Droplet Impact Phenomena Research Articles

High-Speed Stress Field Measurement In A Soft Substrate During Droplet Impact

In this study, we utilized high-speed photoelastic tomography to quantify dynamic stress fields within a soft substrate during droplet impact. This manuscript details the measurement technique, which employs a high-speed polarization camera and the principles of photoelastic tomography. Our method successfully enabled the quantitative measurement of dynamic stress fields in a soft substrate during droplet impact. Additionally, we conducted an analysis of the impact force derived through the spatial integration of the measured stress field. The discussion explored the interplay between the maximum impact force, droplet viscosity, and substrate elasticity. Our findings indicate that the maximum impact force 𝐹max can be expressed as a function with a self-similar variable of 𝑅𝑒 / 𝐶𝑎^2 , where 𝑅𝑒 is Reynolds number representing the droplet inertial force and droplet viscous force and 𝐶𝑎 is Cauchy number representing the ratio of the droplet inertial force and substrate elastic force. This combination, 𝑅𝑒 / 𝐶𝑎^2 , represents the relationship between the relaxation time of droplet and substrate deformation ( 𝜂/ 𝐸), the contact time of the droplet (𝑅 / 𝑉), and the ratio of substrate elastic force to droplet inertial force. Consequently, the maximum impact force 𝑅𝑒 / 𝐶𝑎^2 is determined by the balance between the relaxation and contact times. We believe that our developed method will significantly enhance the understanding of droplet impact phenomena. Furthermore, it holds potential for broader applications in various engineering processes, such as analyzing stress distribution in materials caused by liquid jet impact and studying cavitation bubbles in viscoelastic materials. This method's ability to provide detailed quantitative measurements of dynamic stress fields offers a valuable tool for future research and technological advancements in related fields.

INFLUENCE OF BUBBLE GROWTH AND LIQUID FILM INSTABILITIES ON DROPLET IMPACT PHENOMENA UNDER SATURATED BOILING REGIMES

Evaporation and boiling are processes that occur in many industrial applications involving multiphase flows. For liquid films, however, studies are scarce regarding heat and mass transfer mechanisms and require further research. The main objective of this work is to evaluate bubble formation and detachment, followed by the impact phenomena. Therefore, an experimental setup was built and adapted for this purpose. A borosilicate glass impact surface is placed over a heat source, which consists of an aluminum block with four embedded cartridge heaters that heat the liquid film by conduction. Water and n-heptane are the fluids adopted for the experimental study, as the differences in thermophysical properties allow for a wider range of experiments. Study cases include dimensionless temperatures of θ > 0.6 for similar impact conditions. In terms of bubble formation, n-heptane displays smaller bubble diameters and higher release rates, whereas water exhibits larger bubbles and lower rates. Qualitatively, liquid film temperatures close to the saturation temperature do not reveal a direct influence on the crown development and posterior secondary atomization. For later stages of the impact, the central jet height and breakup are influenced by the film temperature, which is associated with the variation of thermophysical properties.

Target slinging of droplets with a flexible cantilever

AbstractControl of the directional bounce of droplets impacting solid surfaces is crucial for many agricultural and industrial applications. However, for the universal impact process of raindrops on plant leaves, little is known about how the highly coupled and complicated fluid–structure interaction controls the postimpact motion of droplets and endows the leaves with tenacious vitality. Here, we report a leaf‐like superhydrophobic cantilever to flexibly bounce droplets with well‐defined directionality and controllability. Through theoretical modeling and three‐dimensional fluid–solid coupling simulations, we find that the flexible cantilever significantly relieves the impacting forces of raindrops to reduce droplet fragmentation and enhance water repellency. The results further uncover the scaling relations of the droplet bouncing direction with respect to Weber number and cantilever stiffness. By this technique, the seemed disorganized postimpact movements of droplets are programmable and predictable, achieving the goal of where to point and where to hit automatically. This work advances the understanding of natural droplet impact phenomena, opens a new avenue for delicately controlling liquid motion in space with soft materials, and inspires a plethora of applications like soft robots to transport materials and energies, monitor plant growth as well as predict pathogen transmission in plants.

