Water drops in a denser miscible fluid: coalescence, drop shape and penetration depth.

International audienceWhen delicately put in contact with the surface of a denser miscible fluid (as salt-water or glycerin-water mixture), a water drop undergoes a quick coalescence. It then enters the surrounding fluid with a resulting toric shape and a rather high velocity of fall, up to about 20 cm/s. With both effects of buoyancy and viscosity dissipation, the drop progressively slows down until it totally stops and starts rising back. From this inversion of the motion, the toric shape of the drop becomes unstable and gives rise to a succession of destabilizations. We have studied the initial velocity of the drop and its penetration depth, as well as their dependences with the properties of the surrounding fluid (viscosity, surface tension, density difference). In particular, our results reveal that, contrary to a model previously proposed, the initial velocity cannot be simply interpreted as a direct conversion from surface energy to kinetic energy. It seems indeed strongly dependent to the exact nature of the coalescence