Droplet Impact Experiments Research Articles

Heat transfer during droplet impact on a cold superhydrophobic surface via interfacial thermal mapping

AbstractUndesired heat transfer during droplet impact on cold surfaces can lead to ice formation and damage to renewable infrastructure, among others. To address this, superhydrophobic surfaces aim to minimize the droplet surface interaction thereby, holding promise to greatly limit heat transfer. However, the droplet impact on such surfaces spans only a few milliseconds making it difficult to quantify the heat exchange at the droplet–solid interface. Here, we employ high‐speed infrared thermography and a three‐dimensional transient heat conduction COMSOL model to map the dynamic heat flux distribution during droplet impact on a cold superhydrophobic surface. The comprehensive droplet impact experiments for varying surface temperature, droplet size, and impacting height reveal that the heat transfer effectiveness () scales with the dimensionless maximum spreading radius as , deviating from previous semi‐infinite scaling. Interestingly, despite shorter contact times, droplets impacting from higher heights demonstrate increased heat transfer effectiveness due to expanded contact area. The results suggest that reducing droplet spreading time, as opposed to contact time alone, can be a more effective strategy for minimizing heat transfer. The results presented here highlight the importance of both contact area and contact time on the heat exchange between a droplet and a cold superhydrophobic surface.

Impact characteristics of high-temperature alumina droplets on a cold wall: Rebound and adhesion

The adhesion and rebound characteristics of alumina droplets in solid rocket motors are important for accurate prediction of slag deposition. This paper develops an experimental system for high-temperature alumina droplets impacting the wall. The system uses induction furnace to melt the alumina and use argon to squeeze the liquid alumina to obtain high-temperature alumina droplets. The particle diameter of the alumina droplets is 0.2 mm ∼ 6 mm, which can be used to explore the impact characteristics of the alumina drop on the cold wall. The experimental system is used to carry out the droplet impact experiments on the tungsten wall at different temperatures, and the droplet temperature is obtained by the two-color pyrometer. The studies show that the contact line stops moving during spreading when the small micron droplets adhere; while the larger droplets adhere, the contact line stops moving during retracting. When alumina droplets impact with the wall vertically then rebound, two sharp corners will be formed near the contact line. When they impact with the wall obliquely then rebound, the primary sharp corner and the secondary sharp corner will be formed gradually. The regime map of adhesion and rebound is obtained, and it is found that the condensation effect will dominate the adhesion stage when the impact energy of the droplet is small.

Effectiveness of N95 Mask in Preventing COVID-19 Transmission.

N95 mask has emerged as a potential measure to mitigate the airborne transmission of respiratory disease such as COVID-19. Herein, we experimentally investigated the impact and interaction of pure water droplets as surrogate to respiratory droplets with the different layers of a commercially available N95 mask to demonstrate the penetration and passage-capability of respiratory fluids through the different layers. The penetration of an impacting droplet through the mask layers was characterized by elucidating the ejection of secondary droplets from the rear-side surface of the target mask material. In addition, the passage of respiratory fluids through the mask layers was characterized by capillary imbibition of the droplet liquid through the pores, as a function of wettability of the mask material. Droplet impact at Weber numbers We = 208 and 416 has been considered in the present study; the chosen We range corresponds to that of cough droplets realized in real respiratory events. Each layer of the N95 mask is hydrophobic that prevents capillary imbibition through the pores: a sessile droplet placed over the surface exhibits classical diffusion-limited evaporation. Droplet impact experiments on N95 mask layer surfaces reveal that a single layer allows liquid penetration at We = 416; while a combination of five layers, as is the case of a commercially available N95 mask, blocks the penetration completely, consistent with the widely known effectiveness of N95 masks. Herein, we devote special attention to compare the so-obtained efficiency of N95 masks to that of a recently designed two-layer cloth mask containing an intermediate High-Efficiency Particulate Air (HEPA) filter layer (Narayan et al. in Phys Fluids 34:061703, 2022). We conclusively show that the performance of the designed cloth mask is identical to that of a commercially available N95 mask. The assessment of mask effectiveness further includes examination of breathability and comfort by means of passage of air through them. A comparative study has been presented herein for a clear demonstration of effectiveness of different masks in preventing air-borne transmission of COVID-19.

