Droplet Impact Dynamics Research Articles

Breaking the symmetry to suppress the Plateau\u2013Rayleigh instability and optimize hydropower utilization

Droplet impact on solid surfaces is essential for natural and industrial processes. Particularly, controlling the instability after droplet impact, and avoiding the satellite drops generation, have aroused great interest for its significance in inkjet printing, pesticide spraying, and hydroelectric power collection. Herein, we found that breaking the symmetry of the droplet impact dynamics using patterned-wettability surfaces can suppress the Plateau–Rayleigh instability during the droplet rebounding and improve the energy collection efficiency. Systematic experimental investigation, together with mechanical modeling and numerical simulation, revealed that the asymmetric wettability patterns can regulate the internal liquid flow and reduce the vertical velocity gradient inside the droplet, thus suppressing the instability during droplet rebounding and eliminating the satellite drops. Accordingly, the droplet energy utilization was promoted, as demonstrated by the improved hydroelectric power generation efficiency by 36.5%. These findings deepen the understanding of the wettability-induced asymmetrical droplet dynamics during the liquid–solid interactions, and facilitate related applications such as hydroelectric power generation and materials transportation.

Droplet splashing during the impact on liquid pools of shear-thinning fluids with yield stress

The impact of droplets on liquid pools is ubiquitous in nature and many industrial applications. Most previous studies of droplet impact focus on Newtonian fluids, while less attention has been paid to the impact dynamics of non-Newtonian droplets, even though non-Newtonian fluids are widely used in many applications. In this study, the splashing dynamics of shear-thinning droplets with yield stress are studied by combined experiments and simulations. The formation and the propagation of the ejecta sheet produced during the splashing process are considered, and the velocity, the radius, and the time of the ejecta sheet emergence are analyzed. The results show that the non-Newtonian fluid properties significantly affect the splashing process. The ejecta sheet of the splashing becomes easier to form as the flow index reduces, the large yield stress can affect the thickness of the ejecta sheet, and the spreading radius collapses into a geometrical radius due to that the inertia force is the dominant factor in the ejecta sheet propagation.

Evaluating a transparent coating on a face shield for repelling airborne respiratory droplets.

A face shield is an important personal protective equipment to avoid the airborne transmission of COVID-19. We assess a transparent coating on a face shield that repels airborne respiratory droplets to mitigate the spread of COVID-19. The surface of the available face shield is hydrophilic and exhibits high contact angle hysteresis. The impacting droplets stick on it, resulting in an enhanced risk of fomite transmission of the disease. Further, it may get wetted in the rain, and moisture may condense on it in the presence of large humidity, which may blur the user's vision. Therefore, the present study aims to improve the effectiveness of a face shield. Our measurements demonstrate that the face shield, coated by silica nanoparticles solution, becomes superhydrophobic and results in a nominal hysteresis to the underlying surface. We employ high-speed visualization to record the impact dynamics of microliter droplets with a varying impact velocity and angle of attack on coated and non-coated surfaces. While the droplet on non-coated surface sticks to it, in the coated surface the droplets bounce off and roll down the surface, for a wide range of Weber number. We develop an analytical model and present a regime map of the bouncing and non-bouncing events, parametrized with respect to the wettability, hysteresis of the surface, and the Weber number. The present measurements provide the fundamental insights of the bouncing droplet impact dynamics and show that the coated face shield is potentially more effective in suppressing the airborne and fomite transmission.

Impacting-bouncing nanodroplets on superhydrophobic surfaces under electric fields

Because electric fields can significantly modify the morphology of impacting droplets, the impact dynamics of droplets subjected to external electric fields have attracted extensive attention in recent years. Owing to the enhanced viscous effect and the altered viscous dissipation mechanisms, nanodroplets show distinctly different impact behaviors from macroscale droplets. However, it is not clear how electric fields affect the impact dynamics of nanodroplets, especially when the field direction is changed. In this study, molecular dynamics (MD) simulations were performed to reveal the bouncing dynamics of a nanodroplet impacting a hydrophobic surface under electric fields with various field strengths and directions. As compared with the case without an electric field, the bouncing dynamics of the nanodroplet were significantly modified in the presence of the electric fields with tilt angles of α = 0°, 30°, 45°, 60°, and 90°, especially when the field strength was higher than 0.08 V Å −1 . The restitution coefficient, ε b , was enhanced by the electric fields at a non-zero α with strengths larger than 0.08 V Å −1 . Applying an electric field with α = 60° and ≥ 0.08 V Å −1 would lead to the maximum bouncing velocity. The contact time was stretched by a perpendicular electric field when E ≥ 0.08 V Å −1 . When the impact velocity was not sufficient to make a droplet bounce off, an electric field with all directions was capable to cause the opposite. • Bouncing dynamics of nanodroplets subjected to an electric field are studied. • Applying an electric field promotes bouncing. • The electric field with a tilt angle of 60° yields the best bouncing. • A vertical electric field increases the contact time.

