Laser-based measurement of temperature or concentration change at liquid surfaces

This work presents a noncontact laser-based thermoreflectance technique to measure changes in temperature or concentration at a liquid surface. Change s in temperature and, for multi-component liquid mixtures, concentration re sult in a change in the liquid index of refraction, which alters the reflec tivity of the liquid surface. An intensity-stabilized diode laser and silic on photodiode are used to monitor the surface reflectivity. Temperature var iations in I-propanol, methanol, ethanol, and water are measured with uncer tainties of 0.5, 0.7, 0.7, and 3.7 degreesC, respectively. For binary mixtu res, the concentration of a methanol-propanol mixture is measured with an u ncertainty of 0.3 percent. The experimental configuration is noninvasive, s imple in design, and inexpensive. The technique can be used over a wide ran ge of time scales, ranging from micro-seconds to minutes, and can achieve h igh spatial resolution by focusing the probe laser beam to a small spot siz e on the liquid surface.