Experimental investigation of airflow-assisted water droplet impact on horizontal solid surface

In the practice of aircraft icing, airflow-assisted droplet impact is a significant process that needs special attention and further study. In this research, airflow-assisted water droplet impact on a horizontal solid surface is studied experimentally. Effects of the airflow velocity and the droplet release height on the dynamic behaviors of the droplet, including the deformation rate in the airflow, the wetting factor on the surface and the duration of different stages, are analyzed and discussed. Seven kinds of airflow-assisted water droplet impact modes are observed and a regime map is drawn to distinguish the boundary of each mode. The results show that the airflow velocity plays a crucial role in affecting dynamics of the droplet. In the “no breakup - impact” regime, the maximum horizontal deformation rate increases monotonously with the initial gaseous Weber number Weg. Besides, both the maximum and the equilibrium wetting factor keep growing with the increase of Weg. The airflow significantly suppresses retraction of the droplet. In the “breakup - impact” regime, the maximum wetting factor declines slightly with the increasing Weg. This study is significant for understanding the microphysical mechanism inside the airflow-assisted water droplet impact phenomenon.

The impact of temperature on heated liquid films: Crater and jetting impact dynamics

The droplet impact phenomena onto liquid films are a field extensively researched for over a century, which are driven by many practical applications such as heat exchangers, internal combustion engines and spray cooling. Despite the extensive work on wetted surfaces, the influence of temperature on droplet outcome, local evaporation/boiling effects, and liquid film stability has been overlooked in the literature. Therefore, the main objective of this work is to evaluate the influence of the liquid film temperature on the crater and jet dynamics. The experimental setup was designed for this purpose, in which a borosilicate glass surface that contains the liquid film is placed over an aluminium block with embedded cartridge heaters, heating it by conduction. Water, n-decane and n-heptane are the fluids adopted for the experiments due to their differences in thermophysical properties and saturation temperature. Different conditions are considered, which include two dimensionless thicknesses, h∗=1.0 and h∗=1.5, and a range of dimensionless temperatures, θ=0, θ=0.2, θ=0.4 and θ=0.6. Qualitative and quantitative analyses are performed regarding crater evolution, and central jet height and breakup measurements, respectively. Evaporation rate measurements are required due to the influence on the liquid film thickness variation. Qualitative results show that temperature differences promote the formation of recirculation zones near the impact surface and the crater boundaries, as well as the influence on the crater shape and curvature. In terms of the quantitative analysis, the central jet height measurements for the n-heptane and n-decane reveal that higher values of the dimensionless temperature lead to an increase in the jet height, as well as promoting and increasing the occurrence and number of secondary droplets, respectively. Water follows a similar trend with the exception of θ=0.2, which can be explained by a time scale analysis.

Magnetic Control of Water Droplet Impact onto Ferrofluid Lubricated Surfaces

Controlling the impact process of a droplet impacting a liquid film has remained a wide-open challenge. The existing passive techniques lack precise on-demand control of the impact dynamics of droplets. The present study introduces a magnet-assisted approach to control water droplets' impact dynamics. We show that by incorporating a thin, magnetically active ferrofluid film, the overall droplet impact phenomena of the water droplets could be controlled. It is found that by modifying the distribution of the magnetic nanoparticles (MNPs) present inside the ferrofluid using a permanent magnet, the spreading and retraction behavior of the droplet could be significantly controlled. In addition to that, we also show that by altering the impact Weber number (Wei), and the magnetic Bond number (Bom), the outcomes of droplet impact could be precisely controlled. We reveal the role of the various forces on the consequential effects of droplet impact with the help of phase maps. Without the magnetic field, we discovered that the droplet impact on ferrofluid film results in no-splitting, jetting, and splashing regimes. On the other hand, the presence of magnetic field results in the no-splitting and jetting regime. However, beyond a critical magnetic field, the ferrofluid film gets transformed into an assembly of spikes. In such scenarios, the droplet impact only results in no-splitting and splashing regimes, while the jetting regime remains absent. The outcome of our study may find potential applications in chemical engineering, material synthesis, and three-dimensional (3D) printing where the control and optimization of the droplet impact process are desirable.

