Droplet Height Research Articles

Dimensionless analysis of foam stability for application in enhanced oil recovery

The stability of foam during injection into oil reservoirs is critical, especially under high-temperature and high-salinity conditions. This study formulates foam stabilizers using one polymer, two surfactants, and six types of nanoparticles (NPs). Foam stability was assessed with a static setup, examining factors such as interfacial tension (IFT), bubble characteristics, and solution viscosity through dimensionless numbers: Bond number (Bo), Worthington number (We), and Neumann number (Ne). A new formula for dimensionless electrical conductivity was also introduced. Results showed that at optimal concentrations of the four additives, foam stability improved with NPs due to enhanced surface charge from in situ physiochemical reactions, promoting their migration to the fluid interface. Notably, Ne proved more effective than Bo and We in describing foam stability as it accounts for droplet height’s impact on IFT. Acidic NPs demonstrated greater electrostatic force than amphoteric NPs, correlating with improved foam stability reflected in a downward trend in the Ne plot. Additionally, we analyzed the coarsening rate of foam bubbles over time and its relationship to stability. Our findings suggest that dimensionless numbers serve as valuable benchmarks for evaluating foam stability across various additive mechanisms.

Analysis of wettability and contact angle of sodium bicarbonate impregnated wooden surfaces

Scots pine and spruce wood samples were vacuum impregnated with 3, 5, and 9% sodium bicarbonate solution. Contact angle measurements were made by dropping 0.05 mL of pure water and 3% saltwater solution onto the material’s surface. The surface roughness of each sample was measured, and that of Scots pine control samples was higher than that of spruce samples, but the surface roughness values of spruce wood were higher in the samples impregnated with sodium bicarbonate. When pure water or saltwater solution was dropped onto the samples, it was observed that as the waiting times increased, the contact angle value decreased, the droplet height decreased, and the droplet width increased. It was found that the contact angles were higher in the control samples of both tree species than in the samples of 5% and 7%. The contact angles of 9% impregnation solution, pure water, and salt water were higher than the control samples. As the solution ratio increased on the surfaces impregnated with sodium bicarbonate, the contact angle also increased, and the wettability behavior decreased accordingly. Sodium bicarbonate solution is effectively used in the impregnation process for pine and fir woods, making the materials more resistant to water under outdoor conditions. This solution significantly reduces the risk of deformation of wood by increasing the contact angle and reducing its water absorption properties.

Transmission interference fringe (TIF) technique for the dynamic visualization of evaporating droplet

The transmission interference fringe (TIF) technique was developed to visualize the dynamics of evaporating droplets based on the Reflection Interference Fringe (RIF) technique for micro-sized droplets. The geometric formulation was conducted to determine the contact angle (CA) and height of macro-sized droplets without the need for the prism used in RIF. The TIF characteristics were analyzed through experiments and simulations to demonstrate a wider range of contact angles from 0 to 90°, in contrast to RIF's limited range of 0–30°. TIF was utilized to visualize the dynamic evaporation of droplets in the constant contact radius (CCR) mode, observing the droplet profile change from convex-only to convex-concave at the end of dry-out from the interference fringe formation. The TIF also observed the contact angle increase from the fringe radius increase. This observation is uniquely reported as the interference fringe (IF) technique can detect the formation of interference fringe between the reflection from the center convex profile and the reflection from the edge concave profile on the far-field screen. Unlike general microscopy techniques, TIF can detect far-field interference fringes as it focuses beyond the droplet-substrate interface. The formation of the convex-concave profile during CCR evaporation is believed to be influenced by the non-uniform evaporative flux along the droplet surface.

Multiple Self-Powered Sensor-Integrated Mobile Manipulator for Intelligent Environment Detection.

