Droplet Spreading Dynamics Research Articles

Spreading of droplet impact on ribbed superhydrophobic surfaces with varying structure height

Hypothesis:Quantifying droplet spreading characteristics on ribbed superhydrophobic surfaces is challenging due to the complex fluid dynamics, limiting the practical applications. Droplet spreading dynamics are significantly influenced by the diameter ratio (rib diameter to droplet diameter) and height ratio (structure height to rib diameter) during impact. Experiment:Ribbed superhydrophobic surfaces with diameter ratios (0.18 to 0.75) and height ratios (0.125 to 0.5) underwent impact experiments with droplets at varied Weber numbers. Through simulations and analysis, we summarized the patterns of the maximum ribwise spreading coefficient, elucidated the droplet spreading mechanism on ribbed superhydrophobic surfaces. Findings:The findings revealed asymmetry in impact dynamics on ribbed superhydrophobic surfaces in the ribwise and radial directions. The maximum ribwise spreading coefficient initially decreases, followed by increase with increasing diameter ratio, decreasing in a slowing down trend with increasing height ratio. These observations indicated that maximum ribwise spreading coefficient can be minimize with critical ratios. Comparatively, the maximum spreading coefficient at critical height ratios is 72.7% to 92.3% of that on flat surfaces. To precisely forecast maximum ribwise spreading coefficient, two models are introduced for diameter ratios below and above critical range when height ratios below the critical range.

Anisotropic spreading on chemically heterogeneous surfaces: Insights from contact line approach

Studying the spreading of droplets on heterogeneous surfaces is significant for understanding various phenomena, from petroleum engineering to material science, due to its implications for wetting behavior, surface interactions, and functional coating applications. In this work, we investigated the spread of droplets on heterogeneous surfaces using molecular dynamics simulations. The results showed that the contact line approach can predict the equilibrium contact angle of droplet well. We found significant differences in the spreading of droplets on heterogeneous surfaces with different lyophilic pattern sizes, and then we explained the difference in droplet spreading on different heterogeneous surfaces by the proportion of lyophobic and lyophilic regions covered by contact lines. This study emphasizes the importance of contact lines in droplet spreading dynamics and enhances our understanding of the relationship between contact lines and droplet spreading.

Spreading dynamics of droplets impacting on oscillating hydrophobic substrates

Spreading dynamics of droplets impacting on oscillating hydrophobic substrates

Bio-inspired wettability patterning for flexible and wearable electrochemical immunosensors utilizing PDMS-TiO2 coating

The electrochemical sensor strip presented in this work, designed for low-volume point-of-care immunosensing applications and demonstrated using prostate-specific antigen as an example, exhibits remarkable efficiency by requiring only around 2 μL of sample volume. The decrease in sample amount required is facilitated through the application of nature inspired wettability patterning technique using PDMS-TiO2 nanocomposite coating, which incorporates distinct super-hydrophilic and super-hydrophobic regions on the electrode surface. This surface modification facilitates a highly sensitive antigen detection in minimal sample volumes. The sensor strip demonstrates a linear detection range spanning from 0.0001 to 100 ng mL-1, with a limit of detection of 0.0001 ng mL-1. Additionally, the sensor performance remains consistent across varying inclinations of the sensor strip, ensuring its applicability in the field of wearable sensing. Moreover, the incorporation of the PDMS-TiO2 coating enhances the electrochemical performance of the sensor by providing an extended surface area without introducing electrical interference. A wide range of electrochemical methods, such as cyclic voltammetry, linear sweep voltammetry, chronoamperometry, and electrochemical impedance spectroscopy, have been utilized. In support of these experimental findings, simulation studies have explored droplet spreading dynamics upon variation of the droplet size and electrode surface inclination. The developed sensor strip can serve as a promising candidate for practical point-of-care testing applications, expanding its applicability to a broad spectrum of immunosensing applications targeting diverse antigens.

Dynamical Mechanism for Reaching Ultrahigh Voltages from a Falling Droplet

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

Spreading- and evaporation-mediated 2D colloidal assemblies on fluid interfaces

Fluid interfaces exhibit remarkable capabilities and significant application prospects in fabricating two-dimensional (2D) materials and colloidal assemblies. However, the efficient realization of the fluid-mediated self-assembly of colloidal particles has become challenging owing to the complex evolution of spatiotemporal processes and dynamic behaviors at fluid interfaces. In this study, we hypothesized that the superspreading of a volatile droplet directed the ultrafast self-assembly of colloidal particles on an insoluble fluid interface. To validate this hypothesis, employing high-speed and large-field microscopic observation experiments in tandem with theoretical analysis, the time evolution processes of droplet spreading, evaporation, dewetting, and particle assembly were captured, and the multi-scale dynamic behavior of volatile colloidal droplets on an immiscible liquid substrate was studied. Our findings revealed the power law of droplet spreading dynamics at the macroscale, evaporation-induced dewetting of the liquid film and aggregation of colloidal particles at the mesoscopic scale, and colloidal self-assembly under the action of capillary and DLVO forces at the microscopic scale. Moreover, we realized the ultrafast fabrication of 2D colloidal assemblies on liquid interfaces. This work presents a simple, robust, and efficient method for self-assembling and manufacturing 2D-ordered structures and materials based on a platform of fluid interfaces.