Experimental and numerical studies of interaction between liquid droplets with hot surfaces under different conditions

AbstractIn the present paper, experimental and numerical studies for the interaction studies between different liquid droplets and hot metal surfaces were carried out. The obtained results were interpreted using graphs and figures. Droplet impact, spreading, breakup and Leidenfrost phenomenon were observed with the help of a video camera at 100 fps. The interaction studies were done at different surface temperatures and different droplet‐impinging velocities. Three metals and three liquids were selected for the present study. Copper, aluminium and steel were selected as metals with varying roughness factors maintained at different temperatures. At the same time, in liquid entities, methanol, ethanol and kerosene were selected along with water in some portions. The selections of the metals were based on the thermal diffusivity property as it influences the thermal interaction between the droplet and the hot metal surface. The temperature range of the metal surface was kept from 150°C to 425°C, and the impinging velocity range of droplets was in the field of 2.4–4.9 m/s. For computational studies, the Ansys Fluent R3‐2019 version was used. Numerical analysis for the interaction process was carried out using the volume of fluid (VOF) method, solving Navier–Stokes and energy equations in a 2D‐geometry transient simulation. Parameters like surface temperature, heat flux and spreading ratio w.r.t. change in Weber number were investigated and presented in the present paper. Obtained results are useful for better understanding of droplet physics while interacting with hot surfaces which further helps in development of new methods for effective spray system designs. Results show that for a lower value of Weber number, the droplets spread less as compare to high Weber number. Also, spreading ratio of droplets depends only on the surface roughness.

Droplet bouncing on topological nonwetting surfaces via laser fabrication

PurposeThe purpose of this paper is to show that the droplet impact phenomenon is important for the advancement of industrial technologies in many fields such as spray cooling and ink jet printing. Droplet bouncing on the nonwetting surfaces is a special phenomenon in the impact process which has attracted lots of attention.Design/methodology/approachIn this work, the authors fabricated two kinds of representative nonwetting surfaces including superhydrophobic surfaces (SHS) and a slippery liquid-infused porous surface (SLIPS) with advanced UV laser processing.FindingsThe droplet bouncing behavior on the two kinds of nonwetting surfaces were compared in the experiments. The results indicate that the increasing Weber number enlarges the maximum droplet spreading diameter and raises the droplet bounce height but has no effect on contact time.Originality/valueIn addition, the authors find that the topological SHS and SLIPS with the laser-processed microwedge groove array produce asymmetric droplet bouncing with opposite offset direction. Microdroplets can be continuously transported without any additional driving force on such a topological SLIPS. The promising method for manipulating droplets has potential applications for the droplet-based microfluidic platforms.

Experimental Study on Droplet Impact Phenomena on Burning Liquid Pool Surfaces

In this study, the phenomena of water droplet impact on burning methanol and n-heptane pool surfaces were experimentally investigated under various size and velocity conditions of impact droplet. In the n-heptane pool, the temperature increased to the deeper location of the pool and the droplet impact velocity was slower, as compared with those in the methanol pool. These results were caused by the higher heat release rate of the n-heptane pool. However, the impact droplet sizes on the burning methanol and n-heptane pool surfaces appeared to be similar. By visualizing the droplet impact phenomena, the impact pattern maps of burning methanol and n-heptane pool surfaces were constructed and compared with the previous impact pattern maps of their unburned pool surfaces. In the burning and unburned methanol and unburned n-heptane pools, patterns of single jet and splash with secondary jet were observed. On the contrary, in the burning n-heptane pool, patterns of single jet and canopy were observed.

