A capillary-jet instability method for measuring dynamic surface tension of liquid metals

An oscillating‐capillary‐jet method is presented for the measurement of dynamic surface tension of liquids, and in particular, metal melts, which are susceptible to rapid surface degradation, as caused by adsorption of surface‐active elements. The experimental technique captures the evolution of jet swells and necks continuously along the jet‐propagation axis, thus adding temporal information to each image. This method is used in conjunction with an existing linear, axisymmetric, constant‐property model to determine the variation of the instability growth rate, and, in turn, surface tension of the liquid as a function of surface age measured from the exit orifice. The conditions investigated focus on a time window of 2–4 ms from the jet orifice. The surface properties of the eutectic 63% Sn‐37% Pb solder alloy are investigated in a nitrogen atmosphere containing controlled amounts of oxygen (from 8 to 1000 ppm). The capabilities and advantages, as well as the limitations of the method are discussed in the context of the rapid physical and chemical processes affecting molten‐metal capillary‐jet instability and break‐up.