Pinching Dynamics and Satellite Droplet Formation in Symmetrical Droplet Collisions

Abstract

In head-on collisions between two droplets, reflexive separation is frequently formed, showing tentative coalescence followed by disintegration into two primary drops. With higher impact inertia relative to surface tension, characterized by a Weber number (We), more satellite droplets are created between the primary drops. In the symmetric configuration, the existing phenomenological models indicate the absence of satellite droplets at the onset We when the coalesced drops start to break. Supported by experimental and simulation evidence, here we demonstrate the exclusive formation of at least one droplet after pinch of the thread connecting the colliding drops. In accordance with the universal features of a thinning liquid filament approaching singularity as predicted by scaling theories of pinch-off, the mechanism of satellite droplet formation in the symmetrical impact of droplets is clarified. Via slight breaking of the symmetry, no satellite droplet can be observed, thus providing a possible interpretation for the discrepancy in the literature and implications for controlling undesirable drop formation.

Inertial stretching separation in binary droplet collisions

“…Although purely inertial behaviour is seen in the compression phase of head-on collisions (Planchette et al. 2017), increasing within this inertial range shifts the RS regime to significantly higher (from 30 to 190, figure 2 in Huang, Pan & Josserand 2019). As noted above, such a shift is not seen for the C–SS transition.…”

Transitions of bouncing and coalescence in binary droplet collisions

“…To set up the experiment, identical droplets of decane, dodecane, tetradcane or water were generated separately from two glass nozzles with fixed diameters by the conventional drop-on-demand method (Pan et al 2009(Pan et al , 2016(Pan et al , 2019Huang & Pan 2015;Huang et al 2019). The properties of the tested liquids are listed in table 2, including a large range of surface tensions (23.8-72.0 mN m −1 ) and droplet diameters (160-1000 μm).…”

“…Droplet collisions have been extensively studied in past decades (Ashgriz & Poo 1990;Jiang, Umemura & Law 1992;Qian & Law 1997;Estrade et al 1999;Brenn, Valkovska, & Danov 2001;Pan, Law & Zhou 2008;Pan, Chou & Tseng 2009;Zhang & Law 2011;Tang, Zhang, & Law 2012;Kwakkel, Breugem, & Boersma 2013;Huang & Pan 2015;Li 2016;Pan et al 2016Pan et al , 2019Sommerfeld & Kuschel 2016;Hu et al 2017;Al-Dirawi & Bayly 2019;Huang, Pan & Josserand 2019;Chubynsky et al 2020) due to significant relevance to a variety of systems in natural and technological situations. Examples are seen in raindrop formation (Gunn 1965;Strangeways 2006), medical therapy (May 1973;Feng et al 2016), disease transmission (Tellier 2009;Gralton et al 2011) and combustion processes in engines (Chiu 2000;Zhang et al 2016).…”

Suppressing Breakups in Binary Droplet Collisions by Surfactant

“…1(b) illustrates the definitions of the collision parameters, where U is the relative velocity, D the diameter of the droplet,  the projection of the separation distance between the droplet centers in the direction normal to the vector U , l and  the density and surface tension of the liquid, respectively. When droplets collide with sufficiently large We, breakups often occur with satellite droplets production [9][10][11][12][13][14]. For nearly head-on collisions, i.e., B < 0.2, the internal rebounding flows elongate the temporarily coalesced drop and rupture the droplet surface, producing the reflexive separation (IV).…”

“…This separation regime is caused by the coherent motions of the reflective and rotational flows inside the tentatively merged drop. Regarding the influence of liquid properties, physical models based on momentum theory or energy conservation were proposed to describe the transition boundaries between coalescence and separation regimes [7][8][9][14][15][16][17][18][19][20]. Previously, Jiang et al [9] proposed a sliding plate model based on the momentum theory to predict the transition from coalescence (III) to stretching separation (V).…”