Session G01: Drops: Impact, Bouncing, Wetting and Spreading (5:00pm - 5:45pm CST)

The Effect of Raindrop Impact Location on the Dispersal Distance of Splash-Cup Plant Seeds

Splash-cup plants use the kinetic energy from falling raindrops to disperse their seeds. Understanding the biomechanics of splash-cup plants could improve understanding of the evolution of splash-cup plants, provide insight into how foliar diseases are spread, and lead to developments in ink-jet printing technology. In nature, raindrops can impact splash cups in random locations on the cup. Previous studies found that the impact location of a raindrop on a splash cup has a significant effect on the dispersal distance of its seeds, but this relationship has not been studied systematically. We released water drops above 3D printed models of splash cups containing 1 or 5 glass seed mimics. We varied the drop impact location from the center of the cup to past the edge of the cup, and we measured the dispersal distance of the seed mimics. We found that the maximal dispersal distance occurs when the drop impact location is close to the edge of the cup but still inside of the cup. We also investigated the interplay of cup angle (the angle of the cone above the horizontal) and drop impact location on dispersal distance.   

Gradient Submillimeter Groove Induced Directional Rebounding of an Impinging Droplet

Manipulating the liquid droplet lateral motion without any external energy input is a challenge but an essential technology for industrial applications ranging from water harvesting to electronic circuit fabrication. Although the droplet migration following the wettability gradient has been reported, the durability of the artificial chemical heterogeneity is still the biggest obstacle towards real life applications. Here we design a new wettability gradient surface that can drive droplet lateral motion, which is achieved by gradually altering the groove width of the structural topography. The droplet impinging behavior on the surface is investigated via a 3-D Direct Numerical Simulation (DNS) employing the Coupled Level-Set and Volume of Fluid (CLSVOF) surface tracking scheme, the Continuum Surface Force (CSF) method, and a mesh dependent Dynamic Contact Angle (DCA) model. The results show that the droplet lateral motion in the groove vertical direction can be following or against the topographic wettability gradient, which is determined by the coexistence of Cassie and Wenzel states and the unbalanced Young's force. The outcomes are helpful in designing robustness and durability surfaces with topographical wettability gradients for droplet transportation.   

Inclined impact of drops

Here we extend the theory for the case of normal impact of drops\footnote{Gordillo et al. \emph{J. Fluid Mech.} \textbf{866}, 298 (2019).} to predict the time-varying flow field and the thickness of the liquid film created when a spherical drop of a low viscosity fluid, like water or ethanol, spreads over a smooth dry surface at arbitrary values of the angle formed between the drop impact direction and the substrate. Our theoretical results accurately predict the time evolving asymmetric shape of the border of the thin liquid film extending over the substrate during the initial instants of the drop spreading process. In addition, the particularization of the ordinary differential equations governing the unsteady flow when the rim velocity vanishes provides an algebraic equation for the asymmetric final shapes of the liquid stains remaining after the impact, valid for low values of the inclination angle. For larger values of the inclination angle, the final shape of the drop can be approximated by an ellipse whose major and minor semiaxes can also be calculated by making use of the present theory. The predicted final shapes agree with the observed remaining stains, excluding the fact that a liquid rivulet develops from the bottom part of the drop.   

Drop bouncing dynamics on draining films: the influence of the entrained air layer

The characteristic deposition morphology of single droplet impact on smooth liquid films is contingent upon the Weber number (We), the balance of inertial and surface tension effects. We demonstrate previously unexplored dynamics between droplets impacting super-thin films described by the ratio of the liquid film thickness, $H$, to droplets with diameter, $D$ when $H/D$ \textless 0.1. Besides bouncing, delayed merging and early merging cases, a new phenomenon which we call `contact bouncing' associated with droplet impacting thin liquid film of the same substance at moderate to low normal We $=$ 5 \textasciitilde 20 is identified on the draining film (0.004 \textless $H/D$ \textless 0.085) such that the droplets can rebound with negligible mass losses even if the air layer between the droplet and liquid film has been ruptured. To analyze the mechanism of `contact bouncing', we posit that the ability for the droplet to bounce on the draining film is related to the mobility of the liquid film, and the entrained air layer purged by the liquid film initiates the occurrence of the contact. Moreover, normal impact speed as well as the liquid film thickness is presented in the phase diagram including bouncing contact bouncing, delayed merging and early merging regimes.   

