Properties Of Droplets Research Articles

Experimental investigation of the parameters that affect droplet size and distribution for design calculations of two-phase separators

The critical design parameter when sizing a separator is the size of oil droplets in the water phase. This study improves the design of a separator by investigating the parameters that control droplet size, frequency, and distribution.Experimental work was performed to investigate the effect of flow rates and oil layer thickness on these parameters. Experiments were performed using a transparent laboratory separator to allow the measurement of droplet properties. The Design of the Experiment (DOE) method with the Taguchi analysis was applied to investigate statistically if droplet properties are solely a function of the independent variables or if they interact.The findings show that the results can be modelled using Gaussian distributions. Droplet size distribution and the number of droplets produced are functions of the interaction between oil flow rate and oil pad thickness. The oil flow rate dominates the droplet size though layer thickness has a minor effect. The number of droplets (Frequency) increases with both oil and water flow rates but decreases with oil pad thickness. There are clear interactions between all variables resulting in different droplet frequencies for combined effects. The distribution of the droplet sizes is controlled by oil layer thickness, where the spread is seen to rise with thickness. However, interactions between the fluid flows and oil pad thickness give rise to different droplet distributions if either variable were changed on its own.

Enhancing water droplet-based electricity generator by harnessing multiple-dielectric layers structure

Harvesting water energy is promising to relieve the global energy crisis and reach the aim of carbon neutrality. However, few effective technologies can make use of water droplets as a power source efficiently. The droplet-based electricity generator (DEG) with a transistor-inspired design has resulted in enhanced energy harvesting efficiency by orders of magnitude over traditional designs. Despite this, the current DEG generally features a single dielectric layer, limiting its integration with other common objects to achieve "unnoticed" energy harvesting. In this work, we report a novel design featuring multiple dielectric layers-based DEG (M-DEG) that leverages other materials, such as household glass or umbrellas, as the second dielectric layer under the surface triboelectric layer to harvest water droplet energy without interfering with the original function of both. We find that the second dielectric layer enhances the output of M-DEG because of higher equivalent capacitance and charge density. The open circuit voltage and short-circuit current are increased by 90.6% and 68.7%, respectively. The maximal short-circuit current reaches up to record-breaking 17.9 mA. Moreover, a capacitor model for M-DEG is established, which well reveals the influence of the properties of dielectric layers and droplets on the electric output, and accurately predicts the results.

Dynamics of droplets on a thin [EMIm]Ac ionic liquid film

The impact of water and 1-ethyl-3-methylimidazolium acetate ([EMIm]Ac) drops on [EMIm]Ac ionic liquid film was studied. The hydrodynamics of the impact of drops on a water liquid film were compared. A higher Weber number contribute to a larger crown rim diameter and height. The variation trend of the crown rim diameter and height with the liquid film thickness differs much according to the drop and film properties. The crown rim diameter and height have an opposite variation trend with the increasing liquid film temperature. The crown rim diameters with different droplet and liquid film properties were almost the same at the early stage of impact of approximately 4 ms, and then the difference became larger at a later stage. The shape of the generated liquid crown was more regular for drops impact on ionic liquid film. The water droplet impact on a water film has the largest liquid crown rim diameter and height, while the ionic liquid droplet impact on the ionic liquid film has the lowest.

Static and dynamic properties of self-bound droplets of light in hot vapors

The authors explore analogies between self-bound droplet states in Bose-Einstein condensates (BECs) and the propagation of light in nonlinear hot vapor mediums. They establish a mapping between the experimental parameters used in BEC experiments and those needed to observe the analogous phenomenon in hot atomic vapors.

