Millimeter-sized Droplets Research Articles

Morphology of bubble dynamics and sound in heated oil

The interaction between a heated oil bath and water droplets commonly occurs in the kitchen and has important implications for cooking, fire safety, and indoor air pollution. The interplay between the bubble dynamics in a heated oil bath, the generated sound, and the ligament-like expulsion to the surrounding air is examined. We focus on an explosion of a millimeter-sized water droplet in heated oil as a simplified case. We discuss three typical bubble types that can be classified as a function of the stand-off parameter h/R, where h is the distance between the oil surface and bubble and R is the maximum bubble radius. Our data describe the morphology of bubble dynamics inside a heated oil bath and represent those found in the cooking pan. This paper also highlights potential applications of our findings.

Retraction dynamics of low-viscosity nanodroplets: From hydrophobic to hydrophilic surfaces

Because of scale effects, the impact of nanodroplets shows different spreading and retraction dynamics from millimeter-sized droplets, which remains poorly understood. This work investigates the retraction dynamics of low-viscosity water nanodroplets on hydrophobic to hydrophilic surfaces via molecular dynamics (MD) simulations. Two retraction regimes, inertial and capillary regimes, are defined based on whether an obvious rim can be observed in retraction processes. The two retraction regimes successively take place on hydrophobic surfaces, whereas only the capillary regime is observed on hydrophilic surfaces. In the inertial regime, the inertial force in the rim and the viscous forces near the rim entrance and the central film both dominate the retraction. The former makes the retraction velocity scale as We−1/2, whereas the latter causes the scaling of Oh−1/3. Moreover, the spreading factor in the inertial regime decreases from its maximum value to equilibrium value in the same interval of time. Based on these dynamics, an expression for retraction velocity in the inertial regime is proposed. In the capillary regime, the retraction slows down because inertial force weakens significantly. The retraction velocity depends only on the impact velocity and surface wettability. The proposed two expressions of retraction velocities are validated by MD simulations for the impact of nanodroplets on hydrophobic to hydrophilic surfaces at various impact velocities, showing good agreement between them. Furthermore, using the present retraction dynamics, a model of contact time for nanodroplets impacting hydrophobic surfaces is also developed.

Oil Droplet Coalescence in W/O/W Double Emulsions Examined in Models from Micrometer- to Millimeter-Sized Droplets

Water-in-oil-in-water (W1/O/W2) double emulsions must resist W1–W1, O–O and W1–W2 coalescence to be suitable for applications. This work isolates the stability of the oil droplets in a double emulsion, focusing on the impact of the concentration of the hydrophilic surfactant. The stability against coalescence was measured on droplets ranging in size from millimeters to micrometers, evaluating three different measurement methods. The time between the contact and coalescence of millimeter-sized droplets at a planar interface was compared to the number of coalescence events in a microfluidic emulsion and to the change in the droplet size distributions of micrometer-sized single and double emulsions. For the examined formulations, the same stability trends were found in all three droplet sizes. When the concentration of the hydrophilic surfactant is reduced drastically, lipophilic surfactants can help to increase the oil droplets’ stability against coalescence. This article also provides recommendations as to which purpose each of the model experiments is suited and discusses advantages and limitations compared to previous research carried out directly on double emulsions.

The sliding mode and dissipative force of moving nanodroplets on smooth and striped hydrophobic surfaces

The motion of nanodroplets on hydrophobic surfaces is ubiquitous in a broad range of applications such as nanofluidic devices. The dissipative force and sliding mode are two vital factors for the utilization of moving nanodroplets. However, the kinetics of moving nanodroplets is different from millimeter-sized droplets due to their extremely small size, and the dissipative force and sliding mode of nanodroplets have not been clearly understood. In this work, the dissipative force and sliding mode of moving nanodroplets on smooth and striped hydrophobic surfaces are investigated via molecular dynamics simulation. We find that the dissipative force has a more significant effect on nanodroplets than millimeter-sized droplets, and the nanodroplets show a combination of rolling and slipping motion on the hydrophobic surfaces. We report that the dissipative force of moving nanodroplets can be divided into the viscous force and the hysteresis force, and the viscous force is the major resistance to the motion of nanodroplets. We demonstrate that the sliding mode of nanodroplets is jointly determined by the significant slippage and the enhanced viscous effect, and the large shear rate of nanodroplets can promote slipping by increasing the surface slip length. Furthermore, the effects of striped structure on the moving nanodroplets are studied. On the one hand, the striped structure increases the apparent contact angle of nanodroplets and thus reduces the dissipative force. On the other hand, the slip length of the striped surface can be lower than that of the smooth surface under the action of large Laplace pressure of nanodroplets, leading to the increment of the dissipative force and the enhancement of rolling.

