Eur. Phys. J. Special Topics 192, 207-226 (2011)
https://doi.org/10.1140/epjst/e2011-01375-5

Minireview

Star-drops formed by periodic excitation and on an air cushion – A short review

¹ Institut d’Électronique de Microélectronique et de Nanotechnologies, UMR CNRS 8520, Avenue Poincaré, BP. 60069, 59652 Villeneuve d’Ascq, France
² Physics of Fluids Group and J.M. Burgers Centre for Fluid Dynamics, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands

^a e-mail: philippe.brunet@univ-lille1.fr
^b e-mail: J.H.Snoeijer@tnw.utwente.nl

Received: 21 January 2011
Revised: 25 January 2011
Published online: 9 March 2011

Abstract

When simply put on a solid, a liquid drop usually adopts the shape of a spherical cap or a puddle depending on its volume and on the wetting conditions. However, when the drop is subjected to a periodic field, a parametric excitation can induce a transition of shape and can break the drop’s initial axial symmetry, provided that the pinning forces at the contact-line are weak enough. Therefore, a standing wave appears at the drop interface and induces a periodic motion, with a frequency that equals half the excitation frequency. In the first part, we review the different situations where star drops can be generated from various types of periodic excitations. In the second part, we show that similar star drops can occur in a much less intuitive fashion when the drop is put on an air cushion, where no periodic motion is imposed a priori. Preliminary experiments as well as theoretical clues for a hydrodynamic interpretation, suggest that the periodic vibration is due to an inertial instability in the air layer below the drop.