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.

Enhanced fluorescence emitted by microdroplets containing organic dye emulsions

“…These two energies were selected in order to evaluate the effect of laser radiation on droplet's content and they were the limits in terms of beam energy within which modifications of the shape of the droplet took not place. 25 The laser beam was processed so that it could cover the entire droplet and therefore excite a maximum number of dye molecules within it. The fluorescence signal was collected with a VIS optical fiber mounted at 90 with respect to the incident laser beam (along the observation direction in Figure 4(c)) and analyzed with a spectrograph Spectra Pro 750 (Acton Research) coupled with an IMAX 1024 iCCD camera (Princeton Instruments).…”

Fluorescence and Time-Delayed Lasing during Single Laser Pulse Excitation of a Pendant mm-Sized Dye Droplet

“…For each concentration, applied on the droplet was the maximum beam energy which did not distort the droplet shape. Above this energy, unresonant interaction effects (generated by light pressure) [19] took place and produced vibrations or even droplet destruction. In addition, by inducing small vibrations in the droplet, the spherical shape is distorted and the collected signal can be unpredictably altered.…”

“…The beam waist in the interaction plane had the linear dimension around the droplet's diameter (1.24 mm). Since the laser beam is not focused on the droplet, one may use a higher beam energy which does not destroy it, so that in total, a more intense fluorescence emission is produced [14,18,19,22].…”

Smart and green interfaces: From single bubbles/drops to industrial environmental and biomedical applications

“…The interaction of distilled water droplet (3-4 µl volume) with a pulsed laser beam at 532 nm was investigated; the water absorption is very low and the interaction of an aqueous droplet suspended in air with the laser beam is dominated by non-resonant phenomena so that one produces droplet deformations and mechanical vibrations [7,8]. It was shown that the droplets lost material as a consequence of the impact with a laser beam.…”