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Small particle interactions with surfaces dominate processes from industrial manufacturing to micrometeorite impacts in space. For particles that are optically visible a wide range of modeling and studies exist to characterize the mechanical behavior and material properties that govern these interactions. The properties of materials at smaller scales, below microns in size, can differ from bulk measurements and the body of literature examining these smaller interactions lacks the same variety of experimental data. Presented herein is a new tool, the Aerosol Impact Spectrometer (AIS), for the characterization of submicron particles impacting on surfaces. The AIS is composed of a particles source, particle mass spectrometer, linear accelerator, impact target, and post impact detectors. Highly charged particles are generated using an electrospray ionization source before being injected into vacuum through an aerodynamic lens. A subset of the particle beam is selected using a quadrupole deflector and individual particles are injected into a linear electrostatic trap. As particles oscillate in the trap charge detection mass spectrometry is performed to determine individual particle mass and charge before being ejection into a linear accelerator. A series of high voltage elements either accelerates or decelerates each particle to a select final energy before impacting a target surface. The subsequent behavior of the particle is examined with a variety of charge sensitive detectors including measurements of particle sticking and rebound velocity.The rebound velocities over a wide range of initial energies has been demonstrated using polystyrene latex spheres, tin metal particles, and frozen water-ice. The behavior of each species is distinct and further analysis of the tin metal particles shows behavior consistent with non-bulk material properties of the metal. Evidence for water ice melting and fragmentation have also been observed with high impact energies. Finally, the techniques employed to examine particle behavior after impact have also been applied to examine durability in freestanding ultrathin films under particle bombardment and measure the efficacy of large particle detection using a microchannel plate detector.

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., Satellite drops formed on droplet impact hinder applications from inkjet printing to drop energy harvest. Here, authors reveal that patterned-wettability surfaces can break the symmetry of droplet impacts and suppress the Plateau–Rayleigh instability, preventing satellite drop generation and improving the hydropower harvest.

Controlling the drop impact behaviors plays an important role in both designing surface functional materials and improving industrial techniques. Here, the impact dynamics of nanodroplets on "V-shaped" substrates is studied, which is manifested as asymmetrical spreading and retraction behaviors that could be weakened by increasing the angles of the "V" shape, accompanied by the evolution of the shape from leaf-like, stripe-like, to dumbbell-like at v = 20 A/ps. When v increases to 40 A/ps, the previously deposited nanodroplets could be transformed to a rebound one at α = 60 and 90°, which is ascribed to the combined effect of the confinement conditions and the extrusion force. Furthermore, an impact behavior map as a function of impact velocity and angle is depicted, which suggests that a decrease in angles or an increase in the impact velocity is likely to cause the nanodroplets to rebound. More importantly, the results give visible evidence that compared to the flat substrates, the V-shaped structure is advantageous for achieving a fast-rebound behavior even at a low speed of impact, which should be good news for practical applications in many multidisciplinary fields, such as self-cleaning, anti-icing/fogging, pollution prevention, energy storage, and so forth.

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 em...

Improving the impact energy dissipation capacity of functionally graded brittle materials through pore design will help avoid or delay failure. In order to improve the impact energy dissipation capacity of functionally graded brittle materials, pores with specific shapes can be implanted inside them. The effect of pore shape on the impact properties of functionally graded brittle materials was investigated using a lattice-spring model that can quantitatively represent the mechanical properties of functionally graded brittle materials. The calculated results show that the pores with negative Poisson’s ratio such as inner-concave triangle, fourth-order star, and inner-concave hexagon are easy to collapse under the impact, while the square and square-hexagon pores have the strongest resistance to deformation. For all seven pore shapes, the Hugoniot elastic limit of the samples decreased gradually with increasing porosity, and the Hugoniot elastic limit did not change with the change of piston velocity. The propagation velocity of the deformation wave increases with the piston velocity and the velocity of the particle corresponding to the Hugoniot state behind the deformation wave increases accordingly. The principle that pores can enhance the macroscopic impact energy dissipation capacity of functionally graded brittle material samples revealed in this paper will contribute to the prevention of sample impact failure and provide guidance for the optimal design of impact kinetic properties of samples.

This article looks into the effects of the initial knee flexion angle at the contact time on the peak of the impulsive lower limb forces during landing, and how these effects are related to muscular activities. The impact dynamics of drop landing is studied via a musculoskeletal model with eight Hill-type lower-limb muscles. A method is proposed for the representation of two landing strategies: landing with high and low joint stiffness. Then, in each landing strategy, the effect of the initial knee flexion angle on the peak ground reaction force (GRF), the peak knee ligaments force and the peak tibiofemoral contact force is investigated by considering different initial contact postures. It is observed that while landing with a flexed knee decreases the peak GRF in both landing strategies, it decreases the peak tibiofemoral and knee ligaments forces only in landing with low joint stiffness. Specifically, by increasing the initial knee flexion from 0° to 70°, the peak tibiofemoral and knee ligaments forces decrease monotonically by 54% and 82%, in landing with low joint stiffness. For high joint stiffness, however, as the initial knee flexion increases from 10° to 70°, the peak tibiofemoral force is seen to increase monotonically by 42% and the peak knee ligaments force is seen to have a non-monotonic behavior, first decreasing by 42%, and then, increasing by 250%. These results can be considered in training landing strategies to reduce the risk of knee injury.