Regulating droplet impact on a solid hydrophobic surface through alternating current electrowetting-on-dielectric

The phenomenon of droplet impact is commonly found in industrial and agricultural processes. The basic characteristics and theories of a droplet impacting solid walls have been extensively studied, but the regulation of the droplet impact phenomenon has not been adequately examined. This study investigates the regulation of droplet impact on a hydrophobic surface based on alternating current electrowetting-on-dielectric (AC EWOD). When a water droplet impacts a virgin Teflon surface at 1.06 m/s, the phenomenon of partial rebound occurs. When an AC voltage is applied to an electrode pair underneath the Teflon layer, the droplet is stabilized on the hydrophobic surface after impact. To investigate the mechanism of influence of the AC signal on the regulation of droplet impact, the variation in the spread diameter and height of the droplet were characterized at different frequencies and amplitudes of the AC signal. An oscillation in the diameter of the droplet was observed in the retraction stage with the application of AC EWOD, which was the dominant effect in neutralizing the retraction kinetic energy and yielded the rebound inhibition effect. A transition diagram between partial droplet rebound and rebound inhibition was plotted in terms of voltage, frequency, and the Weber number, and theoretical analysis was carried out to determine the retraction kinetic energy dissipated by the viscous force when the AC EWOD signal was applied.

Delayed Leidenfrost phenomenon during impact of elastic fluid droplets

This article highlights the role of non-Newtonian (elastic) effects on the droplet impact phenomenon at temperatures considerably higher than the boiling point, especially at or above the Leidenfrost regime. The Leidenfrost point (LFP) was found to decrease with an increase in the impact Weber number (based on the velocity just before the impact) for fixed polymer (polyacrylamide) concentrations. Water droplets fragmented at very low Weber numbers (approx. 22), whereas the polymer droplets resisted fragmentation at much higher Weber numbers (approx. 155). We also varied the polymer concentration and observed that, up to 1000 ppm, the LFP was higher than that for water. This signifies that the effect can be delayed by the use of elastic fluids. We have shown the possible role of elastic effects (manifested by the formation of long lasting filaments) during retraction in the increase of the LFP. However, for 1500 ppm, the LFP was lower than that for water, but had a similar residence time during the initial impact. In addition, we studied the role of the Weber number and viscoelastic effects on the rebound behaviour at 405°C. We observed that the critical Weber number up to the point at which the droplet resisted fragmentation at 405°C increased with the polymer concentration. In addition, for a fixed Weber number, the droplet rebound height and the hovering time period increased up to 500 ppm, and then decreased. Similarly, for fixed polymer concentrations like 1000 and 1500 ppm, the rebound height showed an increasing trend up to certain a certain Weber number and then decreased. This non-monotonic behaviour of rebound heights was attributed to the observed diversion of the rebound kinetic energy to rotational energy during the hovering phase. Finally, a relationship between the non-dimensional Leidenfrost temperature and the associated Weber and Weissenberg numbers is developed, and a scaling relation is proposed.

Investigation on effect of surface properties on droplet impact cooling of cladding surfaces

Abstract During transients or accidents, the reactor core is uncovered, and droplets entrained above the quench front collides with the uncovered fuel rod surface. Droplet impact cooling can reduce the peak cladding temperature. Besides zirconium-based cladding, versatile accidental tolerant fuel (ATF) claddings, including FeCrAl, have been proposed to increase the accident coping time. In order to investigate the effect of surface properties on droplet impact cooling of cladding surfaces, the droplet impact phenomena are photographed on the FeCrAl and zircaloy-4 (Zr-4) surfaces under different conditions. On the oxidized FeCrAl surface, the Leidenfrost phenomenon is not observed even when the surface temperature is as high as 550 °C with We > 30. Comparison of the impact behaviors observed on different materials shows that nucleate and transition boiling is more intensive on surfaces with larger thermal conductivity. The Leidenfrost point temperature (LPT) decreases with the solid thermal effusivity ( kρC p ). However, the CHF temperature is relatively insensitive to the surface oxidation and Weber number. Droplet spreading diameter is analyzed quantitatively in the film boiling stage. Based on the energy balance a correlation is proposed for droplet maximum spreading factor. A mechanistic model is also developed for the LPT based on homogeneous nucleation theory.

