Development of near-infrared laser-induced capillary wave method to measure viscosity and surface tension

We have developed a non-contact high speed viscosity sensing technique, laser-induced capillary wave (LiCW) method using pulsed volume heating laser of near-infrared wave length. The main idea of the present work is based on the capillary wave induced by volume heating, which behaves more physically simplified than the one induced by surface heating in decay process and has nanometer-scale amplitude even as relatively-small temperature rise. We have derived the new theory for the wave amplitude z (x, z) captured the physics of volume heating by giving the boundary condition of heat conduction into the depth direction. First, we compared the theoretical damping behavior of capillary wave for toluene by volume heating and surface heating. According to the proposed theory, the capillary wave induced by volume heating is formed by only the effect of the thermal expansion with having the negligible effect on the temperature dependence of surface tension. In addition, maximum temperature rise and wave amplitude of water and toluene, absorption length of them are extremely different from each other, was compared between volume heating with surface heating. As a result, it was confirmed that nanometer-scale capillary wave can be induced with the temperature rise of less than mK order by volume heating, which indicates that near-infrared wave length is more applicable to the thremophysical measurement technique as a heating light source. Finally, to demonstrate the validity of the new theory, we have measured viscosities and surface tensions of Newtonian liquids, which showed good agreement within ± 5 % from the reference values.