Experimental and Numerical Study on Dynamic Characteristics of Droplet Impacting on a Hot Tailings Surface

It is quite important to clearly understand the fluid dynamic process of water droplets impacting on a hot extracted titanium tailings surface for the recovery and utilization of tailings. In this research, the finite volume simulations of the droplet impingement were developed by applying the volume-of-fluid method and were validated against experimental results. Droplet-impact experiments were performed by using a high-speed camera. The effects of the Weber number, surface tension value, and contact angle on the spreading factor were quantitatively investigated, and the distributions of pressure, velocity contour, and temperature were analyzed in detail during the impact process. We found that the numerical results of the impact process and spreading factor conform to the experiments. Moreover, the surface tension, contact angle, and Weber number have important consequences for droplet dynamic characteristics. Finally, a new maximum spreading factor model that is governed by Weber numbers is proposed for the extracted titanium tailings surface based on the experimental and numerical results. These findings provide a pathway for controlling dynamic interactions of the droplets impacting on the tailings’ surfaces.

A multi-feature predicting model of crown evolution involving material properties

We propose a new model for the crown evolution after a single drop impacts on a thin film. In contrast with the existing models that mainly focus on the status of well-developed crown or the bottom radius evolution, multiple features are provided in our model, including the temporal evolution of the crown radius, height, velocity, and the characteristic time when the crown reaches its maximum height. Both the fluid viscosity and surface tension are considered in this work rather than ignoring one or both of them as in previous studies, resulting in improved accuracy for crown development prediction. The new model can be applied for droplets of various fluids impinging on relatively thin films (0.1 < δ < 0.67). The reliability of the model is validated via comparing results with five cases of droplet impact experiments.

Toward Surfaces with Droplet Impact Robustness and Low Contact Angle Hysteresis

AbstractSuperhydrophobic surfaces show excellent water repellent performance. However, liquid impalement occurs when a droplet impacts on the surface at high velocity. To achieve higher resistance to liquid impalement at the same scale, a dense array of structures or structures with pinning at the structures top are typically used, but they also lead to increased contact angle hysteresis and higher energy dissipation. Changing the structure side wall shape can help to maintain low hysteresis and withstand high impact energy at the same scale. Droplet contact angle, contact angle hysteresis and droplet impact experiments are performed on both conical and cylindrical pillar surfaces. Comparing conical and cylindrical pillar structures with the same pitch and height, it is found that cylindrical pillars exhibit higher critical Weber number but result in larger contact angle hysteresis and larger liquid residue size when above a critical Weber number. A proper design of conical structures can maintain large contact angle, low hysteresis, strong resistance to impalement, higher number of bouncing and smaller liquid residue size. In addition, the critical Weber number for the conical structures in this work is higher than micro‐patterned pillar surfaces at the same pitch range, implying that we improve the anti‐wetting performance further.

Spreading and freezing of supercooled water droplets impacting an ice surface

The present study conducts supercooled droplet impact experiments on flat ice surfaces with varied impact velocities, initial diameters, droplet supercoolings, and ice surface temperatures. Droplet spreading at room temperature can be described by equating the inertial force to capillary and viscous forces. A universal scaling on maximum spreading can be obtained by interpolating between the laws in the capillary and viscous regimes where the dominant forces are capillary and viscous, respectively. During supercooled droplet spreading on ice surfaces, dendritic solidification occurs, forming a moving ice–water interface and leading to new droplet spreading mechanisms. The maximum spreading factor is sensitive to droplet supercooling, whereas surface temperature has a minor influence on spreading. A new dimensionless number, the icing number, is proposed to scale icing effects on a flow field, refining the law in the viscous regime for supercooled droplets. Nevertheless, icing does not affect the scaling in the capillary regime. The total influence of icing is related to the magnitude of the icing number and the competition between capillary and viscous forces. The icing number can then form a simple criterion for determining whether icing is negligible. The newly derived scaling performs well when modeling the present experimental results.

Improving superamphiphobicity by mimicking tree-branch topography

when a droplet impacts on a superhydrophobic structured surface below a certain impact velocity, the droplet can bounce off completely from the surface. However, above such velocity a fraction of the droplet will pin on the surface. Surfaces capable of repelling water droplets are ubiquitous in nature or have been artificially fabricated. However, as the surface tension of the liquid is reduced, the capability of the surface to remain non-wetting gets hindered. Despite progress in previous research, the understanding and development of superamphiphobic surface to impacting low surface tension droplets remains elusive. It is proposed that multi-layer re-entrant like roughness can further enhance the anti-wetting properties also for low surface tension fluids.In this work, we produce patterned conical micro-structures with lateral nano-sized roughness. Furthermore, the droplet impact experiments are conducted on various surfaces with variable surface tensions (27 mN/m - 72 mN/m) by using droplets with different Weber numbers (2–170).We show that conical microstructures with lateral roughness mimicking tree-branches provides a surface topology capable of absorbing the force exerted by the droplet during the impact which prevents the droplet from pinning on the surface at higher impact velocity even for low surface tension droplets. Our study has significance for understanding the liquid interaction mechanism with the surface during the impact process and for the associated surface design considerations.