A multiple self-powered sensor-integrated mobile manipulator (MSIMM) system was proposed to address challenges in existing exploration devices, such as the need for a constant energy supply, limited variety of sensed information, and difficult human-computer interfaces. The MSIMM system integrates triboelectric nanogenerator (TENG)-based self-powered sensors, a bionic manipulator, and wireless gesture control, enhancing sensor data usability through machine learning. Specifically, the system includes a tracked vehicle platform carrying the manipulator and electronics, including a storage battery and a microcontroller unit (MCU). An integrated sensor glove and terminal application (APP) enable intuitive manipulator control, improving human-computer interaction. The system responds to and analyzes various environmental stimuli, including the droplet and fall height, temperature, pressure, material type, angles, angular velocity direction, and acceleration amplitude and direction. The manipulator, fabricated using 3D printing technology, integrates multiple sensors that generate electrical signals through the triboelectric effect of mechanical motion. These signals are classified using convolutional neural networks for accurate environmental monitoring. Our database shows signal recognition and classification accuracy exceeding 94%, with specific accuracies of 100% for pressure sensors, 99.55% for angle sensors, and 98.66, 95.91, 96.27, and 94.13% for material, droplet, temperature, and acceleration sensors, respectively.

Numerical study of thermocapillary and slip effects on interfacial destabilization under surface acoustic waves

The rapid development of microfluidics has significantly highlighted the role of surface acoustic waves (SAWs) in microfluidic actuation. SAW influences droplet manipulation, inducing interface instability and processes such as droplet splitting, jetting, and atomization, which have been key research focal points. Previous studies have identified a close correlation between these instability mechanisms and three critical parameters: the Marangoni number (Ma), associated with piezoelectric substrate thermal effects; the slip coefficient (β0), related to piezoelectric substrate slip; and the acoustic capillary number (C). Given the intimate link between the aspect ratio (H/L, where H is the characteristic height, and L is the characteristic width of droplets) and atomization size, this study comprehensively investigates the combined effects of these factors on the droplet aspect ratio H/L. Specifically, increases in the acoustic capillary number C and slip coefficient β0 promote reductions in droplet height (H) and outward expansion (L), while the Marangoni number Ma counteracts this expansion, maintaining larger H/L values. This inhibitory effect is particularly pronounced when C and β0 are small but diminishes as their values increase. Additionally, higher values of C and β0 accelerate the convergence of the H/L ratio, whereas Ma decreases the rate of this convergence. Through the coordinated interplay of Ma, β0, and C, multidimensional and fine-tuned adjustments of the droplet aspect ratio H/L over a wide range can be achieved.

Dual residence time for droplets to coalesce with a liquid surface.

When droplets approach a liquid surface, they have a tendency to merge in order to minimize surface energy. However, under certain conditions, they can exhibit a phenomenon called coalescence delay, where they remain separate for tens of milliseconds. This duration is known as the residence time or the noncoalescence time. Surprisingly, under identical parameters and initial conditions, the residence time for water droplets is not a constant value but exhibits dual peaks in its distribution. In this paper, we present the observation of the dual residence times through rigorous statistical analysis and investigate the quantitative variations in residence time by manipulating parameters such as droplet height, radius, and viscosity. Theoretical models and physical arguments are provided to explain their effects, particularly why a large viscosity or/and a small radius is detrimental to the appearance of the longer residence time peak.

Investigating the origin of the far-field reflection interference fringe (RIF) of microdroplets

We show that the reflection interference fringe (RIF) is formed on a screen far away from the microdroplets placed on a prism-based substrate, which have low contact angles and thin droplet heights, caused by the dual convex–concave profile of the droplet, not a pure convex profile. The geometric formulation shows that the interference fringes are caused by the optical path difference when the reflected rays from the upper convex profile at the droplet–air interface interfere with reflection from the lower concave profile at oblique angles lower than the critical angle. Analytic solutions are obtained for the droplet height and the contact angle out of the fringe number and the fringe radius in RIF from the geometric formulation. Furthermore, the ray tracing simulation is conducted using the custom-designed code. The geometric formulation and the ray tracing show excellent agreement with the experimental observation in the relation between the droplet height and the fringe number and the relation between the contact angle and the fringe radius. This study is remarkable as the droplet's dual profile cannot be easily observed with the existing techniques. However, the RIF technique can effectively verify the existence of a dual profile of the microdroplets in a simple setup. In this work, the RIF technique is successfully developed as a new optical diagnostic technique to determine the microdroplet features, such as the dual profile, the height, the contact angle, the inflection point, and the precursor film thickness, by simply measuring the RIF patterns on the far-field screen.

