Deformation Of Liquid Droplets Research Articles

An analytical investigation of liquid droplet deformation in a superposed electric field consisting of an alternating and a constant electric field

An analytical investigation of liquid droplet deformation in a superposed electric field consisting of an alternating and a constant electric field

Surfactant and dilatational viscosity effects on the deformation of liquid droplets in an electric field

HypothesisA conducting droplet suspended in an insulating continuous phase, e.g. an aqueous electrolyte in an oil, is deformed by an applied electric field to nonspherical equilibrium shapes, and can even break-up under strong fields. Many technologies use electro-deformation to manipulate fluid dispersions, with surfactants present on the droplet interfaces forming stabilizing monolayers. While surfactants lower the interface tension which facilitates electro-deformation, the monolayer elasticity resists deformation. High molecular weight surfactants, with large dilatational viscosities, can potentially retard the deformation dynamics. NumericsA boundary integral method simulates the dynamic interfacial deformation of a perfectly conducting droplet in a dielectric in a uniform field. The interface contains an insoluble monolayer which is a Newtonian fluid with constant dilatational viscosity obeying a Langmuir state equation. A range of initial surfactant surface concentrations are studied, with elasticity proportional to concentration. FindingsEquilibrium drop deformations, unaffected by surface viscosity, are strongly resisted by elasticity at high surface concentrations, and field strengths necessary for break-up increase with elasticity. Dilatational viscosity scales with the ratio, κ∗, the surface viscosity (divided by the droplet radius) to the bulk viscosity, and can extend the deformation time. Extended times are described by a time rescaling proportional to κ∗.

Initiation of the Worthington jet on the droplet impact

The deformation of liquid droplets upon impact induces Worthington jets for a certain range of impact velocities. Although the growth of such a jet and its tip velocity are predicted from cases similar to droplet impact, the mechanism behind jet formation is yet to be understood. The present study uses high-speed visualization of droplet impact on a superhydrophobic surface to understand jet initiation in terms of the collapse of an air cavity. Water droplets with diameters of 2.0 and 3.0 mm are generated with the droplet Weber number varying from 2 to 20. The jet velocity is measured from the captured images, from which the maximum velocity is found to be We ∼ 7. The jet velocity at We ∼ 7 is approximately 15 times greater than the impact velocity. Moreover, surface waves are generated upon impact with the solid surface, and they induce an oscillation of the droplet cap as they propagate from the solid–liquid contact line to the top portion of the droplet. Furthermore, we find that the phase of the oscillation is related to the Weber number and greatly influences the jet velocity because it determines the initial conditions for jet generation.

Electrowetting-on-dielectric actuation of a vertical translation and angular manipulation stage

Adhesion and friction during physical contact of solid components in microelectromechanical systems (MEMS) often lead to device failure. Translational stages that are fabricated with traditional silicon MEMS typically face these tribological concerns. This work addresses these concerns by developing a MEMS vertical translation, or focusing, stage that uses electrowetting-on-dielectric (EWOD) as the actuating mechanism. EWOD has the potential to eliminate solid-solid contact by actuating through deformation of liquid droplets placed between the stage and base to achieve stage displacement. Our EWOD stage is capable of linear spatial manipulation with resolution of 10 μm over a maximum range of 130 μm and angular deflection of approximately ±1°, comparable to piezoelectric actuators. We also developed a model that suggests a higher intrinsic contact angle on the EWOD surface can further improve the translational range, which was validated experimentally by comparing different surface coatings. The capability to operate the stage without solid-solid contact offers potential improvements for applications in micro-optics, actuators, and other MEMS devices.