Controlling droplet splashing and bouncing by dielectrowetting

Stopping droplets from bouncing or splashing after impacting a surface is fundamental in preventing cross-contamination, and the spreading of germs and harmful substances. Here we demonstrate that dielectrowetting can be applied to actively control the dynamics of droplet impact. Moreover, we demonstrate that dielectrowetting can be used to prevent droplet bouncing and suppress splashing. In our experiments, the dielectrowetting effect is produced on a flat substrate by two thin interdigitated electrodes connected to an alternating current potential. Our findings show that the strength of the electric potential can affect the dynamic contact angle and regulate the spreading, splashing and receding dynamics at the right time-scales.

Corona splashing triggered by a loose monolayer of particles

In nature, high-speed raindrops often impact and spread on particulate surfaces (e.g., soil, plant leaves with spores or pollen). We study the dynamics of droplet impact on a loosely packed monolayer of particles by combining experimental and mathematical approaches. We find that the presence of mobile particles lowers the critical impact velocity at which the droplet exhibits corona splashing, as the particle area fraction is systematically increased. We rationalize this experimental observation by considering the jamming of frictional particles at the spreading rim. Elucidating the splashing transition of the drop on a particulate bed can lead to a better understanding of soil loss and erosion from falling raindrops.

Dynamic behaviors of impinging viscoelastic droplets on superhydrophobic surfaces heated above the boiling temperature

The impact dynamics of viscoelastic droplets is investigated on a superhydrophobic surface heated beyond the boiling temperature and up to the Leidenfrost temperature. Traditional impact outcomes, including rebound and deposition, are identified and analyzed. While the lower threshold Weber number for droplet rebound decreases with the surface temperature up to the Leidenfrost temperature (but not appreciably at higher temperature) but increases with the aqueous polymer concentration, the upper threshold stays nearly constant. Further analyses on the impact processes reveal that both the surface temperature and the polymer concentration have negligible effects on the droplet spreading, and the maximum spreading factor can be well described by a semi-empirical equation obtained from a simple energy-conservation modeling. By contrast, an increase in the surface temperature or a decrease in the polymer concentration would lead to a speed up of the droplet retraction, resulting in an increase in the restitution coefficient for bouncing droplets due to the suppression of the viscoelastic ligament formation. Moreover, the post-impact oscillation of depositing droplets was analyzed using a damped-harmonic-oscillator model, which shows that both the spring constant and the damping coefficient decrease with the surface temperature, suggesting a decrease in the viscous dissipation and the contact-line friction.

Droplet impact dynamics on single-pillar superhydrophobic surfaces

While ridged, spherical, or cone superhydrophobic surfaces have been extensively utilized to explore the droplet impact dynamics and the possibility of reducing contact time, superhydrophobic surfaces with a single small pillar have received less attention. Here, we report the rebound and splashing phenomena of impact droplets on various single-pillar superhydrophobic surfaces with the pillars having smaller or equal sizes compared to the droplets. Our results indicate that the single-pillar superhydrophobic surfaces inhibit the droplet splashing compared to the flat ones, and the rebound droplets on the former sequentially exhibit three morphologies of top, bottom, and breakup rebounds with the increasing of Weber number, while those on the latter only show the (bottom) rebound. The pillar significantly enlarges the droplet spreading factor but hardly changes the droplet width. Both the relations between the maximum spreading and width factors and the Weber number on all surfaces approximately follow a classical 1/4-power law. Reduction in the contact time is observed for the rebound droplets on the single-pillar superhydrophobic surfaces, dependent on the rebound morphology. Specially, the breakup rebound nearly shortens the contact time by more than 50% with a larger pillar-to-droplet diameter ratio yielding a greater reduction. We provide scaling analyses to demonstrate that this remarkable reduction is ascribed to the decrease in the volume of each sub-droplet after breakup. Our experimental investigation and theoretical analysis provide insight into the droplet impact dynamics on single-pillar superhydrophobic surfaces.

