Experimental investigations of droplet impact on a sessile droplet

Drop-on-drop collisions have garnered great interest due to their common occurrence in applications such as additive manufacturing and spray deposition. When a drop impacts a sessile droplet, the inertia of the impinging droplet converts into the deformation of the droplet and viscous dissipation. Such interactions are often considered to be dominated by inertia and surface tension and are regarded as a surface-driven phenomenon. However, a recent study [Damak and Varanasi, Phys. Rev. Fluids 3, 093602 (2018)] stated the existence of a viscous regime after a critical impact velocity, which prompts reevaluation of the pre-established analogy. The present work expands the scope of analyses with a wide range of impact velocities and with varying viscosity of the droplet and proposes an empirical model with a maximum error of 3.06% to predict the maximum spread of the droplet in drop-on-drop impact. Two different regimes of spreading, namely, inertio-capillary regime and inertio-capillary-viscous regime, were identified. The study highlights a critical velocity after which a regime shift was noticed. A non-dimensional model for critical velocity is proposed to identify the transition between two regimes. The present work produces some insightful speculations about the physics of interfacial interactions in drop-on-drop impacts, based on the experimental observations and the literature.