Droplet Oscillation Research Articles

Droplet impact on inclined substrates under a non-uniform electric field

Droplet impact on inclined substrates under electric fields is a common behavior in electrostatic demisting applications, and understanding the droplet dynamics of this process is important for improving the performance of demisters. This study investigated the droplet impact dynamics on inclined substrates within a non-uniform electric field. Using high-speed imaging, the effects of voltage (U), substrate inclination (θ), and impact velocity (v) on the droplet behaviors were analyzed. The results revealed that at higher voltages, an upward ejection or pinch-off from the liquid column occurred during the recoiling stage, while the maximum dimensionless spreading diameter D*max increased with the voltage up to U ≤ 12 kV, then sharply decreased due to the droplet ejection for U > 12 kV. It was found that the electric field also intensified the droplet oscillation, with the maximum recoiling height H*max positively correlated with U. The secondary droplet ejection volume fraction η increased with the increase in U, decreased with the increase in both θ and D0, and peaked at v = 0.77 m/s. Furthermore, a critical threshold for the ejection or pinch-off and a predictive model for D*max were developed, incorporating electric Bond number (BoE), Weber number (We), and θ. Based on a profound comprehension of the electrohydrodynamic mechanisms governing the droplet impact on inclined substrates, these findings provide appropriate operating conditions to avoid the droplet pinch-off and ejection, improving the efficiency of electrostatic demisters.

Size control and oscillations of active droplets in synthetic cells

Oscillations in the formation and dissolution of molecular assemblies inside living cells are pivotal in orchestrating various cellular functions and processes. However, designing such rhythmic patterns in synthetic cells remains a challenge. Here, we demonstrate the spontaneous emergence of spatio-temporal oscillations in the number of droplets, size, and their spatial distribution within a synthetic cell. The coacervate-based droplets in these synthetic cells sediment and fuse at the cell’s bottom. Through a size control mechanism, the sedimented, large droplets shrink by expelling droplet material. The expelled molecules nucleate new droplets at the top of the synthetic cell, which grow and sediment again. These oscillations are sustained by converting chemical fuel into waste and can continue for hundreds of periods without evidence of fatigue. Strikingly, the period of the oscillation is in the minute’s regime and tunable. The design of oscillating artificial organelles in synthetic cells brings us closer to creating more life-like materials and de novo life.

Crystal nucleation and growth kinetics of acoustically levitated liquid SCN-DC transparent alloys

As an important and promising experimental method for simulating the containerless state in outer space, acoustic levitation provides excellent contact-free condition to investigate solidification process. Meanwhile, the radiation pressure and acoustic streaming caused by nonlinear effects bring various kinds of novel phenomena to crystallization kinetics. In this work, high-speed CCD, low-speed camera and infrared thermal imager were used simultaneously to observe the crystallization process of acoustically levitated SCN-DC transparent alloys. The undercooling ability and solidification process of alloy droplets with different aspect ratios were explored under acoustic levitation state. For hypoeutectic SCN-10wt%DC, eutectic SCN-23.6wt%DC and hypereutectic SCN-40wt%DC alloys, the experimental maximum undercoolings reached 22.5(0.07<i>T</i><sub>L</sub>), 16(0.05<i>T</i><sub>E</sub>) and 32.5K(0.1<i>T</i><sub>L</sub>) respectively and the corresponding crystal growth velocities were 27.91, 0.21 and 0.45 mm/s. In SCN-10wt%DC hypoeutectic alloy, the nucleation mode of SCN dendrite changed from edge nucleation to random nucleation with the increase of undercooling. For SCN-23.6wt%DC eutectic alloy, when undercooling exceeded 12.6K, DC dendrite preferentially nucleated and grew, and then the (SCN+DC) eutectic grew attached to DC dendrite. Moreover, the growth interface of DC dendrite gradually changed from sharp to smooth within SCN-40wt%DC hypereutectic alloy as the undercooling degree rose. The undercooling distribution curve and nucleation probability variation trend were analyzed versus aspect ratio. It was found that as the aspect ratio increased, undercooling of alloy droplet increased firstly, then decreased, and finally remained almost unchanged. Further analysis showed that with the increase of aspect ratio, the cooling rate would rise and thus enhanced the undercooling. However, the increase in surface nucleation rate and the droplet oscillation inhibited deep undercooling of alloy droplet. Therefore, the coupled effects of cooling rate, surface nucleation rate and droplet oscillation determined the undercooling of the alloy. In the case of SCN-40wt%DC hypereutectic alloy, the acoustic streaming and surface oscillation arising from acoustic field were the principal factors intensifying surface nucleation.

