Boiling flow characteristics in microchannels with very hydrophobic surface to super-hydrophilic surface

Boiling flow process plays a very important role to affect the heat transfer in a microchannel. Different boiling flow modes have been found in the past which leads to different oscillations in temperatures and pressures. However, a very important issue, i.e. the surface wettability effects on the boiling flow modes, has never been discussed. The current experiments fabricated three different microchannels with identical sizes at 105 × 1000 × 30000 μm but at different wettability. The microchannels were made by plasma etching a trench on a silicon wafer. The surface made by the plasma etch process is hydrophilic and has a contact angle of 36° when measured by dipping a water droplet on the surface. The surface can be made hydrophobic by coating a thin layer of low surface energy material and has a contact angle of 103° after the coating. In addition, a vapor–liquid–solid growth process was adopted to grow nanowire arrays on the wafer so that the surface becomes super-hydrophilic with a contact angle close to 0°. Different boiling flow patterns on a surface with different wettability were found, which leads to large difference in temperature oscillations. Periodic oscillation in temperatures was not found in both the hydrophobic and the super-hydrophilic surface. During the experiments, the heat flux imposed on the wall varies from 230 to 354.9 kW/m2 and the flow of mass flux into the channel from 50 to 583 kg/m2s. Detailed flow regimes in terms of heat flux versus mass flux are also obtained.