The dynamics of head-on collision between two identical droplets was experimentally and computationally investigated, with emphasis on the transitions from merging to bouncing and to merging again, as the collision Weber number was increased. Experimentally the stroboscopically illuminated microphotographic images of two colliding droplet streams, generated through the ink-jet printing technique, were acquired with adequate temporal resolution of the collision event such that the instant at which the droplets merged for both soft and hard collisions was identified. Using this empirical information as an input, the simulated collision images were found to agree well with the experimental observations and allowed investigation of the collision flow field including the energy budget and the fundamental differences between the soft and hard collisions that lead to merging. It is further shown that the merging instant can be computationally assessed through the use of an augmented van der Waals force to effect merging through rupturing of the surfaces, with the associated Hamaker constant empirically but consistently extracted from the experimental observations.

By using the techniques developed for generating high-speed droplets, we have systematically investigated binary droplet collision when the Weber number (We) was increased from the range usually tested in previous studies on the order of 10 to a much larger value of about 5100 for water (a droplet at 23 m/s with a diameter of 0.7 mm). Various liquids were also used to explore the effects of viscosity and surface tension. Specifically, beyond the well-known regimes at moderate We's, which exhibited coalescence, separation, and separation followed by satellite droplets, we found different behaviors showing a fingering lamella, separation after fingering, breakup of outer fingers, and prompt splattering into multiple secondary droplets as We was increased. The critical Weber numbers that mark the boundaries between these impact regimes are identified. The specific impact behaviors, such as fingering and prompt splattering or splashing, share essential similarity with those also observed in droplet-surface impacts, whereas substantial variations in the transition boundaries may result from the disparity of the boundary conditions at impacts. To compare the outcomes of both types of collisions, a simple model based on energy conservation was carried out to predict the maximum diameter of an expanding liquid disk for a binary droplet collision. The results oppose the dominance of viscous drag, as proposed by previous studies, as the main deceleration force to effect a Rayleigh-Taylor instability and ensuing periphery fingers, which may further lead to the formations of satellite droplets.

The collision between aqueous drops in air typically leads to coalescence after impact. Rebounding of the droplets with similar sizes at atmospheric conditions is not generated, unless with significantly large pressure or high impact parameters exhibiting near-grazing collision. Here we demonstrate experimentally the creation of a non-coalescent regime through addition of a small amount of water-soluble surfactant. We perform a direct simulation to account for the continuum and short-range flow dynamics of the approaching interfaces, as affected by the soluble surfactant. Based on the immersed-boundary formulation, a conservative scheme is developed for solving the coupled surface-bulk convection–diffusion concentration equations, which presents excellent mass preservation in the solvent as well as conservation of total surfactant mass. We show that the Marangoni effect, caused by non-uniform distributions of surfactant on the droplet surface and surface tension, induces stresses that oppose the draining of gas in the interstitial gap, and hence prohibits merging of the interfaces. In such gas–liquid systems, the repulsion caused by the addition of surfactant, as frequently observed in liquid–liquid systems such as emulsions in the form of an electric double-layer force, was found to be too weak to dominate in the attainable range of interfacial separation distances. These results thus identify the key mechanisms governing the impact dynamics of surfactant-coated droplets in air and imply the potential of using a small amount of surfactant to manipulate impact outcomes, for example, to prevent coalescence between droplets or interfaces in gases.

The head-on collision of a droplet onto a liquid layer of the same material, backed bya solid surface, was experimentally and computationally investigated, with emphasis on the transition from bouncing of the droplet to its absorption by the film for given dropletWeber number, We, and the film thickness scaled by the droplet radius, Hf. Experimental results show that while absorption is favoured with increasing We, there exists a range around Hf ≈, 1 over which this tendency is moderated. This local moderation in turn corresponds to a regime, 11 ≲ We ≲ 14, over which increasing Hf from a small value leads to a triple reversalbehaviour of absorption, bouncing, absorption again, and bouncing again. The collision dynamics including evolution of the surface contours of the droplet and film, as well as the energy budgets, were then simulated by using a front-tracking technique. For collisionsleading to absorption and partial absorption, for which part of the absorbed droplet is subsequently ejected from the film, rupture and hence merging of the interfaces were manually imposed at an instant that leads to agreement between the subsequent calculated and experimental images. The simulation satisfactorily identified the different factors influencing the observed non-monotonic response of the collision event.

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