Theoretical Modelling of Droplet Extension on Hydrophobic Surfaces

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

Axial spreading of droplet impact on ridged superhydrophobic surfaces

HypothesisDue to the complex hydrodynamics of droplet impact on ridged superhydrophobic surfaces, quantitative droplet spreading characteristics are unrevealed, limiting the practical applications of ridged superhydrophobic surfaces. During droplet impacting, the size ratio (the ratio of the ridge diameter to the droplet diameter) is an important factor that affects droplet spreading dynamics. ExperimentsWe fabricated ridged superhydrophobic surfaces with size ratios ranging from zero to one, and conduct water droplet impact experiments on these surfaces at varied Weber numbers. Aided by the numerical simulations and theoretical analysis, we illustrate the droplet spreading dynamics and reveal the law on the maximum axial spreading coefficient. FindsThe results show that the droplet spreading and retraction dynamics on ridged superhydrophobic surfaces are significantly asymmetric in the axial and spanwise directions. Focusing on the maximum axial spreading coefficient, we find it decreases first and then increases with increasing size ratios, indicating the existence of the critical size ratio. The maximum axial spreading coefficient can be reduced by 25-40% at the critical size ratio compared with that on flat surfaces. To predict the maximum axial spreading coefficient, two theoretical models are proposed respectively for size ratios smaller and larger than the critical size ratio.

One-step fabrication of flexible superhydrophobic surfaces to enhance water repellency

This paper investigates a one-step fabrication method of flexible superhydrophobic surfaces on silicone rubber substrates, which enhance the repellency to both static and impacting water droplets. Grooved structures with hierarchical roughness were constructed directly on intrinsically hydrophobic silicone rubber surfaces using nanosecond ultraviolet laser. The effects of laser parameters, laser scanning speed and number of scans, on surface morphology and wettability were explored. Results show that the scanning speed and the number of scans jointly affect the roughness of the processed surfaces and further the contact angle of static droplets. Due to the anisotropic wettability, the contact angle perpendicular to grooves is generally larger than that in the parallel direction. When the scanning speed is 540 mm/s and the number of scans is 75, the maximum contact angles in the perpendicular and parallel directions are 156.5°, 155.0°, respectively. One of the silicone rubber based superhydrophobic surfaces was then bent into convex shapes with various curvatures and a series of droplet impact experiments were performed on it. Compared to that on a flat superhydrophobic surface, the contact time of impacting droplets on the curved surface is reduced dramatically. It demonstrates that the flexible surfaces can also have improved repellency to dynamic droplets. This work provides an effective approach for repelling water through fabricating flexible superhydrophobic surfaces.

Is There a Relationship between Surface Wettability of Structured Surfaces and Lyophobicity toward Liquid Metals?

The liquid metal lyophobicity of a rough substrate was, in previous articles, found to be rather independent on the surface wettability. In this article, we scrutinize the impact of surface wettability of a structured (rough) surface on the liquid metal wettability and adhesion. As a model system, a structured diamond coating was synthesized and modified by air plasma. We show that surface wettability (surface free energy) does not play a prominent role for static contact angle measurements and for the liquid metal repelling properties of the diamond coating in droplet impact experiments. In contrast, roll off angles and repeated deposition experiments illustrate that the increased hydrophilicity impacts the long-term liquid metal repellency of our coating. Liquid metal adhered after around 50 deposition/removal cycles on the hydrophilic diamond coating, while no liquid metal adhesion was visible after 100 cycles on the hydrophobic diamond coating, illustrating the fundamental role for the adhesion of liquid metal. The effect of repeated deposition in conjunction with gentle applied force was employed for coating the liquid metal lyophobic (hydrophilic) diamond coating with a thin liquid metal layer. The observed effect may find application in flexible electronics and thermal management systems as a means to improve interfacing of the liquid metal with conductive non-metal coatings.