Self-cleaning surfaces often make use of superhydrophobic coatings that repel water. Here, we report a hydrophobic Si nanospring surface, that effectively suppresses wetting by repelling water droplets. We investigated the dynamic response of Si nanospring arrays fabricated by glancing angle deposition. The vertical standing nanospring arrays were approximately 250 nm tall and 60 nm apart, which allowed the droplets to rebound within a few milliseconds after contact. Amazingly, the morphology of the nanostructures influences the impact dynamics. The rebound time and coefficient of restitution were also found to be higher for Si nanosprings than vertical SI columns. It has been proposed that the restoring force of the Si nanosprings may be responsible for the water droplet rebound and can be explained by considering the droplet/nanospring surface as a coupled spring system. These nanospring surfaces may find applications in self-cleaning windows, liquid-repellent exteriors, glass panels of solar cells, and antifouling agents for roof tiling., Comment: 19 pages, 5 figures, 2 tables

The impact of agglomerates on solid surfaces occurs inevitably during the processing of granular materials. In this work, numerical simulations of the surface impact of wet agglomerates are carried out by introducing liquid bridge interactions into a soft-sphere DEM model, and the instantaneous impact behaviors are discussed quantitatively at the single particle scale. Results indicate that the impact process usually includes four stages, i.e., compression, expansion, fragmentation, and re-agglomeration. The expanding radius is found to follow an exponential-law with time, and the number of fragments follows a power-law with impact velocity. Due to liquid bridge interactions, chain-like structures are observed beyond the impact, similar to those formed during the impact of dense suspension droplets. Moreover, the impact behaviors can be generally classified into four groups with respect to impact velocity, i.e., minor compression without splashing, severe compression with minor splashing, severe splashing with minor fragmentation, and severe fragmentation.

Rocks in deep coal mines are usually in varying degrees of damage state before they are destabilized by impact loads such as rock bursts. For the problem of the mechanical properties and energy evolution of damaged rocks under impact loads, the authors use static loads with different cyclic load thresholds to act on sandstone specimens to make them in distinct degrees of damage. Then, the rock mechanics system (MTS-816) and the Split Hopkinson pressure bar (SHPB) are employed to perform uniaxial compression and impact dynamics tests on sandstones with different degrees of damage. The results show that, from the perspective of mechanical properties, the uniaxial compressive strength and dynamic compressive strength of the damaged sandstone gradually decrease with the increase of the upper limit of the cycle threshold and both obey the growth law of the quadratic function, and the dynamic strength increase factor (DIF) also decreases with the increase of the cyclic load threshold. In terms of energy, with the increment of the cyclic load threshold, the number of cracks in the damaged sandstone is large and the scale is enormous. Due to the effect of cracks, when the incident energy is a fixed value, the transmission energy decreases with the increase of the damage degree and the change law of the reflection energy is the opposite. The systematic study of the dynamic mechanical properties and energy evolution law of the damaged sandstone provides some reference for the prevention and mechanism research of rock bursts.

In this study, we investigated droplet impact dynamics offalling water drops (D0 ~ 2.3 mm ) on slippery oil-infused surfaces and compared them to other features of the surfaces, to elucidate the wettability- and roughness-controlled characteristics. We prepared transparent substrates with the designed characteristics, so it would be feasible to visualize the droplet impact dynamics in detail. A wide range of impact kinetics (We ~ 800 (=ρD0Vi2/σw)) was covered, which gave rise to several types of droplet-impact: gentle spreading, wavy (undulated fingers of spreading edges), droplet break-up, and splashing with small secondary droplets. The basic parameters of the droplet-solid interactions were measured, and events were mapped with respect to the sample surface and impact kinetic conditions. We found that, generally, surface wettability has a major influence on the triple line shape and instability during the impact and retraction process, and thus determines events in of the framework of the dynamic wetting-failure model. Furthermore, while rough conditions promote instability of the impacted droplet, slippery lubricant-infused features tend to dampen perturbations of the spreading/retracting edge.

Regulating the impact dynamics of water droplets on a solid surface is of great significance for some practical applications. In this study, the droplet impingement on a flexible superhydrophobic surface arrayed with micro-scale grooves was investigated experimentally. The surface was curved into cylindrical shapes with certain curvatures from two orthogonal directions, where axial and circumferential grooves were formed, respectively. The effects of curvature diameter and Weber number, as well as the orientation of grooves on droplet spreading and retracting dynamics were analyzed and explained. Results show that the circumferential grooves promote the spreading of a droplet in the azimuthal direction, where the droplet rebounds from the surface with a stretched shape. This mechanism further reduces the contact time of impacting droplets on the superhydrophobic surface compared to the other curving mode.