Coupled lattice Boltzmann method–discrete element method model for gas–liquid–solid interaction problems

In this paper, we propose a numerical model to simulate gas–liquid–solid interaction problems, coupling the lattice Boltzmann method and discrete element method (LBM–DEM). A cascaded LBM is used to simulate the liquid–gas flow field using a pseudopotential interaction model for describing the liquid–gas multiphase behaviour. A classical DEM resorting to fictitious overlaps between the particles is used to simulate the multiple-solid-particle system. A multiphase fluid–solid two-way coupling algorithm between LBM and DEM is constructed. The model is validated by four benchmarks: (i) single disc sedimentation, (ii) single floating particle on a liquid–gas interface, (iii) sinking of a horizontal cylinder and (iv) self-assembly of three particles on a liquid–gas interface. Our simulations agree well with the numerical results reported in the literature. Our proposed model is further applied to simulate droplet impact on deformable granular porous media at pore scale. The dynamic droplet spreading process, the deformation of the porous media (composed of up to 1277 solid particles) as well as the invasion of the liquid into the pores are well captured, within a wide range of impact Weber number. The droplet spreading dynamics on particles is analysed based on the energy budget, which reveals mechanisms at play, showing the evolution of particle energy, surface energy and viscous dissipation energy. A scaling relation based on the impact Weber number is proposed to describe the maximum spreading ratio.

Experimental investigation on the dynamics of a single water droplet impacting wood surface

The dynamic characteristics of water mist droplets hitting the wood surface is one of the important factors affecting the fire extinguishing performance. However, the spreading dynamics between water droplets and wood surfaces during the impacting process have not been adequately characterized in existing literature. This study experimentally investigates the dynamic process of a single water droplet with changing Weber numbers from 66.7 to 1268.5 impacting different solid wood surface from the perspective of fire suppression. Typical impacting phenomena, the phenomena distribution, the droplet spreading dynamics and the maximum spread factor have been analyzed and discussed. Typical phenomena including no splash, transition from no splash to prompt splash, and prompt splash with varying Weber number are observed for the wood surface. The typical phenomena distribution can be distinguished by lgWe and lgRe and the result shows that lgWe < 2.11, 2.11 < lgWe < 2.42, and lgWe > 2.42 correspond to the phenomena of no splash, transition from no splash to prompt splash and prompt splash respectively. The spreading time can be predicted by the expression of ts/(ρRmax3/σ)0.5=6.52∗We-0.92. By analyzing the maximum spreading factor, the model βmax∝(We/Oh)0.198 is found to present good predictions for wood surfaces.

Understanding under-liquid drop spreading using dynamic contact angle modeling

We present numerical modeling and experimental data to investigate droplet spreading dynamics on a substrate submerged in another liquid. Dynamic contact angle equations are formulated and used to simulate the spreading of different oil drops in the presence of water. Drop spreading in the presence of water starts with a viscosity-dominated regime where a scaling law of r∼t is observed. For low viscosity drops, an intermediate inertial regime with scaling r∼t12 is observed after the viscous regime. For high viscosity drops, such an intermediate regime diminishes. At later stages near equilibrium, the spreading dynamics follow Tanner's law for all drops.

Spreading dynamics of a droplet impacts on a supercooled substrate: Physical models and neural networks

The phenomenon of droplets impacting substrate exists widely in nature and industrial production. We investigate the maximum spreading time and the maximum spreading factor of a water droplet impacting a supercooled substrate. By collecting data on the spreading dynamics of a water droplet impacting a supercooled substrate, we compared the suitability of four physical models of maximum spreading time and five physical models of maximum spreading factor to the data. Current physical models more accurately predict the maximum spreading time for a water droplet impacting a supercooled substrate, however, the prediction accuracy of the model for the maximum spreading factor is low. We use the Reynolds number, Weber number, Reynolds number, Capillary number and dimensionless subcooled temperature of the droplet impacting on the supercooled substrate as the input layer, and establish different neural network models to predict the droplet spreading dynamics. We establish Convolutional neural network, and use Spreading pattern and Fingering pattern as output layer to classify and predict impact patterns. In addition, we established an optimized BP neural network with the maximum spreading factor as the output layer for regression prediction.

