Session ZC20: Drops: Impact on Liquids

High-speed drop impact onto moving pools: ejecta sheets and splashing

Droplets often impinge at high-speed onto moving liquid surfaces, transforming droplet impact into a fully 3D problem, up from the typical axisymmetric configuration of normal impact on a static pool. Examples include rain drops landing on oceans and vehicles, in addition to inkjet printing and additive manufacturing. We investigate fast (>2 m/s) droplet impact onto slower-moving (<0.5 m/s) deep liquid pools, whilst also varying the droplet impact velocity and diameter, in addition to fluid properties. Above a certain threshold, we show that pool movement has a dramatic effect on ejecta sheet dynamics in low viscosity conditions, resulting in different post-impact dynamics arising around the circumference of the droplet, and altered splashing behaviour. We present a way to parameterise the impact outcome that accurately classifies the post-impact behaviour for a wide range of fluid properties and dynamic conditions, and naturally recovers the well-known transition due to Reynolds number on static pools. We use this parameterisation alongside observations from higher viscosity conditions to offer insights into the physical mechanisms underpinning impact outcomes on both moving and static deep pools.   

Low speed drop impact onto moving pools: the bouncing to coalescence transition

Normal droplet impacts on a deep fluid bath have been studied in depth using a range of experimental, theoretical, and computational techniques. In particular, the well-defined transitions from bouncing to coalescence, as well as coalescence to splashing, have been extensively mapped due to their relevance in application. However, comparatively less is known about these transitions in the more general scenario where the droplet and bath have a relative horizontal velocity. In this work, we study the bouncing to coalescence transition for a droplet impinging on a deep layer of fluid moving with a constant speed. We achieve a steady tangential bath velocity by using a large diameter annular fluid container spun up to achieve rigid-body rotation with minimal surface disturbances. We demonstrate that the normal impact velocity at which the bouncing to coalescence transition occurs is modified by the presence of a tangential pool velocity, and study its dependence on pool velocity, droplet size, and fluid viscosity. Experimental measurements are compared to direct numerical simulations which resolve the gas layer dynamics, and ultimately allow us to rationalize the observed change in threshold.   

Dynamics of the emerging jet after drop impact

We present detailed measurements of the velocity inside the jet and in the pool below the jet after droplet impact. The motion of the emanating jet is generally considered to be ballistic, but recent findings indicate that other forces are acting on it, which causes the jet to decelerate at a much higher rate. Using combined PIV and shadowgraphy we determine the space-time diagram of the jet that emerges after drop impact. Droplets are released from heights between 100 and 1000 mm. PIV measurements provide measurements of the velocity field inside the jet. These are combined with PIV measurements of the flow in the pool below the jet. Image series are recorded at 12,800 frames per second. It is found that the deceleration of the jet tip may be up to 15 times larger than the gravitational acceleration. The results are compared against a new theoretical model that describes the dynamics of a finite-length slender jet.   

Exploring Droplet Impact and Splashing on Surfactant-Laden Shallow Pools

The interaction between droplets and shallow pools plays a crucial role in various fields, from inkjet printing to coating technologies. Understanding the impact and splashing dynamics of droplets on liquid films is of significant interest for both fundamental research and practical applications. This is particularly true in the presence of surfactants, which can be either naturally occurring, purposely added, or undesired contaminants. In this presentation, we study droplet impact on surfactant-laden shallow pools, endorsing the effects of surfactant-induced Marangoni stresses. We study these phenomena to unravel the intricate mechanisms governing droplet impact and splashing behaviour. Furthermore, we explore the influence of surfactant properties on the interfacial dynamics, including the surfactant-induced local reduction of surface tension and the adsorption/desorption of surfactant between the interface and the bulk. Our findings shed light on the complex interplay between droplets, shallow pools, and surfactants, paving the way for enhanced control and optimisation of various processes involving droplet impact and shallow pools.   

Impact and bouncing of Leidenfrost Jets

The Leidenfrost effect consists of a liquid droplet surfing over its own evaporating vapor layer. It is now widely studied, essentially because of its analogy with a drop resting on a perfectly super-hydrophobic substrate. This effect has also been evidenced in systems different from the original Leidenfrost droplets: for example, drops over granular materials, sublimating solids over a heated solid substrate, or a droplet over a heated liquid. Surprisingly, the case of liquid jets has been studied solely for macroscopic jets impinging on heated plates at normal incidence to characterize the heat transfer and subsequent cooling of the plate at the contact boiling transition. In this study, we show for the first time that the Leidenfrost effect exists for millimetric impinging jets. The influence of several parameters, jet radius and velocity, incident angle and substrate temperature are experimentally investigated. We mainly focus on the minimal thickness of the vapor cushion and on its typical area over which the jet bounces on its own vapor layer. We also show that a directed percolation-like transition appears just before the jet enters contact boiling on the heated substrate.   