Spreading of droplets impacting different wettable surfaces at a Weber number close to zero

Six different surfaces ranging from superhydrophilic to superhydrophobic were fabricated. Droplet impact experiments were conducted on these surfaces with water droplets 2.25 mm in diameter at a Weber number close to zero to study the effects of surface wettability on the impact process and post-impact oscillation. Droplet impact on all surfaces, except for superhydrophobic surfaces, is dominated by a spreading stage and no evident retraction is observed. Using the inertia-capillarity time tc = ρR03/σ as characteristic time, the spreading stage on all surfaces at We < 0.5 was calculated to be 2.25 ± 0.11tc; further, it could be divided into inertial and viscous spreading stages at 1 ± 0.11tc based on droplet height evolution. A semi-empirical correlation for calculating the evolution of spreading factors on different surfaces was fitted by a piecewise exponential function using experimental data, and a deviation of ±20% was observed between the fitted and experimental data.

Experimental investigation on the Leidenfrost phenomenon of droplet impact on heated silicon carbide surfaces

Due to its superiority in suppressing hydrogen generation under severe accident conditions, silicon carbide (SiC) has been regarded as one of the promising candidates among the diverse accident tolerant fuel (ATF) claddings. Droplet impact experiments are conducted on preheated CVD-SiC surfaces with different roughness and the polished sintered-SiC and stainless-steel surface. The effects of surface roughness, contact angle and thermal properties on the impact behavior and cooling efficiency are discussed. The experiments are carried out in the range of 10 < We < 120 and Tsurf < 460 °C. The observed droplet impact phenomena on all the surfaces are categorized into five regimes, i.e. deposition, rebound with secondary atomization, breakup with secondary atomization, rebound and breakup. Deposition corresponds to nucleate boiling. Rebound and breakup are the hydrodynamic phenomena of film boiling. The rest two correspond to transition boiling. Droplet breakup can be induced thermally or mechanically in the transition boiling. Surface roughness can enhance droplet breakup both in the film and transition boiling. The small contact angle also promotes breakup. The thermal property of surfaces has little effect on the critical Weber number for droplet breakup, but affect the Leidenfrost point temperature (LPT) and CHF temperature significantly. The rupture of liquid film induced by roughness reduces the stagnant pressure beneath the droplet and lower the LPT. The droplet spreading dynamics are analyzed quantitatively, and developed an empirical model for droplet maximum spreading factor. It is found that the LPT model based on homogeneous nucleation mechanism coincides well with present experimental results.

A numerical study of spray/wall impingement based on droplet impact phenomenon

A new spray/wall impingement model is purposed based on droplet impact experiments and droplet morphology dynamics. Various splashing modes are summarized and analyzed based upon earlier research on droplet impact and conservation laws. Corona splash and prompt splash are distinguished in the new spray/wall impingement model taking the incident and wall conditions into consideration. Morphology dynamics of droplet impingement is analyzed and coupled into the conservation equations, which helps to obtain more accurate dynamic results of splashing droplets. The new wall impact model is validated in a constant volume combustion system under different conditions. Two different fuels, including n-tridecane and diesel, are adopted for spray impingement validations. It is found that this new model shows good performance in predicting impinging spray properties, including radius penetration and splashing height, in wide jet conditions.

Two-Pronged Jet Formation Caused by Droplet Impact on Anisotropic Superhydrophobic Surfaces

When a liquid droplet impacts a superhydrophobic surface with anisotropic surface patterning in the form of alternating ribs and cavities, the rebounding droplet may exhibit a unique two-pronged jet emission. Droplet impact experiments with 11 different fluids of viscosity that varied by more than three orders of magnitude were conducted, and this paper quantifies the Capillary number, Ca, and Ohnesorge number, Oh, ranges over which the two-pronged phenomenon occurs. For Oh &gt; 0.0154, the behavior was never observed, while at lower values of Oh, the behavior occurs for an intermediate range of Ca that depends on Oh.

Characterization of the Topography and Wettability of English Weed Leaves and Biomimetic Replicas

The topography and wettability of the underside of English weed (Oxalis pes-caprae) leaves and of their biomimetic replicas are investigated. Polyvinyl siloxane molds were cast from the leaves and then filled with an epoxy pre-polymer to produce replicas. The particular topographical structures of leaves and replicas were evaluated by optical microscopy and Scanning Electron Microscopy (SEM) analysis. The static wettability of leaves and replicas was assessed by contact angle measurements, while the dynamic wettability was characterized by estimating contact angle hysteresis and studying the dynamic behavior of impacting water droplets. A smooth glass slip and its replica were used as control surfaces. The replica moulding method used was able to transfer the characteristic pattern of irregular 100 μm − 200 µm × 60 µm convex papillae interspersed with stomata of the original leaf to the epoxy replicas. The static contact angle of 143° ± 3° and the contact angle hysteresis of 2° indicate that the underside of the English weed leaf is close to superhydrophobic. The lower contact angles (130° ± 4°) and higher hysteresis (31°) observed for the replica when compared with the original leaves were associated to an inaccurate replication of the chemistry and structures of the three-dimensional wax projections covering the plant surface. Also, trichomes in the original leaves could not be accurately reproduced due to their flexibility and fragility. Differences in wetting behavior were also evident from droplet impact experiments, with rebound regimes prevailing in the original leaves and regimes characterized by higher adhesion and larger dissipation predominating in the replicas. Nevertheless, the morphological features of the leaf transferred to the replica were sufficient to promote a clear hydrophobic behavior of the replica when compared with the smooth epoxy reference surface.

