Deformation Of Water Droplets Research Articles

Dynamic characteristics of corona discharge generated under rainfall condition on AC charged conductors

By synchronous measurement of corona current and the water droplet deformation process on a conductor surface, different types of corona discharge are visualized when AC voltage is applied on a line-ground electrode system. The corona characteristics are closely related to the applied voltage and water supply rate. With the increase of AC voltage, the positive Taylor cone discharge firstly appears and then disappears, replaced by the dripping and crashing discharge. Furthermore, the number of pulses in each pulse train increases with the increase of applied voltage. The mechanism of the transfer from the positive Taylor cone discharge to the dripping and crashing discharge is found to be related to the oscillation process of the water droplet. The water supply rate also has a great influence on the characteristics of corona currents. The number of positive pulse trains increases linearly when the water supply rate gets larger, leading to a higher audible noise and radio interference level from the AC corona, which is quite different from that of the DC corona. The difference between the AC and DC coronas under rainfall conditions is analyzed finally.

Probing Mechanical Properties of Water–Crude Oil Interfaces and Colloidal Interactions of Petroleum Emulsions Using Atomic Force Microscopy

Atomic force microscopy (AFM) is frequently used to elucidate complex interactions in emulsion systems. However, comparing results obtained with “model” planar surfaces to curved emulsion interfaces often proves unreliable, because droplet curvature can affect adsorption and arrangement of surface-active species, while droplet deformation affects the net interaction force. In the current study, AFM was used to study the interactions between a colloidal probe and water droplet. Force magnitude and water droplet deformation were measured in toluene solutions of asphaltene or bitumen at different concentrations and varying droplet aging time. Interfacial stiffening and an increase in particle–droplet adhesion force were observed upon droplet aging in bitumen solution. As reported in our previous study (Kuznicki, N. P., Harbottle, D., Masliyah, J., and Xu, Z. Dynamic Interactions between a Silica Sphere and Deformable Interfaces in Organic Solvents Studied by Atomic Force Microscopy. Langmuir 2016, 32 (38), 9...

Rotating Arm-Based Experimental Study on Droplet Behavior in the Shoulder Region of an Aircraft Aerodynamic Surface

An experimental study has been performed on water droplet deformation in the shoulder region of an airfoil. The experiments have been carried out in a rotating arm facility 2.2 m long and able to rotate up to 400 rpm (90 m/s). A blunt airfoil model (chord length equal to 0.468 m) was placed at the end of the arm. A droplet generator was used to generate a stream of water droplets with an initial diameter of 1000 μm. An imaging system was set up to record the trajectories and deformations of the droplets in three different regions close to the airfoil shoulder. The base flow field was characterized using a particle image velocimetry system. The experiments show that droplet deformation results in the shoulder region of the airfoil are different from those pertaining to the leading edge region. In particular, droplets in the shoulder region tend to rotate to the direction of the incoming airfoil which generates an interference effect between the droplets that make up the stream. These differences have been quantified applying an existing theoretical model specifically developed for the leading edge region to the results obtained in the present study.

Investigation of the electric field driven self-propelled motion of water droplets on a super-hydrophobic surface

Super-hydrophobic coatings have recently drawn considerable attention in research and applications towards self-cleaning materials. This paper presents experiments and analysis of water droplet behaviors on glass, silicone rubber, and super-hydrophobic surface in an ac electric field applied parallel to the surface. Experimental results show that a water droplet tends to move on a super-hydrophobic surface while it tends to stretch or deform on glass and silicone rubber surfaces. A physical model of the water droplet deformation and motion mechanism is presented. An electrostatic force acting as the domain driving force plays a key role in water droplet motion on super-hydrophobic surface. The electrostatic stress acting along the external profile of a water droplet cross section was simulated using a surface integral of the Maxwell stress tensor. The simulation for surfaces with different wetting properties are in good agreement with the experimental results. Super-hydrophobic coating demonstrates a prospective electric field enhanced self-cleaning property.

The effect of gas and water droplet temperature on characteristics of water-droplet deformation at moderate velocities of droplet movement

An experimental study has been performed on the effect of the temperature of gas and water droplets (3–6 mm in size) on deformation of water droplets that move with a moderate velocity (up to 5 m/s). A high-speed videocamera (recording at up to 105 fps) and a cross-correlation videocomplex were used. The droplet temperature varied in the range of 285–360 K. For gaseous mediums, air at a temperature of 280–300 K and condensed substance combustion products at a temperature about 1100 K were used. Typical deformation cycles were found. Their length, duration, and amplitude were calculated. The time a shape of droplet transforms into a new shape and the time the droplets retain their shape as they move through a gas medium have been determined. Approximating correlations for the basic characteristic of deformation under various heating or cooling conditions in a water droplet–gas system were derived.

