Phenomenon Of Droplets Research Articles

Puffing and Microexplosion Behavior of Water in Pure Diesel Emulsion Droplets During Leidenfrost Effect

ABSTRACTThe microexplosion evolution phenomenon of single droplets of water in pure diesel emulsion under Leidenfrost effect has been studied. The tested emulsions were stabilized with a blend of commercial surfactants with three different water contents of 9%, 12%, and 15%. A high speed camera synchronized with backlight technique was used to capture the evolution of microexplosion and puffing. Three different droplet diameters of approximately 2.6 mm, 2 mm, and 0.2 mm were analyzed. It was found that the tendency of microexplosion and puffing frequency was influenced by the droplet diameter. Coalescence was the dominating factor in inducing microexplosion in bigger droplets. It was observed that the child droplets ejected from the parent droplet undergoes further puffing processes. The size of the secondary droplets after microexplosion were also found to be slightly influenced by the parent droplet size.The waiting time for microexplosion and puffing were compared for different droplets size.

Super-Hydrophobic/Icephobic Coatings Based on Silica Nanoparticles Modified by Self-Assembled Monolayers.

A super-hydrophobic surface has been obtained from nanocomposite materials based on silica nanoparticles and self-assembled monolayers of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) using spin coating and chemical vapor deposition methods. Scanning electron microscope images reveal the porous structure of the silica nanoparticles, which can trap small-scale air pockets. An average water contact angle of 163° and bouncing off of incoming water droplets suggest that a super-hydrophobic surface has been obtained based on the silica nanoparticles and POTS coating. The monitored water droplet icing test results show that icing is significantly delayed by silica-based nano-coatings compared with bare substrates and commercial icephobic products. Ice adhesion test results show that the ice adhesion strength is reduced remarkably by silica-based nano-coatings. The bouncing phenomenon of water droplets, the icing delay performance and the lower ice adhesion strength suggest that the super-hydrophobic coatings based on a combination of silica and POTS also show icephobicity. An erosion test rig based on pressurized pneumatic water impinging impact was used to evaluate the durability of the super-hydrophobic/icephobic coatings. The results show that durable coatings have been obtained, although improvement will be needed in future work aiming for applications in aerospace.

Physical and Numerical Investigations of Mould Flux Entrainment into Liquid Steel

Abstract This paper presents results of model tests, performed in order to analyze phenomenon of slag droplets entrainment into steel in mould, during continuous casting process. The carried out studies took the form of laboratory experiments using physical model, in which – using similarity criteria – the behaviour of interfacial boundary liquid steel-liquid slag has been simulated using water and silicon oils, differing in physicochemical properties. Additionally, based on PIV (Particle Image Velocimetry) measurements and numerical simulations, vector flow field and values of critical velocities, at which observed the occurrence of interfacial boundary silicon oil-water instability have been identified. Based on the carried out investigations, results, that illustrate relationship between critical entrainment velocity and physicochemical properties of oils have been presented.

ChemInform Abstract: Organic Reactions in Microdroplets: Reaction Acceleration Revealed by Mass Spectrometry

AbstractReview: [77 refs.

Puddle jumping: Spontaneous ejection of large liquid droplets from hydrophobic surfaces during drop tower tests

Large droplets and puddles jump spontaneously from sufficiently hydrophobic surfaces during routine drop tower tests. The simple low-cost passive mechanism can in turn be used as an experimental device to investigate dynamic droplet phenomena for drops up to 104 times larger than their normal terrestrial counterparts. We provide and/or confirm quick and qualitative design guides for such “drop shooters” as employed in drop tower tests including relationships to predict droplet ejection durations and velocities as functions of drop volume, surface texture, surface contour, wettability pattern, and fluid properties including contact angle. The latter is determined via profile image comparisons with numerical equilibrium interface computations. Water drop volumes of 0.04–400 ml at ejection speeds of −0.007–0.12 m/s are demonstrated herein. A sample application of the drop jump method is made to the classic problem of low-gravity phase change heat transfer for large impinging drops. Many other candidate problems might be identified by the reader.

Kerberos: A three camera headed centrifugal/tilting device for studying wetting/dewetting under the influence of controlled body forces

Despite the significant effort spent over the years to study wetting and dewetting phenomena of droplets on solid substrates under applied forces, there are still many unresolved issues. To name a few, a concise theory for the two dimensional contact line is missing. Also, a controversy has recently arisen for the effect of droplet size on the tangential force required for the inception of sliding. It is clear that to resolve such issues and to better understand the underlying processes a new generation of experimental techniques and results are needed. In this spirit, a novel device, Kerberos, is proposed and constructed here. Kerberos offers two major innovative features compared to prior techniques: (a) it allows simultaneous rotation and tilting of the droplet supporting plate (these mechanisms have been employed separately in the past) and (b) three Wi-Fi cameras (viewing the droplet from X–Y–Z directions) follow the rotation/tilting motion. The above features permit on one hand the independent control of normal and tangential forces applied to the droplet and, on the other, allow real time 3D impressions of the droplet shape. Indicative results manifest the wealth of new experimental data accessible by Kerberos which were out of reach by other techniques. The theoretical background of droplet behavior in the new device is discussed, making assumptions that lead to closed form solutions. Such a model is useful for the design of experiments with the new device.

