Session G31: Drops: Impact, Bouncing, Wetting and Spreading I

Impact forces of water drops falling on superhydrophobic surfaces

A falling liquid drop, after impact on a rigid substrate, deforms and spreads, owing to the normal reaction force. Subsequently, if the substrate is non-wetting, the drop retracts and then jumps off. As we show here, not only is the impact itself associated with a distinct peak in the temporal evolution of the normal force, but also the jump-off, which was hitherto unknown. We characterize both peaks and elucidate how they relate to the different stages of the drop impact process. The time at which the second peak appears coincides with the formation of a Worthington jet, emerging through flow-focusing. Even low-velocity impacts can lead to a surprisingly high second peak in the normal force, even larger than the first one, namely when the Worthington jet becomes singular due to the collapse of an air cavity in the drop.

    

Inertia and viscosity dictate drop impact forces

A falling liquid drop, after impact on a rigid substrate, deforms and spreads, owing to the normal reaction force. Indeed, during this process, we observe a maximum in the normal reaction force owing to the inertial shock experienced by the drop. Naturally, the time to reach this maximum is governed by the inertial timescale. Subsequently, if the substrate is non-wetting, the drop retracts and jumps off. We have recently shown that even the jump-off is associated with a maximum in the normal reaction force [Zhang et al. 2022, arXiv preprint arXiv:2202.02437]. The time at which the second peak appears is set by the inertio-capillary timescale, independent of the material and flow properties, such as the drop's viscosity and impact velocity. However, these properties dictate the magnitude of this peak. Furthermore, the second force peak also coincides with the formation of a Worthington jet which disappears at high viscosity owing to an increase in viscous dissipation that enervates internal momentum leading to poor flow focusing during retraction. In this work, we characterize both these peaks in the normal reaction force and elucidate how they relate to the different stages of the drop impact process (impact, spreading, retraction, and take-off).   

Contact formation and front propagation beneath impacting hemispherical bodies through air

Before an object can contact a surface during impact, it must drain the air beneath it. For droplet impacts and soft solids, the air will fail to drain, and instead compress. Due to the low viscosity of the air, the impact process takes place over fleeting timescales and diminutive length-scales, and is typically obscured by the impacting body, making direct observation difficult or impossible. Here we us a combination of the Virtual Frame Technique and Frustrated Total Internal Reflection that enables the direct visualization of contact formation and front propagation during impact. We find that the physical properties of the fluid or material involved in the impact process, including the capillary velocity in fluid droplets and the Rayleigh velocity in elastomer impact have non-trivial consequences for the impact process - both the droplet and the elastomer skate on a nanometer-scale film of air during the highest velocity impacts, but only the elastomer demonstrates a transition from elasticity- to inertially-dominated impact regimes. Perspectives on contact front instability will be discussed for both liquid droplets and soft elastic hemispheres.   

Impact of a liquid marble on a flat plate

A millimetric droplet of fluid coated in a hydrophobic powder, also known as a liquid marble, is released onto a flat surface. The marble behaves as a soft elastic solid during contact and can rebound multiple times before eventually dissipating its initial impact energy. Curiously, it is observed that coefficient of restitution depends non-monotonically on the impact speed. Experimental measurements for the coefficient of restitution and contact time are compared to the predictions of a weakly viscous droplet rebound model, solved using a kinematic match condition. The model is then extended to study the case where the impact surface oscillates vertically and captures a period-doubling cascade to chaos observed in experiment. Ongoing and future work will be discussed.   

Droplet Jump from Particle Bed

Droplet rebound from a bed of mobile solid particles is central for understanding a range of phenomena including the cleaning of surfaces via the impact of water and the erosion caused by rain. We investigate droplet rebound from shallow particle beds in the unique low-gravity environment of a drop tower. In our experiments a droplet is deposited on a thin layer of particles, at the start of the drop test the droplet jumps from the particle bed in response to the step reduction in gravity. As the droplet leaves the particle bed, it takes a fraction of the particles with it. Droplet jump properties such as contact time and velocity are considered. Characteristics of droplet jump from a particle bed are compared to droplet jump from a superhydrophobic surface.