Analysis of the impact dynamics of water-in-oil composite droplets on metal plates

Composite droplets composed of immiscible liquids often have unique applications in many situations, where splashing and recoil of the composite droplet upon impact with plane surfaces are important. Due to the different properties of the constituent liquids, composite droplets will have different flow morphology than single-phase droplets. This research employs an injection method to generate composite droplets and investigates the impact dynamics of these droplets on a horizontal copper plate at room temperature using high-speed cameras. Four impact heights (H = 10 cm, 15 cm, 20 cm, 25 cm) and seven water volume fractions (α = 0.0625, 0.125, 0.1875, 0.25, 0.3125, 0.375, 0.4375) were considered. The results show that the double squeezing of the oil phase by the water phase and the copper plate is the main reason for the droplet splashing phenomenon. The degree of droplet splashing and the probability of the droplet generating a satellite droplet are directly proportional to the impact velocity. When the impact height is constant, the extent of splashing decreases with an increase in α. Additionally, the maximum value of the jetting height for composite droplets is mainly influenced by the volume of satellite droplets during the recoil process. Overall, the maximum relative jetting height of composite droplets shows an initial increasing trend followed by a decreasing trend with an increase in α. At α = 0.1875 and 0.25, there is a negative correlation between the relative jetting height and impact height. This implies that the maximum value of the relative jetting height is primarily controlled by the volume of the satellite droplets during the recoil process.

Electrical behavior and water phase transformation in multi‐walled carbon nanotube/polytetrafluoroethylene composite films

AbstractMulti‐walled carbon nanotubes (MWCNTs) with excellent physiochemical properties have been recently applied to energy/power generation and storage systems encompassing polymer electrolyte membrane fuel cells (PEMFCs). However, there is a lack of investigations of the properties of MWCNT composites themselves under the actual operating conditions of systems. In this paper, MWCNTs/polytetrafluoroethylene (PTFE) composites were fabricated as electric and hydrophobic components for energy/power systems. The electrical properties, heating behavior, hydrophobicity, and water evaporation behavior of the composites are characterized within the operating voltage ranges of PEMFC. With 50% MWCNT content, the conductivity and the contact angle were 11.83 S/cm, and the contact angle 128.1°, respectively. The MWCNT/PTFE composite film showed a linear temperature rise with voltages and rapid temperature saturation within 40 s. The rate in the initial heating range was measured at 0.5°C/s for the voltage of 0.5 V. The voltage increase induced an exponential decrease in the height of water droplets on the composite surface. The evaporation rate increased almost 10‐fold, from 0.013 mm/min at 0.0 V to 0.12 mm/min at 1.0 V. The proposed characteristics of the functional composite provide insights into the material requirements for electrochemical devices, including PEMFC.Highlights Multi‐walled carbon nanotube/polytetrafluoroethylene composites can be applied to energy storage and power generation. Key properties of the composites are proposed for polymer electrolyte membrane fuel cell (PEMFC) operating conditions. The composite has a fast heating by high conductivity and temperature uniformity. Water‐vapor transformation under PEMFC operating voltage is quantified.

Numerical Investigations of the Kinetic Behavior of Adhering Droplets on the Inclined Windshield in Airflows

A theoretical foundation for implementing surface self-cleaning can be provided by analyzing the motion of adhering droplets in airflow. When driving in rainy circumstances, self-cleaning windshield technology can efficiently guarantee driver safety. In this study, the CLSVOF method is employed to simulate a three-dimensional wind tunnel model, enabling an investigation into the dynamics of droplets adhering to a windshield under the influence of airflow. Subsequent analysis mainly focuses on the impacts of wind velocity and droplet size on the motion patterns and morphological characteristics of the droplets. The temporal evolution of the forces acting on the droplets is examined, along with a comparative analysis of the predominant forces driving droplet motion against other forms of resistance. The results demonstrate that the motion patterns of the droplets can be broadly categorized into three phases: accelerated decline, forces equilibrium, and accelerated climb. As wind speed increases, there is a noticeable reduction in the wetting length Ld, while the height of the droplets H and the dominant force influencing their motion shift from gravitational component Fgsinα to wind traction force Fwind. Moreover, an increase in droplet size accentuates the lag in changes to wetting length, droplet height, and the contact angle.