The dynamics of a metal droplet impacting on a substrate surface has been studied numerically in this report. Numerical solutions of the Navier–Stokes and energy equations show the evolution of the droplet as it spreads upon impact with the substrate while simultaneously undergoing solidification. The interplay of different forces including inertia, viscous, and surface tension, coupled with solidification of the molten material in layers lead to complex flow dynamics. The change in density and viscosity owing to a change in temperature resulting from the cooling process is found to influence the spreading of the droplet significantly. The model was exercised for three different materials, namely, aluminum, copper, and nickel to determine the final splat radius as well as the spreading time. The surface tension forces as well as solidification rates were found to be the dominant factors in determining the above parameters as well as the shape of the splat during spreading. The results were found to be in good agreement with an existing analytical model.

A droplet impacting on inclined surfaces yields more complex outcomes than on normal impact and the effect of the inclining angle on the impact dynamics is still in controversy. Here, we show that a drop impacting on inclined superhydrophobic surfaces exhibits an asymmetric rebound with a distinctive spreading and retraction along the lateral and tangential directions. Meanwhile, there is an obvious contact time reduction with the increase of the inclining angle and impact velocity. We demonstrate that the contact time reduction is attributed to the asymmetric drop spreading and retraction, which endows a fast drop detachment. Simple analyses are presented to interpret this phenomenon, which is in a good agreement with the experimental results.

Droplet deposition on superhydrophobic surfaces has been a great challenge owing to the shortness of the impact contact time. Despite recent research progress regarding flat superhydrophobic surfaces, improving deposition on ubiquitous wired and curved superhydrophobic leaves remains challenging as their surface structures promote asymmetric impacts, thereby shortening the contact times and increasing the likelihood of droplet splitting. Here, we propose a strategy to solve the deposition problems based on an analysis of the impact dynamics and a rational selection of additives. Combining the prominent extension property of flexible polymers with surface tension reduction of the surfactant, the well-chosen binary additives cooperatively solve retention and coverage problems by limiting the fragment and enhancing local pinning and wetting processes at a very low usage. This work advances the understanding of droplet deposition by rationally selecting additives based on the impact dynamics, which is believed to be useful in a variety of spraying, coating, and printing applications.

Mixed-wettability surfaces have been extensively adopted to manipulate impact dynamics of droplets. In this work, a three-dimensional numerical model is developed to investigate the splitting behaviors of droplets when they impact a hydrophobic strip decorated on a hydrophilic background surface. The volume of fluid (VOF) method incorporated with mesh-refinement technique and dynamic contact angle model is employed to capture the shape evolution of droplets accurately. Dynamic behaviors of impact droplets are analyzed for various strip widths, strip hydrophobicities, and surface hydrophilicities at three typical Weber numbers. The results show that the droplet can be successfully split into two symmetric small droplets for all the three Weber numbers, so long as the strip width and wettability contrast between the strip and background surface are properly designed. The non-axisymmetric spreading and retraction of the film triggered by the strip lead to a shorter spreading and a faster retraction on the hydrophobic strip than the hydrophilic surface, which causes the formation of a liquid bridge on the strip and reduces the viscous dissipation on the strip. Thus, the non-axisymmetric spreading and retraction dynamics as well as the reduced viscous dissipation are responsible for the droplet splitting. A simplified theoretical model is developed to predict the splitting or no splitting behavior on the basis of energy conservation principle, which agrees with the present simulations well. Finally, the simulated results are assembled in phase diagrams to obtain the criterion for the droplet splitting from the critical strip width and the critical wettability contrast between the strip and background surface.

A synthesizable interfacial epoxy-polyurea-hybridized matrix (IEPM), composed of chemical bonded nanostructures across an interface width ranging between 2 and 50 μm, is a candidate for dialing-in molecular vibrational properties and providing high-impact dynamics resistance to conventional fiber(x)-reinforced epoxy (F/E), engendering an x-hybrid polymeric matrix composite system (x-IEPM- t

Enhancement of drop deposition is advantageous in various fields, such as painting, cooling, and agricultural spraying. We present a method to suppress the bounce magnitude of a drop on nonwetting solid surfaces via asymmetrical shaping. Numerical results show that the asymmetrical drop shape can substantially alter the hydrodynamics, thereby leading to a reduction in bounce height by nearly 55% on hydrophobic surfaces compared with the spherical shape. To elucidate how the shape affects the bounce magnitude, we scrutinize the asymmetrical hydrodynamic features in spreading and retraction and the quantitative analysis of momentum for varying geometric parameters, revealing that enhancement of asymmetry in the horizontal momentum plays a vital role in suppressing bounce magnitude. The asymmetrical drop impact can provide implications for exploiting dynamic scenarios of drops, such as fusion of sprayed drops in unequal size and drop manipulation in microfluidics.

