Unresonant interaction of laser beams with microdroplets

Abstract

The interaction of distilled water microdroplets (volumes of 3-4μl) with pulsed laser beams emitted at 532nm is described. At 532nm the distilled water absorption is very low and the interaction of a water bead with the laser radiation is dominated by unresonant phenomena. Following the collision of the laser beam with a microdroplet in suspended/ hanging/pendant position in air, deformations and mechanical vibrations of the droplets are produced. The conditions in which the droplets lose material as a consequence of the impact with laser beams are also explored. The effects produced on the droplet were studied pulse by pulse and depend on: droplet’s content, beam wavelength, power and focusing conditions, irradiation geometry and adhesion of the bead to the capillary on which it is suspended. The laser pulses energies were varied in four steps: 0.25mJ, 0.4mJ, 0.7mJ and 1mJ. The pulse full time width was 5ns and the typical focus diameter on the droplet was 90μm; the beam had a relatively low divergence around the focus point. The microdroplets and the modification/evolution of their shapes are visualised by recordings performed at 10kframes/second. Following a microdroplet interaction with the laser beam one may also produce at a controlled moment in time nanodroplets propagating at high (probably supersonic) speeds and microdroplets propagating at slower speeds. One may also produce pendant droplets of smaller dimensions than the initial one as well as micro/nano gas bubbles in the pendant droplet’s material/volume. In a second set of experiment was recorded at high speed the behaviour of the microdroplets of Rhodamine 6G in distilled water at resonant interaction with similar laser pulses, at the same power levels. The optical phenomena considering that the microdroplets contents are Newtonian liquids which dominate the beads behaviour at interaction with the laser beams, are discussed

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