Coupling Process between Droplet and Iron Investigated by Reactive Molecular Dynamics Simulations.

The droplet-to-iron electrochemical reaction is common in nature and industrial production, and it causes damage to the economy, safety, and the environment. The electrochemical reaction of droplet-to-iron is a coupling process of wetting and corrosion. Presently, investigations into electrochemical reactions mainly focus on the corrosions caused by a solution, and wetting is rarely considered. However, for the droplet-to-iron electrochemical reaction, the mechanism of charge transfer in the process is still unclear. In this paper, a reactive molecular dynamics simulation model for the droplet-to-iron electrochemical reaction is developed for the first time. The electrochemical reaction of droplet-to-iron is studied, and the interaction between droplet wetting and corrosion on iron is investigated. The effects of temperature, electric field strength, and salt concentration on the electrochemical reaction are explored. Results show that droplet wetting on the iron surface and the formation of a single-molecular-layer ordered structure are prerequisites for corrosion. The hydroxyl radicals that penetrate the ordered structure acquire electrons from iron atoms on the substrate surface under the action of Coulomb forces and form iron-containing oxides with these iron atoms. The corrosion products and craters lead to a reduced droplet height, which promotes droplet wetting on iron and further intensifies the droplet-to-iron electrochemical reaction.

Extremely large water droplet impact onto a deep liquid pool.

Most studies of droplet impact on liquid pools focus on droplet diameters up to the capillary length (0.27cm). We break from convention and study extremely large water droplets (1 to 6cm diameter) falling into a pool of water. We demonstrate that the depth and width of the cavity formed by large droplet impact is greatly influenced by the deformed shape of the droplet at impact (i.e., prolate, spherical, and oblate), and larger droplets amplify this behavior by flattening before impact. In particular, the maximum cavity depth is a function of the Froude number and axis ratio of the droplet just before impact. Further, the cavity depth is more dependent on the droplet height than width, and the maximum cavity diameter is independent of the droplet height. In general, we observe that more oblate droplets result in decreasing cavity depths for a fixed liquid volume. This is because an increase in horizontal droplet diameter results in a reduced impact energy flux and therefore reduced cavity depth.

A novel ADE system with tunable droplet size and ejection height

Acoustic droplet ejection (ADE) has been proven to move liquids in droplet form from a container to mid-air. However, the droplet sizes produced using the traditional ADE setups are proportional to their heights if the physical framework is unchanged. This limitation obstructs further employment of ADE technology since having the exact sizes of droplets at a specific location or different sizes at different locations is required for many applications. To overcome this limitation and enable more possible applications of ADE, this study proposed an innovative ADE configuration that could manipulate the size and height of the ADE droplets with only electrical signals. To achieve this, a low-voltage driving period and a pinhole structure were added to create a water mound before ejection. First, simulations were conducted to validate the proposed method and find the parameters of the novel ADE setup. After that, a driving circuit featuring a high-voltage pulser and a field-programmable gate array was built. However, a 3D spherical resin model was printed to focus the acoustic wave on the water surface, and a cover with a pinhole was added to create water mound. To observe the behavior of the droplets, a recording system and detection algorithm were developed to capture and identify the dimension/height of the droplets, respectively. Finally, the proposed ADE configuration successfully manipulated the droplet size at the same ejecting height under three driving voltages (65, 70, and 75 V) and pinhole diameters (3.7, 4, and 4.4 mm).

Electric manipulation on deformation of ionic ferrofluid sessile droplets.