Impact dynamics of a charged droplet onto different substrates

Impact behaviors of an electrically charged water droplet on different solid substrates and subsequent dynamic mechanisms were experimentally investigated in this study. Droplets were generated from a metal capillary by a syringe pump with a constant diameter of about 2.2 mm. The capillary was directly connected to a high voltage direct current power supply, while a lower counter ring electrode was grounded. A high-speed camera was utilized to visualize the droplet impact morphology. The influences of the droplet charge density, substrate wettability, and surface temperature were analyzed. The results showed that the impact on hydrophilic surfaces exhibited a greater spreading diameter but a smaller recoiling height than that on a hydrophobic surface, which was attributed to the increased viscous dissipation on the substrate. In addition, compared with a neutral droplet, the maximum spreading diameter of a charged droplet was found to be improved by about 8.4%, where the enhancing effects were proportional to the droplet charge ratio. This was due to the weakening effects of the Coulomb repulsion on the liquid surface tension. Moreover, the impact of charged droplets on a hot copper substrate in three different boiling regimes, called convection, nucleate and film boiling, was also discussed. Finally, a model of the maximum spreading ratio of a charged droplet based on the Weber number, charge ratio, and wettability was established. This study demonstrated that the free charges in a droplet was able to influence its impact behaviors, which would hold great promise for some related technologies.

Surface Acoustic Waves to Control Droplet Impact onto Superhydrophobic and Slippery Liquid-Infused Porous Surfaces.

Superhydrophobic coatings and slippery liquid-infused porous surfaces (SLIPS) have shown their potentials in self-cleaning, anti-icing, anti-erosion, and antibiofouling applications. Various studies have been done on controlling the droplet impact on such surfaces using passive methods such as modifying the lubricant layer thickness in SLIPS. Despite their effectiveness, passive methods lack on-demand control over the impact dynamics of droplets. This paper introduces a new method to actively control the droplet impact onto superhydrophobic and SLIPS surfaces using surface acoustic waves (SAWs). In this study, we designed and fabricated SLIPS on ZnO/aluminum thin-film SAW devices and investigated different scenarios of droplet impact on the surfaces compared to those on similar superhydrophobic-coated surfaces. Our results showed that SAWs have insignificant influences on the impact dynamics of a porous and superhydrophobic surface without an infused oil layer. However, after infusion with oil, SAW energy could be effectively transferred to the droplet, thus modifying its impact dynamics onto the superhydrophobic surface. Results showed that by applying SAWs, the spreading and retraction behaviors of the droplets are altered on the SLIPS surface, leading to a change in a droplet impact regime from deposition to complete rebound with altered rebounding angles. Moreover, the contact time was reduced up to 30% when applying SAWs on surfaces with an optimum oil lubricant thickness of ∼8 μm. Our work offers an effective way of applying SAW technology along with SLIPS to effectively reduce the contact time and alter the droplet rebound angles.

Impact Dynamics and Freezing Behavior of Surfactant-Laden Droplets on Non-Wettable Coatings at Subzero Temperatures.

The present study investigates the impact and freezing behavior of the droplets of surfactant solutions on non-wettable coatings at very low temperatures of -10 to -30 °C. Our goal is to elucidate the critical role of concentration, molecular weight, and ionic nature of surfactants on these phenomena. To achieve this goal, we used sodium dodecyl sulfate (anionic), hexadecyltrimethylammonium bromide (cationic), and n-decanoyl-n-methylglucamine (nonionic) at four concentrations ranging from 0 to 2 × CMC (critical micelle concentration). We captured the impact-freezing of the droplets on superhydrophobic alkyl ketene dimer coatings using a high-speed camera at 5000 frames per second. The results show that the ability of the droplets to spread and retract on the coatings is a function of concentration, ionic nature, and molecular weight of the surfactants, as well as the temperature-dependent viscosity of the solutions. Additionally, surfactant-laden droplets generally demonstrated an accelerated freezing compared to pure water. This might be due to the fact that the presence of surfactants can promote both heterogeneous ice nucleation from within the liquid and a larger solid-liquid interfacial area by filling the air pockets of the surface, leading to enhanced heat transfer. The behavior of the cationic surfactant at certain concentrations was, however, an exception leading to a freezing delay, for which a mechanism will be proposed.

Droplet Impact on Surfaces with Asymmetric Microscopic Features.