Impact dynamics of high Weber number drops on chemically modified metallic surfaces

Hydrophobic/superhydrophobic metallic surfaces prepared via chemical treatment are encountered in many industrial scenarios involving the impingement of spray droplets. The effectiveness of such surfaces is understood through the analysis of droplet impact experiments. In the present study, three target surfaces with aluminum (Al-6061) as base material–acid-etched, Octadecyl Trichloro Silane (OTS) coated, and acid-etched plus OTS-coated–were prepared. Experiments on the impact of inertia dominated water drops on these chemically modified aluminum surfaces were carried out with the objective to highlight the effect of chemical treatment on the target surfaces on key sub-processes occurring in drop impact phenomenon. High speed videos of the entire drop impact dynamics were captured at three Weber number (We) conditions representative of high We (We>200) regime. During the early stages of drop spreading, the drop impact resulted in ejection of secondary droplets from spreading drop front on the etched surfaces resembling prompt splash on rough surfaces whereas no such splashing was observable on untreated aluminum surface. Prominent development of undulations (fingers) were observed at the rim of drop spreading on the etched surfaces; between the etched surfaces the OTS-coated surface showed a subdued development of fingers than the uncoated surface. The impacted drops showed intense receding on OTS-coated surfaces whereas on the etched surface a highly irregular receding, with drop liquid sticking to the surface, was observed. Quantitative analyses were performed to reveal the effect of target surface characteristics on drop impact parameters such as temporal variation of spread factor of drop lamella, temporal variation of average finger length during spreading phase, maximum drop spreading, time taken to attain maximum spreading, sensitivity of maximum spreading to We, number of fingers at maximum spreading, and average receding velocity of drop lamella. Existing models for maximum drop spreading showed reasonably good agreement with the experimental measurements on the target surfaces except the acid-etched surface.

Dynamic Impact Behavior of Water Droplet on a Superhydrophobic Surface in the Presence of Stagnation Flow

The following study investigates splashing of impinging water droplets on superhydrophobic surfaces with and without the presence of a stagnation flow. Droplets were accelerated by either gravity or gravity and co-flow. By changing the height and the air flow velocity different combinations of stagnation flow and droplet velocity were created. The spreading diameter, spreading velocity and contact time were studied for different air and droplet speeds. It was clearly observed that for a fixed impact velocity (i.e. constant Weber number), the presence of the stagnation flow promotes splashing and formation of satellite droplets. Consequently, for the co-flow droplet impact experiments, the mass of the recoiled droplet is significantly smaller than that of the impinging droplet in still air.

The observation of air bubble entrapment for water droplets impinging on parafilm surface

The formation of an entrapped air bubble for a water droplet impinging on a parafilm surface was studied by using a high-speed video camera system. The images were taken sequentially immediately after the droplet impingement. The relaxation of impinging droplets from both the top and side views was monitored, and a formation mechanism of the entrapped air bubble was proposed in this work. An investigation of droplets with different sizes (D0) and impact velocities (Vi) was performed, and it was found that the formation of the entrapped air bubble was observed only at (i) Vi=0.35–0.56m/s for droplets with D0=2.14mm, (ii) D0=1.93–2.14mm for Vi=0.57±0.02m/s, and (iii) D0=2.30–2.46mm for Vi=0.52m/s.The fluid flow inside the droplet during the formation of air bubble entrapment was examined by performing two sets of droplet impact experiments: (i) droplets with various Vi at a fixed droplet size and (ii) droplets with various volumes at a fixed Vi. Once the water droplet touched the parafilm surface, the drop spread, and the central cone of the impinging droplet descended. After a few milliseconds, the central cone disappeared, an air cavity formed, and the droplet started to retract. The retraction generated an inward, radial flow mainly to the top and bottom of the central air cavity. A crucial condition for the formation of an entrapped air bubble in this work is the inward flow over the top of the air cavity closes earlier than the upward movement of the fluid below the central cavity.