Fundamentals and Manipulation of Bare Droplets and Liquid Marbles as Open Microfluidic Platforms

Microfluidics, as one of the most valuable analytical technologies of this century, has played an important role in various fields. Particularly, out-of-channel microfluidics, often referred to as open microfluidics (OMF) has recently drawn wide research attention among scholars for its great potential in convenient manual intervention. Much recent research has been geared toward bare droplets and particle-armed droplets (namely liquid marbles, LMs), which could serve as independent systems in OMF. Their versatile applications include but are not limited to nanomaterials preparation, energy harvesting, cell culture and environment monitoring. These applications are mainly attributed to the excellent independence, low reagent consumption and short reaction time of separate droplets and LMs. In addition, more operation features, such as diverse handling options, flexible controllability and high precision, further enable droplets and LMs carrying small liquid biochemical samples to be manipulated in an open environment freely. Considering the emergence of important research on bare droplets and LMs, this paper systematically reviews the state of the art in the fundamentals and manipulation of the two novel platforms under the frame of OMF. First, the intrinsic property of bare droplets on solid substrates, especially on superhydrophobic ones, is discussed, followed by the formation mechanism of nonwetting LMs and the effect of coating particles on LMs’ performance. Then, friction obstacles and actuation principles raised in driving droplets and LMs are further analyzed theoretically. Subsequently, several classical types of manipulation tasks for both droplets and LMs, namely transportation, coalescence, mixing and splitting, are discussed with a focus on key techniques to accomplish the tasks aforementioned. Finally, the fundamental and manipulation similarities and differences between bare droplets and LMs are summarized and future developments towards droplet- or LM-based microreactors and microsensors are recommended accordingly.

Quantum liquid droplets in Bose mixtures with weak disorder

We study the properties of self-bound liquid droplets of three-dimensional Bose mixtures in a weak random potential with Gaussian correlation function at both zero and finite temperatures. Using the Bogoliubov theory, we derive useful formulas for the ground-state energy, the equilibrium density, the depletion, and the anomalous density of the droplet. The quantum fluctuation induced by the disorder known as the glassy fraction is also systematically computed. At finite temperature, we calculate the free energy, the thermal equilibrium density, and the critical temperature in terms of the disorder parameters. We show that when the strength and the correlation length of the disorder potential exceed a certain critical value, the droplet evaporates and is eventually entirely destroyed. We calculate the density profiles of this exotic state by means of numerical simulations of the corresponding generalized disorder Gross-Pitaevskii equation. Our predictions reveal that as the strength of the disorder gets larger, the atomic density varies rapidly in the plateau region. We point out in addition that the peculiar interplay of the disorder and the repulsive Lee-Huang-Yang corrections play a pivotal role in the collective modes of the self-bound droplet.

Collision regimes and dynamic behaviors of a viscous droplet impacting on a spherical particle at high temperatures

Understanding droplet–particle collision behaviors is essential for the pyrohydrolysis process of pickling liquor, where the pickling liquor is sprayed and dried into particles. This study systematically investigated the collision characteristics between a viscous droplet and a heated stainless steel particle whose temperature ranged from 100 to 700 °C. The results indicate that the vapor thrust promotes droplet rebound in smaller spreading diameters but induces disintegration in larger spreading diameters under the film boiling regime. The collision regime map is summarized, and the transition thresholds of collision patterns are significantly increased with increasing liquid viscosity. The spreading factor and contact line velocity are strongly affected by the particle temperature at high liquid viscosities. In addition, the hydrophobic nature of particle surface in film boiling regime is favorable for droplet spreading. The dynamic contact angle significantly depends on the particle temperature and droplet properties. The dimensionless contact time shows a power law dependency on the Weber number in the rebound pattern, but it is almost a constant in the disintegration pattern.

Study on droplet wettability of low surface tension working medium based on special-shaped microstructure surface

The wetting properties of droplets on microstructure surfaces directly determine the hydrophobic properties of the surface, and the size effect is an important part of the study of droplet wettability on microstructure surfaces. Taking reentrant special-shaped microstructure and doubly-reentrant special-shaped microstructure as the research objects, the influences of many geometric parameters of microstructure on the wetting characteristics of droplet with low surface tension were obtained by numerical simulation and theoretical analysis, and the concept of critical intrinsic contact angle was proposed. It is found that the two kinds of special-shaped microstructures can suspend the droplet with low surface tension, and the droplet finally presents a large apparent contact angle. However, there are three kinds of wetting transition (Cassie state →Wenzel state transition) on the surface of the doubly-reentrant special-shaped microstructure, so the wetting regulation mechanism of the surface with the same geometric parameters is different from that of the reentrant special-shaped microstructure. In addition, by analyzing the energy barrier at the pinning point, it is further proved that the doubly-reentrant special-shaped microstructure has better performance of inhibiting the wetting transition of droplet.