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 initial stages of the impact dynamics. Here, we experimentally study drops impacting at low velocities onto surfaces textured with asymmetric (tilted) ridges. We found that the difference between impact velocity and the capillary speed on a solid surface is a key factor of spreading asymmetry, where the capillary speed is determined by the friction at a moving three-phase contact line. The line-friction capillary number Caf = μfV0/σ (where μf,V0, and σ are the line friction, impact velocity, and surface tension, respectively) is defined as a measure of the importance of the topology of surface textures for the dynamics of droplet impact. We show that when Caf ≪ 1, the droplet impact is asymmetric; 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 ridges. When Caf ≫ 1, the geometric details of nonsmooth surfaces play little role.

Comparison of cryogenic and non-cryogenic droplet impact dynamics at low velocities using OpenFOAM

Comparison of cryogenic and non-cryogenic droplet impact dynamics at low velocities using OpenFOAM

Freezing morphologies of impact water droplets on an inclined subcooled surface

Freezing of impact water droplets is ubiquitous in nature. Prior studies mostly focus on the freezing shapes of droplets impinging perpendicularly to a cold surface. In this work, we investigate how the frozen morphologies of impact water droplets are formed on a subcooled inclined (45 °) surface. To enhance the coupling between droplet impact dynamics and solidification, we hereby conduct the experiments on superhydrophilic surfaces under various substrate temperatures (-45 °C ≤ Ts ≤ -25 °C) and droplet impact velocities (1.33 m/s ≤ V0 ≤ 3.96 m/s) where the cooling rate is significantly improved. Intriguingly, we discover four types of frozen droplet morphologies, namely elliptical cap, half ring + cap Ⅰ, half ring + cap Ⅱ, and half ring + single ring, depending upon the impact velocity and substrate temperature. The formation of such morphologies resulted from the competition between the timescales associated with droplet solidification and impact hydrodynamics are appreciably altered by the inclined impact due to symmetry breaking as compared to the normal impact. To unravel the underlying physics, based on scaling analyses we propose a phase diagram to show how frozen morphologies are controlled by droplet impact and freezing related timescales, and find that such phase diagram can corroborate with the experimental findings.

Numerical study on surface-heterogeneity-induced anisotropic impact dynamics of droplet

Droplet impact on flat surfaces with heterogeneity is expected to trigger unusual impact dynamics, such as anisotropic spreading and directional rebounding, which in turn could be utilized in the control of droplet motion in many industrial processes. The present work illustrates a numerical study on droplet impact on a flat superhydrophobic substrate coated with a heterogeneous rectangle area, the influence of different positions and wettability of this heterogeneous area on the impact dynamics is investigated. The simulation results show the wettability of the heterogeneous area can impose significant influence on the spreading, retracting, and bouncing processes, especially when the heterogeneous area is located on a site other than the initial contact point, the bouncing of droplet is more likely to happen, and the contact time also becomes shorter. The clear pictures and analysis of the impact dynamics could advance the knowledge of how to control droplet motion by surface coatings.

Theoretical Modelling of Droplet Extension on Hydrophobic Surfaces

In this work, droplet impact dynamics on hydrophobic surfaces under different wettability and impact conditions are examined numerically. Multi-phase finite volume simulations are carried out by employing volume-of-fluid scheme and are validated against experimental results. Numerous droplet impact experiments are performed by varying Weber number, Reynolds number and substrate contact angle. Sticking, bouncing and splitting regimes are identified. It is observed that, for the same Reynolds number, the droplet attaches, rebounds or fragments depending on the Weber number. A semi-analytical representation using a single mass-spring-dashpot model is employed for the prediction of droplet height evolution during spreading and retraction. The spring and damping coefficients required for the model are estimated from the empirical correlations for contact time and restitution coefficients obtained from the simulations. Even though the Reynolds number is found to influence the coefficient of restitution significantly, it is found to have negligible influence on contact time. The theoretical forecasting exhibited good agreement with simulated results and it gives a significant reduction in computational time compared to the conventional full-field continuum simulations.

Effects of the surface wettability of nanoparticles on the impact dynamics of droplets

Herein, we report an experimental investigation on droplet impact dynamics with a particular interest in the effects of hydrophilicity and hydrophobicity of particles on the impact dynamics. Remarkable contact line pinning and receding breakup were observed when hydrophobic particles were present in the droplets, while hydrophilic particles showed little effect on the dynamics. Furthermore, the maximum retraction height and vertical oscillatory relaxation time for the droplet were restrained by hydrophobic particles. Scanning electron microscopy and rheological analyses revealed that these phenomena may be related to the adsorption and deposition of hydrophobic particles near the three-phase contact line, which leads to an increase in local viscosity and friction near the edge of the drop. In addition, the contact line pinning is positively associated with the Weber number and particle concentration. Our work provides a novel approach for droplet impact dynamics control, particularly for suppressing the oscillation and contraction of droplets.

