Water Droplet Impact Research Articles

Dynamics of water droplet impact on a textured heated and tilted surface

The study reports the experimental results of dynamics of water droplet impact on smooth and rough tilted substrates made of stainless steel with different temperature of the surfaces. The influence of the direction of the texture and the temperature of the surface on the characteristics of droplet impact and sliding was studied. The tilt angle of the substrate was varied over a wide range. The Weber number was 100.It was established that the temperature of the surface did not influence the droplet/surface interaction modes. However, at the tilt angle of the substrate more than 20°, the surface temperature began to have an effect on the characteristics of the droplet sliding. The distance a water droplet travelled on the surface from the moment of contact with the substrate to the moment it came to a complete stop (droplet pinning) increased. The greatest influence of the temperature and tilt angle of the surface was observed when the droplet slid over the substrate in the direction not along but across the scratches. In this case, the distance a water droplet travelled on the surface increased significantly.

Numerical and theoretical modeling of water droplet impact on hydrophilic and superhydrophobic cones

The phenomenon of droplet impingement on solid surfaces is prevalent in various natural and industrial contexts. Research on impact dynamics on conical surfaces keeps emerging, with superhydrophobic cones receiving more attention than hydrophilic ones. This study systematically investigates water droplet impact dynamics on both hydrophilic and superhydrophobic cones using a two-phase numerical solver under different Weber numbers (We) and cone angles (φ). Three distinct phases are identified in the We–φ map to describe the different outcomes on each type of cones. Generally, deposition occurs ultimately on hydrophilic cones, whereas rebounding is observed on superhydrophobic ones. The maximum spreading area βAmax on hydrophilic cones depends only slightly on φ but consistently increases with We, following a scaling law of We0.5 at higher We. In contrast, on superhydrophobic cones, βAmax increases significantly with both We and φ, and the exponent in the scaling laws of βAmax with respect to We increases notably as φ increases. Three characteristic times are defined to describe important motion states on both types of cones. Corresponding scaling laws for each time with We are established. Two theoretical models are developed to predict the maximum spreading position for droplets on hydrophilic cones and the rebound position on superhydrophobic cones, respectively. Gravitational potential energy is included in the energy budget for both models, and an auxiliary viscous dissipation due to spontaneous spreading is accounted for the hydrophilic case. Satisfactory agreement between the theoretical and numerical results is achieved.

A durable superhydrophobic surface with bud-particle structure prepared by one-step spray method

Superhydrophobic surfaces play an essential role in numerous applications such as anti-fouling, waterproofing, and drag reduction due to their water-repellent. However, for most fabrication methods, there are high requirements for the preparation process and conditions. Meanwhile, the superhydrophobic surfaces generally have the problem of insufficient durability. In this study, a one-step method to fabricate a stable superhydrophobic surface by spraying is proposed. A new bud-particle structure is obtained by modifying micron and nano Aluminium hydroxide(Al(OH)3) particles, which helps the coated surface to demonstrate excellent superhydrophobicity with the water contact angle ∼157.0° and rolling angle ∼2.0°, and it shows great repellency to water droplet impact. Due to the compatibility of Al(OH)3with the pretreatment substrate and the reinforcing effect of nanoparticles, the coating maintained the superhydrophobicity well in the sandpaper wear and the ultrasonic damage test, which reflects the outstanding robustness of the coating.Moreover,benefiting from the control of the wetting state by micronand nanoparticles doping,the coating showed an excellent underwater drag reduction effect in the rheometer experiment, with a drag reduction rate of ∼67.7 %. The one-step spraying method proposed is a simple and convenient way to create the surface with excellent superhydrophobicity and durability, which has great potential applications in improving metal surface oxidation resistance and underwater reducing drag.

