The onset of fragmentation in binary liquid drop collisions

Abstract

International audienceBinary collisions of drops of immiscible liquids are investigated experimentally at well-defined conditions of impact. In the experiments we vary all relevant properties of an aqueous and an oil phase, the impact parameter, the drop size and the relative velocity. The drops observed after the collisions exhibit three main phenomena: full encapsulation, head-on fragmentation, and off-centre fragmentation. The regimes characterized by these phenomena replace the ones observed in binary collisions of drops of the same liquid: coalescence, reflexive separation, and stretching separation. Our aim is a universal description of the two fragmentation thresholds of such collisions. Based on the capillary instability and an energy balance, we establish for head-on collisions a scaling law for the evolution of the threshold impact velocity with the properties of the liquids and the droplet size. The fragmentation threshold for off-centre collisions is compared to established models from the literature, which appear unsatisfactory. Introducing an effective impact parameter, which accounts empirically for the deformation and rotation of the drops upon impact, we describe this fragmentation threshold in a universal way. For both fragmentation thresholds, the agreement between experimental data and their theoretical representation is very good. Our work yields new insight into binary collisions of drops and proposes a perspective to develop a more general description with implications for binary collisions of drops of a single liquid as well

Water colliding with oil

“…An interesting variation on this theme, which mixes the mechanics of impacts with the physics of wetting (owing to the presence of three phases) was considered by Planchette, Lorenceau & Brenn (2012), who looked at the collision of two drops of different chemical nature (oil and water). Rebound and fragmentation are also observed, but because water and oil do not mix, the product of the collision can be a drop of liquid 1 fully encapsulated by a shell of liquid 2, as sketched in figure 1(a).…”

“…The shell itself can be solidified by polymerizing the oil, making the process relevant in cosmetics or in pharmaceutical applications. Hence a natural question, raised by Planchette et al (2012) is: can we make these objects in a dynamical way, taking advantage of the collision, but without breaking up the liquids? Günter Brenn invented a wonderful tool for this kind of study: he designed a drop gun, which shoots at regular intervals calibrated drops (of a fraction of millimetre in size), at a well-defined velocity (in the range of metres per second).…”

“…Beyond their intrinsic relevance, these questions are also practical: Planchette et al (2012) found that fragmentation occurs if the aspect ratio of the cylinder is larger than π, that is, the criterion discovered by Plateau (and discussed by Delaunay and Rayleigh) for the instability of a liquid cylinder. It is remarkable that this 'static' criterion remains valid in a dynamical situation, where internal flows and elasticity arising from the ends of the cylinder might modify it.…”

“…In Planchette et al (2012)'s work, oils have a viscosity ∼10 times larger than water, and glycerol can be added to the water, so that the distortion of the mixed oil/water drop implies a non-negligible dissipation of energy. Planchette et al use the viscosity of the liquid to prevent the development of the cylinder (in figure 1, the viscosity of the system decreases from figures 1b to 1d).…”

“…It is certainly useful to tackle these problems in a more fundamental way, allowing us to quantitatively understand these complex situations. This of course leads not only to a better control of the process of encapsulation, but also to new solutions: Planchette et al (2012) report for example that even when fragments are made, oil can be transferred to the different drops of water, so that multiple encapsulated drops are simultaneously achieved. More fundamentally, these experiments make it possible to perform controlled impacts in air, a situation simpler than on a solid, which helps to build a solid ensemble of data on which to base the very active numerical and theoretical research on these subjects.…”