Numerical investigation of large bubble entrapment mechanism for micron droplet impact on deep pool

Water droplet impacting into a deep liquid pool is one of the most well-known flow phenomena in fluid mechanics. As a ubiquitous natural aeration process, the coalescence of water droplets in lakes and ponds and the subsequent bubble entrapment are one of the most notable ways of gas-liquid exchange in nature, and it is of great significance for underwater sound transmission, aquatic ecosystems and chemical process. The shape of an oscillating droplet in impact under different surrounding medium and initial condition is a key factor for the subsequent cavity formation and bubble entrapment. In this study, the adaptive mesh refinement technique and volume of fluid (VOF) method are applied to the study of the water droplet impact phenomena. Five kinds of deformed micron water droplets with different aspect ratios and impact velocities of 4 m/s and 6 m/s are selected to investigate the influences of drop deformation and impact velocity on the bubble entrapment, capillary wave propagation, and vortex ring evolution. The results show that at low impact velocities (<i>Fr</i> = 75, <i>We</i> = 64.4, <i>Re</i> = 1160, <i>V</i><sub>i</sub> = 4 m/s), the shape of water droplet does not cause the cavity formation and bubble entrapment to change significantly. However, under higher impact velocity (<i>Fr</i> = 112.5, <i>We</i> = 145, <i>Re</i> = 1740, <i>V</i><sub>i</sub> = 6 m/s), deformed droplet with an aspect ratio of 1.33 coalesces with the pool, and large bubble entrainment occurs. The large bubble entrapment is affected mainly by the vortex ring generated under the free surface at the neck between the droplet and the pool. The vortex ring penetrates more deeply before it pulls the free surface to generate a rolling jet at the upper interface of the cavity. The rolling jets then contact the center of the cavity and collapse to entrain a large bubble. At the end of the bubble entrapment phenomenon, the cyclone inside the cavity pushes the sidewall of the cavity continuously, and effectively increases the lateral volume of the bubble, which plays a vital role in the bubble entrainment process. In the initial stage of the impact, the flatter the shape of the droplet, the greater the curvature of the jet generated on the neck between the droplet and the pool, the greater the strength of the vortex ring generated under the free surface. However, the vortex ring formed by the oblate-shaped water droplet is generated too close to the free surface, and the early free surface pulling reduces the strength of the vortex ring, thus the vorticity maximum value decays relatively fast.

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.

Droplet Impact Phenomena in Fluidized Bed Coating Process with a Wurster Insert

The aim of this study is to determine favorable process conditions for the coating of placebo tablets. Tablets made of microcrystalline cellulose are coated with hydroxypropyl cellulose polymer and Advantia™ Prime polymeric mixture film in lab-scale fluid-bed environment with a Wurster tube. In order to determine favorable process conditions (concentration, Wurster tube position, inlet air temperature, and atomization pressure), evaluation factors expressing process efficiency were calculated. Stereomicroscopy analysis provided good results with respect to the coating layer adherence and consistency. Results showed that the increased number of the coating cycles contributes to the desired featureless film morphology, when sufficiently high temperature and pressure are applied, thus resulting in high intra- and intertablet uniformity. Additionally, this paper analyzes the coating process from a mechanistic perspective of the underlying phenomena occurring on a tablet surface. Provided diagrams can help efficiently in detecting proper conditions that will result in coated tablets with strictly defined aimed properties. Process and formulation properties synergically result in a preferential occurrence of a deposition mechanism for all experiments conducted. Moreover, collision is found as a prevalent impact regime for the coating process studied. Finally, our intention here is to correlate hydrodynamic conditions and droplet breakup occurrence with a droplet diameter.