A review of droplet bouncing behaviors on superhydrophobic surfaces: Theory, methods, and applications

The phenomenon of droplet bouncing on superhydrophobic surfaces has received extensive attention in the academic and industrial fields, as it is critical for various engineering applications, such as anti-icing, spray cooling, and metal quenching. In this review, the research of droplet bouncing behaviors is comprehensively introduced from the bouncing mechanism, research methods, and potential applications. The bouncing mechanism is related to three aspects: droplet properties, surface characteristics, and ambient conditions. Among them, droplet size, impact velocity, gas film, surface morphology, surface temperature, and applied electric field are frequently concerned. Surface wettability is critical for droplet bouncing behaviors, which affects the droplet movement on the surface. Momentum and mass distribution are essential to reduce contact time, which can be achieved through the surface morphology design. The manipulation of solid–liquid contact lines and surface tension can achieve directional droplet transportation. In addition, typical droplet bouncing experiments are presented, and experimental studies of single and successive droplets in recent decades are collated. Volume of fluid, the lattice Boltzmann method, and molecular dynamics are described, which are typical simulation methods for droplet bouncing dynamics at different scales. Potential engineering applications such as surface self-cleaning, digital microfluidics, and enhanced heat transfer have been developed through theoretical foundations and research methods. Finally, the conclusions and the possible future research directions are outlined. The durability of superhydrophobic surfaces becomes a bottleneck for engineering applications. The life cycle research perspective may be applied to future studies.

Modifying the Properties of Microemulsion Droplets by Addition of Thermoresponsive BAB* Copolymers.

Oil-in-water (O/W) microemulsions (ME) typically feature a low viscosity and exhibit ordinary viscosity reduction as a function of temperature. However, for certain applications, avoiding or even reverting the temperature trend might be required. This can be conceived by adding thermoresponsive (TR) block copolymers that induce network formation as the temperature increases. Accordingly, various ME-polymer mixtures were studied for which three different block copolymer architectures of BAB*-, B2AB*-, and B(AB*)2-types were employed. Here, "B" represents a permanently hydrophobic, "A" a permanently hydrophilic, and "B*" a TR block. For the TR-block, three different poly(acrylamide)s, namely poly(N-n-propylacrylamide) (pNPAm), poly(N,N-diethylacrylamide) (pDEAm), and poly(N-isopropylacrylamide) (pNiPAm), were used, which all exhibit a lower critical solution temperature. For a well-selected ME concentration, these block copolymers lead to a viscosity enhancement with increasing temperature. At a polymer concentration of about 22 g L-1, the most pronounced enhancement was observed for the pNPAm-based systems with factors up to 3, 5, and 8 for BAB*, B2AB*, and B(AB*)2, respectively. This phenomenon is caused by the formation of a transitory network mediated by TR-blocks, as evidenced by the direct correlation between the attraction strength and the viscosity enhancement. For applications requiring a high hydrophobic payload, which is attained via ME droplets, this kind of tailored temperature-dependent viscosity control of surfactant systems should therefore be advantageous.

High speed measurement of liquid properties from behavior of micro droplets on vertically oscillating substrate

In this study, we introduce a practical improvement to our experimental method to measure the mechanical properties of small liquid droplets by observing their oscillation on solid substrates. Instead of the electric tweezer system used in our previous study, the oscillation was applied by the mechanical motion of the substrate. Such instrument could be applied in the future use of inkjet which forms a droplet. In addition, an optical knife-edge method was employed to detect droplet deformation with high time resolution. We demonstrated the ability of the system to measure the adsorption of evaporated ethanol molecules on a water droplet.

The super elongation complex (SEC) mediates phase transition of SPT5 during transcriptional pause release.