Insight into rime ice accretion on an aircraft wing and corresponding effects on aerodynamic performance

ABSTRACTA method based on the Eulerian two-phase flow theory to numerically simulate three-dimensional rime ice accretions on an aircraft wing is presented in this paper. The governing equations for supercooled droplet motion under Eulerian framework are established using the droplet pseudo-fluid model. A permeable wall boundary condition is proposed to simulate the phenomenon of droplets impinging on the wing in solving the governing equations for droplets. The local droplet collection efficiency is readily obtained from the droplet flowfield solution in the control volume adjacent to the wing surface. The rime ice accretion can be simulated under the assumption that the droplets freeze immediately as they impinge on the wing surface since the environment temperature is low enough (typically below –15°C). A method to build the ice shape is proposed based on the assumption that ice grows in the direction normal to the wing surface. The rime ice accretion on a GLC-305 swept wing model under some specific conditions has been simulated to validate the present method. Furthermore, different flight conditions, namely, different angles of attack and different angles of sideslip, have been dealt with to investigate their effects on rime ice accretion as well as the corresponding aerodynamic effects.

Self-Propelled Hovercraft Based on Cold Leidenfrost Phenomenon.

The Leidenfrost phenomenon of liquid droplets levitating and dancing when placed upon a hot plate due to propulsion of evaporative vapor has been extended to many self-propelled circumstances. However, such self-propelled Leidenfrost devices commonly need a high temperature for evaporation and a structured solid substrate for directional movements. Here we observed a “cold Leidenfrost phenomenon” when placing a dry ice device on the surface of room temperature water, based on which we developed a controllable self-propelled dry ice hovercraft. Due to the sublimated vapor, the hovercraft could float on water and move in a programmable manner through designed structures. As demonstrations, we showed that the hovercraft could be used as a cargo ship or a petroleum contamination collector without consuming external power. This phenomenon enables a novel way to utilize programmable self-propelled devices on top of room temperature water, holding great potential for applications in energy, chemical engineering and biology.

Dynamic Behaviors and Energy Transition Mechanism of Droplets Impacting on Hydrophobic Surfaces

The wettability of hydrophobic surfaces and the dynamic behaviors of droplets impacting on hydrophobic surfaces are simulated using a lattice Boltzmann method, and the condition for the rebound phenomenon of droplets impacting on solid surfaces is analyzed. The results show that there is a linear relationship between the intrinsic contact angle and the interaction strength of fluid-wall particles. For hydrophobic surfaces with the same intrinsic contact angle, the micromorphology can increase the surface hydrophobicity, especially the hierarchical micromorphology. The dynamic behaviors of droplets impacting on solid surfaces are affected by the wettability. The surface hydrophobicity is stronger, and the rebound phenomenon occurs easier. If the droplet’s kinetic energy is greater than the sum of the surface energy and the minimum conversion gravitational potential energy when the spreading and shrinking finish, the rebound phenomenon will occur. As the hydrophobic surface’s viscous dissipation is much smaller than the hydrophilic surface’s, the droplet still has high kinetic energy after the spreading and shrinking, which is advantageous to rebound for droplets.

Passive mixing enhancement of microliter droplets in a thermocapillary environment

Fast and efficient mixing of reagents and bioassays is of great importance in micro total analysis systems and in particular, open-surface microfluidics. This is the first study of droplet collision and mixing phenomenon of levitated droplets on an immiscible thin liquid substrate undergoing Marangoni convection. The mixing percentage was determined by monitoring the changing color during chemical reaction that occurs in the resultant coalesced droplet. The thermocapillary effect of the liquid substrate plays a significant role in mixing in that the liquid surface velocity and the droplet velocity increase with an increase in the surface temperature gradient. This, in turn, increases the mixing of the coalesced droplet, which is further enhanced when the drop size is smaller. Since the collision is convection dominated, an appropriate time scale is proposed, which provides a universal correlation of the non-dimensional mixing time in terms of Reynolds number.