Experiment and Simulation Investigation on the Characteristics of Diesel Spray Impingement Based on Droplet Impact Phenomenon

In order to study the characteristics of diesel spray impinging in small-size internal combustion (IC) engine, the laboratorial platform of spray impingement is used to visualize the droplet impact phenomenon. A high-speed camera is used to record the whole process of the spray. An experimental research is accomplished under the impingement phenomenon of diesel spray on the wall with a fixed injection volume of 25 mg, injection pressure of 40 MPa, wall-injector distance of 40mm and orifice diameter of 0.26 mm. The computational fluid dynamics (CFD) simulation is carried out and the turbulence kinetic energy (TKE) distribution diagram is obtained. The results show that the maximum spray radius is 29.3 mm and maximum spray height is 10.2 mm at 2.8 ms. The spray impingement phenomenon presents the “blanket” shape. The simulation results are similar to the experimental results with the average error of the spray height within 3%.

A coupled Euler-Lagrange CFD modelling of droplets-to-film

ABSTRACTIn this paper, a droplet to film interaction model technique is presented. In the proposed approach, the liquid and gas continua are modelled using an enhanced Volume-of-Fluid (VoF) technique while the droplets are tracked using a Lagrangian framework and are coupled to the Eulerian phases using source terms. The eventual target application is an aeroengine bearing chamber in which oil is found as droplets, shed from the bearings, splashing on impact, separated from wall surfaces at obstacles or simply re-entrained, and as a continuum oil film coating the bearing chamber outer walls which it also cools. In finite volume Computational Fluid Dynamics (CFD) techniques, a prohibitively large number of cells would be required to describe the details of the droplet impact phenomenon. Based on published correlations, the splashing droplets are created and tracked as Lagrangian particles. The flowing film and the gas continua are handled with an enhanced VoF technique.

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.

Impact of Viscous Droplets on Superamphiphobic Surfaces.

The impact of a liquid droplet on a solid surface is one of the most common phenomena in nature and frequently encountered in numerous technological processes. Despite the significant progress on understanding the droplet impact phenomenon over the past century, the impact dynamics, especially the coupling effects between the properties of a liquid and surface wettability on the impact process, is still poorly understood. In this work, we experimentally investigated the impact of viscous droplets on superamphiphobic surfaces, with the viscosity of liquids ranging from 0.89 to 150 mPa s. We showed that an increase in liquid viscosity will slow down the impact process and cause bouncing droplets to rebound lower and fewer times. The critical impact velocity, above which droplets can rebound from the superamphiphobic surface, was found to linearly increase with the liquid viscosity. We also showed that the maximum spreading factor increases with Weber number or Reynolds number but decreases with the liquid viscosity. Scaling analyses based on energy conservation were carried out to explain these findings, and they were found to be in good agreement with our experimental results.

Droplet impact behavior on heated micro-patterned surfaces

Impact behavior of droplets on a surface is an intriguing research topic, and its control should be very useful in diverse industrial applications. We investigated the impact behavior of water droplets on the textured and chemically treated surface of silicon and obtained the impact mode map on the parameter plane subtended by the Weber number (up to 85) and temperature (up to 320 °C). The patterns comprise of micropillars (14 μm in height) in square lattice with a lattice constant of 10 and 20 μm, and the surface was further made superhydrophobic by coating with graphene nanosheets. Six distinct impact modes are identified. It was found that the impact mode map can be dramatically altered by modifying the texture and chemistry of the surface, and the observations are well explained with regard to heat transfer, vapor/bubble generation and vapor flow beneath the droplet. Instability in the droplet arising from the mismatch between vapor generation rate and exhaust conditions is the dominant factor in determining the impact mode. Our results revealed more facts and features of the droplet impact phenomenon and can be very useful for target-oriented surface design towards precise control of droplet impact behavior on heated substrates.