Transitions of bouncing and coalescence in binary droplet collisions

In droplet impacts, transitions between coalescence and bouncing are determined by complex interplays of multiple mechanisms dominating at various length scales. Here we investigate the governing parameters comprehensively experiments scaling analyses, providing a unified framework for understanding predicting outcomes when using different fluids. Specifically, while had not been observed in head-on collisions water drops under atmospheric conditions, it was found our to appear on increasing diameter sufficiently. Contrarily, always impacts alkane drops, disappear decreasing The variations related gas draining dynamics inter-droplet film suggest an easier means controlling as compared alternating ambient pressure usually sought. analysis further shows that given Weber number, enlarging or fluid viscosities, lowering surface tension contributes larger characteristic minimum thickness film, thus enhancing bouncing. key dimensionless group $(O{h_{g,l}},\;O{h_l},\;{A^\ast })$ is identified, referred two-phase Ohnesorge number liquid Hamaker constant, respectively. Our thickness-based model indicates ${h^{\prime}_{m,c}} > 21.1{h_{cr}}$ , where ${h^{\prime}_{m,c}}$ maximum value $({h_{m,c}})$ ${h_{cr}}$ critical thickness, occurs both off-centre collisions. That is, $1.2O{h_{g,l}}/(1 - 2O{h_l}) \sqrt[3]{{{A^\ast }}}$ fully developed regime occurs, thereby yielding lower efficiency. transitional universally be 4.