Release of promoter-proximally paused RNA Pol II into elongation is a tightly regulated and rate-limiting step in metazoan gene transcription. However, the biophysical mechanism underlying pause release remains unclear. Here, we demonstrate that the pausing and elongation regulator SPT5 undergoes phase transition during transcriptional pause release. SPT5 per se is prone to form clusters. The disordered domain in SPT5 is required for pause release and gene activation. During early elongation, the super elongation complex (SEC) induces SPT5 transition into elongation droplets. Depletion of SEC increases SPT5 pausing clusters. Furthermore, disease-associated SEC mutations impair phase properties of elongation droplets and transcription. Our study suggests that SEC-mediated SPT5 phase transition might be essential for pause release and early elongation and that aberrant phase properties could contribute to transcription abnormality in diseases.

Sequence-dependent fusion dynamics and physical properties of DNA droplets.

Liquid-liquid phase separation (LLPS) of biopolymer molecules generates liquid-like droplets. Physical properties such as viscosity and surface tension play important roles in the functions of these droplets. DNA-nanostructure-based LLPS systems provide useful model tools to investigate the influence of molecular design on the physical properties of the droplets, which has so far remained unclear. Herein, we report changes in the physical properties of DNA droplets by sticky end (SE) design in DNA nanostructures. We used a Y-shaped DNA nanostructure (Y-motif) with three SEs as a model structure. Seven different SE designs were used. The experiments were performed at the phase transition temperature where the Y-motifs self-assembled into droplets. We found that the DNA droplets assembled from the Y-motifs with longer SEs exhibited a longer coalescence period. In addition, the Y-motifs with the same length but different sequence SEs showed slight variations in the coalescence period. Our results suggest that the SE length greatly affected the surface tension at the phase transition temperature. We believe that these findings will accelerate our understanding of the relationship between molecular design and the physical properties of droplets formed via LLPS.

Water droplet evaporation in atmospheric air stream

Evaporation of a water droplet in air is a natural phenomenon that causes the droplet and air properties to change over time. In this paper, time-dependent variation of air and water droplet properties in a flow is examined with the developed numerical model, in the range of conditions that concern air-conditioning and respiratory-based medical events and applications. An experimental setup is built and the model is validated with experiments carried out for 27 cases. Input parameters are chosen as the inlet air temperature, relative humidity, droplet temperature and diameter, and air–water mass mixing ratio. Output properties are determined as the outlet air temperature, relative humidity, mass droplet ratio, and droplet temperature to define the application case in the simplest way. Model results of 432 different cases are used to develop correlations with R20 values above 0.984, to present the effects of co-variation of input parameters. The results show that the latent heat demand of droplet evaporation is the main factor in the decrease of air temperature, while droplet temperature has almost no effect. As the air becomes saturated with moisture, evaporation almost stops and droplets carried by air flow remain. The droplet diameter affects only the process speed by reducing the processing time by the square of the reduction ratio. The developed numerical model and correlations allow the rapid determination of properties change in air–water stream over time, so it will provide great convenience to researchers dealing with water droplet evaporation in an atmospheric air stream.

EFFECT OF THERMOPHYSICAL AND DIELECTRIC PROPERTIES OF A LIQUID DROPLET ON CONTINUOUS MOTION IN AN ELECTRIC FIELD

The present study investigates the role of thermophysical and electrical properties of various liquid drops on their continuous motion over a Polydimethylsiloxane (PDMS) coated electrode with DI water as a reference. Droplet motion is achieved in an electric field around an active electrode when a ground wire is placed horizontally in an open-electrowetting-on-dielectric (EWOD) device. A charged-coupled device (CCD) camera is used to record the drop shapes and displacement of the moving droplet at 120 fps. Using image processing tools, the velocity of the droplet is determined from a time sequence of its centroid position. The dynamic contact angle of the drop is determined from the tangent drawn over the air-liquid interface. Liquids of interest include ferrofluid and a surfactant solution in water, as well as glycerin for droplet volumes in the range of 2-10 μl with voltages within 170-270V<sub>DC</sub>. Simulations are carried out in a 2D Cartesian coordinate system within COMSOL Multiphysics<sup>®</sup> software. The drop is taken to spread immediately after application of voltage following the Young-Lippmann equation and is accompanied by continuous motion. The interfacial forces arising from the electric field are calculated in terms of the Maxwell's stress tensor (MST). The electrostatic force is a source term in the Navier-Stokes equations treated using a fully coupled approach. Interface shapes of ferrofluid and surfactant droplets do not show significant departure from moving water droplets. As the concentration of the ferrofluid increases, surface tension decreases, and the droplet speed increases. The extent of spreading of a surfactant solution is higher, thus generating a higher interfacial area for the electric field, leading to a higher droplet velocity. In glycerine, high viscosity and low permittivity increase wall resistance, which is an important factor in reducing the velocity attained during continuous motion. In comparison with a water droplet, the glycerin drop moves slower, surfactant solution is faster, and the ferrofluid drop has characteristics similar to water. Droplets of glycerin, aqueous solutions containing surfactant, and ferrofluid show the greatest deformation.

