Impact Velocity Research Articles

Ultrafast bounce of particle-laden droplets.

The rebound of liquid droplets on solid surfaces exhibits behavior reminiscent of elastic spheres, albeit with distinct contact dynamics. While the rapid detachment of droplets from surfaces holds significant relevance for various applications, previous endeavors relying on engineered surfaces can only reduce the contact time to several milliseconds, primarily due to capillary effects dominating droplet bounce. Here, we present ultrafast rebound by designing heterogeneous core-shell droplets encapsulating a particle (DEP), which achieves an unprecedentedly short contact time of 0.3 ms and 0.05 ms with polydimethylsiloxane and glass particles, respectively. This remarkable contact-time reduction is universally applicable to diverse systems, including both water and oil droplets, elastic and rigid particles, super-repellent and superlyophilic surfaces, and is effective across a wide range of impact velocities. Beyond exhibiting liquid-like dynamics, DEP manifests solid-like behavior owing to asynchronized motions between the particle and the droplet, which effectively breaks down the dominance of capillarity. With systematic experimental and analytical studies, we delineate contact times in three bouncing regimes and identify critical conditions governing regime transitions. DEP amalgamates the bouncing dynamics of both solids and liquids, offering a robust and versatile strategy for tailoring contact time to suit diverse applications involving solid-liquid composite systems.

Vehicle Collision Analysis of the Reinforced Concrete Barriers Installed on Bridges Using Node-Independent Model

This paper focuses on improving vehicle collision simulations using finite element analysis (FEA) to examine interactions between reinforced concrete barriers and bridge decks. Three scenarios are explored: treating the barrier as a rigid body, analyzing reinforcement with a fixed base, and including the bridge deck’s cantilever portion beneath the barrier. Except for the rigid body model, a node-independent approach models the complex interactions between reinforcement and concrete in barriers on the bridge deck. This study evaluates barrier strength, occupant risk, and post-collision vehicle safety. Strength is assessed by examining stress in reinforcement and concrete, while occupant risk is measured using Theoretical Head Impact Velocity (THIV) and Post-impact Head Deceleration (PHD). Vehicle trajectory during collisions is also analyzed for stability. The results show significant differences in stress distribution and failure patterns when the bridge deck is considered compared to scenarios without it. Occupant risk evaluations suggest more flexible responses when the bridge deck is included. However, vehicle trajectory post-collision showed no significant differences across scenarios. These findings indicate that modeling efficiency varies based on evaluation criteria, suggesting a more realistic and effective approach for assessing barriers on bridges.

A Pulsed Bubble‐Driven Efficient Liquid‐Solid Triboelectric Nanogenerator

AbstractHarnessing energy from underwater bubbles has garnered significant attention, particularly for powering off‐grid circuitry. However, the efficiency of bubble‐driven liquid‐solid interface charge transfer remains low. This research unveils a phenomenon: accelerated bubble slippage enhances liquid‐solid interfacial charge transfer. Building upon this discovery, a pulse bubble‐based power generation technique is proposed, achieving an energy density of 24.2 mJ L−1 generated by pulsed bubbles. The crux of pulse bubble power generation lies in the precise control of impact velocity. By meticulously regulating the impact kinetic energy of bubbles, the accumulated potential energy of multiple small bubbles is converted into instantaneous pulse kinetic energy. A typical pulse bubble is controlled within a 72 ms timeframe, unleashing a surge of energy that can directly illuminate 400 light‐emitting diodes. This approach represents a groundbreaking advancement in underwater energy harvesting technology, dramatically expanding its potential applications.

Mechanical and dynamic characteristics for the CFRP, GFRP, and hybrid composites exposed to HCl environment