Bouncing off the Walls: The Influence of Gas-Kinetic and van der Waals Effects in Drop Impact

The remarkable discovery that reductions in ambient gas pressure can suppress splashing [Xu et al., Phys. Rev. Lett. 94, 184505] prompted a flurry of experimental analyses aimed at elucidating the influence of air films during drop impact. This culminated in recent experimental observations revealing that when the impact speed is below a critical threshold, mm-sized drops no longer wet the solid but are able to skate over air nanofilms and rebound from surfaces of any wettability [Kolinski et al., Europhys. Lett. 108, 24001 (2014); de Ruiter et al., Nat. Phys. 11, 48 (2015)]. In this talk, I will demonstrate that the bouncing-wetting threshold can only be accurately predicted by accounting for micro-physics that is usually neglected - namely, gas kinetic effects (GKE) and van der Waals (vdW) forces. An efficient FEM computational model incorporating these effects is developed and shown to reproduce experimental results. To isolate GKE, the pressure dependence of the threshold is mapped and provides experimentally verifiable predictions. There are two principal modes of contact leading to wetting and both are associated with a vdW-driven instability of the film. This research has been recently published in [Phys. Rev. Lett. 124, 084501].   

Dynamic Wetting Regimes in Droplet Impact on Micropatterned Surfaces

The main objective of this study is to explore the dynamic behavior of a droplet impact at low Weber ($We$) numbers on hydrophobic micropatterned surfaces and to obtain insight into all possible dynamic wetting regimes. A series of continuum simulations has been conducted for a 1 $\mu$L water droplet at $We<30$ on micropatterned surfaces, whose roughness size is on the order of 25 $\mu$m. We examined three surfaces with different solid area fractions of $\phi = 0.04, 0.0443, 0.0625$ but with a similar surface roughness ratio ($r\approx 1.75$), so that their static wetting states are all Cassie-dominant. In total, we find 6 different dynamic wetting regimes, i.e., Cassie, Cassie rebound, temporary penetration rebound, Wenzel, Wenzel to Cassie, and Wenzel rebound. For the surfaces with a smaller $\phi$, more possible wetting regimes have been observed, and hence, the final wetting state is dependent on the initial impact velocity. Moreover, the regime boundaries have been evaluated in terms of $We$ and $\phi$. Our results show that at a given impact velocity, the solid microstructure plays an important role in impact dynamics and determines the final wetting state.   

Rayleigh-Taylor Instability in Drop Impact Experiments

When a liquid drop strikes a deep pool of a second liquid, an impact crater opens while the drop liquid decelerates and spreads on the surface of the crater. If the density of the drop is larger than the surrounding, we find that the drop-pool interface becomes unstable, producing mushroom-shaped plumes growing radially outward. We interpret this instability as a Rayleigh-Taylor instability associated with the approximately radial deceleration of the drop-pool interface. We investigate experimentally how changing the density contrast and the Froude number affect the instability and the growth of the resulting mixing layer. Using backlighting and Planar Laser-Induced Fluorescence methods, the position of the air-liquid interface, the mixing layer thickness and the instability wavelength are obtained. An energy conservation model for the mean crater radius is derived and compared with the experiments. Then, the mixing layer dynamics is explained by a model initially governed by the geometrical expansion of the crater, and then by the density-driven instability between the drop and the pool. Finally, the measured instability wavelength is compared with an approximate linear stability analysis of a spherical, viscous, and radially accelerated fluid sphere into a less dense fluid.   

Evolution of interface deformations for a drop impacting a viscous thin film

Surfaces coated with thin and viscous liquids provide hydrodynamic lubrication desired in many industrial and technological applications. However, these liquid films deform when subject to external forces eg. during drop impact, which could deteriorate the surface coating. In the present work, we perform experiments of a water drop impacting a thin silicone oil film in an ambient air environment. In the considered~low impact velocity regime, the water droplet rebounds from the coated surface due to air cushioning between the drop and the silicone oil surface. In order to better understand the coupling between the deformation of the water-air and the oil-air interface, we investigate a) the narrow air layer profiles sandwiched between the impacting drop and the underlying thin film during the impact and b) the thin film deformations immediately after the rebound of the drop. The air layer profiles and thin film deformations are measured independently using color interferometry and digital holographic microscopy techniques, respectively. We discuss the influence of film thickness, film viscosity, and drop impact velocity on the obtained air layer profiles and thin film deformations.   

High-Speed Droplet Impact Onto Deformable Substrates: Analysis And Simulations

The impact of a high-speed droplet onto a substrate is a highly non-linear, multiscale phenomenon and poses a formidable challenge to model. In addition, when the substrate is deformable, such as a spring-suspended plate or an elastic sheet, the fluid-structure interaction introduces an additional layer of complexity. We present two modeling approaches for droplet impact onto deformable substrates: matched asymptotics and direct numerical simulations. In the former, we use Wagner's theory of impact to derive analytical expressions which approximate the behaviour during the early stages of the impact. In the latter, we use the open source volume-of-fluid code Basilisk to conduct direct numerical simulations designed to both validate the analytical framework and provide insight into the later times of impact. Through both methods, we are able to observe how the properties of the substrate, such as elasticity, affect the behaviour of the flow. We conclude by showing how these methods are complementary, as a combination of both can lead to a thorough understanding of the droplet impact across timescales.   