Droplet impact dynamics on gradient textures at room temperature have been intensively studied in the past decade. The wetting gradient surfaces are reported to direct the droplet motion. In this work, we show an undirectional rebound behavior of impacting droplets on gradient grooved surfaces at high temperature. The impinging droplet rebounds towards the denser microstructure region at contact boiling state, but towards to the sparser region when at Leidenfrost film boiling state. Our analysis indicates that the non-directional rebounding direction is attributed to the unbalanced Young's forces or vapor pressure difference between the droplet and the substrate. In contact boiling regime, the droplet rebounds towards the denser region due to the unbalanced Young's force. However, in the Leidenfrost film boiling region, adequate vapor generated underneath the droplet impedes the direct contact and larger vapor pressure in the denser region causes the droplet to rebound oppositely. This study is envisioned to promote the understanding of droplet dynamics at high temperature and provide promising applications in various systems where regulating droplet motion is needed.

Due to the excellent impact resistant performance of steel-plate concrete (SC) structure compared with the conventional reinforced concrete (RC) structure, SC structure is preferred to be used in the design of external walls of nuclear island buildings for new nuclear power plants (NPPs). This study aims at evaluating the effect of material and geometric parameters of SC containment on its impact resistant performance, thus the numerical simulation and sensitivity analysis of SC containment subjected to malicious large commercial aircraft attack are conducted based on the force time-history analysis method. The results show that: (1) the impact resistant performance of full SC containment is better than that of half SC containment, (2) for relatively thin full SC containment, the impact response and concrete damage can be significantly reduced by the enhancing of concrete strength grade or the increasing of steel plate thickness, (3) for the thicker full SC containment, concrete strength grade has only a slight influence on the impact displacement response, and the increasing of steel plate thickness has no significant effect on mitigating the impact displacement response. However, the increasing of steel plate thickness can effectively reduce its plastic strain, and the decreasing of strength grade of steel plate may obviously increase its plastic strain, and (4) concrete thickness plays a decisive role on the improvement of impact resistance, which is more effective than the enhancing of concrete strength grade. Resultantly, this paper provides a reference and guidance for the design of SC structure external walls of nuclear island buildings against a large commercial aircraft.

International audience; We investigate the impact of composite objects. They consist of a soft layer on top of a rigid part with a hemispherical impacting end. The coefficient of restitution (e) of such objects is studied systematically as a function of the mass ratio and of the nature of the materials. For rather elastic materials, the coefficient of restitution is a nonmonotonic function of the mass ratio and exhibits important variations. The dynamics of the impact can be characterized by several bounces depending on the ratios between the four timescales at play. These include the duration of contact of the rigid part with the substrate and the time for the elastic waves to travel back and forth in the soft layer. In that sense, describing these projectiles requires one to take into account both the Hertzian theory of contact and the elastic waves described by Saint-Venant's approach.

In the present pandemic time, face masks are found to be the most effective strategy against the spread of the virus within the community. As aerosol-based spreading of the virus is considered as the primary mode of transmission, the interaction of masks with incoming droplets needs to be understood thoroughly for an effective usage among the public. In the present work, we explore the interactions of the droplets over the most commonly used 3-ply surgical masks. A detailed study of the wetting signature, adhesion, and impact dynamics of water droplets and microbe-laden droplets is carried out for both sides of the mask. We found that the interfacial characteristics of the incoming droplets with the mask are very similar for the front and the back side of the mask. Further, in an anticipated attempt to reduce the adhesion, we have tested masks with a superhydrophobic coating. It is found that a superhydrophobic coating may not be the best choice for a regular mask as it can give rise to a number of smaller daughter droplets that can linger in air for longer times and can contribute to the transmission of potential viral loads.

In the present investigation, we explore the morphological evolution of a ferrofluid drop impacting on a solid substrate in the presence of a vertical magnetic field. The morphological evolution of the droplet is quantified by the change in its height, diameter, and contact line velocity. Two types of liquid drop are used in the present investigation viz. a water-based ferrofluid drop and a glycerol-based ferrofluid drop. The glycerol-based ferrofluid droplet acts as a viscoelastic drop. A comparative analysis of the impact dynamics is carried out between two different liquid drops in the presence/absence of a magnetic field. It is shown that the effect of fluid elasticity brings about a controllability on the impact dynamics in the presence of a magnetic field. The experimental investigations revealed that the viscoelastic ferrofluid drop in the absence of a magnetic field encounters a negligible recoiling effect. However, in the presence of a vertical magnetic field, the drop experiences significant recoiling of the contact line. Also, it is shown that the maximum deformation encountered by the drops is directly related to the strength of the applied magnetic field.