Liquid droplet behavior and instability in a polymer electrolyte fuel cell flow channel

A combined theoretical and experimental study of the influence of controllable engineering parameters, including surface PTFE (Teflon™) coverage (ranging from 5 to 20 wt.%), channel geometry, droplet chord length and height, and operational air flow rate, on liquid droplet deformation at the interface of the diffusion media (DM) and the gas flow channel was performed. The theoretical model reasonably predicts the measured contact angle hysteresis and identifies conditions under which the droplet tends toward an unstable state. The results presented in this study indicate that operational conditions, droplet height, chord length, channel size and level of surface hydrophobicity of the DM directly affect the droplet instability. Based on the analytical force balance model, the critical Reynolds number at which a droplet of given dimensions tends towards instability (i.e. may be removed from the channel) is predicted. Experimental data in both the stable and unstable regions as defined by the critical Reynolds number curve generally corresponds to the stability predictions. Empirical correlations relating surface tension and DM PTFE content were developed. At high flow rates, the surface hydrophobicity of the DM surface enhances efficacy of droplet removal, and may help to prevent local channel flooding. However, at low flow rates, hydrophobicity of the DM surface has only a minimal impact on efficacy of droplet removal and therefore high PTFE content may not be necessary, or desirable, since it also increases the electrical resistance of the system. It is also suggested that for constant air velocity and droplet size and shape, reduced channel height is preferred for effective water droplet removal where practical.

Effect of Liquid Droplet Deformation on the Displacement Due to Airstreams.

The author describes the effect of deformation of a liquid droplet on its displacement due to airstreams. Experiments are conducted using a 60mm×150mm crosssectional and 500-mm-long horizontal air-suction wind tunnel. Uniformly sized liquid droplets produced by the longitudinal vibration of a nozzle are used. The liquid droplets exposed to airstreams are deformed;that is, their size normal to the airstream direction is changed. The projection area of a droplet in relation to airstream direction is expected to greatly influence the displacement of a droplet and the drag;therfore, the longitudinal droplet diameter is measured precisely. The author formulated an equation that quantifies dimensionless droplet displacement with dimensionless exposure time. The equation can be used to predict displacement of droplets with slight deformation. Therefore, the appropriate deformation rate of the droplet applicable to the equation is investigated in detail. The relationship between Weber number and maximum / minimum dimensionless longitudinal droplet diameter is also surveyed.

Deformation of liquid droplets during collisions with hot walls: Experimental and Numerical Results

AbstractMeasurements of droplet deformation during wall impingement were performed for ethanol droplets and water droplets with diameters ranging from 100 to 200 μm. The wall temperature is well above the Leidenfrost temperature of the droplet liquid. With monodisperse droplet streams and a special illumination technique, slow motion images of the phenomena can be obtained. Measurements with high temporal resolution below 1 μs are possible using a standard video camera. The experimental results are compared with numerical results, which were obtained by solving the three‐dimensional Navier‐Stokes equations for incompressible fluids including surface tension effects. The fluids are treated with the volume‐of‐fluid method and the free surface is modeled according to the continuum‐surface‐force model. Numerical and experimental results show good agreement.

Atomization of Liquid Droplets Due to Air Streams. 1st Report. Displacement of a Liquid Droplet.

The deformation, breakup, and displacement of liquid droplets due to air streams are described. Experiments are conducted using a horizontal air-suction wind tunnel. Uniform liquid droplets produced by a longitudinal vibration of a nozzle are used. Shattering processes of liquid droplets are observed using a stroboscope. The deformation of a liquid droplet varies with airstream velocity. The typical critical breakup pattern of a droplet in this experiment is a bag breakup. Time variation of droplet displacement are investigated in detail. Under a constant airstream velocity, the displacement of a droplet increases as its diameter decreases. When deformation of a droplet is increased to some level, the droplet is moved irregularly, The relationship between dimensionless time and dimensionless displacement is presented for droplets with small deformation.

Analysis of liquid droplet deformation in a gas flow

The small perturbation method is used to obtain equations describing the dynamics of a liquid droplet in a flow of ideal incompressible gas. The stability criteria and droplet disintegration time are determined.

Observation of Liquid Droplet Deformation due to Liquid Jet Impact as Possible Mechanism of Molten Fuel Coolant Interaction

Observation of Liquid Droplet Deformation due to Liquid Jet Impact as Possible Mechanism of Molten Fuel Coolant Interaction