The Science of Mocha Diffusion

Mocha diffusion is a well-known technique for generating beautiful, flower-like patterns on ceramic surfaces. A thin coating of wet, clay "slip" is first applied to the surface, followed by drops of dark ink with additives such as ethanol or vinegar. The result is a complex fingering pattern. Although one may intuit that Marangoni forces drive the spreading of the ink, we have found that the rheology and thickness of the underlying wet slip plays a critical role. Most importantly, the slip must be shear thinning. We have performed a number of experiments with both clay slip and other fluid systems to explore this phenomena. In the laboratory, we use shear-thinning solutions of sodium alginate or cornstarch and water as the sub-fluid, and drops of food coloring as the spreading fluid. There is an optimal consistency for the sub-phase that enhances fingering. Our preliminary rheology measurements suggest that a competition between Marangoni stress and the onset stress for shear thinning can qualitatively explain the appearance of fingers, but a quantitative mechanism for the initiation of the instability is not yet known. We will also show how mocha diffusion can be used as a great tool for teaching the beauty of fluid mechanics in K-12 classrooms (even virtually).   

Droplet dynamics in a stagnation-point flow

The application of airflow normal to a droplet-laden substrate is commonplace in drying and cleaning processes that are part of coating and printing applications. Recent studies by Hooshanginejad et al. have demonstrated that a partially wetting droplet can exhibit complex behaviors when subject to a stagnation-point flow of air. Depending on the droplet size, and magnitude and position of the air jet, the droplet is observed to oscillate in place, split into two, or depin from the substrate. In order to rationalize the experimental findings, we build a 2D lubrication model which implements the effects of potential airflow and a moving contact line using a precursor film and disjoining pressure. In this talk, we discuss the theoretical formulation and some preliminary model results in conjunction with the experimental findings.   

Non-Newtonian Drop Impacts: Spread and Retraction on Micropillars

Studies of drop dynamics are essential for controlling and optimizing drop deposition on any surface. Droplet interactions with micro-patterned surfaces such as a pillar array are important for numerous microfluidic and industrial applications. Drop spreading and retraction on patterned surface has been studied widely for Newtonian fluids, but impacts of non-Newtonian fluids on patterned surfaces have not been studied in detail [1-2]. Drop impact of non-Newtonian fluids is of significant interest owing to the common use of such fluids in industrial and biological processes [1]. In this study , the effect of surface patterning on drop spreading and retraction has been studied for both Newtonian (glycerol) and non-Newtonian (carbopol) aqueous solutions, and preliminary results will be discussed. We aim to investigate the underlying mechanism between surface patterns and drop dynamics for various non-Newtonian solutions. [1] N. Laan, K. G. de Bruin, D. Bartolo, C. Josserand and D. Bonn, Physical Review Applied 2, 044018 (2014). [2] S. Robson and G. R. Willmott, Soft Matter 12, 4853 (2016). -/abstract- Authors: Santhosh Kumar Pandian Miguel Balzan Geoff   

Image Classification for Splash Detection using Artificial Neural Network (ANN)

It is important to detect whether splashing occurs during a drop impact on a solid surface for various applications both in nature and industry. Although recent high-speed video technology has enabled time-resolved observations for the study of drop impact, splash detection still heavily relies on a frame-by-frame inspection with human eyes. This study classified the images of spreading and splashing drops using artificial neural network (ANN). A feedforward neural network (FNN) was trained and achieved 100{\%} for test accuracy and 1.0 for Area Under the Curve (AUC). The visualized weight matrix in the hidden layer of the trained FNN resembled the image of a splashing drop. Remarkably, the weight matrix also showed that other than the presence of ejected pieces from the drop, the shape of the impacting drop is another important judging criterion for the trained FNN. For images of splashing drop, the computed values before activated by sigmoid function in the output layer is proportional to the corresponding Weber number. This showed that although not being included in the training, Weber number of a splashing drop can be predicted by the trained FNN based on the splash intensity captured in the image.   