In the current study, the low-velocity impact (LVI) characteristics of carbon fiber-reinforced polymer (CFRP), glass fiber-reinforced polymer (GFRP), and hybrid composites before and after the corrosive environment exposure were investigated. In this regard, CFRP, GFRP, and hybrid composites were subjected to LVI and tensile loadings after being kept in a 10% diluted HCl environment for 1 week and 1 month, and the impacts of the corrosive environment on the composites’ dynamic and mechanical responses were determined by comparing the outcomes of the control and aged specimens. LVI tests for CFRP, GFRP, and hybrid composites were carried out by transferring 25.2 and 11.2 J impact energy to the specimens with two impact velocities of 3 and 2 m/s, respectively, and thus, the effects of impact energy and hybridization were investigated. Moreover, tensile tests were conducted for the control and aged specimens with 2 mm/min crosshead speed, and thus, the effects of fiber material, hybridization, and corrosive environment on the mechanical properties were determined. The study found that CFRP composites had higher stiffness than GFRPs, whereas hybrid composites exhibited dynamic responses between CFRP and GFRP, as expected. On the other hand, it turned out that the composites absorbed most of the impact energy, which was interpreted as being absorbed by the damage of fiber-reinforced composites, which stand out with their brittle characteristics. Furthermore, it was discovered that the damage severity elevated as expected with a longer aging time, which was attributed to the corrosive liquid attacking the fibers, matrix, and fiber/matrix interfaces and reducing strength. It was also observed that, as predicted, the corrosive effects generally resulted in a reduction in tensile responses, including ultimate strain and tensile strength.

Droplet impact dynamics on the surface of super-hydrophobic BNNTs stainless steel mesh.

The 'gas‒liquid‒solid' mechanism annealing method was used to create a superhydrophobic boron nitride nanotube (BNNT) stainless steel mesh in a tube furnace at 1250°C in an NH3 environment. Fe powder was used as a catalyst, and B:B2O3 = 4:1 was used as the raw material. The water droplets on the surface of the superhydrophobic material had a contact angle of approximately 150° and a slide angle of approximately 3°. By using molecular dynamics (MD) simulation technology, a three-dimensional braided physical model of nanodroplets and superhydrophobic BNNT mesh surfaces with the same contact angle and rolling angle was prepared via the function weaving method. The Weber number (We) was used as the entrance point to establish the relationship between macroscale experimental studies and nanoscale MD simulation analysis on the basis of these efforts. A study was conducted on the dynamic behaviour of droplets impacting a superhydrophobic BNNT filter surface. We suggest that the wettability, substrate structure, and impact velocity are connected to the impact dynamic behaviour of droplets on the basis of the data obtained at various scales. The findings demonstrate that when the droplet impact velocity increases, several droplet phenomena-such as impact-rebound, impact-spread-rebound, and impact-spread-breaking-polymerisation-spatter-appear on the substrate surface sequentially. The mechanism of impact behaviour at various scales is explained in light of these events. Furthermore, a better theoretical model is proposed to assess the droplet wetting transition at the nanoscale. This model accurately predicts the boundary Weber number that starts the wetting transition. Moreover, the connections among the impact velocity, spreading diameter, and contact time (or We) are examined. The tendencies found via MD simulations match the outcomes of the experiments. Our discoveries and outcomes broaden our understanding of how droplet impact affects the dynamic behaviour of superhydrophobic surfaces. A scientific foundation for examining the dynamic behaviour of droplets is provided by combining simulations and experiments.

Numerical modeling and experimental validation of low velocity impact of woven GFRP/CFRP composites

Low-velocity impact of 2D woven glass fiber reinforced polymer (GFRP) and carbon fiber reinforced polymer (CFRP) composite laminates was studied experimentally and numerically. Hybrid laminates containing blocked layers of GFRP/CFRP/GFRP with all plies oriented at 0° were investigated. Relatively high impact energies were used to obtain full perforation of the laminate in a low-velocity impact setup. Numerical simulations were carried out using the in-house transient dynamics finite element code, Sierra/SM, developed at Sandia National Laboratories. A three-dimensional continuum damage model was used to describe the response of a woven composite ply. Two methods for handling delamination were considered and compared: (1) cohesive zone modeling and (2) continuum damage mechanics. The reduced model size achieved by omission of the cohesive zone elements produced acceptable results at reduced computational cost. The comparison between different modeling techniques can be used to inform modeling decisions relevant to low velocity impact scenarios. The modeling was validated by comparing with the experimental results and showed good agreement in terms of predicted damage mechanisms and impactor velocity and force histories.