Derivation of Generalized Young’s Equation for Wetting of Cylindrical Droplets on Rough Solid Surface

<p class="1Body">The surface tension depends on the radius of curvature of the liquid-vapor interface. For nano-scale wetting phenomena of cylindrical droplets, we should consider the curvature effects of the surface tension and the line tension. However, previous works have not analyzed the influence of the curvature effects of the surface tension. In this paper, we discuss the influence of the curvature effects of the surface tension on the contact angles based the Kim-Lee-Han-Park equation. The hydrophilic wetting of cylindrical droplets on rough and chemically homogeneous non-deformable substrates were studied by methods of thermodynamics. A generalized Young’s equation for wetting of cylindrical droplets on chemically homogeneous and rough non-deformable substrates was derived based on the thermodynamic equilibrium conditions. This equation reduces to the Wenzel equation if we ignore the influence of line tension. For contact angles of cylindrical droplets with sufficiently large radii, a generalized Young’s equations were derived considering the curvature effects of the surface tension.</p>

Nonlinear Relationship between the Radii of Droplets and the Contact Angle of Wetting

<p class="1Body">Wetting abilities are important in many industrial applications, for instance, the wetting abilities of electrolytes on electrodes plays a key role in improving the specific energy density of supercapacitors and lithium-ion batteries. For nano-scale wetting phenomena, we should consider the curvature effects of the surface tension and the line tension. However, previous works have not analyzed the influence of the curvature effects of the surface tension. In this manuscript, the nano-scale wetting phenomena of spherical droplets on smooth non-deformable substrates were studied by methods of thermodynamics. The total Helmholtz free energy total and the grand potential of this system are calculated. A generalized Young’s equation for wetting of spherical droplets with large enough radius is derived. It is shown that there exists a nonlinear relationship between the contact angle and the radii of droplets or the line tension.</p>

Wetting and evaporation phenomena of water droplets on textured surfaces

When water droplets are placed on a surface, the contact angle depends on the surface conditions, and the subsequent evaporation of the droplets depends on the wetting phenomena. In this study, we investigated the wetting and evaporation of droplets of deionized water from surfaces with various intrinsic contact angles and surface morphologies (θ0=34°, 58° and 109°, 1.00<fW<2.00, 0.02<fC–B<1.00). For each droplet, the apparent contact angle and radius of the contact line were measured, and the wetting state was visualized with synchrotron X-ray radiography. Comparing between the experimental data and the results of a number of wetting models, the models were experimentally validated. On the basis of the wetting phenomena, the evaporation rate depending on the surface condition is clarified in the experimental ranges.

Simulation of Droplet Impacting on Elastic Solid with the SPH Method

The phenomenon of droplet impacting on solid surfaces widely exists in both nature and engineering systems. However, one concern is that the microdeformation of solid surface is difficult to be observed and measured during the process of impacting. Since the microdeformation can directly affect the stability of the whole system, especially for the high-rate rotating components, it is necessary to study this phenomenon. Aiming at this problem, a new numerical simulation algorithm based on the Smoothed Particle Hydrodynamics (SPH) method is brought forward to solve fluid-solid coupling and complex free surface problems in the paper. In order to test and analyze the feasibility and effectiveness of the improved SPH method, the process of a droplet impacting on an elastic plate was simulated. The numerical results show that the improved SPH method is able to present more detailed information about the microdeformation of solid surface. The influence of the elastic modulus of solid on the impacting process was also discussed.

Surface formation phenomena of DHA-containing emulsion during convective droplet drying

Encapsulation of docosahexaenoic acid (DHA) protects this environment-sensitive substance from oxidation and also masks its odor. In this study, DHA was encapsulated with octenyl succinic anhydride modified (OSA) starch and whey protein concentrate. Single droplets of emulsions with different compositions were suspended on a glass filament to observe the dissolution behavior of dried and semi-dried particles. At each drying stage, the drying process were stopped to monitor changes in the semi-dried particle’s morphology, either by attaching an additional water droplet or by keeping the semi-dried particle suspended in the drying chamber without air flow. Based on observations in the above-mentioned test, the crust formation phenomena of emulsion droplets with different compositions were compared at various drying stages. The microstructures of dried particles containing DHA were examined by scanning electron microscopy (SEM). The surface oil coverage and distribution were studied by observing the surface structure before and after oil extraction. The results obtained in this study indicate the underlying particle formation mechanisms during drying of DHA-containing emulsions formed with different wall materials.

Numerical and analytical study of the impinging and bouncing phenomena of droplets on superhydrophobic surfaces with microtextured structures.

The dynamics of droplets impinging on different microtextured superhydrophobic surfaces are modeled with CFD combined with VOF (Volume of Fluid) technique. The method is validated by experimental data and an analytical model (AM) that is used to predict the penetrating depth and the maximum spreading diameter of an impinging droplet. The effects of geometrical shapes and operating conditions on the spreading and bouncing behaviors of impinging droplets are investigated. Six surfaces with different shapes of pillars are considered, namely, triangular prism, square pillar, pentagonal prism, cylindrical pillar, and crisscross pillar surfaces. The bouncing ability of an impinging droplet on textured surfaces can be illustrated from three aspects, namely, the contact time, the ranges of velocities for rebound and the penetrating depth of liquid in the maximum spreading stage. The surface with crisscross pillars exhibits the best ability to rebound, which can be attributed to its large capillary pressure (PC) and its special structures that can capture air in the gaps during the impinging process.