Droplet spreading over a non-Newtonian liquid film

We investigate the spreading dynamics of a liquid lens over a thin fluid layer. We consider the case of a liquid subphase which exhibits non-Newtonian behaviour (described by the Ostwald--de Waele constitutive equation) and we examine the spreading of both clean and surfactant-laden droplets. In the limit of both a thin droplet and a thin subphase, we employ lubrication theory to derive a coupled system of evolution equations for the interface positions and the surfactant monomer interfacial and bulk concentrations and the resulting governing equations are solved numerically using the ?nite element method. The results of an extensive parametric analysis to examine the effects of the physical parameters and rheological characteristics on the flow will be discussed.~   

Viscous Bouncing

Water drops impacting a superhydrophobic surface exhibit bouncing due to the inherent repellency of the substrate. This repellency persists even when the liquid viscosity is increased by two orders of magnitude. We show a way to predict the limiting bouncing viscosity. Furthermore, we discuss the variation of the contact time and elasticity of the rebound until viscosity eventually suppresses it entirely.   

A hydrodynamic analog of Friedel oscillations

Impurities on the surface of a metal may lead to the emergence of wavelike statistical patterns in the surrounding electron sea known as Friedel oscillations. We demonstrate that, despite its vast difference in scale, a classical hydrodynamic pilot-wave system may exhibit strikingly similar statistical behavior. Through experiments and simulations, we study the wave-mediated interaction between a liquid drop self-propelling on the surface of a vibrating fluid bath and a submerged circular well, that plays the role of an impurity, or topological defect, in the medium. The well induces a self-excited attractive force that draws the drop inwards along an Archimedean spiral, before it crosses over the well and departs along a straight radial path. The drop is thus scattered relative to its incoming direction. Oscillations in the drop speed emerge in its outgoing trajectory due to the waves induced by the drop's resonant interaction with the well. By considering an ensemble of particle trajectories, we demonstrate the emergence of localized wavelike statistics in the otherwise uniform histogram of the particle position, an effect strongly reminiscent of Friedel oscillations. The emergent statistical behavior is rationalized in terms of a wave-mediated interaction mechanism.   

Capillary wave instability driven air film rupture during drop impact on smooth surfaces.

The stability of the interstitial air film underneath a droplet plays an important role in drop contact dynamics upon impacting a smooth surface. A stable air film leads to droplet bouncing whereas an unstable air film results in droplet contacting the surface and consequent spreading or splashing depending on the impact velocity. Apart from the previously reported film and kink contact modes, here we present the theoretical and experimental evidence for a dimple failure mode of air film driven by a capillary wave instability, for a liquid droplet impacting onto an atomically smooth, lubricated surface. The dimple failure occurs beyond the inertial-capillary time scale and the contact is initiated when the dimple inverts at the central of the droplet. The effects of drop impact velocity and viscosity on the dimple failure mode are explored. While low viscosity droplets exhibit bouncing, dimple failure, and kink failure with increasing impact velocity, the dimple failure mode is absent in higher viscosity droplets due to viscous damping of the capillary wave.   

Power-Law Transition of Drop-impact Crater Collapse

The collapse of immiscible drop-impact craters is studied with two simultaneous ultra-high-speed video cameras. Similar to previous same-liquid impacts$^{\mathrm{1}}$, fastest jets emerge from a dimple at the bottom of the crater which contracts without bubble pinch-off. Different from the capillary-inertial collapse of a drop, where the neck radius scales as $R$\textasciitilde $t^{\mathrm{2/3}}$, the pure inertial collapse follows a power law of $R$\textasciitilde $t^{\mathrm{1/2}}$, where we find an exponent \textasciitilde 0.55 which is explained by a slow logarithmic approach$^{\mathrm{2}}$. For the pinch-off case, we discover a power-law transition from capillary-inertial to inertial when approaching the singularity for both immiscible and miscible liquid impacts, with a cross-over time \textasciitilde 100 $\mu $s before the pinch-off. The capillary-inertial part has a prefactor $C=R$/($\sigma t^{\mathrm{2}}$/$\rho )^{\mathrm{1/3}}=$1.75$\pm $0.2 based on pool properties, while the prefactor for inertial collapse, $C_{\mathrm{inertia}}=R$/(\textit{DUt})$^{\mathrm{0.5}}=$0.30$\pm $0.04, is found. Capillary waves are found to mold the air-dimple into different collapse shapes, such as bamboo and telescopic forms. The finest jets are only 12 $\mu $m in diameter and the normalized jetting speeds are up to one order of magnitude larger than for jets from bursting bubbles. The singular jets show the earliest cross-over into the inertial regime. The fastest jets can pinch off a toroidal micro-bubble from the cusp at the jet base. 1. Thoroddsen \textit{et al}., \textit{J. Fluid Mech.}, \textbf{848}$, $R3 (2018). 2. Eggers \textit{et al}., \textit{Phys. Rev. Lett.}, \textbf{98}, 094502 (2007). 3. Yang \textit{et al.}, Submitted to \textit{J. Fluid Mech.} (2020).   