A note on the perforation of thin concrete slabs by rigid projectiles

We analyzed the perforation data from several works where thin concrete slabs were impacted by rigid long rods. The data includes the ballistic limit velocities of these projectile/slab pairs, as well as the residual velocities of the projectiles. We found that the experimental ballistic limit velocities can be accounted for by the perforation model that we published previously. On the other hand, we had to update our model to account for the residual velocities in these tests. The experimental data in these works showed that more concrete mass was ejected from the back face of these slabs than we assumed in our earlier model. Moreover, the ejected mass in these experiments was found to decrease with impact velocity. These features are in contrast with our earlier model’s assumptions, and we had to update the model as discussed here.

Development of a method for the impact performance characterization of commonly worn helmets in rodeo bull-riding

As the rodeo sport of bull-riding increases in popularity, dangers to bull-rider athlete safety continue to pose serious health risks. Despite frequent occurrences of head trauma in the sport, current test methods for evaluating helmets commonly worn in bull-riding fail to consider rotational head acceleration, which is correlated to concussive events in impact sports. Additionally, the second most prevalent mechanism contributing to head trauma in bull-riding is bull kick impacts, which have yet to be accurately modeled in a test method. Researchers aim to enhance bull-rider athlete protection through the development of a helmet test method that simulates bull kicks with a pneumatic linear impactor. The proposed test method considers impacts at two impact velocities and six impact locations using a flat and Equestrian Hazard Anvil. Two commonly worn helmets in bull-riding are evaluated in this study, and an adequate sample size is determined from analyzing variation between repeated impacts. The proposed test method is statistically validated, and researchers show that impacts at higher kicking velocities and impacts with the flat anvil result in increased rotational head accelerations. Ultimately, this study emphasizes the need for helmets designed specifically for the unique impacts of bull-riding by simulating bull kicks through pneumatic linear impactor testing to reduce rotational head kinematics and hoof penetration.

Outcomes of Sub-Neptune Collisions

Observed high multiplicity planetary systems are often tightly packed. Numerical studies indicate that such systems are susceptible to dynamical instabilities. Dynamical instabilities in close-in tightly packed systems, similar to those found in abundance by Kepler, often lead to planet–planet collisions. For sub-Neptunes, the dominant type of observed exoplanets, the planetary mass is concentrated in a rocky core, but the volume is dominated by a low-density gaseous envelope. For these, using the traditional perfect merger assumption (also known as the “sticky-sphere” approximation) to resolve collisions is questionable. Using both N-body integration and smoothed-particle hydrodynamics, we have simulated sub-Neptune collisions for a wide range in realistic kinematic properties such as impact parameters ( b′ ) and impact velocities ( vim ) to study the possible outcomes in detail. We find that the majority of the collisions with kinematic properties similar to what is expected from dynamical instabilities in multiplanet systems may not lead to mergers of sub-Neptunes. Instead, both sub-Neptunes survive the encounter, often with significant atmosphere loss. When mergers do occur, they can involve significant mass loss and can sometimes lead to complete disruption of one or both planets. Sub-Neptunes merge or disrupt if b′<bc′ , a critical value dependent on vim /v esc, where v esc is the escape velocity from the surface of the hypothetical merged planet assuming perfect merger. For vim/vesc≲2.5 , bc′∝(vim/vesc)−2 , and collisions with b′<bc′ typically leads to mergers. On the other hand, for vim/vesc≳2.5 , bc′ ∝ vim /v esc, and the collisions with b′<bc′ can result in complete destruction of one or both sub-Neptunes.

Investigations of high-speed projectile impact on symmetric sandwich structures containing solid propellant with core perforations

The response of solid rocket motors (SRMs) to high-speed fragment impacts is crucial for their safety design and operational use in scenarios such as rocket launches and space applications. The visualized Burn to Violent Reaction (BVR) test is used to observe intense reactions induced by high-speed projectile impacts. Employing a two-stage light gas gun and optical diagnostic techniques including high-speed schlieren imaging and direct photography, the impact-induced deflagration/explosion behavior, and reaction growth behavior were investigated. The damage mechanisms of the casing and propellant samples were assessed, and the reaction growth and afterburn effects of the impact-induced fragment cloud were quantitatively analyzed. The results indicate that the ignition delay time is inversely correlated with the impact velocity, decreasing from ms to μs scale. Across a wide range of velocities (1050–2058 m/s), higher projectile velocities induce more sustained and vigorous combustion reactions within the propellant. Furthermore, increasing the propellant air gap to 7.8 cm does not trigger further reactions under the studied configurations. The reaction mechanisms are closely linked to the characteristics of the fragment cloud induced by the impact. The developed Smoothed Particle Hydrodynamics (SPH) method, incorporating material constitutive models, ignition criteria, and reaction growth models, was used to study the influence of projectile velocity on the reaction mechanisms. The simulation results were compared with experimental data, demonstrating satisfactory accuracy.

