Dynamics of squeezing fluids: Clapping wet hands

Abstract

Droplets splash around when a fluid volume is quickly compressed. This phenomenon has been observed during common activities such as kids clapping with wet hands. The underlying mechanism involves a fluid volume being compressed vertically between two objects. This compression causes the fluid volume to be ejected radially and thereby generate fluid threads and droplets at a high speed. In this study, we designed and performed laboratory experiments to observe the process of thread and drop formation after a fluid is squeezed. A thicker rim at the outer edge forms and moves after the squeezing, and then becomes unstable and breaks into smaller drops. This process differs from previous well-known examples (i.e., transient crown splashes and continuous water bells) in aspects of transient fluid feeding, expanding rim dynamics, or sparsely distributed drops. We compared experimental measurements with theoretical models over three different stages; early squeezing, intermediate sheet-expansion, and later break-up of the liquid thread. In the earlier stage, the fluid is squeezed and its initial velocity is governed by the lubrication force. The outer rim of the liquid sheet forms curved trajectories due to gravity, inertia, drag, and surface tension. At the late stage, drop spacing set by the initial capillary instability does not change in the course of rim expansion, consequently final ejected droplets are very sparse compared to the size of the rim.

Free radially expanding liquid sheet in air: time- and space-resolved measurement of the thickness field

“…Note also that this configuration is the non stationary version of a Savart sheet formed through the impact of a continuous liquid jet on a small disk at high Reynolds number (Clanet & Villermaux 2002;Clanet 2007;Bremond et al 2007). Such a free radially liquid sheet expanding in air and generating fluid threads and droplets at a high speed is also observed when a small volume of fluid is quickly compressed between two objects (Gart et al 2013). Rozhkov et al (2002Rozhkov et al ( , 2004 on the one hand and Villermaux & Bossa (2011) on the other hand proposed a theoretical model to describe the hydrodynamics of the free radially expanding liquid sheet, that lead to a same radial velocity field, u(r, t) but to different predictions for the radial sheet thickness field, h(r, t) and the sheet diameter, d(t).…”

Fluid dynamics of diving wedges

“…Our model bears similarity with previous models developed in the context of moving fluid sheet [14]. But it goes further in that it takes into account the sheet stretching and includes original contributions such as Venturi-induced suction.…”