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Using the thermoreflectance imaging method, the temperature profile and transient response of thermally tunable ridge waveguides with laterally supported suspension are investigated. This method has a high accuracy in the temperature measurement. The experimental data convincingly confirm a uniform temperature distribution along the waveguide except the initial 30 μm long sections near the two longitudinal edges. The 10%–90% rising time and 90%–10% falling time of the device transient thermal response are also measured to be ∼48 μs and ∼44 μs, respectively, regardless of different waveguide lengths and at different heating powers. In addition, the delay time of the waveguide transient thermal response is revealed to be 1.3 μs by comparison between experiment and simulation.

A unique liquid film lift-off during a falling water droplet impacting on a heated titanium oxide nanotube surface has been experimentally revealed through a high speed visualization system. It is suggested that the Leidenfrost point on the nanotube surface has been significantly delayed, as compared to that on the bare titanium surface. Such delay is inferred to be a result of the increase in the surface wettability and the capillary effect by the nanoscale tube structure. By measuring the liquid lift-off distance from the substrate surface, a droplet lift-off is typically divided into four stages, namely, first contact, first lift-off, second contact, and second lift-off. The residence time at each stage is quantitatively evaluated. As the surface temperature increases, the duration time is significantly reduced for both the first contact and the first lift-off stages. Published version

A point heat sink is produced by impinging a high frequency microscale droplet stream onto a superheated copper substrate. Although the overall target surface area is larger than the liquid-solid interface by two or three orders of magnitude, the thermal energy is mainly removed through the point heat sink rather than the rest dry area. Therefore, the spherical conduction patterns in the solid materials are observed with a “nozzle-shifting” method which requires only two temperature probes. The temperature gradient in the vicinity of the impingement stagnation point is tremendously high, suggesting that the liquid-solid interface temperature is significantly lower than the far-field bulk temperature of the substrate. Moreover, the liquid-to-solid heat transfer is measured, which agrees well with the theoretical prediction. The maximum interface heat flux can reach around 80 W/mm2. It is insensitive to the substrate temperature in a relatively wide temperature range, which brings conveniences to the potential industrial applications. NRF (Natl Research Foundation, S’pore) Published version

The flow field in a droplet can be distorted as the light passes through a convex liquid-vapor interface in the measurement of Particle Image Velocimetry. We have developed a solution from ray tracing to recover the distortion flow in the droplet. A three-dimensional surface model has been applied to a self developed ray tracer to recover the distorted images. By tilting the camera upwards between 10° and 20° in angle, about 95% of the full internal flow vectors of a pendant droplet, especially near the interface, can be recovered to the real positions for further analysis.