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Liquid droplets impinging upon a solid surface are present in many diverse and important engineering and scientific applications. Examples include impingement cooling of surfaces, condensation phenomena in which liquid droplets fall and strike a surface, vigorous mixing of gas-liquid systems, and in manufacturing, e.g., pouring a liquid material onto a cooled surface or mold to form objects. The accurate measurement of the solid-liquid interface temperature of a liquid droplet impinging on a solid surface is extremely difficult to do using traditional, contact-based techniques, however. This work presents a laser-based technique that is capable of resolving the transient interface temperature change as a liquid droplet strikes a surface. The measurement records changes in reflected light from the liquid-solid interface, which is correlated to temperature change. In this work, the substrate is BK7 glass or quartz. The liquid falls from a height of several centimeters, while the laser beam is sent through the transparent solid material from the underside for the measurement. Results are presented for water and glycerin, with good agreement found between predicted and measured temperature histories. Additional features of the technique include extremely fast temporal resolution, which is limited only by the speed of the light-capturing electronics. A temporal resolution of 1 microsecond is readily obtainable with inexpensive electronics. Also, the use of a high-quality Gaussian laser beam from, e.g., a HeNe laser or a single-mode fiber can be focused to a spot size ranging from several millimeters to 10 microns in diameter at the liquid interface, thus providing a wide variety of spatial resolutions, including those in the microscale. Finally, an array of interrogation beams can be employed simultaneously to monitor the spatial temperature history of a droplet as it spreads and cools once striking the surface. Both present and future applications of the technique will be discussed in the talk.

Optical Technique to Measure Transient Interface Temperature During Droplet Impingement on a Cooled Surface