Dynamics of viscous adhesion: from elongated capillary bridges to fingering instability

Two thin viscous layers adhere to one another through the formation of a liquid bridge that grows and expels the air separating the adhesive layers. The bridge feeds on the liquid layers, sometimes leading to a fingering instability. Unexpectedly, the fingers are formed by the liquid whose viscosity is higher than the surrounding air. To explore this instability, we first address the model problem of a single finger of wetting liquid bridging a bath to an overhanging beam. We show that the finger dynamics strongly depends on the liquid viscosity, the depth of the pool, and the gap separating the liquid surface from the beam. We then describe the fingering instability that occurs when putting two coated surfaces in contact, and present how the fingers dynamics is influenced by the gap between the solid surfaces, the amount of liquid available, and the physical properties of the liquid.   

Instabilities of spreading and wetting fronts of impacting drops

We experimentally observed the instabilities of both the spreading and wetting fronts of impacting drops of different physical properties and impact parameters. Both the instabilities were simultaneously observed with high-speed backlighting and total-internal-reflection (TIR) imaging. The backlighting method is suitable for the observation of the dynamics of spreading rim, while TIR for the contact line. Both the wave lengths of the instabilities were quantitatively evaluated with the Fourier transform and were compared. As a result, we found that both the wavelengths were in accordance with each other for water, while for glycerol-water solutions the wavelengths showed no correlation We also found that the criteria for the wetting instability cannot be determined only by capillary number of the contact line dynamics. Moreover, we found that the wettability between a solid surface and an impacting drop greatly affects the width of thin air film region located in between the spreading and the wetting fronts.   

Capillary Rebound: Droplets Bouncing on a Fluid Bath

The rebound of droplets impacting a fluid interface is studied both experimentally and theoretically. Millimetric drops are generated using a custom 3D-printed drop-on-demand (DOD) generator and impact a deep bath of the same fluid. Measurements are compared directly to the predictions of a quasi-potential model that resolves the time-dependent bath interface shape, droplet trajectory, and droplet deformation. The drop is modeled as a damped harmonic oscillator and its dynamics are directly coupled to the response of the interface through a single-point kinematic match condition which we demonstrate to be an effective and efficient model in certain parameter regimes. The influence of the physical parameters on the drop trajectory, restitution coefficient, and contact time is elucidated, with good agreement between the experiment and theory. This relatively efficient model is then readily extended to capture other physical scenarios, such as the impact of superhydrophobic spheres on a fluid interface. Ongoing and future work will be discussed.   

Drop Impact onto Polarized Dielectric Surface for Controlled Coating

Control of surface wettability by means of electrowetting or electrowetting-on-dielectric (EWOD) are among the most effective methods of active enhancement of surface wettability by liquids. The effectiveness of application of the electric field during drop impact is of importance for variety of coating and spraying technologies. Electrohydrodynamics of drop impact onto a dielectric surface with electrodes embedded in it is experimentally investigated. Water drop impact onto stretched Teflon and un-stretched Parafilm surfaces is studied. The results indicate that drop spreading on such non-wettable surfaces can be significantly enhanced with the electric field application. In particular, drop rebound can be suppressed by the electric forces.   

Dynamic Electrowetting-on-Dielectric (DEWOD): From Ab-initio to Macroscale Modeling.

In this contribution, we present the results of a combined density functional theory (DFT) calculations of water adsorption on dielectric surfaces and the phase-field modeling (PFM) of droplet impact and spreading. The DFT calculations are performed using VASP (Vienna Ab initio Simulation Package) code whereas the PFM is developed in Comsol Multiphysics software package employing phase-field, laminar flow and electrostatics modules. The DFT calculations are performed to reveal the dominant water adsorption sides, energetics and the electron density profile on Teflon and parafilm surfaces. The PFM macroscopic calculations are performed to model water droplet impact onto a dielectric hydrophobic Teflon and parafilm surface. Three cases of droplet impact are studied, namely, the impact onto a surface with no voltage applied, and the impacts onto the surfaces with 8.5 kV and 10 kV applied. The modeling results are directly compared to our own experimental measurements of droplet impact onto Teflon and Parafilm surface without and with an applied voltage as well as static contact angle measurements.   