Analysis of droplet behavior and breakup mechanisms on wet solid surfaces

The behavior and dynamics of droplet spreading are pivotal phenomena that exert a profound influence on numerous scientific disciplines, technological advancements, and natural processes. This study was conducted with the aim to investigate factors influencing the shape and geometry of a liquid droplet on a solid surface using the lattice Boltzmann method (LBM). LBM as a mesoscale numerical fluid simulation has gained increasing popularity among the most favorable numerical methods for simulating multi-phase/multi-component fluid flow in complex geometries. Accordingly, parameters dependency, surface tension, two-phase diagram, and wettability were evaluated in the LBM, and stable and calibrated forms were used for the droplet simulations. Also, an equation was obtained to determine the contact angle in the LBM system with a determination coefficient of 0.988. Then, droplet behavior was examined for its dependency on wettability, interfacial tension, and line tension. The results showed droplets breakup in a certain interfacial tension at high adhesive force. These breakups were due to the force balance in the triple line. They were not monotonic and first decreased and then increased the volume of the droplets.

Experimental investigation of the influence of nanoparticles on droplet spreading dynamics and heat transfer during early stage cooling

With the increasing interest in improving high-performance cooling systems, research on droplet cooling requires more complex methods for enhancing heat transfer. Hence, we investigate the effect of nanoparticle presence and concentration on droplet spreading dynamics and heat transfer during the early stages of spreading on a heated surface. For this purpose, water and three water-based TiO2 nanofluid droplets (0.2, 0.5, and 1 wt%) are tested at four release heights impinging on a sapphire substrate below boiling point (80 °C). With an emissive TiAlN coating in the infrared (IR) domain, the surface temperature of the substrate is measured via a high-speed IR thermal camera. Droplet spreading is simultaneously monitored with two high-speed black/white cameras. The thermal and rheological properties are experimentally characterized for accurate results and to investigate only the nanoparticle presence in the fluid. The captured temperature fields are analyzed by solving the inverse heat conduction problem. We observe that all nanofluid droplets spread on a heated surface marginally less than water droplets. To the best of the authors’ knowledge, this study is the first to reveal that during the early stages of droplet spreading, the presence of nanoparticles and the resulting change in viscosity primarily influence heat transfer through the modification of the droplet spreading diameter, rather than changes in thermal properties.

Investigating the dynamics of droplet spreading on a solid surface using PIV for a wide range of Weber numbers

Investigating the dynamics of droplet spreading on a solid surface using PIV for a wide range of Weber numbers

Measuring the Adhesion Force and the Spreading Radius between Droplets and a Solid Surface during Short-Time Spreading

When a droplet contacts a solid surface, the liquid spreads over the solid surface to minimize the total surface energy. This phenomenon is widespread in industrial production and nature, so research on droplet spreading is of great significance. Here, the adhesion force and the spreading radius during droplet spreading can be quantified using a highly sensitive photoelectric method. It is possible to study droplet spreading from two dimensions at the microscale. The adhesion force is measured by an optical lever, and the spreading radius is measured by an ultrafast electrical method. The measurement method allows the force resolution and the space-time resolution to reach the nanonewton lever and the nanosecond lever, respectively. We obtain the maximum spreading radius and the maximum adhesion force during short-time spreading through our technique. Moreover, we numerically simulate the droplet spreading process through the lattice Boltzmann solver and confirm the observed results. This study provides a new experimental technique for studying droplet spreading dynamics from multiple perspectives, which can deepen our understanding of droplet spreading and provide guidance for the development of new techniques.

A generalized scaling theory for spontaneous spreading of Newtonian fluids on solid substrates

HypothesisThere exists a generalized solution for the spontaneous spreading dynamics of droplets taking into account the influence of interfacial tension and gravity. ExperimentsThis work presents a generalized scaling theory for the problem of spontaneous dynamic spreading of Newtonian fluids on a flat substrate using experimental analysis and numerical simulations. More specifically, we first validate and modify a dynamic contact angle model to accurately describe the dependency of contact angle on the contact line velocity, which is generalized by the capillary number. The dynamic contact model is implemented into a two-phase moving mesh computational fluid dynamics (CFD) model, which is validated using experimental results. FindingsWe show that the spreading process is governed by three important parameters: the Bo number, viscous timescale τviscous, and static advancing contact angle, θs. More specifically, there exists a master spreading curve for a specific Bo and θs by scaling the spreading time with the τviscous. Moreover, we developed a correlation for prediction of the equilibrium shape of the droplets as a function of both Bo and θs. The results of this study can be used in a wide range of applications to predict both dynamic and equilibrium shape of droplets, such as in droplet-based additive manufacturing.

