engleski

Laser generation of regular wavy patterns by nonlinear instability of a metal nanolayer

We report on an experimental study of the formation of structures caused by surface-tension-driven instability of a molten chromium layer generated by a single KrF laser pulse of nanosecond duration. The molten metal layer was formed in a 500 nm thick Cr film deposited on a glass substrate in a confined configuration. The strong lateral thermal gradient  Ty generates a surface tension gradient and causes the formation of instabilities on a liquid Cr film with a very high aspect ratio  = 13000, and of a low Prandtl number of Pr ~ 0.01. Two types of patterns, the Left inclined (Low wave-number), and the Right inclined (High wave-number) wavy-like instabilities with respect to the direction of  Ty, were observed. As the laser energy increases from 38 to 100mJ the inclination angle of the waves alternates with the transition of the right inclined to the left inclined, and vice versa. The analysis shows that the formation of inclined wavy structures occurs because the ratio, W of the Rayleigh number, Ra and the Marangoni number, Ma (W = Ra/Ma), takes values W > 1. The oscillation of inclined waves is governed by the Biot number, decreasing from the intermediate values of Bi ~ 49 to the low values Bi ~ 3 - 4, as the energy of the laser pulse is increased.

Flow instabilities; Surface tension driven instability; Nonlinearity; bifurcation and symmetry breaking

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