Complex immiscible drop impact morphology

We present an experimental study of the various impact morphologies which can emerge when a heavy perfluorocarbon (PP1) drop impacts on a water pool. PP1 is immiscible with water and has larger density, but much lower surface tension. The drop deforms into a thin hemispheric layer covering the surface of the impact crater. The subsequent crater collapse leads to a plethora of different bubble or drop-entrapment phenomena, via dimple-formation at its bottom. We build a regime diagram, using different drop sizes and impact velocities, to classify these phenomena, such as air or water entrapped inside the PP1 liquid, or the dimple formation. Capillary waves form novel dimple-shapes, with bamboo-like or telescopic surfaces [1]. Focus is on the formation of the bamboo-like multi-dimples and the evolution process to the telescopic dimples. The width and the depth of the telescopic dimples are studied with the impact velocity. The collapse of these dimples can produce very fine dimples emerging at high speeds, the so-called singular jets. We characterize their velocity, thickness and the number of secondary droplets. REFERENCES: 1. Yang, Z. Q., Tian, Y.S. and Thoroddsen, S. T., `Multitude of dimple shapes can produce singular jets during the collapse of immiscible drop-impact craters'. Submitted to Journal of Fluid Mechanics (2020).   

Not Participating

Drop impacts are ubiquitous and relevant to many important natural and industrial processes. Although the kinematics of drop impact have been extensively studied experimentally due to the fast advance of high-speed photography techniques, the dynamic aspects of drop impacts remains largely unexplored. We investigate the pressure and shear stress distributions of drop impacts via a newly-developed technique, high-speed stress microscopy. By combining laser-sheet illumination, high-speed photography, and traction force microscopy, we track the fast movements of fluorescent particles embedded in elastic gels under the impact of liquid drops. The measurements enable us to obtain strain fields of the elastic gels induced by the impact. The temporal evolution of impact pressures and shear stresses of liquid drops can then be extracted based on the strain-stress relation of continuum mechanics. Our study on the pressure distribution confirms the key prediction of the self-similar theory and simulations, where the maximum impact pressure occurs near the contact line, rather than the center of impacting drops. The temporal evolution of the drag force can be deduced by integration of the shear stress. The information is crucial for mitigating impact-induced damages on solid substrates.   

Drop Impact on Heated Nanotextures

Drop impact on a heated surface is a ubiquitous phenomenon and plays a vital role in various applications, such as coating, cooling and combustion. We experimentally study and unravel the phase diagrams of different impact outcomes on heated (nearly regular) nano-pillars for the first time, under wide ranges of impact velocity ($V$) and surface temperature ($T_s$). Water drops can deposit, spread, rebound, or break-up with atomizing on the heated nanostructures as $V$ and $T_s$ are increased. We find a significant influence of nanostructures on the impact dynamics by generating particular events in specific parameter ranges. For example, events of splashing, gentle central jetting, and violent central jetting are observed on and thus triggered by the heated nanostructures. The maximum spreading factor, $\beta$, displays two separate trends on heated surfaces for the low-$We$ and high-$We$ ranges. Furthermore, a model is proposed by balancing the droplet dynamic and vapor pressure to predict the dynamic Leidenfrost temperature ($T_L^D$). Compared with the flat surface, the $T_L^D$ for $We \approx 10$ is decreased by the high-roughness nanotextures.   

Inertial effects on sphere settling through a liquid-liquid interface

Settling of a spherical particle toward an interface separating two liquids is encountered in many fields, from geophysics to engineering applications, where it is pivotal to understand, characterize and predict the floating/sinking outcome. In this study, we characterize the outcome of sphere settling for varying sphere size, density and drop height in the Reynolds number range of 40-180 based on sphere approach velocity and radius. The sphere motion and interface deformation are tracked by high-speed imaging. It has been shown previously that a theoretical model based on static conditions could predict critical conditions for floating/sinking transition of a sphere under dynamic conditions; except when the lower fluid was more viscous than the upper fluid and the sphere's Reynolds number (based on sphere velocity and radius) in the upper fluid was $\ge $ 1. In such cases, a downward `history' force from a collapsing sphere wake aided sinking. However, in this study, we found that the sphere inertia could significantly alter the floating-sinking transition condition also when the lower fluid is less viscous than the upper one and the Bond number (based on density difference between fluids and sphere radius) is $\ge $1.   

Spreading of viscous drops on a liquid-infused solid

Textured materials infused by oil, also known as liquid-infused solids (LIS), have special dynamical properties, in particular, because the liquid trapped in the texture lends them liquid-like properties with very little adhesion towards water and the possibility to promote slip. We revisit the classical problem of spreading of a droplet (water or glycerol), but on these special surfaces that are hemi-solid, hemi-liquid. We observe that a millimetric water drop spreading on a LIS (typically in a time scale of 10 milliseconds) is always slower than that on a solid - the more viscous the oil infused, the slower the spreading. Conversely, glycerol drops spread much faster on infused surfaces - the lower the viscosity of the oil, the faster the glycerol drop spreads, and always spreads faster than on a solid. We study experimentally the physics of such unique spreading behaviors and the parameters that govern it.   