Identification of Supercooled Cloud Water by FY-4A Satellite and Validation by CALIPSO and Airborne Detection

Cold clouds are the main operation target of artificial precipitation enhancement, and its key is to find a supercooled cloud water area where the catalyst can be seeded to promote the formation of precipitation particles and increase precipitation to the ground. Based on the multi-spectral characteristics of the Fengyun-4A (FY-4A) satellite, a methodology for identifying supercooled cloud water is developed. Superimposed by a cloud top brightness temperature of 10.8 µm, a combination of 0.46 µm, 1.6 µm, and 2.2 µm red–green–blue (RGB) composites are used to identify the cloud phase and to obtain the real-time supercooled cloud water distribution every 5 min and in a 2 km resolution for the whole coverage of China. Based on the RGB composition, the supervised machine learning method K-mean clustering was applied to classify the cloud top phase. The results were validated extensively with Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP). It is worthwhile to highlight that the corresponding hit rate reached 87% over the full disk domain for both the summer and winter seasons. Furthermore, on 29 November 2019, microphysical properties were measured, and the data of supercooled cloud droplets and ice crystals were obtained using YUN-12 transport aircraft in Taiyuan. After simultaneously matching the satellite with an airborne track, the cloud particle image data were obtained near the cloud top and within the clouds during the climb and descending stages of the flight. The phase obtained from the microphysical properties of supercooled cloud droplets and ice crystals was compared with cloud phase results identified by FY-4A and Moderate Resolution Imaging Spectroradiometer (MODIS) cloud phase products. The case study and comparison show that (1) the supercooled water clouds and ice particles identified by FY-4A are in good agreement with those from the airborne measurement at the cloud top and within the cloud and (2) the positions and shapes of water clouds and ice clouds identified by FY-4A correspond well with MODIS cloud phase products. However, there is a small deviation in the extent of ice clouds, which is mainly located in the transition area between ice clouds and water clouds. The extent of ice clouds identified by FY-4A is slightly larger than that of MODIS products. Combined with airborne detection, the comparison shows that the ice clouds identified by the FY-4A satellite are consistent with aircraft detection. The supercooled cloud water identified by FY-4A can meet the needs of the operational precipitation enhancement of cold clouds, improve operational effectiveness, and promote the application of satellite technology for weather modification.

Computational study of dynamics of confined droplets under electric field: effect of contact angle

PurposeThe contact behaviors of droplets on confined surfaces influence significantly their dynamics and morphological transition induced by the electric field. This paper aims to delve into the electric stress, electric field distribution, flow field and evolution of droplet neck to understand the underlying mechanisms.Design/methodology/approachElectrohydrodynamics of droplets in confined environment is numerically analyzed based on finite volume method (FVM) combining with volume-of-fluid (VOF) method for two-phase interface capturing. Numerical solutions are obtained through solving electrohydrodynamics model coupling fluid dynamics with electrostatics.FindingsIt was found that the droplet neck with high interfacial curvature undergoes different transition depending on the contact angle. At large domain height, the droplets on the surfaces with the contact angle of θ < 90° tend to break up into smaller droplets adhered on top and bottom surfaces. The detachment of droplets is identified when the contact angle is much greater than 90°. Notably, the droplets at θ = 90° exhibit asymmetrical shape evolution, but for other cases there is symmetrical shape of droplets during transition process. With decreasing the domain height, no obvious deformation through driving the contraction of the droplet neck is observed.Originality/valueIt remains unclear how the electric field parallel to the surfaces affects the shape transition and electrohydrodynamics of confined droplets when changing the contact angle. In this paper, the authors study the electrohydrodynamics of droplets in confined space when the electric field is exerted parallel to contact surfaces. In particular, the authors consider the effect of the surface wettability on the droplet deformation. The problem is solved through FVM combining with the VOF method to implement the capturing of two-phase interfaces. The results indicate that the electrohydrodynamic behaviors of droplets are sensitive to the contact properties of droplets on the surfaces, which has not been reported in previous works.