Spreading of graphene oxide suspensions droplets on smooth surfaces

Understanding and predicting the spreading of droplets on solid surfaces is crucial in many applications such as printed electronics and spray coating where the fluid is a suspension and in general non-Newtonian. However, many models that predict the maximum spreading diameter usually only apply to Newtonian fluids. Here, we study experimentally and theoretically the maximum spreading diameter of graphene oxide suspension droplets impacting on a smooth surface for a wide range of concentrations and impact velocities (5≤We≤700, 30≤Re≤2000). As the particle concentration increases the rheological behavior changes from a viscous fluid to a shear-thinning yield stress fluid and the maximum spreading diameter decreases. The rheology for all concentrations is well described by a Herschel–Bulkley model that allows us to determine the characteristic viscosity and corresponding Reynolds number Re during spreading. Analogous to Newtonian fluids, the spreading ratio follows the Re1/5 scaling in the viscous spreading regime. Furthermore, we use this characteristic viscosity to develop an energy balance model that takes into account the viscous dissipation and change in surface energies to find the maximum spread diameter for a given impact velocity. The model contains one non-dimensional parameter α that encodes both the dynamic contact angle during spreading and the droplet shape at maximum spread. Our model is in good agreement with our data at all concentrations and agrees well with literature data on Newtonian fluids. Furthermore, the model gives the correct limits in the viscous and capillary regime and can be solved analytically for Newtonian fluids.

New insights into impact-induced removal of the deposited droplet

This paper presents a comprehensive investigation into the collision dynamics of equal and unequal-sized nanodroplets on a flat surface using molecular dynamics simulations, revealing new insights into scaling laws and energy dissipation mechanisms. The simulations, conducted with the Large-Scale Atomic/Molecular Massively Parallel Simulator software, involved an initially stationary droplet on the surface and a suspended droplet with varying diameter ratios (λ) and impact velocities. The results show that at low Weber numbers (We &amp;lt; 24.15), the droplets tend to deposit after impact, while at higher Weber numbers (We ≥ 24.15), they undergo spreading and retraction, ultimately rebounding. The study reveals that the dimensionless contact time (t*) and maximum spreading factor (βmax*) in collisions between droplets of different sizes do not follow the same scaling relationship observed in single nanodroplet impacts. By redefining the Weber and Reynolds numbers (Re), the new scaling relationships t* ∼ We2/3Re−1/3λ−1/3 and βmax* ∼ We2/3Re−1/3λ−1/3 are proposed and validated. This work represents a further in-depth study of previous research on single nanodroplet impact, introducing for the first time the diameter ratio in unequal droplet impacts into the variation patterns of contact time and maximum spreading diameter. Moreover, these findings highlight the importance of revisiting and potentially revising classical theories to accommodate the unique physical phenomena that emerge at smaller scales.

Numerical Simulation and Experimental Analysis of Aerosol Deposition: Investigating Nozzle Effects on Particle Dynamics and Deposition

Aerosol deposition (AD), also known as vacuum kinetic spray (VKS), is used to deposit dense ceramic films onto a substrate surface at room temperature. One predominant factor influencing the overall deposition process is the nozzle geometry, which significantly impacts the quality, shape, and thickness of the coating. To evaluate the effect of nozzle geometry in AD, particle trajectory and impact velocity were investigated via computational simulation. A Eulerian-Lagrangian model was used to simulate the gas flow, particle in-flight behavior, as well as particle deposition characteristics on a flat substrate. Three common nozzle geometries in AD were utilized: a converging-diverging nozzle with a slit cross-section (CD-Slit), a converging-barrel nozzle with a slit cross-section (CB-Slit), and a converging-diverging round (CD-Round) nozzle. Moreover, suitable drag coefficient and Nusselt number correlations were used to account for compressibility and rarefaction effects on particle dynamics and heat transfer. The simulation results were compared to the deposition pattern obtained experimentally. The results demonstrate that shape of the nozzle has profound effect on the particle impact velocity and deposition pattern. The CD-Round nozzle provides uniform, thinner coatings ideal for extensive surfaces at a slower deposition rate. In contrast, the CB-Slit nozzle is optimized for maximum coating thickness in narrow, thick linear patterns. The CD-Slit nozzle achieves high deposition rates with uniform coatings and a distinctive cat-ear shaped pattern.

