THERMOCAPILLARY FLOW EXCITED BY FOCUSED NANOSECOND LASER-PULSES IN CONTAMINATED THIN LIQUID-IRON FILMS

A new method is established to determine surface tension and mechanisms of evaporation of liquid metals in a wide range of high temperatures. It is based on a combination of high-speed transmission electron microscopy imaging of flow in nanosecond laser pulse molten films and computer simulation. The technique was applied to iron films with native oxides to investigate the effects of surface active impurities in a melt with transient temperatures and gradients up to 4000 K and 5x10(8) K/m, respectively. Such melts show a shear flow with direction changing once or twice during 1-2 mu s after a 20 ns laser pulse, which cannot be simulated using table values for the temperature coefficient of the surface tension and the vapor pressure. Instead, evaporation is negligible, and the flow of the liquid is mainly driven by a fast changing gradient of the surface tension caused by a time-varying distribution of temperature and dissolved surface active oxygen atoms. Current site coverage models, giving the surface tension as function of temperature and impurity content for static liquids, successfully can be applied to liquids moving on the nanosecond/micrometer scale, too. However, the number of active surface sites can be vastly reduced by short-lived oxide covers, e.g., in iron with native oxides down to 3.5% of the total number of surface sites. (C) 1995 American Institute of Physical.