Thermally driven Marangoni effects on the spreading dynamics of droplets

The influence of the thermally-induced Marangoni stresses on the falling and spreading dynamics of a droplet surrounded by another liquid phase is numerically studied. A water droplet is released in a three-dimensional (3D) domain filled with oil. The droplet descends, comes at an apparent resting position above an oil film close to the bottom surface, and eventually wets the solid surface when the underneath film ruptures. A linear vertical temperature gradient is applied in the solution domain (with the bottom surface as the cold side) which imposes a surface tension gradient across the oil-water interface. The Marangoni source term is added to the momentum equation coupling the momentum and the energy equations. It is assumed that the dynamic contact angle changes according to the Kistler relation during the spreading. The Volume of Fluid (VOF) method is used to capture the interface between the phases. The solver is validated for both the falling and spreading phases. During the buoyancy-driven falling regime, the droplet retains its spherical shape at the low Reynolds number O(1) and finite Bond number O(0.001). It is revealed that the spreading rate of the droplet is a decreasing function of Marangoni number (Ma). Unlike the isothermal systems, where the bottom side of the droplet becomes slightly flattened at the resting stage, the Marangoni stress imposes an upward force on the droplet which elongates the shape of droplet in the temperature gradient direction. Consequently, the ultimate spreading radius at the equilibrium state is smaller at larger Ma. The slow rate of spreading and also the small wetted area at large temperature gradients adversely affect the heat transfer rate from the liquid to the cold plate where the local convective heat transfer coefficient and the average Nusselt number (Nu) are decreasing functions of the Marangoni stress.

Arrested Dynamics of Droplet Spreading on Ice.

We investigate the arrested spreading of room temperature droplets impacting flat ice. The use of an icy substrate eliminates the nucleation energy barrier, such that a freeze front can initiate as soon as the droplet's temperature cools down to 0 °C. We employ scaling analysis to rationalize distinct regimes of arrested hydrodynamics. For gently deposited droplets, capillary-inertial spreading is halted at the onset of contact line freezing, yielding a 1/7 scaling law for the arrested diameter. At low impact velocities (We≲100), inertial effects result in a 1/2 scaling law. At higher impact velocities (We>100), inertio-viscous spreading can spill over the frozen base of the droplet until its velocity matches that of a kinetic freeze front caused by local undercooling, resulting in a 1/5 scaling law.

Comparative Study on the Spreading Behavior of Oil Droplets over Teflon Substrates in Different Media Environments.

This paper comparatively investigated the spreading process of an oil droplet on the surface of highly hydrophobic solid (Teflon) in air and water media using a high-speed imaging technology, and analyzed their differences in spreading behavior from the perspective of empirical relations and energy conservation. Furthermore, the classical HD and MKT wetting models were applied to describe the oil droplet spreading dynamics to reveal the spreading mechanism of oil droplets on the Teflon in different media environments. Results showed that the entire spreading process of oil droplets on Teflon in air could be separated into three stages: the early linear fast spreading stage following , the intermediate exponential slow spreading stage obeying , and the late spreading stage described by . However, the dynamics behavior of dynamic contact angle during the oil droplet spreading on Teflon in water could be well described by these expressions, and . Clearly, a significant difference in the oil droplet spreading behavior in air and water media was found, and the absence of the intermediate exponential spreading stage in the oil–water–Teflon system could be attributed to the difference in the dissipated energy of the system because the dissipation energy in the oil–water–solid system included not only the viscous dissipation energy of the boundary layer of oil droplet, but also that of the surrounding water which was not included in the dissipation energy of the oil–air–solid system. Moreover, the quantitative analysis of wetting models suggested that the MKT model could reasonably describe the late spreading dynamics of oil droplets (low TPCL velocities), while the HD model may be more suitable for describing the oil droplet spreading dynamics at the early and intermediate spreading stages (high TPCL velocities).

AI predicts water droplet spreading dynamics and icing patterns on cold surfaces

The techniques could lead to improvements in aircraft anti-icing, spray-freeze drying, and influenza vaccine storage.

Universal Aspects of Droplet Spreading Dynamics in Newtonian and Non-Newtonian Fluids.

Droplet impacts are common in many applications such as coating, spraying, or printing; understanding how droplets spread after impact is thus of utmost importance. Such impacts may occur with different velocities on a variety of substrates. The fluids may also be non-Newtonian and thus possess different rheological properties. How the different properties such as surface roughness and wettability, droplet viscosity, and rheology as well as interfacial properties affect the spreading dynamics of the droplets and the eventual drop size after impact are unresolved questions. Most recent work focuses on the maximum spreading diameter after impact and uses scaling laws to predict this. In this paper, we show that a proper rescaling of the spreading dynamics with the maximum radius attained by the drop and the impact velocity leads to a unique single and thus universal curve for the variation of diameter versus time. The validity of this universal functional shape is validated for different liquids with different rheological properties as well as substrates with different wettabilities. This universal function agrees with a recent model that proposes a closed set of differential equations for the spreading dynamics of droplets.