Lattice Boltzmann simulation of water droplet impact and freezing on inclined supercooled surfaces with different roughnesses and wettabilities

Water droplet's impact and subsequent freezing on inclined supercooled surfaces, having micro-pillar structures with three different types of wettabilities (hydrophilic, neutral, or hydrophobic) are investigated numerically by a three-dimensional, multi-relaxation time, pseudo-potential liquid-vapor phase-change lattice Boltzmann method. Effects of volume expansion of water at 0 °C as well as the advancing contact angle of the droplet on real rough surfaces are considered in this model. Temporal variations of droplet morphological changes after impact and subsequent freezing on the supercooled inclined surfaces under various wall temperatures are illustrated. Simulation results indicate that after impacting on the supercooled rough inclined surfaces at a given Weber number, a water droplet can form fully penetrated elliptical ice morphologies and stretched stream-like ice morphologies attached on the supercooled surface, or partially and fully rebounding from the supercooled surface depending on the wettability of the surface, the surface inclination, and the degree of supercooling. It is shown that elliptical ice patterns are formed on hydrophilic substrates where droplets do not recoil after spreading, while stream-like ice patterns are mostly formed on neutral substrates where droplets are gradually stretched due to the gravity because droplets are attached on the inclined surface. On the other hand, partial and full rebounds of droplets occur on hydrophobic surfaces having high inclination angle where droplets undergo strong recoiling and rebounding after spreading on rough surfaces. At low degrees of supercooling (i.e., higher substrate temperatures), partial rebounds occur because a portion of the water droplet freezes on the substrate during contact with the supercooled substrate; otherwise, fully rebounding occurs. These results elucidate the effects of wettability, roughness, temperature and inclined angle of the wall on ice morphologies. Additionally, effects of Stefan number (substrate temperature) on the four different ice morphologies of a water droplet (D0 = 100 and Pr = 13.5) after its impact on supercooled inclined rough surfaces at a Weber number of We = 112 (Reynolds number of Re = 164.9) are illustrated.

Successive rebounds of obliquely impinging water droplets on nanostructured superhydrophobic surfaces

The impact and rebound of water droplets on superhydrophobic surfaces frequently happen in nature and also in a number of industrial processes, which has thus stimulated strenuous efforts to explore the underlying hydrodynamics. Despite that massive achievements have been made over the past decades, existing works are mostly focusing on the short-time bouncing dynamics after a single impact; however, the long-term, successive droplet rebounds, which are practically more important, only received very limited attention. In this work, we perform an experimental investigation on the impact of water droplets on inclined nanostructured superhydrophobic surfaces at low Weber numbers, where massive complete rebounds arise. It was found that an obliquely impinging droplet would undergo many impacts on the superhydrophobic surface, accompanying with sliding on the surface, jumping in air, and complex shape evolutions. Based on the kinematic analyses, we demonstrate that the droplet motion on the surface can be decomposed into a perpendicular impact, which is dominated by the capillary and inertial forces, and a translational motion under the drive of gravity. By contrast, the jumping motion after droplet rebound is solely governed by the gravitational force, yet relevant droplet characteristics are affected by the energy loss during the impact on superhydrophobic surface, which sets the maximum height that the droplet rebounds to. In addition, three distinct shape evolution modes–namely, oscillation, rotation and their combination–were identified on jumping droplets, and the direction of a rotational droplet can be altered via the following impingement on the superhydrophobic surface.

An analytical model for ice accretion on the engine strut surface

To predict flight icing more widely and practically, an ice accretion numerical framework that incorporates both the water droplet splash and the ice crystal sticking is developed. By proposing a deformation hypothesis, we deduce the modified energy conservation expression and the force balance relation for water droplet impingement. Subsequently, a new threshold determination and the probabilities for the droplet splash and ice crystal sticking are obtained, which are applicative across a wide range of Weber number after the validation. Through the interface tracking for a single droplet with the volume of fluid method, the droplet impingement dynamics are further explored, and the results of interaction with the wall serve the boundary treatments of droplet impingement in the discrete phase model. Additionally, the probability statistics method is employed to determine the parameters of the secondary droplets. Through the dynamic mesh technique, the retentive water droplets and the collected ice crystals are transformed into the accumulated ice in real time to update the ice accretion on the strut surface. Results demonstrate that the diameter, velocity, and content of droplets or crystals play significant roles in the impingement and the icing phenomena. Based on our numerical model, the predictions show that the ice accretion on the engine strut is influenced by flight parameters and environmental conditions, providing crucial guidance for the icing protection processes.