Release characteristics of core droplet from a double-emulsion droplet

Double-emulsion droplets have been applied in biological, chemical, and medical fields due to their excellent encapsulation and release properties of active substances. However, their application and popularization are limited by a poor understanding of the double-interface characteristics. In this work, the core release characteristics from double-emulsion droplets were studied by numerical simulations. The coupled level set and volume of fluid method was used to track the interface motion, and the continuous surface tension was applied to describe the interfacial tension force. It was found that the initial core release may be interrupted by the retraction of the middle phase when the core droplet diameter is large, the interfacial tension is high, or the external droplet diameter is small. The retraction is induced by the pressure increase behind the droplet in the middle phase. Four core release types were found: release at one time (Type I), release at two times on the left side (Type II), partial release (Type III), and release on both sides at different times (Type IV). Due to rising interfacial energy and reduction of the middle-phase resistance, the release type will gradually transform from Type I to Type II, III, or IV with increasing core diameter, increasing interfacial tension, and decreasing external diameter. Finally, a phase diagram summarizing the four release types of the core release was obtained. The results of this study have profound guiding significance for the application and understanding of the release properties of double-emulsion droplets.

Stiffness-dependent active wetting enables optimal collective cell durotaxis

The directed migration of cellular clusters enables morphogenesis, wound healing and collective cancer invasion. Gradients of substrate stiffness direct the migration of cellular clusters in a process called collective durotaxis, but the underlying mechanisms remain unclear. Here we unveil a connection between collective durotaxis and the wetting properties of cellular clusters. We show that clusters of cancer cells dewet soft substrates and wet stiff ones. At intermediate stiffness—at the crossover from low to high wettability—clusters on uniform-stiffness substrates become maximally motile, and clusters on stiffness gradients exhibit optimal durotaxis. Durotactic velocity increases with cluster size, stiffness gradient and actomyosin activity. We demonstrate this behaviour on substrates coated with the cell–cell adhesion protein E-cadherin and then establish its generality on substrates coated with extracellular matrix. We develop an active wetting model that explains collective durotaxis in terms of a balance between in-plane active traction and tissue contractility and out-of-plane surface tension. Finally, we show that the distribution of cluster displacements has a heavy tail, with infrequent but large cellular hops that contribute to durotactic migration. Our study demonstrates a physical mechanism of collective durotaxis, through both cell–cell and cell–substrate adhesion ligands, based on the wetting properties of active droplets. Substrate stiffness influences cellular cluster migration through collective durotaxis. Now, the underlying mechanism of this process is explained by considering the wetting dynamics of the clusters.

DNA Droplets: Intelligent, Dynamic Fluid.

Breathtaking advances in DNA nanotechnology have established DNA as a promising biomaterial for the fabrication of programmable higher-order nano/microstructures. In the context of developing artificial cells and tissues, DNA droplets have emerged as a powerful platform for creating intelligent, dynamic cell-like machinery. DNA droplets are a microscale membrane-free coacervate of DNA formed through phase separation. This new type of DNA system couples dynamic fluid-like property with long-established DNA programmability. This hybrid nature offers an advantageous route to facile and robust control over the structures, functions, and behaviors of DNA droplets. This review begins by describing programmable DNA condensation, commenting on the physical properties and fabrication strategies of DNA hydrogels and droplets. By presenting an overview of the development pathways leading to DNA droplets, it is shown that DNA technology has evolved from static, rigid systems to soft, dynamic systems. Next, the basic characteristics of DNA droplets are described as intelligent, dynamic fluid by showcasing the latest examples highlighting their distinctive features related to sequence-specific interactions and programmable mechanical properties. Finally, this review discusses the potential and challenges of numerical modeling able to connect a robust link between individual sequences and macroscopic mechanical properties of DNA droplets.