Study of Rapid Heat Transfer Phenomena of Nano-second

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The experimental and theoretical study of laser pulse heating of metal surfaces was conducted in water to make clear the physics of heat and fluid flow phenomena in nano-second order. In the experiment employing mercury as the test surface, it was found that the shock wave due to phase change of water and mercury is generated just after laser irradiation and, at the next stage of milli-seconds, a semi-spherical vapor bubble is formed and , in the last stage of a few seconds, the deformation of mercury surface is followed. In contrast, in case of heating in air, plasma of mercury appears in several micro-seconds after the irradiation accompanied by shock wave without deformation of mercury surface. In the experiment employing the silicon solid surface in water, a strong shock wave appears due to the phase change of water which travels in water with the speed of sound velocity. The phase change may be understand to occur when the surface temperature of silicon reaches the homogeneous nucleation temperature of water. In the case of heating of silicon surface in air, the shock wave is observed but this is mainly due to the expansion of air. In order to explain those experimental results, simple model of shock wave generation is proposed basing on the facts that (1)heat transfer is governed by usual parabolic heat conduction (non-Fourier and quantum effects are not significant) ; (2) liquid convection can be neglected but compressibility of water becomes important ; (3) phase change takes place when the metal surface reaches the homogeneous nucleation temperature of water, which yields the shock wave. This model explain well the dependence of shock wave strength on laser irradiation power.