Binary Droplet Wetting States on Structured Surfaces

The interaction between liquid and solid substrates is of great importance and offers an interesting and complex phenomenon of interest to many industrial and everyday applications. In this work, we report on the different wetting behaviors observed upon binary mixture droplet deposition on structured surfaces. The extent of wetting and the wetting regimes when depositing pure water, ethanol and their mixtures on micro-structured surfaces have been found to be highly dependent on the concentration of the mixture and the spacing between pillars. For pure water on short spaced structures, the droplet rests in the Cassie-Baxter (CB) mode, whereas as the spacing between pillars increases droplets typically sit in an intermediate regime or partial wetting regime (where part of the droplet rests above the structures while other regions penetrate within the structures). In the case of pure ethanol or low surface tension mixtures, droplets tend to be in the Wenzel regime. Moreover, for different binary mixtures, hemi-wicking and intermediate regimes have also been noticed depending on the different spacing. Our work is focusing on studying these different wetting regimes, providing a universal wetting regime map for binary mixture on structured surfaces and giving a theoretical and practical explanation for them.   

Surface impingement of high-speed micron-sized diesel drop trains: splashing characteristics and secondary droplets

This work investigates surface impingement of high-speed micron-sized diesel drop trains using computational simulations. The drop trains serve as a simplified analog to approximate fuel sprays. The drop size and impact velocity represent engine-relevant fuel injection conditions. The 3D simulations include impingements onto initially dry and wetted stainless steel substrates, where the effects of impingement frequency were quantified. The transition from depositing to splashing was identified and the effects of pre-existing film thickness were investigated. Using a robust algorithm, secondary droplet characterization was performed on simulation results to obtain distributions of secondary droplet size, velocity and trajectory angle. The results were compared to spray-wall interaction (SWI) sub-models commonly used in Lagrangian-Eulerian solvers. The comparison reveals the SWI sub-models suffer from significant inaccuracy under engine-relevant conditions, highlighting the need for further study of high-speed micron-sized fuel drop impingement.   

On the displacement dynamics and shape oscillation of an impacting water droplet on silicone oil pool at low weber number

Water droplet impact on liquid silicone oil pool was studied experimentally and theoretically using high speed imaging techniques. Interfacial studies related to immiscible liquids has a huge application in industries as well as medical fields. This study primarily focuses on the interfacial dynamics between the impacting water droplet and the silicone liquid bed with primarily focus on displacement dynamics and shape oscillations of the water droplet. The impact weber number was varied from 2 to approximately 20 in a controlled manner. The displacement of the water droplet was tracked using shadowgraphy and perspective view imaging at approximately 5000-10000 frames per second which was further corroborated using theoretical analysis. The forces affecting the displacement of the droplet are the drag forces, buoyancy forces and the weight of the droplet. However modelling the drag and buoyancy is quite challenging due to the deformation and shape oscillations of the droplet. At lower weber number several interesting phenomena were observed. The droplet oscillations leads to a transient air cavity formation just on the top of the droplet while it is sinking in the silicone oil. The air cavity collapses and pinches off forming an air bubble which gets entrapped in the oil medium.   

Evaporation Driven Droplet Spawning on Microdecorated Surfaces

The dynamics of solid-liquid interactions are of central importance in numerous scientific endeavours and are especially pertinent to the innovation of sophisticated microfluidic platforms. Emerging fluidic manipulation principles, for example, offer new perspectives on the outstanding challenges in designing microfluidic-based diagnostic and therapeutic technologies. In this work, we reveal an intriguing wetting phenomenon of an evaporating binary liquid, distinguished by the spontaneous formation of droplets from a liquid-imbibed microdecorated surface. Upon deposition, the water-ethanol mixture invades the topographical features, forming a finite (reservoir) droplet bound by a thin liquid film -- otherwise known as the hemi-wicking state. Shortly thereafter, additional mini-droplets spontaneously emerge from the liquid film. To explain this atypical behaviour -- occurring naturally under ambient conditions -- we consider the evolving physicochemical properties of the binary mixture, driven by selective evaporation of the more volatile component. The emerging picture reveals the complex interplay of the underlying principles, allowing us to establish the physical criteria conducive to the formation of spawned droplets.   

Selective droplet transport over asymmetric sawtooth surface microstructures

Manipulating the motion of small droplets is of significant interest for a broad range of applications including microfluidics, digital lab-on-chip platforms, spray painting and coating. In this study, water droplets of a few microliters were placed on a surface patterned with micron sized asymmetric sawtooth ridges. When the surface is subjected to symmetric horizontal oscillations, the droplets may move to a well-defined direction. The travel speed is shown to be strongly dependent on droplet volume, oscillation frequency, and surface pattern properties. The maximum travel speed could be estimated theoretically, and these were found to be in reasonable agreement with experiments for the smaller droplets (2 $\mu $l). We predicted how the droplet travel speed depends on droplet volume, oscillation frequency, and the estimated wetting resistance derived from the contact line friction and the detailed surface geometry. It is found that the droplet travel speed is significant in a rather narrow frequency range around the eigenfrequency. The frequency range of droplet transport was found to be narrower for larger droplets (6 $\mu $l), thus making the transport more selective. Altogether we demonstrate a selective droplet transport at a controllable speed.   