Oscillations of a water droplet on a horizontally vibrating substrate

Oscillations of a water droplet on a horizontally vibrating substrate

Exploring droplet oscillation dynamics in surface tension measurements

This study builds upon prior research by exploring droplet oscillation dynamics for surface tension determination using a drop-on-demand high-temperature droplet generator. Computational fluid dynamics (CFD) simulations were conducted to analyse frequency shifts over time, comparing two different materials with consistent results. The findings suggest potential for developing correction factors for oscillations with larger initial deformations. Additionally, frequency shifts relative to evolving aspect ratios of droplets starting with higher initial deformations were compared. Corrective measures can be applied, particularly beneficial for short-term measurements based on image analysis with minimal overall frequency shift. Slight asymmetry in oscillation with increasing aspect ratio could be accredited to droplet cross-sectional geometry or energy availability for returning prolate droplets to a spherical state. Experimental results indicated minimal frequency shift within a measurement duration of up to 40 ms, affirming the adequacy of using a fitted sine function without a time-dependent frequency term for overall frequency determination. A dimensionless criterion can be used to filter out unsuitable droplets. A temperature-dependent surface tension trend for AlCu10 alloy consistent with literature findings is introduced.

Visualization and quantitative study on film boiling heat transfer of a salt droplet on the hydrophobic substrate

Visualization and quantitative study on film boiling heat transfer of a salt droplet on the hydrophobic substrate

Suppression strategy for metal droplet overlapping fusion defects caused by droplet impact dynamics under coaxial shielding gas

Suppression strategy for metal droplet overlapping fusion defects caused by droplet impact dynamics under coaxial shielding gas

Probing the contact time of droplet impacts: From the Hertz collision to oscillation regimes.

Droplet rebound on nonwetting surfaces is a common phenomenon. However, the underlying physics regulating the contact time remains unclear. In this work, we investigate droplet impacts on superamphiphobic surfaces through experiments and theoretical analyses. By analyzing the spreading and retraction of droplet impinging processes over a wide range of Weber numbers (We), it is revealed that droplet impacts experience three regimes as We is varied, which are denoted as the Hertz collision (We<1), transition (1<We<10), and oscillation (We>10) regimes. In the Hertz collision regime, the droplet impinging process is temporally symmetric, i.e., the spreading time, t_{S}, and the retraction time, t_{R}, are almost the same. Furthermore, t_{S} and t_{R} decrease with increasing We and follow a power-law dependence, which is different from previous theories. In the transition regime, t_{S} remains dominated by the Hertz collision, while t_{R} is governed by droplet oscillation. In the oscillation regime, t_{S}, t_{R}, and the total contact time, t_{C}, become independent of We. These three regimes are valid for both monophase and compound droplets. The findings in this work advance the understanding and offer a clear picture of droplet impact dynamics.

Experimental investigation of contact time of bouncing droplet on vibrating substrates

The observation of an elastic substrate self-driving droplet to produce a “springboard effect” provides new enlightenment to the application of elastic materials in the anti-icing area. The droplet–substrate dynamic of a water drop impacting a superhydrophobic elastic substrate is experimentally investigated at different Weber (We) numbers and beam stiffness. For water drop, the spreading dynamic is not affected by the We number and beam stiffness since the inertial action is dominant, and the elastic action of the beam is relatively small, while the receding dynamic is closely related to the parameters. For elastic substrate, the vibrating deflection increases with the increase in the We number and reduction of the stiffness, while the vibrating frequency is only dependent on its stiffness. Based on this, the rebound dynamic of the droplet is discovered dependent on the scale relationship between the droplet and substrate oscillation period. Finally, a relation of the contact time of a droplet impacting elastic substrates, which is verified to hold for a large range of We numbers, beam stiffness, and droplet sizes, is established. The discoveries may contribute to the design of a droplet–elastic substrate system to achieve desirable contact time, providing a theoretical basis to forecast the performance of droplet–substrate systems by employing elastic materials.

Resonant oscillation of droplets under an alternating electric field to enhance solute diffusion

This study investigates a novel microfluidic mixing technique that uses the resonant oscillation of coalescent droplets. During the vertical contact-separation process, solutes are initially separated as a result of the combined effects of diffusion and gravity. We show that the application of alternating current (AC) voltage to microelectrodes below the droplets causes a resonant oscillation, which enhances the even distribution of the solute. The difference in concentration between the top and bottom droplets exhibits frequency dependence and indicates the existence of a particular AC frequency that results in a homogeneous concentration. This frequency corresponds to the resonance frequency of the droplet oscillation that is determined using particle tracking velocimetry. To understand the mixing process, a phenomenological model based on the equilibrium between surface tension, viscosity, and electrostatic force was developed. This model accurately predicted the resonance frequency of droplet flow and was consistent with the experimental results. These results suggest that the resonant oscillation of droplets driven by AC voltage significantly enhances the diffusion of solutes, which is an effective approach to microfluid mixing.

An improved smoothed particle hydrodynamics method for modeling multiphase flows

Multiphase flows are prevalent in both natural and engineered systems. The study of multiphase flow problems using numerical simulation is challenging due to the presence of high nonlinearities and moving interfaces. In this paper, an improved multiphase smoothed particle hydrodynamics (SPH) model is proposed for simulating multiphase flows. In the improved multiphase SPH model, an improved interface repulsive force model is proposed to reduce the interpenetration of particles at the multiphase interface and make the multiphase interface smooth and clear, and an improved kernel gradient correction is introduced to optimize the computational results. In addition, the particle shifting technology is applied to make the particle distribution uniform. Five numerical examples including the Rayleigh–Taylor instability, non-Boussinesq lock-exchange problem, square droplet deformation, single bubble rise, and circular droplet oscillation are investigated to verify the correctness and effectiveness of the improved multiphase SPH model. The results demonstrate that the improved multiphase SPH approach is effective in modeling multiphase flows.