PVDF-HFP encapsulated WS2 nanosheets in droplet-based triboelectric nanogenerators for possible detection of human Na+/K+ ion concentration

Here we report the liquid–solid interaction in droplet-based triboelectric nanogenerators (TENG) for estimation of human Na+/K+ levels. The exploitation of PVDF-HFP encapsulated WS2 as active layer in the droplet-based TENG (DTENG) leads to the generation of electrical signal during the impact of water droplet. Comparison over the control devices indicates that surface quality and dielectric nature of the PVDF-HFP/WS2 composite largely dictates the performance of the DTENG. The demonstration of excellent sensitivity of the DTENG towards water quality indicates its promising application towards water testing. In addition, the alteration in output signal with slightest variation in ionic concentration (Na+ or K+) in water has been witnessed and is interpreted with charge transfer and ion transfer processes during liquid–solid interaction. The study reveals that the ion mobility largely affects the ion adsorption process on the active layer of PVDF-HFP/WS2 and thus generates distinct output profiles for diverse ions like Na+ and K+. Following that, the DTENG characteristics have been exploited to artificial urine where the varying output signals have been recorded for variation in urinary Na+ ion concentration. Therefore, the deployment of PVDF-HFP/WS2 in DTENG holds promising application towards the analyse of ionic characteristics of body fluids.

On the role of surface morphology in impacting-freezing dynamics of supercooled droplets

A thorough understanding of droplet impact and freezing is vital in preventing ice accretion on many outdoor devices. This simulation-based study investigated the effect of surface morphology on the impacting-freezing process of a supercooled droplet. Also, the variations of Weber number and supercooling temperature were studied numerically. The droplet impact and freezing process were simulated with the volume of fluid method and freezing model. A more accurate simulation was achieved by modeling the supercooled droplet and the dynamic contact angle. At the given ranges of the input parameters, the main factors that guaranteed droplet rebounding after collision were determined. The supercooling temperature and the groove width should be above 266 K and less than 0.21 mm, respectively. The droplet should also maintain its cohesion and integrity during impact. Creating grooves on a surface is novel and paves a new way to understand the impact and solidification of water droplets in supercooled conditions.

Impact damage testing based on high-speed continuous water jet aircraft coatings

When an aircraft passes through a rainy area at high speed, the coating on the front edge of the fuselage will be continuously eroded by raindrops, causing the coating to wear, crack or even peel off. This paper uses carbon fiber T300 material as the base material, and at the different impact speeds and impact numbers, water cutting equipment was used to simulate the erosion of the coating caused by the continuous impact of water droplets. The damage morphology of samples at different damage stages was observed by digital microscope and Scanning Electron Microscope (SEM), and the damage evolution curve was established to analyze and reveal the damage behavior and damage mechanism of rain erosion. The results show that the degree of damage experienced an increasing trend with the increase of impact numbers and speed, until circular peel damage was formed; no damage occurred during the incubation period, and the curvature of the damage evolution curve increased significantly after the expansion period and eventually showed a stable expansion trend. The mechanical properties of the coating material were the main influencing factors of its rain corrosion resistance. Moreover, the axially symmetric unsteady contact problem of droplets impacting the surface of a solid deformable body was studied. And the contact area was determined based on the iterative algorithm boundary positioning method. A mathematical model and closed mathematical formula describing the unsteady interaction between a droplet and a solid deformable obstacle were proposed.