Droplet impact on asymmetric microstructures

The impact of liquid drops on a rigid surface is central in cleaning, cooling and coating processes in both nature and industrial applications. However, it is not clear how details of pores, roughness and texture on the solid surface influence the spreading stages of the impact dynamics. Here, we experimentally study drop impacting onto surfaces textured with asymmetric (tilted) ridges and quantitatively discuss the influence of the asymmetric microstructures with the maximum spreading radius of the droplet. We define the line-friction capillary number $Ca_{f}={\mu_f V_0}/{\sigma}$ (where $\mu_f$, $V_0$ and $\sigma$ are the line friction parameter, impact velocity and surface tension, respectively) as a measure of the importance of the topology of surface textures for the dynamics of droplet impact. We show that when $Ca_f \ll 1$, the contact line speed in the direction against the inclination of the ridges is set by line-friction, whereas in the direction with inclination the contact line is pinned at acute corners of the ridge. When $Ca_f\gg 1$, impact inertia of the droplet entirely governs spreading and the geometric details of non-smooth surfaces play little role.   

When does a viscous drop stop bouncing?

Processes involving liquid drop impact on solid substrates abound in nature. For example, drops bouncing-off a repellent (superamphiphobic) substrate. As the drop spreads, its initial kinetic energy is transformed into surface energy. The recoiling stage follows spreading after the liquid has reached its maximal extent, where surface energy is transferred back into kinetic energy. A part of the system's initial kinetic energy is lost throughout the process because of the viscous dissipation: Inside the drop, in the air boundary layer around the drop, and in the thin air layer between the drop and the substrate.\\ Recently, [Jha et al. 2020, DOI: 10.1039/d0sm00955e] showed that drops with viscosity as high as 200 times that of water could bounce-off from repellent surfaces. Here, we delineate this bouncing to no-bouncing transition in the Weber (inertia vs. surface tension) - Ohnesorge (viscosity vs. surface tension) numbers regime map. We also show that the liquid drops' viscous dissipation is as vital as other dissipation modes. As the Ohnesorge number increases, this dissipation dominates and controls the transition mentioned above. The present work provides information on the physics of the bouncing of viscous drops and closure about viscous dissipation's modes and location.   

Aerosol saliva droplet dynamics on surfaces around omni-phobic coatings

In the event of the recent coronavirus outbreak, NIH studied viruses being deposited from an infected person onto everyday surfaces in a household or hospital setting, such as through coughing or touching objects. Such studies are vital to understanding the nature of virus spread and can help in determining methods and materials that can lead to safety upgrades in infection prone scenarios. Studies have shown that viruses become inactivated and proteins lose activity upon exposure to air-water interfaces. However, when the viruses are in a three-part system consisting an aqueous medium, a surface and air referred to as a triple-phase-boundary system, stronger inactivation is expected. We are experimenting with 10 different highly touched surfaces to study the effect of a hydrophobic coating on the saliva droplet dynamics on coated surfaces, determining its contribution to the virus spread. The experiment includes using electrostatic spraying technology for coating and observing the effect of different surface morphologies due to the charged spraying on the droplet dynamics. We also test a nano-particle based disinfectant (Nanoxen) and its time effective anti-viral activity on these surfaces.   

Spreading of Volatile Low-Viscosity Drops on a Solid Dry Surface

We present the results of our investigation of the spreading of volatile low-viscosity drops (methanol, ethanol, and 2-propanol) spreading on a dry glass surface at room temperature under three atmospheric conditions: in ordinary air at 1 atm pressure, in an atmosphere of air mixed with each fluid's own saturated vapor at 1 atm, and along the coexistence curve in an atmosphere of each fluid's respective saturated vapor pressure. We observe that the spreading follows a power law, with the drop footprint radius $r$ growing as a function of time $t$, with $r \sim t^a$. The growth law is highly dependent on the surrounding atmosphere. For large aspect ratio “pancake” drops of these fluids in air at 1 atm, the exponent $a \sim 1/4$, in contrast to nonvolatile viscous fluids whose exponent is typically $1/10$ (Tanner's law) or $1/8$, depending on the regime. If the surrounding atmosphere is saturated with the volatile fluid's vapor (with or without the presence of air) the growth exponents revert back to conventional values near $1/8$. We present a model to explain this variation in spreading exponents.