated to get amplified by high degree of roughness and patterning. Trapping of air in the surface asperities has been cited as the possible reason. The present work focuses on the study of effect of surface chemistry and roughness on flow behavior close to solid surfaces. Superhydrophobic surfaces have been generated by novel methods and wet-etching has been used to generate well-defined patterns on silicon surfaces. For flow characterisation, a micrometre resolution Particle Image Velocimetry (micro-PIV) facility has been developed and flow measurements have been carried out with a spatial resolution of less than 4 µm. It has been found from the experiments that flow of water on smooth surfaces, with or without chemical modification, conforms to the no-slip within the resolution limits of experiments. Deviation is observed in case of rough and patterned hydrophobic surfaces, possibly because of trapped air in asperities. Total Internal Reflection experiments, used to visualise the air pockets, confirmed the trapping of air at asperities. Diffusion of air out of the crevices seems to be the limiting factor for the utility of these surfaces in under-water applications.