Cooling by impinging droplets has been the subject of several studies for decades and still is, and, in the last few years, the potential heat transfer enhancement obtained thanks to nanofluids’ use has received increased interest. Indeed, the use of high thermal conductivity fluids, such as nanofluids’, is considered today as a possible way to strongly enhance this heat transfer process. This enhancement is related to several physical mechanisms. It is linked to the nanofluids’ rheology, their degree of stabilization, and how the presence of the nanoparticles impact the droplet/substrate dynamics. Although there are several articles on droplet impact dynamics and nanofluid heat transfer enhancement, there is a lack of review studies that couple these two topics. As such, this review aims to provide an analysis of the available literature dedicated to the dynamics between a single nanofluid droplet and a hot substrate, and the consequent enhancement or reduction of heat transfer. Finally, we also conduct a review of the available publications on nanofluids spray cooling. Although using nanofluids in spray cooling may seem a promising option, the few works present in the literature are not yet conclusive, and the mechanism of enhancement needs to be clarified. ispartof: Energies vol:14 issue:1 status: published

A study shows that during oblique impact of a particle onto a liquid surface, nonaxisymmetric wetting of the particle causes a nonaxisymmetric distribution of fluid pressure and shear stress along the particle surface. This leads to a deviation of the dominant forces at different impact stages from the particle direction of motion. The particle then follows a different trajectory from that of the impact direction while a viscous moment is generated that rotates the particle.

Currently, it appears that there is a lack of understanding related to the role of SSF, in the two-phase behavior of the deceleration history, which is an issue discussed recently in the impact dynamics field. This paper analytically and numerically focuses on the effect of SSF on the projectile deceleration characteristic of concrete-like targets. Firstly, the penetration process according to the two-phase feature of the projectile deceleration is revised, where analytical results indicate that the SSF has a phased feature corresponding to the two-phase behavior of the deceleration history. Furthermore, a series of numerical simulations are conducted to understand the role of SSF more clearly. Simulation results show a similar conclusion to the analyses of the two-phase penetration process, at the range below a certain critical striking velocity, adding friction can reproduce the experimental data, when exceeding the critical striking velocity, the simulated results without considering friction are closest to the experimental data. Hence, it could be gained that the role exchange between the SSF and the dynamic term contributes to the two-phase penetration behavior for concrete-like materials. This indicates that the sensitivity of SSF to the penetration process is one of the factors driving the two-phase feature.

In a piezoelectric raindrop harvester equipped with spoon the thickness of the water layer modifies the impact dynamics and has an important effect on the performance. To analyze this phenomenon, series of experimental tests carried out in the laboratory with different values of water layer depth are presented and discussed. Results show that the increase in the water depth decreases the resonance frequency. The generated voltage increases with water depth until it reaches an optimum value. Further increases are not beneficial, since large spills take place. Both open circuit voltage and on load voltage are considered.

Ice adhesion and accretion on power lines is a severe problem that can pose a threat to the electric power transmission, and this icing phenomenon is significantly related to the impact dynamics of freezing rain droplets. In the current paper, this impacting process was studied by using computational fluid dynamics, and the model was verified by an experiment with a high-speed camera. The detailed droplet impacting processes on the surface of a very commonly used overhead power line (the ACSR-type cable) were analyzed. The effects of surface wettability (θ = 67–135 deg) and initial droplet impact velocity (We = 22–219) on the evolution of the liquid–solid contact area during the whole process and the volume of the residual liquid on the power line surface after impact were studied. Meanwhile, the influence of the surface structure of the ACSR power line on the droplet impact dynamics was analyzed. Results show that the capturing of impacting droplets can be enhanced by the grooved structures on a hydrophilic ACSR power line surface, while differently the expelling of impacting droplets can be enhanced by these grooved structures on a hydrophobic ACSR power line surface. By analyzing the possible influence of the surface structure of an ACSR power line on the phase transition of impacting droplets, these grooved structures could facilitate the formation of ice nucleation which can finally make the ice adhesion and accretion on an ACSR power line is more serious than that on a traditional smooth cylindrical power line.

Droplet impact on a miscible liquid film finds several applications in scientific and engineering problems. Here, we report that how pre-existing perturbations of a target liquid film, in the form of surface waves, change the impact dynamics and splashing phenomena. For this purpose, we release dyed water droplets and study droplet impact dynamics on a wavy water film, where the waves atop the liquid film are generated by imposing low-frequency two-dimensional (2D) lateral (horizontal) vibrations on glass substrates. Methylene blue-dyed droplet was employed to facilitate the observation of the details of mixing phenomenon given its importance in some applications such as casting homogeneous thin-film coatings. The results are compared with the impact on a quiescent/smooth liquid surface. Splashing is found to be more pronounced for the wavy surface given the additional energy imparted by vibrations and waves. An increase in the crown diameter is also observed for the vibrated wavy liquid film. It is observed that mixing is enhanced significantly in the presence of vibrations and surface waves. The interaction between the pre-existing surface waves and the waves formed after the droplet impact is also studied.

