Measurements of heat transferred and residence time of a droplet on a hot surface

This dissertation focuses on experimental studies of thermal transport between solid and liquid, especially between Pt(or CFx)-coated Si and water droplet, using an ultrafast pump-probe method, time-domain thermoreflectance (TDTR), combined with two-photon absorption (TPA) thermometry. I developed the technique to measure both i) the heat transfer (the amount of thermal energy transferred from hot surface to the water droplet) and ii) the residence time using the same apparatus when water droplet was in contact with a hot Si surface.;I achieved a sub-msec time resolution for simultaneous measurements of the near-surface temperature (using TPA) and the effective thermal conductance (using TDTR) of the solid-liquid interface. I studied the droplet impact on both hydrophilic (Pt-coated Si) and hydrophobic (CFxcoated Si) surfaces. For the smooth hydrophilic surfaces, the amount of thermal energy transferred decreased beyond 150 °C due to droplet shattering while the residence time monotonically decreased as temperature increased. The heat flux calculated from the heat transfer and the residence time approached ~500 W cm-2 at 210 °C, which was comparable or exceeded the reported values of the critical heat flux in typical water boiling experiment. However, it only existed for a short time, on the order of 10 msec.;For the patterned hydrophobic surface, I also studied the heat transfer and the kinetics of liquid-to-vapor phase transformation when the water droplet bounced off the hot surface. I found that the residence time from TDTR measurements was up to 40 times shorter than that from high-speed camera imaging; the trapped vapor at the ridge quickly moved to the center of the pattern. I also found that the contribution to heat transfer by evaporation was non-negligible at T>130 °C while the contribution to heat transfer by conduction decreased with temperature due to the short residence time.;In addition, I extended the pump-probe system to the measurement of true contact area. I studied adhesion between Pt-coated Si and PDMS with pyramids array according to humidity; the humidity affects the capillary portion between Si and PDMS. Assuming that the contact area between surfaces was proportional to the effective thermal conductance of PDMS, I measured the effective thermal conductance with varying the distance between surfaces at dry (50%) environments; the difference between two conditions was reported without further quantitative analysis.