Numerical Simulation of Water Film Flow and Breakup on Anti-Icing Surface

The flow and morphological characteristics of liquid water on the icing and anti-icing surfaces of aircraft are closely related to the icing characteristics and anti-icing surface temperature distribution. To predict the flow and breakup characteristics of a water film, a 3D model of continuous water film flow and a model of water film breakup into rivulets on an anti-icing surface were constructed based on the icing model, and the corresponding methods for solving the models were developed. Using the NACA0012 airfoil as a simulation object, the changing characteristics of height and velocity for a continuous water film with time and the morphological characteristics of rivulets formed from the breakup of a continuous water film were simulated numerically. The results indicate that, with an increase in inflow velocity, the time required for the water film to completely cover the surface and reach stability decreases. Downstream in the water droplet impact zone, the calculated values of continuous water film height align well with experiments, as well as the stream height at the continuous water film rupture location with the experimental values. With the reasonable contact angle, the calculation error of the stream width is within 10%.

Extremely large water droplet impact onto a deep liquid pool.

Most studies of droplet impact on liquid pools focus on droplet diameters up to the capillary length (0.27cm). We break from convention and study extremely large water droplets (1 to 6cm diameter) falling into a pool of water. We demonstrate that the depth and width of the cavity formed by large droplet impact is greatly influenced by the deformed shape of the droplet at impact (i.e., prolate, spherical, and oblate), and larger droplets amplify this behavior by flattening before impact. In particular, the maximum cavity depth is a function of the Froude number and axis ratio of the droplet just before impact. Further, the cavity depth is more dependent on the droplet height than width, and the maximum cavity diameter is independent of the droplet height. In general, we observe that more oblate droplets result in decreasing cavity depths for a fixed liquid volume. This is because an increase in horizontal droplet diameter results in a reduced impact energy flux and therefore reduced cavity depth.

Numerical investigation of dynamic icing of wind turbine blades under wind shear conditions

In the wind shear icing environment, the periodically varying inflow conditions experienced by wind turbine blades can significantly enhance the dynamic icing characteristics, which poses a great challenge to wind turbine icing research. To investigate this phenomenon, the Improved Multi-Shot Icing Computational Model (IMSICM) is utilized. In IMSICM, an efficient dynamic icing computational frame is designed by using the 3D Free Wake Lifting Line method coupled with the 2D Viscous and Inviscid Interaction method to calculate the flow field. Meanwhile, the Lagrangian method is applied to compute the water droplet impingement information and the Messinger Model is used to simulate the ice growth. After validation of IMSICM by the icing wind tunnel experimental results, the wind shear icing processes of the NREL 5 MW wind turbine under different wind shear conditions are analyzed. The results show that different from the linear growth behavior of Rime ice, the nonlinear features of Glaze ice development are significant. The wind shear effect has a greater impact on the Glaze ice shape, mainly manifested as the ice protrusions behind the main ice horn are suppressed. This phenomenon is mainly due to the combined effect of water impingement shadowing and heat transfer reduction.

Water Droplet Reflection Approximation using the Linear Least Square Method from Experimental Data

The impact of water droplets on droplet time is a phenomenon in nature that is implemented in the industrial world, where the contact time between droplets and surfaces affects the transfer of mass, momentum and energy. To manipulate and reduce the contact time of influencing droplets, previous publications reported surface microadjustments influencing droplet surface interfaces. In this final project simulation, water droplets will be discussed using a mechanical energy model and linear least squares method. In the first part the author will show that surface elasticity also influences the impact of droplets, where droplets on the surface of taro leaves can cause reflections again. In the second part the author will explain mechanical energy modeling to create a simulation of the model that influences water droplets. In the third part the author gets the results of the linear least square method modeling. The discussion of the linear least square method model will be related to problems in the industrial world.

Straightforward Fabrication of Double Roughness Structures on a Microcrystalline Film of a Diarylethene Derivative.

Double roughness structure mimicking the surface of a lotus leaf was prepared using a newly synthesized diarylethene having a six-membered perfluorocyclohexene ring. The cubic-shaped crystals of the open-ring isomer, with sizes of approximately 7 μm, appeared immediately following solution casting. Upon UV irradiation, each cubic crystal was covered with needle-shaped crystals of the closed-ring isomer to form double roughness structures within 1 h. This structure could bear the continuous impact of water droplets.

