Levitation of a drop over a moving surface

Abstract

We investigate the levitation of a drop gently deposited onto the inner wall of a rotating hollow cylinder. For a sufficient velocity of the wall, the drop steadily levitates over a thin air film and reaches a stable angular position in the cylinder, where the drag and lift balance the weight of the drop. Interferometric measurement yields the three-dimensional (3D) air film thickness under the drop and reveals the asymmetry of the profile along the direction of the wall motion. A two-dimensional (2D) model is presented which explains the levitation mechanism, captures the main characteristics of the air film shape and predicts two asymptotic regimes for the film thickness h 0 : For large drops h 0 ∼ Ca 2/3 κ −1 b , as in the Bretherton problem, where Ca is the capillary number based on the air viscosity and κ b is the curvature at the bottom of the drop. For small drops h 0 ∼ Ca 4/5 (aκ b ) 4/5 κ −1 b , where a is the capillary length.

Droplet levitation over a moving wall with a steady air film

“…The lubrication pressure depends on the shape of the air film (Neitzel & Dell'Aversana 2002). The film shape, including the free surface of the droplet, is determined by the local balance of the lubrication pressure and the surface tension and hydrostatic pressures (Lhuissier et al 2013). However, to the best of the authors' knowledge, such balances have not been experimentally verified for a wide range of parameters.…”

“…Isothermal non-coalescence phenomena between a droplet and a wall or a liquid surface can occur under various conditions. Examples include droplet levitation over moving solid walls (Neitzel et al 2001;Smith & Neitzel 2006;Lhuissier et al 2013;Saito & Tagawa 2015;Gauthier et al 2016), atomically smooth horizontal walls (de Ruiter et al 2015), inclined walls (Hodges, Jensen & Rallison 2004;Gilet & 262 E. Sawaguchi, A. Matsuda, K. Hama, M. Saito and Y. Tagawa 1 mm U FIGURE 1. Side view of a levitating droplet over a moving wall.…”

Mechanism of Contact between a Droplet and an Atomically Smooth Substrate

“…When a droplet lands on a substrate, a thin layer of air will be trapped in between [1][2][3][4][5][6][7][8][9][10][11][12], which significantly affects the dynamics of the droplet [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. Basic fluid mechanics tells us that it is very difficult to push fluid out of a narrow gap [29].…”

Droplet impact onto moving liquids

“…rain droplets falling on moving bodies of water, high-speed inkjet printing or spraying/painting onto moving substrates which may be dry or already wet (Derby 2010). Experimental and theoretical studies of liquid impact onto moving solid substrates are very scarce, but have demonstrated that splashing or bouncing can be controlled, and even partially suppressed, by adjusting the speed of the substrate (Bird, Tsai & Stone 2009;Lhuissier et al 2013). On the other hand, from the theoretical point of view, some light has been shed on related splashing problems such as angled impact of solid bodies onto liquid surfaces or oblique water-entry events, where asymptotic theories may be applicable to the problem at hand (Korobkin 1988;Howison, Ockendon & Oliver 2004;Moore et al 2012;Sun & Wu 2013).…”