Numerical investigation on super-cooled large droplet icing of fan rotor blade in jet engine

Icing (or ice accretion) is a phenomenon in which super-cooled water droplets impinge and accrete on a body. It is well known that ice accretion on blades and vanes leads to performance degradation and has caused severe accidents. Although various anti-icing and deicing systems have been developed, such accidents still occur. Therefore, it is important to clarify the phenomenon of ice accretion on an aircraft and in a jet engine. However, flight tests for ice accretion are very expensive, and in the wind tunnel it is difficult to reproduce all climate conditions where ice accretion can occur. Therefore, it is expected that computational fluid dynamics (CFD), which can estimate ice accretion in various climate conditions, will be a useful way to predict and understand the ice accretion phenomenon. On the other hand, although the icing caused by super-cooled large droplets (SLD) is very dangerous, the numerical method has not been established yet. This is why SLD icing is characterized by splash and bounce phenomena of droplets and they are very complex in nature. In the present study, we develop an ice accretion code considering the splash and bounce phenomena to predict SLD icing, and the code is applied to a fan rotor blade. The numerical results with and without the SLD icing model are compared. Through this study, the influence of the SLD icing model is numerically clarified.

Wettability model for various-sized droplets on solid surfaces

The wetting phenomenon is crucial for the formation of stable liquid films on solid surfaces. The wettability of a liquid on a solid surface is characterized by the Young equation, which represents an equilibrium condition of a droplet at the three phase contact line. In general, the surface force in the vertical direction on a solid surface is ignored because of the resistance of the solid surface. However, considering the adhesion energy of the droplet rather than the force balance at the contact line, the vertical component of the surface force can be expected to be an important factor during wetting. Based on this concept, an analytical model is developed herein by considering the energy balance including adhesion forces acting not only in the horizontal but also in the vertical direction, in addition to the effect of gravity on the droplet. The validity of the developed model is then evaluated by experimental observation of the wetting phenomena of droplets on low- and high-surface-energy solids. Existing data are also used for evaluation of our model. The developed model describes the wetting phenomena of droplets with sizes ranging from nano- to millimeters under all experimental conditions and exhibits universality. In addition, on the basis of our model, the line tension is discussed. The results indicate that the line tension approach may be considered as a method to explain wetting phenomena by considering gravitational potential and other macroscopic parameters as a single parameter (i.e., line tension).

Modeling and experimental study of vapor phase-diffusion driven sessile drop evaporation

Modeling the evaporation of a water droplet spraying into a variable parameter chamber is a crucial step in ice production. It can result in a better understanding of vacuum ice production or dehumidification ice production procedure. This study developed numerical diffusion-control partial differential equations to describe droplet behavior at different chamber parameters. This study also theoretically examines the cooling phenomenon of water droplets due to evaporation in an evaporation chamber. Initial injection diameters, relative humidity of the evaporation chamber, temperature and pressure are some of the parameters considered. The prediction of the model was in agreement with experimental data. Results show that the smaller the initial radius of the droplet in the saturated evaporation chamber, the faster the droplet freezes, even though both have the same equilibrium temperature. Low relative humidity can significantly reduce the droplet flash time. The temperature of the droplet changes a lot at different initial injection diameter, relative humidity and pressure of the evaporation chamber. Different initial temperatures hardly affect the temperature variation of the droplet.

Behavior of the plasma characteristic and droplet transfer in CO2 laser–GMAW-P hybrid welding

The plasma behavior and metal transfer in CO2 laser + GMAW-P hybrid welding have been investigated. A 650 nm laser, in conjunction with the shadow graph technique, is used to observe the metal transfer process. The effect of the mutual distance and laser power on the metal transfer has been discussed. The results indicate that the laser-induced plasma plume have a significant impact to the arc shape, resistance, electrode melting, droplet formation, detachment, and impingement onto the workpiece. The laser-induced plasma changes the conductive path and forces affecting on the droplet. High laser power and short distance between laser beam and arc (DLA) reduce the pulse base time (PB) of the voltage of phase, increase the droplet detachment time (PD) and the pulse current time (PP) of the voltage of phase, and it also lead to an upward and inward force near the bottom of the droplet. As a consequence, the droplet formation time is increased, and eventually an off-axis droplet phenomenon is deduced. The vapor jet force induced by the the keyhole plasma acts on the droplet as a retention force; this force decreases when the DLA becomes larger and increases when the laser power becomes higher. The observation may help in understanding the weld characteristics with respect to variation in mutual distance and laser power which may be beneficial in using the main process parameters to produce desired weld quality.