Active electric-driven droplet manipulation in digital microfluidics constitutes a promising domain owing to the unique and programmable wettability inherent in sessile ionic droplets. The coupling between the electric field and flow field enables precise control over wetting characteristics and droplet morphology. This study delves into the deformation phenomena of ionic sessile ferrofluid droplets in ambient air induced by uniform electric fields. Under the assumption of a pinned mode throughout the process, the deformation is characterized by variations in droplet height and contact angle in response to the applied electric field intensity. A numerical model is formulated to simulate the deformation dynamics of ferrofluid droplets, employing the phase field method for tracking droplet deformation. The fidelity of the numerical outcomes is assessed through the validation process, involving a comparison of droplet geometric deformations with corresponding experimental results. The impact of the electric field on the deformation of dielectric droplets is modulated by parameters such as electric field strength and droplet size. Through meticulously designed experiments, the substantial influence of both field strength and droplet size is empirically verified, elucidating the behavior of ionic sessile droplets. Considering the interplay of electric force, viscous force, and interfacial tension, the heightened field intensity is observed to effectively reduce the contact angle, augment droplet height, and intensify internal droplet flow. Under varying electric field conditions, droplets assume diverse shapes, presenting a versatile approach for microfluidic operations. The outcomes of this research hold significant guiding implications for microfluidic manipulation, droplet handling, and sensing applications.

A Visualization Experiment on Icing Characteristics of a Saline Water Droplet on the Surface of an Aluminum Plate

When the offshore device, such as an offshore wind turbine, works in winter, ice accretion often occurs on the blade surface, which affects the working performance. To explore the icing characteristics on a microscale, the freezing characteristics of a water droplet with salinity were tested in the present study. A self-developed icing device was used to record the icing process of a water droplet, and a water droplet with a volume of 5 μL was tested under different salinities and temperatures. The effects of salinity and temperature on the profile of the iced water droplet, such as the height and contact diameter, were analyzed. As the temperature was constant, along with the increase in salinity, the height of the iced water droplet first increased and then decreased, and the contact diameter decreased. The maximum height of the iced water droplet was 1.21 mm, and the minimum contact diameter was 3.67 mm. With the increase in salinity, the icing time of the water droplet increased, yet a minor effect occurred under low temperatures such as −18 °C. Based on the experimental results, the profile of the iced water droplet was fitted using the polynomial method, with a coefficient of determination (R2) higher than 0.99. Then the mathematical model of the volume of the iced water droplet was established. The volume of the iced water droplet decreased along with temperature and increased along with salinity. The largest volume was 4.1 mm3. The research findings provide a foundation for exploring the offshore device icing characteristics in depth.

Lattice Boltzmann simulation of droplet solidification processes with different solid-to-liquid density ratios

Droplet solidification is widespread in nature and industry. The difference in droplet density before and after solidification has a significant impact on the solidification characteristics of the droplet. In this research, a lattice Boltzmann model considering that difference is developed to simulate the droplet solidification process and is validated by comparing the simulation results with the analytical solutions and the experimental data. The triple contact line variation, solidification time, and final droplet profile are analyzed with a special emphasis on the effect of solid-to-liquid density ratio. As the density ratio increases, the triple contact line tends to go up but the droplet height decreases, leading to a shorter solidification time. In the final droplet profile, a tip or plateau is observed when the density ratio is smaller or larger than unity and the plateau radius increases at a larger density ratio. The effects of Stefan number (St), contact angle (CA), and Bond number (Bo) under different density ratios on the solidification process are further investigated. The increasing St and Bo, or decreasing CA reduces the solidification time. The Bo and CA mainly affect the solidification process by modifying the initial droplet profile. This work can provide a better understanding of the droplet solidification mechanisms and related applications.

EXPERIMENTAL INVESTIGATION OF THE RESUSPENDED NANOFLUID DROPLET EVAPORATION AT PERIODICALLY VARYING ELECTRIC FIELD

The addition of nanoparticles can significantly increase the evaporation rate of droplets. However, there is still no consistent conclusion on whether the factor promoting droplet evaporation is the motion of suspended nanoparticles or the deposited nanoparticles changing the structure of the solid surface. Therefore, the fully deposited surfaces were prepared by drying 0.005-0.015 vol.% Al<sub>2</sub>O<sub>3</sub>-water nanofluids and the resuspension process of nanoparticles was investigated by applying periodically varying electric field. The mechanism of nanoparticle influence on the droplet evaporation process was investigated by measuring the contact angle, droplet height, and contact surface radius. The results show that the motion of suspended nanoparticles promotes the droplet evaporation. The evaporation time of droplets on the nanoparticle-deposited surface is longer than that on the nondeposited surface without electric field, while the trend is opposite in the presence of the electric field. After the electric field is applied, the nanoparticles are resuspended into the droplet due to the instability of the deposition layer structure. The motion of nanoparticles leads to an increase in the droplet evaporation rate, and the enhancement effect is optimal when the electric field switching frequency is 90 Hz.