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 drop impacting at low velocities onto surfaces textured with asymmetric (tilted) ridges. 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, 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 $Ca_f \sim 1$, the pinning is only temporary until the liquid-vapor interface reaches to the next ridge where a new contact line is formed. Finally, when $Ca_f\gg 1$, the geometric details of non-smooth surfaces play little role., Comment: 12 pages, 7 figures, under consideration for Physical Review Fluids. Revised February 04 2021

In the applications of composite honeycomb sandwich structures, they often are severely impacted, resulting in partial penetration or complete perforation. To study the mechanical response ...

International audience; Liquid drop impact dynamics depends on the liquid–substrate interaction. In particular, when liquid–solid friction is decreased, the spreading of the impacting drop lasts longer. We characterise this effect by using two types of superhydrophobic surfaces, with similar wetting properties but different friction coefficients. It is found that, for large enough impact velocities, a reduced friction delays the buildup of a viscous boundary layer, and leads to an increase of the time required to reach the maximal radius of the impacting drop. An asymptotic analysis is carried out to quantify this effect, and agrees well with the experimental findings. Interestingly, this novel description complements the general picture of drop impact on solid surfaces, and more generally addresses the issue of drag reduction in the presence of slippage for non-stationary flows.

While the droplet impact dynamics on stationary superhydrophobic surfaces has been extensively studied, the dynamic behaviors of impact droplets on moving superhydrophobic surfaces have received less attention. Here, we report the droplet impact dynamics on a moving superhydrophobic surface. We show that compared to the stationary surface, the moving superhydrophobic surface breaks the symmetry in both droplet spreading and retracting. Specifically, the shear force exerted by the moving surface acting on the impact droplet enlarges the maximum spreading in the moving direction, and thus, the droplet contact time is reduced. The contact time of impact droplets was examined thoroughly under the effects of the droplet impact (normal) and the wall moving (tangential) Weber numbers. We provide a scaling analysis to explain how the contact time depends on the normal and tangential Weber numbers. Our experimental investigation and theoretical analysis provide insight into the droplet impact dynamics on moving superhydrophobic surfaces. Ministry of Education (MOE) Published version This research was funded by the Ministry of Education of Singapore via Tier 2 Academic Research Fund (No. MOE2016-T2-1-114).

Because of the great threat of supercooled large water droplets (SLDs) to the safety of aircraft and outdoor devices, the mechanism of rapid freezing upon impact has attracted considerable attention of researchers in recent years. As an important factor in the impact dynamics and icing of supercooled water droplets, the effect of wettability on the impact-freezing of SLDs has not been systematically investigated, which is important to the investigation of icing predictions and anti-icing design. In this study, an experiment was designed to observed the impact and freezing of supercooled water droplets on surfaces with different wettabilities (from hydrophilic to superhydrophobic). The shapes and spreading ratios of frozen droplets were compared to evaluate the effect of surface wettability on icing. The impact/retraction, nucleation, and ice growth during the impact of supercooled water droplets were investigated. The coupled process of retraction and icing was considered to reveal the reason for the frozen spreading ratio differences on different surfaces. The findings were as follows: (1) Contrary to the common sense notion that the spreading area of frozen droplet always depends on the wettability of surface, the mean frozen spreading ratio of a supercooled water droplet (diameter of 2.3 mm and impact velocity of 2.2 m/s) is nearly independent of wettability (wettability-independent region) below −15 ° C ; while above −10 ° C the mean frozen spreading ratio only depends on the wettability (wettability-dominated region). Between these temperatures there is a linear transition region of mean frozen spreading ratios. (2) The transition from wettability-dominant icing to wettability-independent icing is due to the relative changes of the icing time and retraction time. When the mean icing time is much larger than the retraction time, most of the droplets freeze after the completion of retraction. However, when the mean nucleation time is smaller than the retraction time, the supercooled droplet freezes without any retraction. (3) A theoretical model is presented to predict the mean frozen spreading ratio of supercooled water droplets impacting different surfaces. With this model, the contribution of the surface properties on the rapid icing of SLDs is discussed.

Controlling the impact of water droplets on nonwetting surfaces is difficult due to their superior liquid repellency. Herein, we experimentally demonstrate that the impact dynamics of water droplets on superhydrophobic surfaces can be effectively altered by adding a very small amount of high molecular weight polymer. The presence of polymer chains in water enhances droplet–surface interactions and liquid viscoelasticity, which couple with the hydrodynamic effect and thus affects the dynamic behaviors of impinging droplets. Whereas various impact phenomena with altered transitional boundaries in the phase diagram have been observed for droplets of aqueous polymer solutions at low concentrations, only deposition was identified for aqueous polymer droplets with high concentrations. Nevertheless, the polymer addditive does not influence droplet spreading but siginificantly slows down droplet retraction and increases the contact time of rebounding droplets on the superhydrophobic surface. Moreover, the induced...