Water sprays formed by impinging millimeter-sized droplets on superhydrophobic meshes

Impinging a millimeter-sized liquid droplet on a leaky substrate—such as a porous mesh—can cause the formation of many small droplets from the ligament fragmentation. Although this phenomenon has been widely considered as a desirable strategy to produce liquid sprays of monodisperse droplets, the underlying mechanism has not yet been completely elucidated, and the spray needs detailed characterization. Herein, we experimentally investigate the atomization phenomena occurring in the recoiling and spreading stages of impinging water droplets on superhydrophobic meshes. We show that the spray formed during droplet recoiling is stimulated by the longitudinally symmetric air cavity collapse on the superhydrophobic mesh, and thus the size of the spray formation area on the mesh is almost identical to the size of the simultaneously generated upward jet. By contrast, the water spray produced during droplet spreading is expelled under the action of the inertia-induced hydrodynamic pressure, and the size of spray formation area on the mesh exhibits a power-law dependence on the Weber number; yet, the pore geometry restricts it to take a constant value when the Weber number is sufficiently high. By performing statistical analyses on the spray droplet sizes, we further demonstrate that the mean sizes of spray droplets are mainly set by the mesh pore sizes, but the complex atomization dynamics leads to a broad size distribution, which is beyond the expectation.

Electrowetting-on-dielectric-induced nanodroplet splitting between two parallel plates

Manipulation of nano/microdroplets has recently received a great deal of attention because of its promising applications. In this study, electrowetting-on-dielectric (EWOD)-induced nanodroplet splitting between two parallel plates is investigated via molecular dynamics (MD) simulations. The results show that the splitting and non-splitting dynamics closely depend on the charge density applied to electrodes. There exists a critical charge density, above which successful splitting is observed. The plate gap and electrode size are found to significantly influence the critical charge density. Furthermore, MD simulations also demonstrate that the criterion of splitting for millimeter-sized droplets fails to predict nanodroplet splitting. On the basis of two simple geometric relations between the droplet and electrode sizes, a new criterion, 1/2 ≤ R0/l ≤ (3/π − 1/4)1/2, is proposed to estimate whether nanodroplets can be split, where R0 is the initial wetting radius of nanodroplets on the plates when no charge is applied to electrodes, and l is the electrode size. The proposed criterion is in good agreement with MD simulations.

Effect of Mach number on droplet aerobreakup in shear stripping regime

The present experimental study investigates the shear stripping breakup of single droplets in subsonic and supersonic gaseous flows. In contrast to most research that places emphasis on the Weber number (We), we focus on the individual effects exerted by flow Mach (M∞) and Reynolds numbers (Re). Millimeter-sized droplets made of either ethylene glycol or water are exposed to shock-induced flows. Shadowgraph and schlieren images of the breakup process are recorded by an ultra-high-speed camera. The experimental We is constrained at 1100, while M∞ is varied from 0.3 to 1.19 and Re from 2600 to 24,000. A systematic analysis of the experiment series reveals that the breakup pattern alters with M∞ although a constant We is maintained. The classical stripping behavior with fine mist shed from the peripheral sheet changes to rupture of multiple bags along the periphery at M∞ = 0.63, and further to stretching of ligament structures from the leeward surface at M∞ = 1.19. The corresponding breakup initiation is delayed and the resultant fragments are sized less uniformly and distributed over a narrower spread. In terms of the early-stage deformation, droplets experience less intense flattening and slower sheet growth at higher M∞. The change of Re introduces additional variations, but only to a minor extent.Graphical abstract

Mixed Lagrangian-Eulerian Simulation of Interaction between a Shockwave and a Cloud of Water Droplets

We present a numerical model suitable for simulation of shock or blast waves passing through a cloud of water droplets. The model takes into account the droplet breakup, radiation, and evaporation effects. The gas phase (a mixture of air and water vapor) is solved within an Eulerian framework with a set of compressible transport equations. The disperse phase (water droplets) is represented by a number of Lagrangian parcels of a specified size and mass distribution. The model has been verified with experimental data. The results show that for large (millimeter-sized) droplets with high Weber numbers, the breakup model is the most important part for accurate representation of the wave-droplet interaction phenomena, while for very fine droplets (1 $$\mu$$ m or less), the evaporation effects are the strongest in the shockwave mitigation process. The radiative heat flux increase due to high droplet emissivity is found to be possible for an intermediate size water droplets (about 10–20 $$\mu$$ m) in the case of continuous heat release.