Improving polymer electrolyte membrane fuel cell performance and preventing flooding by exciting gas flow

Improving polymer electrolyte membrane fuel cell performance and preventing flooding by exciting gas flow

The coalescence and oscillation of eutectic gallium indium alloy droplets

The coalescence and oscillation of eutectic gallium indium alloy droplets

Measuring the density, viscosity, and surface tension of molten titanates using electrostatic levitation in microgravity

Rare earth and barium titanates are useful as ferroelectric, dielectric, and optical materials. Measurements of their thermophysical properties in the liquid state can help guide melt processing technologies for their manufacture and advance understanding of fragile liquids' behavior and glass formation. Here, we report the density, thermal expansion, viscosity, and surface tension of molten BaTi2O5, BaTi4O9, and 83TiO2-17RE2O3 (RE = La or Nd). Measurements were made using electrostatic levitation and droplet oscillation techniques in microgravity, which provide access to quiescent liquid droplets and deep supercooling of 510–815 K below the equilibrium melting points. Densities were measured over 900–2400 K. Viscosities were similar for all four compositions, increasing from ∼10 mPa s near 2100 K to ∼30 mPa s near 1750 K. Surface tensions were 450–490 dyn cm−1 for the rare earth titanates and 383–395 dyn cm−1 for the barium titanates; surface tensions of all compositions had small or negligible temperature dependence over 1700–2200 K. For solids recovered after melt quenching, x-ray microtomography revealed the fracture mechanics in crystalline products and minimal internal porosity in glass products, likely arising from entrapped gas bubbles. Internal microstructures were generally similar for products processed either in microgravity or in a terrestrial aerodynamic levitator.

Rebound of oscillating droplets on non-superhydrophobic surfaces

Rebound of oscillating droplets on non-superhydrophobic surfaces

Formation mechanism and elimination method of joint defects in lattice structures printed by metal droplet deposition manufacturing

Formation mechanism and elimination method of joint defects in lattice structures printed by metal droplet deposition manufacturing

Marangoni Convection Velocity in Nonlinear Hanging-Droplet Vibration Phenomena

The Marangoni effect involves a mass transfer along an interface between two phases owing to the gradient of the interfacial tension. The flow caused by this phenomenon is called Marangoni convection, a complex phenomenon that involves mass transfer processes, such as surfactant adsorption/desorption processes, solvent dissolution phenomena, and viscous dissipation processes. Therefore, the strength of the convection depends on the various thermodynamic and physical properties of the fluids. In this study, we experimentally investigated the relationship between the Marangoni convection generated inside a hanging oil droplet and the interfacial tension of the oil droplet in an aqueous phase by the particle image velocimetry method. This convection velocity depended on the initial value of the interfacial tension in the oil–water interfacial tension oscillation phenomenon accompanied by the expansion and contraction of the hanging drop. Additionally, the droplet oscillation frequency decreased as the Marangoni convection velocity increased. Furthermore, continuous convection, which is unlike Marangoni convection, was observed within this spontaneously expanding and contracting hanging-droplet system. This buoyant convection was caused by the mutual dissolution of the hanging-droplet oil phase and the surrounding aqueous phase.

Generation and Suppression of Pendant Droplet Oscillation in Electron Beam Directed Energy Deposition

Generation and Suppression of Pendant Droplet Oscillation in Electron Beam Directed Energy Deposition

Investigating the dynamic characteristics of oil droplets spreading on solid surfaces in water

The use of thermoplastic-reinforced pipes for fluid recovery is becoming increasingly common because the oil field enters the high-water-cut stage in this process. Investigating frictional drag in case of oil-water flow in nonmetallic gathering pipelines is important because the dynamics of oil droplets on the inner surfaces of the pipes have a significantly influence on the drag. In this article, we experimentally and theoretically investigate the dynamic characteristics of the spreading of oil droplets with varying diameters from 600 to 900 μm and viscosities on surfaces with different degrees of wettability in water. The durations of oscillation and spreading of oil droplets on the surfaces submerged in water were found to be dependent on their diameter and viscosity. Because the prevalent equations used for calculations in this context cannot adequately describe the durations of oscillation and spreading of the oil droplets on surfaces in water, we corrected them based on experimental data. Finally, we examined the dimensionless maximum diameter of spreading of the oil droplets on the surfaces in water, and found that it was influenced by the diameter of the oil droplets and the wettability of the surface. We developed a theoretical model to represent the behavior of the oil droplets as they spread in order to predict the dimensionless maximum diameter of spreading, and verified it by using experimental data. And the results showed that the deviations between the experimental results and those of the model were generally within ±8%.

Computation of aluminum droplet ejection and flight in microgravity

Computation of aluminum droplet ejection and flight in microgravity