Exploring mechanisms of asymmetric droplet impact dynamics on roughness gradient surface

Droplet impact on surfaces with varying roughness and wettability is a common phenomenon in both natural and industrial environments. While previous studies have primarily examined asymmetric droplet rebound driven by impact velocity or Weber number, the influence of surface structure and associated impact mode transitions has received less attention. In this study, molecular dynamics simulations and detailed analyses are employed to investigate the mechanisms governing droplet rebound on nanopillar arrays with gradient distributions. Results reveal that nanopillar height significantly influences rebound direction, with two distinct directional transitions occurring as the height increases. Additionally, the effects of surface structure and Weber number on impact patterns, rebound velocity, and contact time are systematically evaluated, with contact angle calculations shedding light on the underlying force mechanisms. A phase diagram is developed to illustrate the relationship between rebound direction, Weber number, and nanopillar height. The study further extends the analysis to substrates with bidirectional gradient distributions, demonstrating consistency with single-directional gradient results and validating the broader applicability of the findings. This research provides critical insights into droplet dynamics on roughness gradient surfaces, emphasizing the role of nanopillar height and impact mode in controlling droplet behavior and highlighting potential applications in the design of structured array surfaces.

Dynamic impact analysis of solution droplets in frost-free air-source heat pumps: Transition from droplets to liquid film

This study investigates the dynamic propagation of solution droplets in frost-free air source heat pump systems, focusing on calcium chloride and ethylene glycol solutions. The research examines three key stages: droplet impact on a surface, collision with stationary droplets, and impact on a stabilized liquid film. Key findings include that increased surface contact angle and solution concentration inhibit droplet spreading, while higher impact velocity enhances it. Droplet retraction is influenced by surface tension and adhesion, with calcium chloride droplets showing variable retraction and ethylene glycol droplets consistently retracting less as concentration increases. Collisions with stationary droplets lead to behaviors such as merging, spreading, and rebound, with higher velocity accelerating these processes. In the final impact stage, a stable liquid film formation results in crown sputtering and oscillation, where sputtering height is inversely related to concentration but increases with impact velocity. These insights contribute to optimizing droplet management in air treatment systems.

Tunable impact resistant materials: Synergistic enhancement of paraffin/MRP/re-entrant structures

In the realm of safety protection, there is a critical demand for materials capable of superior energy absorption and adaptability across diverse application scenarios. Here, this study presents integrated two-phase negative Poisson’s ratio (NPR) composites with adjustable mechanical properties sensitive to magnetic field, temperature and impact load. It is achieved by substituting the vacancy of re-entrant structures (frame phase) with soft paraffin/magnetorheological plastomer (MRP) fillers (matrix phase). The paraffin/MRP exhibits temperature-sensitive properties, enhanced magnetorheological (MR) effect (up to 4178%) and shear thickening (ST) effect (up to 2365%). Through quasi-static compression and dynamic impact tests, the composites demonstrate remarkable adaptability in impact resistance. Specifically, the first peak force exhibits responsiveness to changes in magnetic flux density, temperature, and impact velocity, with respective variations of 80.6%, 68.2%, and 30.3%. The design strategy paves the way for tunable impact resistant materials with potential for extreme environment buffering applications.