Laser-induced Ejection of a Millimeter-sized Liquid Droplet from a Metal Surface by IR Laser Irradiation at Three Different Pulse Durations

Laser-induced Ejection of a Millimeter-sized Liquid Droplet from a Metal Surface by IR Laser Irradiation at Three Different Pulse Durations

Efficient water scavenging by cooling superhydrophobic surfaces to obtain jumping water droplets from air

Dew collection is significant in harvesting water and relieving water shortages in arid regions. However, current methods for collecting dew or steam are mainly focusing on the millimeter-sized droplets condensed on the superhydrophobic surfaces. Here, we present a concept for harvesting micro droplets that can spontaneously bounce on the cooling superhydrophobic aluminum surface with randomly micro-nano composite structures, which were fabricated by using a two-step surface structural process. Moreover, an integrated device has been developed, which consists of a triboelectric nanogenerator and the superhydrophobic aluminum sheet. We experimentally explained that the triboelectric nanogenerator, which provides an external electric field by converting wind energy to electric energy with DC voltage pulse peaks of about 60 V, can be utilized to enhance the collection capacity of the jumping water droplets.

Bubble formation in freezing droplets

Water droplet icing on solid surfaces has been intensively studied in recent years but still with many mechanisms unrevealed. Here, we report a bubble formation phenomenon that always occurs in freezing droplets, from millimeter-sized sessile droplets to microscale condensed droplets, during icing and condensation frosting. In the second stage of icing (the first stage is nucleation and recalescence), air dissolved in liquid water is separated out in the ice front, forming many isolated bubbles. Coupled with the droplet icing physics, a theoretical model was proposed to predict the bubble formation and elucidate its influencing factors. Due to the bubble formation, the final ice droplet is actually a porous medium, rather than a complete solid ice. These results bring new insight into icing physics, helping modify icing models and improve anti-icing and deicing techniques.

Microdrop impact on soft substrates at low Weber numbers

In this work, we experimentally investigate the impact of microdrops on diverse soft substrates at low Weber numbers. We show that the dynamic behaviors of the impinging microdrops are very different from that of macroscopic droplets reported in the literature. Whereas various impact phenomena have been identified for millimeter-sized water droplets in previous studies, we only observed regular deposition for droplets of a few tens of micrometers in the experiment. In contrast to the significant slowdown of the receding millimeter-sized droplets, no effects of substrate stiffness on the impact process have been identified at microscale. These different dynamics are explained by the different dynamic response of soft viscoelastic materials during droplet impact over different timescales. Finally, the analysis of the post-impact droplet oscillation with the theory for damped harmonic oscillators reveals that the damping coefficient is also size-dependent while the spring constant is not. A simple scaling argument was proposed to understand this finding.

Numerical investigation on impingement dynamics and freezing performance of micrometer-sized water droplet on dry flat surface in supercooled environment

In-flight icing usually occurs when supercooled droplets impact on the cold surface of an aircraft, which should be concerned for its adverse effects on aerodynamic performance. To fundamentally elucidate the detailed mechanism of aircraft icing, a mathematical model on the impingement dynamics and solidification of a supercooled water droplet was developed and further validated with previous experimental results. Considering the effects of surface tension, wall adhesion and contact line dynamics, the coupled volume of fluid and level-set method was used to track the air–water interface, with the solidification issue solved by the enthalpy-porosity method. The temporal evolutions of the water phase, flow velocity, temperature and heat flux distributions were tracked and analyzed after the phase transition of supercooled water occurred. High impact velocities and millimeter-sized droplets were considered in this study to make the results more applicable to in-flight icing. As concluded, the spreading ratios of the droplets mainly distribute in the range of 0.8 ± 0.1 corresponding to LWC = 1.0 g/m3. Besides, the transient heat transfer between the solid surface and droplets could be fitted by a logarithmic function after appropriate dimensionless processing, which was proportional to the 1.5th power of the liquid fraction. Contributions of this work could be an effort to understand the microphysical phenomenon in the aerodynamic icing process.