Sliding of water droplets onto a textured metal surface

Technologies for catching fluid from a steam mixture in the form of small droplets using the lattices or metal plates located at a certain angle are used in many industrial installations. Conducting experimental studies of the processes of collision and draining the drops of fluid on the surfaces of the metal is relevant for increasing the efficiency of gathering fluid from the steam-air mixture in industrial plants. Accordingly, the purpose of this work was to analyze the effect of the texture of the metal surface on the characteristics of the process of spreading water drops after their fall. The experiments were carried out at the setup, which is based on the shadow technique. The height of liquid droplets and the angle of the substrate inclination were varied. To assess the effect of texture on the characteristics of the spreading of water drops on the surface of brass substrates, microgrooves were applied by a grinder. It was found out that after dosing, the drop stretched along the grooves. The left and right contact angles (LCAs and RCAs), measured perpendicular to the grooves, increased by 15% compared to the angles measured on the polished surface of the substrate. It was established that after pinning the drops on the tilted surface, the difference between the LCAs, measured in parallel and perpendicular to the grooves, increased by more than 35%.

Optimization analysis of droplet dust removal mass on tilted superhydrophobic surface based on regulating PV tilt angle and drop height

This study proposes a method for adjusting the mass of droplet dust removal based on droplet height and photovoltaic tilt angle, utilizing a model of droplet motion on inclined superhydrophobic surfaces. A droplet's motion on dusty and inclined hydrophobic surfaces is analyzed using a high-speed digital imaging system. The impact of drop height and photovoltaic tilt angle on the mass of dust removed by the droplet is experimentally investigated. The findings reveal that the mass of droplet dust removal increases quickly with the rise of drop height and tilt angle. The modification of the tilt angle yields a more pronounced enhancement in dust removal efficiency. At an angle of 40°, the drop height rises from 0.5 cm to 10 cm, leading to an extra 6.6 mg of dust removal per drop. At a height of 0 cm, the mass increased by 10.62 mg after raising the angle from 10° to 40°. Droplet impact on the surface is prone to shattering. The emitted minute droplets will adhere to the surface and form a cohesive ash band, with a visible light transmission rate of merely 67 %. When a droplet rolls on a tilted superhydrophobic surface, there's a dust removal threshold. Once exceeded, the rolling shape tends to become ellipsoidal. The experimental and model predictions of droplet dust removal mass are of the same order of magnitude and differ by a few micrograms. Therefore, the theoretical model can be used to improve the efficiency of droplet dust removal by adjusting the drop height and photovoltaic angle.

Simulation of a sessile nanofluid droplet freezing with an immersed boundary-lattice Boltzmann model

The freezing process of a sessile nanofluid droplet has been reported to behave differently from a pure sessile water droplet in terms of freezing dynamics and the final shape of the ice droplet. When nanoparticles are added to the water droplet, instead of forming a pointed tip on the top, the completely frozen droplet exhibits a flat plateau shape. To investigate this unique scenario, we developed a lattice Boltzmann (LB) model that combines the multiphase solidification model (MSM) with the immersed boundary method (IBM). The MSM is based on our previous work of simulating the freezing of a pure droplet, while the IBM is used to handle the interaction forces between the suspended particles and the different phases in the freezing droplet. Using this LB model, we succeeded in simulating the formation process of the frozen plateau shape. The simulation takes into account the dynamics of the dispersed particles, including their expulsion from the propagation freezing front and their segregation, which brings the liquid water to the edge. We compared the simulated freezing shape profiles with the experimental images and found that the shape forms in a similar manner. We then used the developed model to explore more cases, considering the effects of droplet contact angles and particle volume concentration. Results show that the distribution of particles and final droplet height depend on the surface wettability, as the freezing front exhibits a concave/convex shape on hydrophilic/hydrophobic surfaces, resulting in different particle separation distributions on the freezing front interface. Furthermore, our simulation results confirm the experimental conclusion that the plateau size on the frozen top increases with particle concentration and appears to be independent of the initial droplet contact angle.