This chapter gives an overview of the interfacial dynamics of complex fluids, with focus on non-Newtonian drop impact phenomena and non-Newtonian sprays. After a general introduction about Newtonian drops and sprays, the impact dynamics of viscoelastic and viscoplastic drops on both homothermal and heated surfaces is discussed. Finally, capillary instabilities and the atomisation process of non-Newtonian fluids are described.

We experimentally studied droplet impact dynamics onto wing feathers of kingfishers. Distilled water droplets with a fixed diameter of 2.06 mm were used as drop liquid and the initial impact velocities of droplets varied from 0.28 m·s−1 to 1.60 m·s−1. Two high-speed cameras were utilized to capture the impact process of water droplets onto the wing feather surface from both horizontal and vertical directions. Two states of the feathers (elastic and inelastic) were considered to study the influence of elasticity. At the entire impact velocity range we studied, regular rebound, bubble trapping and jetting, partial pinning and partial rebound of droplets on inelastic wing feather surface were observed as the initial impact velocity increased. However, only one dynamic behavior (regular rebound) was found on the elastic wing feather surface. The elasticity plays a more important role in the direction difference of droplet spreading than wing feather microstructure. The contact time of water droplets on the elastic wing feather surface was less than that on the inelastic surface within the range of Web numbers from 1.06 to 36 under test conditions.

Controlling drop dynamics on solid surfaces is an important challenge. In many strategies for efficient drop deposition, drop dynamics is generally assumed to be axisymmetrical. We demonstrate shape-dependent impact dynamics that can considerably modify the dynamics by deforming the drop into a truncated spherical shape at the impact moment. Experimental and numerical studies show the exceptional rim dynamics that lead to reduced bounce heights compared with spherical drops. We investigate the impact dynamics of truncated spherical drops as a function of the truncation depth, surface wettability, and impact velocity numerically. The bounce height of the truncated drop reduces by 56% below spherical drops. To elucidate the mechanism for the reduction in the bounce height, we conduct the horizontal and vertical momentum analyses of truncated drops. The truncated drop impact can potentially open up new opportunities for enhancing drop deposition in practical applications, such as surface coating and spray cooling.

Using the lattice Boltzmann method, a numerical study was conducted to investigate the oblique impact of two successive droplets on a flat surface. The focus was placed on the effects of surface inclination, lateral/longitudinal offset, the impact dynamics of the two droplets and the subsequent dynamics of the combined droplet. The evolution of the topology, contact lines and spread factor of the two droplets under various conditions was compared and analyzed. It was found that, compared to single droplet impact, the impact of successive droplets shows quite different dynamics due to the involved coalescence process. The surface inclination causes asymmetric spreading of the droplets. The increase in surface inclination leads to faster downward spreading and reduced lateral spreading. The non-zero offset between the two droplets further enhances this asymmetry. Furthermore, the intermixing between the two droplets during the oblique impact was also examined. It was observed that the surface inclination changes the mass distribution of the combined droplet.

The rotary-percussive ultrasonic drill (RPUD) employs vibrations on two sides of a piezoelectric stack to drive a drill tool to achieve simultaneous rotary-percussive motion. It has the advantages of being small and requiring low power, low axial load, and small holding torque, making it suitable for extra-terrestrial rock sampling, especially for a minor planet with a weak gravitational field. This paper presents the impact dynamics prediction of the percussive system of RPUD, which is composed of a piezoelectric actuator, a free mass, and a drill tool. Considering the vibration of the RPUD and the weight on bit, the interactions between these three components before and during drilling are analyzed separately. The effects of various parameters (i.e., the coefficient of restitution between the actuator and the free mass, the damping ratio of the RPUD, the weight of the free mass, and the weight on bit) on the contact force between the free mass and the drill tool, a number of collisions per second, kinetic energy transferred to the drill tool per second, and reacting force are simulated. Simulation results show that the free mass converts the high-frequency harmonic vibration of the actuator into lower frequency impacts on the drill tool. Furthermore, the contact force and the kinetic energy transferred to the drill tool per second can be enhanced by increasing the coefficient of restitution, free mass, and weight on bit or decreasing the damping ratio of the RPUD.

Poor performance in the condensers in power plants and chemical plants is due to the fact that condensed water is deposited on the heat transfer pipes. The dynamics of condensed water droplets forming on the surface of heat transfer pipes have a significant effect on the heat transfer efficiency of heat exchangers. In the present study, a numerical approach using a coupled level-set and volume of fluid (CLSVOF) method was adopted to investigate the impact of water droplets on cylindrical pipes. The numerical model was verified by an experiment, and both sets of results showed qualitative and quantitative agreements. The effects of the surface wettability, impact velocity and relative size of the droplet to the pipe on the droplet impact dynamics are systematically investigated. Moreover, the regularities of the contact area between the liquid and the pipe during the impacting process as well as the volume of residual liquid remaining on the pipe post-impact are also analyzed; these two parameters are the key factors which affect the heat transfer efficiency of heat transfer pipes, and they cannot be acquired very accurately using experiments.