Reduction in the contact time of droplet impact on superhydrophobic surface with protrusions

Previous studies have pointed out that adding a single protrusion on the superhydrophobic surface can effectively reduce the contact time of droplets, which is of practical importance in applications like anti-icing, self-cleaning, and anti-frost. However, the droplet impact dynamics and the mechanism for contact time reduction are still far from completely understood. Therefore, in this paper, via a three-dimensional pseudopotential lattice Boltzmann model coupled with a modified curved boundary scheme (which satisfies mass conservation), the droplet impact dynamics is simulated in a wide range of Weber numbers (0 < We≤ 67.7), protrusion size (10 ≤ W ≤ 30), and protrusion shape (triangle, square, and circle), with particular interest in understanding their effects on the contact time. We demonstrate that the variation of contact time depends on the bouncing type as well as the retraction way. Among three possible bouncing types observed in the simulation, including non-break bouncing, two droplets bouncing, and three droplets bouncing, two droplets bouncing has less contact time and could be facilitated by increasing We and using triangle protrusion. However, with increasing of W or using square protrusion, the bouncing type may change from two droplets bouncing to three droplets bouncing, and the deformation of fragments may become definitely different, leading to a long contact time. In addition, a transition from twice-retraction to once-retraction can be obtained by increasing We or making the protrusion shape sharp. When the fragments resulting from droplet splitting only retract once on the bottom wall, the contact time can be effectively reduced.

Droplet impact dynamics on an aluminum spinning disk

Droplet impact on a spinning surface has been observed in different industries and plays an important role in the performance of industrial systems. In the current study, the dynamics of water droplet impact on a hydrophilic spinning disk is investigated. An experimental setup is designed in a way that droplet diameter, impact velocity, disk rotational speed, and location of impact are precisely controlled. While the droplet diameter is fixed in the present study, other mentioned parameters are changed and their effects on the droplet behavior are discussed. High-speed imaging is used to record the droplet dynamics under various operating conditions. It is demonstrated that after impact, droplet spreads on the surface due to a high adhesion between water and the hydrophilic substrate. It is indicated that the wetted area is a function of time, impact velocity, disk rotational speed, and centrifugal acceleration. Furthermore, depending on the mentioned parameters, different phenomena such as rivulet formation, fingering, and detachment of secondary droplet(s) are observed. In the angular direction, in general, the wetted length increases as time passes. However, in the radial direction, the droplet first spreads on the surface and reaches a maximum value, and then recedes until a plateau is attained. At this instant, a bulk of liquid, which is called wave in this study, moves radially outward from the inner boundary of the droplet toward its outer boundary due to the effect of centrifugal force. Once the wave reaches the outer boundary, depending on its size and momentum, fingers or rivulets are formed, and small droplet(s) may detach. The process is analyzed comprehensively, and different empirical correlations for wetted lengths in radial and angular directions, secondary droplet formation, number of fingers, the onset of fingering, and wave velocity are developed.

Impact dynamics of Newtonian and viscoelastic droplets on heated surfaces at low Weber number

The mechanism of contact between liquid droplets and hot surfaces is important and attracts many researches recently. Herein, we experimentally investigate the contact of Newtonian and viscoelastic droplets between gradually heated surfaces at low Weber numbers, which has not been explored. To present a detailed analysis of contact line dynamics, experiments were performed across a surface temperature ranging boiling conditions to above Leidenfrost temperature (100°C−300°C), while various impact phenomena with increasing surface temperature have been observed for Newtonian and viscoelastic droplets. We demonstrate that the polymer additives significantly affect the dynamic contact angle and contact radius on heated hydrophilic surface. In the Leidenfrost regime, the increased velocity causes considerable reduction on contact time of water droplets, especially at the breaks-up mode. However, it does not influence the contact time of impinging polymer droplet. Aqueous polymer droplets with high molecular weight and high concentration suppress secondary atomization, splashing and break-up, but promote droplet foaming and the generation of viscoelastic filaments. The results illustrate how the polymer additives, surface temperature and impact velocity affect the impact outcomes, and original impact phase diagrams are proposed finally.