Features of water droplet deformation during motion in a gaseous medium under conditions of moderate and high temperatures

Using cross-correlation video recording facilities, the features of deformation of water droplets with characteristic dimensions of 3–6 mm are studied experimentally during their motion in a high temperature (about 1100 K) gaseous medium (in combustion products) with velocities of up to 5 m/s. In addition, experiments for water droplets moving with identical initial velocities and dimensions in air at moderate (about 300 K) air temperatures are carried out. Typical shapes of droplets and cyclical character of their variation during motion in the considered gaseous media are established. The numerical values of the main characteristics of the isolated “deformation cycles” (amplitude, times, and durations) are determined, and their susceptibility to high temperatures of the gaseous medium, droplet dimensions, and droplet motion velocities is revealed.

Linear dynamics modelling of droplet deformation in a pulsatile electric field

A linear dynamic model of water droplet deformation in the presence of an electric field has been developed. Analytical solutions of the differential equation of motion are provided with different waveforms as forcing terms, namely in the case of half-sinusoidal, square and sawtooth waves. The main dimensionless groups are identified as a result of this analysis. The predictions of the model are compared with some data of droplet deformation available in the literature. The calculations based on this model show that the waveform affects the response of the droplet to the electric field stimulus. Resonance is possible only when the droplets are sufficiently large (i.e. for Ohnesorge number less than 1). The oscillation amplitude decreases rapidly with the electric field frequency. A qualitative comparison with some experiments of droplet-interface coalescence available in the literature has also been addressed, suggesting a correlation between the formation of secondary droplets and the amplitude of oscillation of the mother droplet. The outcomes of this analysis can be useful for the selection of the best operating conditions to improve the electrocoalescence process efficiency, as they can provide guidelines to the choice of the most suitable electric field parameters.

Coalescence behaviour of water droplets in water‐oil interface under pulsatile electric fields

In this research, the deformation of water droplets in sunflower oil-interface under pulsatile electric field was studied experimentally. Three types of coalescence were observed: (i) complete coalescence, (ii) incomplete coalescence and (iii) no-coalescence. The first type is desirable because of leaving no secondary droplets. The second type that produced secondary droplets which caused by necking process, due to extreme elongation of droplets (mostly small droplets), was undesirable; because the small droplets were more difficult to coalesce and remove. The no-coalescence was caused by very fast coalescence and extensive pushing of droplet into the continuous phase. In this work the process was operated with the utilization of a batch cylindrical separator with high voltage system. The lower part of the cylinder was filled with the aqueous phase and its top part was filled with sunflower oil to form an interface between the two phases. The effects of electric field strength, frequency, and waveform types were investigated. It was found that, the ramp-ac waveform was the best waveform, avoiding the production of secondary droplets and in this case the frequency also played an important role.

Role of the electric double layer in the ice nucleation of water droplets under an electric field

Figuring out the mechanism of ice nucleation on charged aerosols or in thunderstorms is of fundamental importance in atmospheric science. However, findings on whether the electric field promotes or suppresses heterogeneous ice nucleation are conflicting. In this work, we design an apparatus and test the influence of the electric field on ice nucleation by freezing a series of deionized water droplets resting on solid surfaces with an electric field perpendicular to the substrates. Results show that ice nucleation is obviously promoted under the electric field and is independent of the field direction. Theoretic analyses show that the promotion is due to the reduction of Gibbs free energy which can be partially rationalized by the electric field sustained in the electric double layer at the solid-water interface, with strength about two orders higher than that of the external electric field. Moreover, water-droplet deformation under the electric field is not expected to be the cause of the ice-nucleation promotion.

Direct numerical simulation of two-phase turbulent flow in fuel cell flow channel

For high-performance low-temperature fuel cells (e.g. hydrogen proton exchange membrane fuel cell for powering vehicles), significant amount of reactant needs to be supplied, leading to turbulent two-phase flow, which is largely ignored in previous studies. In this study, a direct numerical simulation (DNS) model of the two-phase turbulent flow in fuel cell flow channel is developed with a modified volume-of-fluid (VOF) approach for tracking the air/water interface. The turbulent flow inlet of the two-phase DNS model is obtained from a validated single-phase DNS model. By resolving the whole range of spatial and temporal scales of turbulence, the results of the two-phase DNS model show that the deformation of water droplet is asymmetric and broken into small pieces/films, and is significantly different from the laminar and the corresponding k−ε models. It is suggested that the turbulence effect on the two-phase transport in fuel cell flow channel is significant and needs to be considered for water management by using the DNS model.