Self-driven transport and heat transfer characteristics of a droplet on a wettability-gradient surface by front-tracking method

Droplet manipulation is a multidisciplinary field with broad applications across various industries. It holds significant potential in areas such as microfluidics, oil–water separation, water harvesting, and heat transfer. However, there is still a lack of comprehensive knowledge regarding droplet migration on restricted surfaces. In this study, we conducted a numerical simulation using the front-tracking method to investigate the heat transfer associated with droplet migration on a cold plate with a wettability gradient. We examined the effects of relative temperature differences, surface wettability, low initial impact velocities (We≤10), and wettability constraints (the width of the wettability stripe capable of driving droplet movement) on various droplet-related heat transfer characteristics and the resulting temperature field distribution. Our key findings indicate that as the temperature difference between the droplet and the surface increases, the heat flux experienced by the droplet after deposition also increases. Additionally, the decline in the heat flux curve during the descending phase becomes more significant. The surface contact angle plays a crucial role in the heat transfer dynamics during droplet migration. Droplets reach thermal equilibrium more quickly on hydrophilic surfaces with smaller contact angles. Higher initial impact velocities initially cause droplets to rebound on the surface, leading to more pronounced fluctuations in transient heat flux during the impact phase. However, as droplets transition from the rebound phase to the migration phase, the impact velocity's influence diminishes. Additionally, the restricted wettability (W*) affects the droplet-surface heat transfer through variations in the wetting area. We observed a fourfold difference in the relative wetting area between W*=0.4 and W*=2.5 in the final stage.

Analyzing rain erosion using a Pulsating Jet Erosion Tester (PJET): Effect of droplet impact frequencies and dry intervals on incubation times

Accelerated laboratory testing is essential to understand the rain erosion behavior of coated samples applied to the leading edge surface of a wind turbine blade. This study investigates the impact of droplet impact frequencies and dry intervals on the incubation time for damage on polyurethane-coated samples using a Pulsating Jet Erosion Tester (PJET). A novel theoretical model for water slug volume is introduced, allowing for a more accurate comparison across different impact velocities and frequencies. The effect of dry intervals on coating performance is quantified, revealing that longer dry intervals and shorter pre-dry rain exposure can significantly increase the number of impacts a coating can withstand before damage. The study challenges the traditional continuous impingement testing by demonstrating that dry intervals can extend incubation time by a factor of three to five. Additionally, this paper proposes a recalibrated approach to PJET testing, which better mimics the cyclic nature of real-world rainfall, leading to improved predictive models for material degradation. The findings emphasize the importance of considering the visco-elastic behavior of coatings and the role of intermittent rain exposure in erosion testing, offering invaluable insights for designing future PJET test parameters.

Coupling effects of loading rate and temperature on mode I dynamic fracture characteristics of ductile cast iron

Engineering structures made of ductile cast iron (DCI) have a potential risk of failure due to extreme service environments such as high velocity impacts and sub-zero temperatures. Therefore, it is of great importance to investigate the dynamic fracture behavior of DCI under the coupling effect of rate and temperature. In this paper, two sets of impact velocities (5 m/s and 13.5 m/s), and four sets of temperatures (20 °C, −40 °C, −60 °C, and −80 °C) were specially designed to investigate the coupling effect on the mode I dynamic fracture toughness (DFT). The results show that DFT is positively correlated with impact velocity at 20 °C, −40 °C and −60 °C. However, at −80 °C, the rate effect is reversed. Moreover, DFT decreases with decreasing temperature regardless of impact velocity. With microscopic analysis, the phenomenon of ductile–brittle transition (DBT) was observed in the failure of the material, and it’s verified by dynamic tensile tests. The ductile–brittle transition temperature (DBTT) of DCI is determined as −39.7 °C by comparing the DFT with the strain energy density (SED) characterization method.

Characteristics of Particle-Wet Wall Impact Damage: Effects of Liquid Film Parameters

Particle impact damage significantly affects the safe and stable operation of energy and chemical process equipment. During particle transportation, a liquid film may form on the wall, influencing the damage mechanism caused by particle effect. This study investigates the effects of liquid film parameters, specifically viscosity and thickness, on wall damage. Hydroxypropyl Methylcellulose (HPMC) solution was applied as a liquid film, and the effect of the liquid film's viscosity and thickness was examined through impact damage experiments using a particle (45 steel) and a wall (1060 aluminum alloy) with the liquid film. Results indicate that the presence of a liquid film can inhibit wall damage. Increased liquid film viscosity and thickness reduce wall damage, but they affect the turning points of damage characteristic parameters differently concerning impact angle and velocity. Finally, the study establishes correlation between dry and wet impact by introducing the effect coefficient of liquid film. This reveals the alteration rules of the influence coefficient and proposes a wet impact damage model, providing a reference for the damage prediction in process equipment.