Water droplet impact dynamics comparison on solid and hollow square micropillared substrates

We experimentally investigate microliter-sized water droplet impact on solid and hollow square micropillared polydimethylsiloxane substrates. Micropillared substrates with different values of pitch (34, 47, and 62 μm) and hole sizes (0, 3, 6, and 10 μm) of pillars are fabricated using soft lithography following direct laser writer maskless photolithography. We observe that hollow micropillared substrates exhibit increased hydrophobicity as compared to the solid micropillared substrates. Interestingly, we find that hydrophobicity is further enhanced as the hole size is increased. To understand the impact dynamics, we perform high-speed visualization to acquire the transient evolution of the impacting droplets. Based on the impact velocity (0.22–0.62 m/s), pitch, and hole size, we identify various regimes, namely, non-bouncing, partial bouncing, and complete bouncing. At a given impact velocity and pitch value, non-bouncing and bouncing regimes are observed for solid and hollow micropillared substrates, respectively. We find that the hollow micropillared substrate exhibits higher values for capillary pressure, impalement pressure, and the energy barrier associated with the Cassie–Baxter to Wenzel transition toward the impacting droplets. This is due to a decrease in the solid fraction owing to the incorporation of circular holes in pillars. The analysis shows the energy loss due to viscous dissipation decreases with an increase in hole size, which enhances the bouncing fate possibility. The fundamental insights gained from this study can be effectively leveraged by modulating the surface morphology to realize the desired droplet impact characteristics for various potential applications such as self-cleaning and energy harvesting.

Evolution of the impact force of supercooled water droplets with different shapes on a superhydrophobic cold surface

We conducted a numerical study on the evolution of the impact force of supercooled water droplets with different shapes when they do not fully rebound on a superhydrophobic cold surface. The evolution of peak impact forces and their characteristic times with Weber number (We) were focused. The presence of a cold surface had a relatively less influence on the evolution of the first peak impact force but had a significant influence on the second peak impact force when We > 60. The sudden increase in the second peak impact force was closely related to the formation and subsequent collapse of cylindrical-like structures inside impinging water droplets. To incorporate the quantitative influence of droplet shapes, we introduced correction factors based on the aspect ratio (AR) in the fitting expressions for peak impact forces and their characteristic times. Specifically, we utilized AR (AR ≤ 1.0) and AR1/6 (AR >1.0) when 3 < We < 40, or AR4/3 (AR ≤ 1.0) and AR1/2 (AR > 1.0) when 40 < We < 100 as correction factors for the first peak impact force. Moreover, we employed AR−2 as the correction factor for the first peak characteristic time and AR−1/3 for the second peak characteristic time. These corrections extended the applicability of the fitting expressions to supercooled water droplets with different shapes.

Experimental study on contact time of a water droplet impact under controlled surface temperature

Bouncing droplets on superhydrophobic surfaces is one of the potential methods used for anti-icing. The surface supercooling is a significant parameter influencing the bouncing dynamic. A droplet impacting cold superhydrophobic surfaces is investigated via experimental methods. The influence of the surface supercooling and the Weber number on the impact dynamic is elucidated. Intriguingly, the surface supercooling shows no influence on the spreading time, and the initial retraction time as the heat exchange can be ignored in these processes, while it shows a strong influence on the late retraction time as it can lead to the wetting transformation. To further quantitatively describe the influence of surface supercooling, the relationships of the retraction rate in the late retraction are developed, considering the changes in the receding contact angle caused by the supercooling degree. Finally, the relationship of the contact time is established over a range of Weber numbers (We = 49.37–70.53), surface supercooling (ΔT = 15–24 °C), and droplet sizes (D0 = 2.2–3.2 mm). This work is the first to establish the relationship of the droplet contact time on cold superhydrophobic surfaces, which can provide a quantitative method to calculate the contact time for anti-icing.