When is a sphere flat?

Water impact events are characterized by large, short-lived impulses during the early stages of water entry.  For sphere impacts, the analytical force model of Shiffman and Spencer from 1945 has been repeatedly confirmed by experiments for a range of sphere radii r.  The transient force is driven by added mass as a growing surface area of the sphere becomes wetted.  On the other extreme, the transient force on a flat disk impacting water is so fast that the impact physics are altered (Jain et al. 2021).  However, as a sphere looks increasingly disk-like as , this raises the question of when and how sphere impact behavior transitions to disk impact behavior.  Here, we vary the nose curvature on a cylindrical body from hemispherical (r = R_cyl) to flat (r => infinity) and experimentally measure the water impact forces on free-falling bodies.  We show that the classical sphere model of Shiffman and Spencer explains the measured forces until the nose curvature reaches .  Near this nose curvature, the body forms a depression on the water surface prior to impact, resulting in a trapped air layer whose dynamics play a significant role in the impulsive force.  Because this depression has finite curvature, we hypothesize that the largest impact force will not be on a perfectly flat nose, but rather a nose with slight positive curvature, which is supported by our experiments.  We thus explain the mechanisms by which sphere impactors behave as flat disks and demonstrate the implications for water entry forces.   

Bouncing-to-merging transitions of droplet impact on heated liquid film

Droplet impact on a liquid film is ubiquitous in numerous natural

and industrial processes. In many of these applications, the impacting

droplet and the impacted film contain liquids at different temperatures.

In this talk, we will discuss the findings from our study on droplet (radius,

R) impact on a heated liquid film of varied thickness (H). Using n-

tetradecane as our test liquid, we conducted a series of experiments where

the temperature difference (ΔT ) between the liquid film and the droplet

was varied from 0 to 60°C. The primary focus of this work is to identify

changes in the transition between bouncing to merging outcomes as the

film temperature increases. We will first compare the regime diagrams

where bouncing and merging outcomes were mapped in a Weber number

and normalized film thickness (H^* = H/R) space for various ΔT . The

regime maps show that, for deep pool (H^* >> 1) and shallow pool (H^∗ ≈

1) limits, ΔT exhibits varied degrees of influence on the bouncing and

merging transitions. Large ΔT , however, renders merging easier across

all H^* and beyond a critical temperature difference (ΔT ≥ 60°C for n-

tetradecane) the bouncing regime essentially disappears. Subsequently, to

explain the observed dynamics, we will discuss the role of evaporation and

vapor layer adjacent to the heated liquid film on the interfacial merging.   

Impact of non-spherical droplets onto a liquid pool

Droplets larger than the capillary length either released from a needle or generated from any other mechanism are not perfectly spherical and show significant variations in their axis ratio after release. Variations in the droplet axis ratio may significantly affect the depth of cavity penetration, shape, and formation-retraction time. We investigate crater evolution and crater shape formed by the impact of very large non-spherical droplets onto a deep liquid pool theoretically as well as experimentally. Both a droplet impact energy balance and a potential flow model are proposed to predict the effect of droplet axis ratio on the crater shape. Experimental results show that the droplet shape at impact significantly change the crater shape and evolution time. Prolate droplets make deep-narrow craters and oblate droplets make shallow-wide craters. The time of cavity formation and retraction is short for prolate and long for oblate shaped droplet impacts. The energy model reveals that the cavity aspect ratio scales parabolically with the droplet axis ratio. The potential flow model predicts cavity evolution time for different droplet shapes with good experimental agreement.   

Capillary retraction of a slender fiber into a driven and confined microfluidic droplet

The elastocapillary deformation of flexible objects by a liquid interface has been widely studied from an experimental and theoretical point of view. It has been shown that the equilibria for these systems depends on the surface tension between all three phases as well as the bending modulus of the deformable object. Depending on the liquid/flexible object combination, a plethora of self-assembled structures including coiled droplets, bundled hair and capillary origami have been observed. If these equilibria are well described in the literature, the dynamical approach towards such equilibrium in the presence of a surrounding hydrodynamic flow has been less reported.

This study therefore aims to explore a visco-elasto-capillary relaxation starting from a model non-equilibrium state. For this purpose, the dynamic retraction of a slender fiber into a moving droplet was observed. We have experimentally witnessed such a phenomenon in droplet/fiber pairs prepared in biphasic microfluidic devices under pressure-driven flow. We observed two distinct dynamics depending on the sense of retraction of the fiber compared to the direction of the external-phase flow, and depending on the relative viscosity of the droplet and external phases. Using a minimal model considering an energy balance between the surface energy of the fiber and viscous dissipation, the observed capillary retraction dynamics can be described.