16 th Int. Symp on Appl. Laser Techniques to Fluid Mechanics, Lisbon, Portugal, July 09 – 12, 2012 2.2.1 3D characterization of the inner flow in an oscillating drop V. Palero 1,* , J. Lobera 1 , P. Brunet 2 , M. P. Arroyo 1 1: Grupo de Tecnologías Ópticas Laser (TOL), I3A-Universidad de Zaragoza C/ Pedro Cerbuna, 12, 50009-Zaragoza, España 2: Laboratoire Matière et Systèmes Complexes, Paris, France * Correspondent author: palero@unizar.es Keywords: Digital in-line holography, droplet dynamics, free surface instabilities The main objective of this work is to study the three-dimensional movement inside an oscillating liquid drop by means of digital in-line holography. A liquid drop was squeezed between two parallel glass plates separated a distance of 2.25 mm. The glass was treated to be slightly hydrophobic (contact angle around 90º, with hysteresis around 10º). The plates were attached to the vertical axis of a mechanical shaker connected to a function generator that produces vibrations in a wide range of frequencies. A scheme of this set-up is shown in figure 1. The drop was illuminated from below with an expanded laser beam. A mirror redirected the light to a high speed camera. Digital in-line holograms were recorded with the droplet oscillating at a frequency of 55 Hz. As the vertical acceleration is increasing, different transitions can be observed for a flat squeezed drop. Firstly, the contact line is pinned (figure 2a). As the oscillation amplitude increases, the contact line is unpinned but the drop is still axi-symmetric (figure 2b). At higher accelerations, the drop loses its axial symmetry (faceted shape, figure 2c) and takes the shape of a star (figure 2d). The 3C-3D velocity vector maps have been obtained for these droplet states. For the state shown in figure 2c, the drop shows azimuthal standing waves and oscillates between two different configurations in one period T e , the time between the two configurations being T e /2. Fig. 1 Optical set-up This study showed for the first time how internal structures of the flow can be related to the appearance of the break-up of axisymmetry of a drop shape induced by external vertical vibrations. Although such faceted and star shapes are qualitatively predicted from a simple pendulum-like model, more specific mechanisms in a hydrodynamics point of view should benefit from such internal visualizations. Furthermore in an applied prospective, the prescription of external vibrations at appropriate frequency can be used as a simple way to induce mixing in weakly-pinned drops, thanks to the appearance of such star shapes that produces vortices in the fluid flow. (a) (b) (c) (d) Fig. 2 Drop states under shaking Acknowledgements Authors wish to thank Spanish Ministerio de Ciencia e Innovación and European Comission FEDER program (project DPI2010-20746-C03-03), to Gobierno de Aragón – Fondo Social Europeo (project PI044/08 and Laser Optical Technology research group, T76) and COST-ESF (action P21). References 1. Noblin X, Buguin A Brochard-Wyart F (2004) Vibrated sessile drops: Transition between pinned and mobile contact line oscillations, Eur. Phys. J. E 14, (2004) 395-404. 2. Noblin X, Buguin A Brochard-Wyart F (2005) Triplon modes of puddles, Phys. Rev. Lett 94, 166102. 3. Palero V, Arroyo MP, J. Soria, Digital holography for micro-droplet diagnostics (2007) Exp. Fluids 43, 185-195.