The impact dynamics of a ferrofluid droplet on a tempered glass surface in the presence of a vertical magnetic field has been investigated experimentally. The results show how precise control of the impact dynamics of a ferrofluid droplet can be obtained.

In line with recent efforts and developments in emerging printed electronics, using solution-processed coatings, we studied the impact dynamics and deposition of single and multiple polymeric aqueous and isopropanol (IPA)-diluted PEDOT:PSS solution droplets, across seven orders of magnitude timescale. The solution properties and release height of droplets from a needle were varied to generate Weber numbers in the range of 94–510, with two Ohnesorge numbers of 0.0108 and 0.0195, for aqueous and IPA-diluted PEDOT:PSS solution droplets, respectively. The former droplet on FTO glass substrate is partially wetting, whereas the latter is fully wetting, generating different phenomena in the prolonged wetting and drying time. The solutions showed shear-thinning behavior at high shear rates, but viscosity immediately reached a saturated limit at higher shear rates and, therefore, the fluids behaved as Newtonian fluids during impact. Among the results, the addition of IPA resulted in improved spreading of PEDOT:PSS in the wetting phase, with wetting trend following the Tanner’s law. We then assessed the prediction power of existing models to predict maximum spreading, taking into account the role of measured static and dynamic contact angles during spreading. Multiple droplets (2, 5, and 15) were impacted nearly simultaneously and formed lines and films. We examined the bridge formation, spreading length growth, and shape evolution of multiple coalescing droplets. We also correlated the formed surface area with the number of coalescing droplets and discussed the ideality of the shape of the formed film. The results of this study will help to pave the way for large-scale manufacturing of organic coatings using droplet-based methods.

International audience; Fused silica samples were subjected to laser induced shock loading. Laser flux was varied in order to obtain different amounts and characteristics of damage in the samples. Three dimensional damage and fracture maps of two identical samples impacted by high and low laser flux values were obtained using both optical microscopy and X-ray computed micro-tomography. Three prevalent fracture and damage patterns were identified. Peridynamic approach was used to simulate the laser impact conditions on the samples in order to explain the causes of the observed fracture and damage morphologies. A proprietary shock physics code, ESTHER, was used to calculate the transient kinetic energy imparted to the samples based on the experimental laser flux values. The kinetic energy values were then integrated over time and provided target values to match for the peridynamic impact conditions. The main fracture patterns were captured by peridynamic simulations with reasonable quantitative accuracy. Explanations for initiation and propagation of each of the fracture patterns were presented based on the peridynamic dynamic fracture simulations. Limitations of the computational approach and recommendations for future work is provided.

Targeted investigation of the impact fracture performance of PVB laminated glass, which is a combined analytical and numerical method for impact dynamics prediction, is presented. In this method, the relationship between the impact force and the post-cracking response of laminated glass is developed in the framework of impact dynamics to present a prediction formulation. The formulation is utilized to be applied to predict vehicle impact speed based on the crack patterns propagated on the automotive laminated glass due to the pedestrian head impact. To prepare a numerical set-up, an automotive laminated glass finite element model is proposed by inserting cohesive elements on all common surfaces of glass layers and glass-PVB interfacial. The prediction model is validated well by comparison with numerical crash patterns and ten real vehicle–pedestrian accident cases.

In this study, the spreading and retraction dynamics of impacting droplets on lubricant-infused PTFE surfaces were investigated through high-speed imagery. Superhydrophobic Polytetrafluoroethylene (PTFE) surfaces with randomly rough microstructures were prepared by sanding PTFE. Several silicone oils with different viscosities were infused into the structures of superhydrophobic PTFE surfaces. A glycerin and water solution was used for the impacting droplets. The viscosity ratio between the impinging droplet and infused oil layer was varied from 0.06 to 1.2. The droplet impact dynamics on lubricant-infused surfaces were found to change as the viscosity of the infused silicone oil layer was decreased. These changes included an increase in the spreading rate of the droplet following impact, an increase to the maximum spreading diameter, and an increase to the retraction velocity after the droplet reached its maximum diameter. These variations in the impact dynamics were most significant as the viscosity ratio became larger than one and are likely due to the reduction of viscous losses between the oil and water phases during the spreading and retraction of the impacting droplet. Using a scaling analysis which takes into account the role of energy dissipation in the impact dynamics, all the data for the maximum diameter of the droplet on lubricant-infused PTFE surfaces were found to collapse onto a single master curve. Finally, measurements of the dynamic advancing and receding contact angle were made during spreading and retraction of the droplet. These measurements showed the expected Cox-Voinov-Tanner scaling of contact angle for the high oil viscosity, low viscosity ratio lubricant infused surfaces. However, like the superhydrophobic surface, little changes in either the dynamic advancing or receding contact angle were observed for droplets spreading on the surface infused with the lowest viscosity oil.