Dynamical Mechanism for Reaching Ultrahigh Voltages from a Falling Droplet

AbstractRecent experiments have demonstrated an extremely high output voltage of 260 V by impinging droplets on an electret surface with top and back electrodes. However, the mechanism underlying the generated high voltage remains elusive. Through high‐speed imaging of the impinging water droplet, it is found that the recorded voltage is proportional to the change rate of contact length l between the droplet and the top electrode over time, that is, dl/dt. By combining an electrostatic model with multi‐physics simulations, the time‐dependent charge distribution on the top electrode is analytically described as a function of several key factors pertaining to the spreading dynamics of droplet and the geometrical dimensions of device. These results show that the high voltage originates from the accumulation of charge in the spreading droplet and the ensuing instantaneous discharge upon high‐speed contact with the top electrode. Using the model‐optimized parameters, the experimentally generated voltage from a falling 64 µL droplet with a 2 mm salt can be boosted to 1030 V, giving rise to an energy conversion efficiency as high as 3.3%. This conversion efficiency will further increase by 14 times if the surface charge density of the electret can be enhanced to its limit using state‐of‐the‐art fabrication techniques.

Research on the Methods for Obtaining Droplet Impingement Characteristics in the Lagrangian Framework

The impact of supercooled water droplets is the cause of aircraft icing, and the acquisition of impingement characteristics is the key to icing prediction and the design of ice protection systems. The introduction of water droplet collection efficiency is required to obtain the characteristics for the Lagrangian method. In this work, a traditional flow tube method, a local flow tube method, and a statistical method are established to calculate the local collection efficiency, based on Lagrangian droplet trajectories. Through the numerical simulations of the air–droplet flow field around an NACA 0012 airfoil, the accuracies of the three methods in regard to collection efficiency are verified. Then, these three methods are applied to obtain the results for water droplet trajectories and the collection efficiency of an S-shaped duct, a 2D engine cone section and an icing wind tunnel. The results show that the distributions of water droplet collection efficiency obtained by the three methods are consistent and the three methods are all feasible when the water droplets do not overlap or cross before hitting the aircraft surfaces. When the water droplets are shadowed by upstream surfaces or blown by air injection, the droplet trajectories might overlap or even cross, and the local collection efficiencies obtained by the traditional flow tube method, local flow tube method, and statistical method might differ. The statistical method is relatively accurate. However, not all the droplet impingement characteristics obtained by the three methods are different due to these effects, and the non-crossing of the droplet trajectories is not a necessary condition for the use of the flow tube method. The effects of trajectory crossings are analyzed and discussed in detail in different situations for the three methods. This work is helpful for understanding and accurately calculating the droplet impingement characteristics and is of great significance for simulations of the aircraft icing process and anti/de-icing range.

The impact, freezing, and melting processes of a supercooled water droplet onto a cold slippery liquid-infused porous surface

In the present study, we report for the first time the observations of the impact, freezing, and melting processes of a supercooled water droplet onto a cold slippery liquid-infused porous surface (SLIPS). A parameter study of the droplet supercooling degree and the SLIPS temperature was carefully performed. Moreover, the impact process of a water droplet onto the SLIPS under the room temperature conditions was also measured for comparisons. The results showed that the impact kinematics of a water droplet on a SLIPS under the room temperature conditions was significantly different from those of the supercooled water droplet impinging onto a cold SLIPS. Under the same water droplet temperature conditions, the spreading factors rapidly increased to their maximum values at a similar rate, regardless of the change of the SLIPS temperature. Besides, under the same SLIPS temperature conditions, the maximum spreading factor of the water droplet at room temperature was significantly greater than those of the supercooled water droplet cases. When the temperatures of the water droplet and SLIPS were relatively high, freezing process started after the recoiling process had already finished for a certain period of time. However, when the temperatures of the water droplet and SLIPS were relatively low, freezing process could happen during the supercooled water droplet recoiling process. In addition, under the same water droplet temperature conditions, the decrease of the SLIPS temperature could result in the increase of the average energy dissipation rate of the water droplet.