Unresonant interaction of laser beams with microdroplets

Abstract

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