Deformation of Water Droplets at Various Initial Temperatures During Motion Through Cooled Air

Experimental investigations exploring the integral characteristics of the deformation of water droplets at various initial temperatures (280 to 360 K) in air at 275–300 K were conducted. Sizes (diameters) and motion velocities of the droplets were varied between 3 and 6 mm and 0 to 5 m/sec, respectively. High-speed (up to105 frames per second) video cameras, cross-correlation measuring systems, and panoramic optical methods (PIV and IPI) were used. The main characteristics of droplet “deformation cycles,” as well as their shapes and existence times were determined. The influence of droplet sizes and motion velocities on the characteristics of their deformation for the conditions corresponding to operation of modern cooling towers, evaporators, separating machines, irrigators, and other devices typical in gas-vapor-droplet technological cycles was established.

Influence of the Temperature of Gases on the Deformation Characteristics of Moving Water Droplets

An experimental analysis of the influence of the temperature of gases on the deformation characteristics of water droplets moving through them is carried out. High-speed (up to 105 frames per second) photography with video cameras and cross-correlation complexes, as well as panoramic optical methods of recording the motion parameters of gaseous and liquid flows, were used. The laws governing the movement of water droplets through gases with a temperature of about 1100 K and in air at relatively low (to 280 K) and moderate (about 300 K) temperatures have been studied. The principal characteristics of the deformation of water droplets (duration, extension, and amplitudes of "deformation cycles") have been established. The differences in the characteristic shapes of droplets during their motion in gaseous media (at different temperatures of the latter) have been determined. The times during which droplets retain a certain shape in the deformation process have been calculated.

Experimental characterization of water droplet deformation and breakup in the vicinity of a moving airfoil

An experimental study is presented on the deformation and breakup of water droplets in the vicinity of an incoming airfoil. The experimental campaign was carried out at the INTA rotating arm facility in the context of the INTA-NASA collaboration. In this experimental setup, the flow past the droplets accelerates (in the reference frame of the droplets) which is a situation similar to the one found in actual aircraft flight. This is in contrast to most of the previous more fundamental experimental studies in which the flow past the droplet has a constant velocity. During the tests, the velocity of the incoming airfoils ranged between 50 m/s and 90 m/s. Three different symmetric airfoil models were tested. They all had a spanwise length of 0.2 m. The chord lengths were 0.690 m, 0.468 m and 0.199 m respectively. The leading edge radii of curvature were 0.103 m, 0.070 m, and 0.030 m respectively. The undisturbed droplet diameters were in the range from 364 μm to 1075 μm. The base flow field was characterized using a Particle Image Velocimetry technique. The droplets were tracked using a high speed imaging system. The rate of deformation of the droplets was measured and the deformation and breakup processes were recorded and correlated to the parameters that defined the base flow. It was found that, within the range of tested experimental conditions, the favored breakup mechanism was of the so-called “bag and stamen” type. However, flow acceleration tended to anticipate significantly the onset of this breakup mechanism as compared to previous experimental studies in which flow velocity was constant. This is important, for instance, for researchers that develop theoretical and numerical models of droplet-gas interaction for aircraft simulation purposes.

Water droplet deformation in gas stream: Impact of temperature difference between liquid and gas

Experimental investigations were conducted to study the macroscopic behaviors of water droplets (diameter from 3mm to 6mm and velocities from 0.5m/s to 5m/s) deformed during falling down in a gas stream. Various temperatures of gases (275–1100K) and water droplets (275–360K) were considered. The investigations were conducted by using optical cross-correlation technics to measure the displacement velocity of droplets and the liquid convection as a function of time. High-speed (105frames per second) video camera allowed to record the shape deformations. Two deformation modes were identified. The relationships between the shapes of droplets, their velocities and sizes were established. The impact of the temperature difference across the liquid–gas interface on the deformations characteristics was accurately analyzed. The main deformation properties (amplitude, length and period of one shape oscillation cycle) were evaluated at different temperatures of the surrounding gas and of the droplets; the velocities and sizes of droplets were measured and correlated with the shapes of droplets and their periods of oscillation.

Experimental determination of times, amplitudes, and lengths of cycles of water droplet deformation in air

The regularities of deformation of water droplets moving in air at the moderate Weber numbers (We < 7) are experimentally investigated with the use of high-speed cross-correlation recording equipment at frame delays of less than 100 ns. Deformation cycles with characteristic times, amplitude, lengths, and droplet forms are established. The effect of the droplet velocity on the main characteristics of the deformation cycles is determined.

Water droplet deformation under the motion in gas area with subsonic velocities

The experimental investigation of water droplet deformation (characteristic dimensions 3 ÷ 6 mm) when moving through the gas area (air) with 1÷ 5 m/s velocities was carried out. The high-speed (delay time between the frames is less than 100 ns) tools of cross-correlation videorecording were used. A cyclic change nature of the droplet shapes (from close to spherical to conditionally ellipsoidal) was found. The characteristic times of “deformation cycles” and the maximal deviations of droplet dimensions relative to initial dimensions were determined. The influence of droplet velocities and its dimensions on the deformation characteristics was determined.

Experimental investigation of mixtures and foreign inclusions in water droplets influence on integral characteristics of their evaporation during motion through high-temperature gas area

Experimental investigation of influence of mixtures and foreign inclusions in water droplets on integral characteristics of their evaporation during the motion through high-temperature (more than 1000 K) gas area has been held. Investigations have been conducted with the usage of two-phase and heterogeneous mixtures diagnostics optical methods “Particle Image Velocimetry” and “Interferometric Particle Imaging”. It has been established that salt admixtures injection with sufficiently large relative concentration (γ = 0.1) influences moderately on droplet evaporation characteristics (evaporation velocities are decreased by 5–7%) when water droplet sizes are less than 0.5 mm. Salt admixtures influence is intensified (evaporation mass velocities of water droplets with admixtures and without of them differ by 11–18% at droplet sizes 1–3 mm) with droplet size increase. It has been indicated that foreign solid particle injection in droplets leads to considerable (by several times) intensification of droplet heat-up and evaporation processes. The thermophysical characteristics of these foreign solid particles significantly differ from water. Conditions of significant deformation and breakage of water droplets with solid inclusions have been indicated at intensive evaporation.

The Distinguishing Quality of Water Droplets on Insulating Surface under AC and DC Electric Field Stress

This paper presents the comparison of water droplets on insulating surface under alternating current (AC) and direct current (DC) electric field. Besides that, it is demonstrated about the insulator deterioration under both electric field stressed due to an ageing and partial discharge (PD) phenomenon. The vital parameters factors are water droplets conductivity, droplet volume, surface roughness and droplet positioning that they cause to occur the electric field intensification. The field is intensified at the interface between the droplet, air and insulating material. Thus, the PD occurred due to electric field intensification increases with the deformed droplet. The deformation of water droplet under AC electric field stress is more intense than DC field. The electrostatic forces change the droplet shapes and spread them along the electric field direction. The local electric field intensification provokes the PD giving way to reduction of hydrophobicity of insulator surfaces. In addition, the PD activity could appear as a trigger for a surface breakdown. And the localized arcs cause damage to insulating material then finally leads to deterioration of insulation materials and the pollutant contamination.

Periodic Deformation of Microsize Droplets in a Microchannel Induced by a Transverse Alternating Electric Field

When a water droplet is suspended in an immiscible, insulating liquid medium of essentially infinite dimensions and is subjected to a periodically alternating electric field, it will undergo periodic deformation at a frequency twice that of the field. This work examined the periodic deformation and bursting of microsize water droplets in silicone oil moving through a PTFE microchannel with an inner diameter nearly twice that of the droplets, under a periodically alternating electric field normal to the direction of flow. When the sinusoidal electric field was either relatively weak or had a low frequency, the droplets displayed periodic deformation alternating between a prolate ellipsoidal along the electric field direction and a sphere. In contrast, droplets alternated between prolate and oblate shapes when the electric field strength was high or the frequency of the electric field was high. When the applied electric field strength was increased, a constriction formed in the central portion of droplets which had deformed into a prolate ellipsoid. When the electric field strength was further increased, droplets broke into two or more smaller droplets as the result of the inertial force acting on the water in the droplet and the electrostatic force at the interface of the droplet and the surrounding liquid. The majority of the small droplets formed by such breakups subsequently merged into single droplets over the course of time.

An experimental investigation on hydrodynamics of charged water droplets in dielectric liquid medium in the presence of electric field

An experimental study was carried out to investigate the motion, deformation and breakup of charged water droplets in the presence of electric field. A typical nozzle-ring electrode apparatus was conducted consisting of a nozzle employed as high voltage electrode and a metal ring as grounded electrode. Experiments were performed in silicone 100cSt oil as an insulating medium. Under influence of D.C. electric field the motion and deformation of conducting water droplet and its detachment from the tip of nozzle were experimentally studied. The droplet behavior was recorded by high speed camera precisely and its responses to deformation influenced by electric field were digitally extracted by image processing of the sequential frames. Three parameters were defined in order to characterize the detachment of drop from nozzle and its deformation result from electric and hydrodynamic forces. Results show that increase in applied voltage leads to an increase in frequency of drop formation and subsequently droplet size declining. Moreover the motion of produced drops occurs in particular branches for voltages more than 3.5kV. The number of these branches increases with applied voltage. The maximum amount of deformation increases in higher voltages leading to drop break-up in 4.5 and 5.5kV applied voltages. The results can be helpful for better understanding of the phenomena taking place during droplet detachment in dripping mode and reveal the drop behavior (e.g. deformation) in electrospraying apparatus geometry which has not been studied before.