On the mechanism of the micro-droplet etching of silicon wafers for texturization

Polished silicon wafers were used for the present study to remove the effect from surface mechanical damages. It was shown that conventional HF-HNO3-H2O bath wet etching cannot generate any texture on the polished silicon, while the micro-droplet etching (MDE) imposed by vapor from the similar bath can do. By examining the attack of millimeter-sized single droplet of the acidic solution to polished silicon, it was found that preferential attack at edge of the droplet occurred. When large amounts of micro-droplets were condensed on polished silicon, such preferential attack will generate the roughness required by a texture. W-shaped micro-pits were found on the MDE etched polished silicon, confirming the edge-attack mechanism. It agrees with the general phenomenon of preferential corrosion of a solid at gas-liquid-solid triple phase boundary zone.

Introduction to Droplet-Based Millifluidic Chemistry Using a Macroscopic-Droplet Generator

This activity introduces droplet-based millifluidics and the usage of aqueous droplets as independent chemical microreactors for reaction-kinetics studies. Students build their own droplet generators from common macroscopic glassware and connecting parts to create trains of millimeter-sized droplets in which a redox reaction takes place. The activity allows students to work on reaction kinetics, hydrodynamics at low Reynolds numbers, and image analysis at the macroscopic scale.

Combustion of rocket-grade kerosene droplets loaded with graphene nanoplatelets—A search for reasons behind optimum mass loadings

Nanofluid-type fuels formulated by mixing minute quantities of graphene nanoplatelets (GNPs) with Isrosene, a refined form of kerosene utilized in semi-cryogenic engines by the Indian Space Research Organization (ISRO), were found to improve the convective heat transfer characteristics by 49% in a previous study. In this study, the combustion characteristics of such nanofluid-type fuels, such as initial mass burning rates, burning rate constants, and flame stand-off ratios, were determined for millimeter-sized droplets under ambient conditions. The impact of the specific surface area of the nanoplatelets and their relative mass loading in the nanofluid was determined. While the specific surface area did not have any impact on the combustion characteristics, the relative mass loading had a strong impact on the burning rate constants, with an increment of approximately 13.5% over Isrosene for a mass loading of 0.2%. There also existed an optimum value of mass loading, beyond which the burning rate constants. This anomalous behavior, which had earlier been conjectured to be due to various physico-chemical reasons, was attributed primarily to the gas phase conduction phenomenon, corroborated by the analysis of the temperature profiles in the gas phase, acquired using an infrared imaging camera.

Transient evolution of the heat transfer and the vapor film thickness at the drop impact in the regime of film boiling

When a drop impinges onto a wall heated above the Leidenfrost temperature, a very thin vapor film is formed at the interface between the liquid and the solid substrate. This vapor layer modifies the impact behavior of the drop and induces a significant decrease in heat transfer. A model is proposed for the growth of this vapor layer and its resistance to the heat transfer. The main assumptions are as follows: (i) a uniform but time varying thickness of the vapor film, (ii) a quasi-steady Poiseuille flow inside the vapor film, and (iii) a constant wall temperature. Heat energy and momentum balances are employed to obtain an ordinary differential equation describing the evolution of the vapor film thickness during the drop impact. For droplets injected at a temperature sufficiently lower than the saturation temperature, this equation predicts that the impact velocity has no influence on the thickness of the vapor film. This latter is solely governed by the local heat flux transferred to the liquid, which predominates over the heat flux used for liquid evaporation. An accurate description of the droplet heating is therefore required to complement this model. As an attempt, this description is based upon a one-dimensional analysis, which includes some effects due to the complex fluid flow inside the spreading droplet. Finally, the theoretical model is validated against experiments dealing with millimeter-sized ethanol droplets. Two optical measurement techniques, based on laser-induced fluorescence and infrared thermography, are combined to characterize the heat transfer as well as the thickness of the vapor film.

Laser-Induced Ejection of Millimeter-Sized Liquid Droplet from Metal Surface with a 1 Joule/Pulse ns-Laser

Laser-Induced Ejection of Millimeter-Sized Liquid Droplet from Metal Surface with a 1 Joule/Pulse ns-Laser

Determining the surface dilational rheology of surfactant and protein films with a droplet waveform generator

Understanding rheological properties of surfactant and protein films plays a crucial role in a variety of industrial and research areas, such as food processing, cosmetics, and pharmacology. To determine the surface dilational modulus using drop shape analysis, one needs to measure the dynamic surface tension in response to a sinusoidal oscillation of the surface area of the droplet. Despite many applications of drop shape analysis in studying interfacial rheology, oscillation of the droplet surface area is usually controlled in an indirect manner. Existing methods are only capable of controlling volume oscillations of the droplet rather than its surface area. We have developed an arbitrary waveform generator (AWG) to directly oscillate the surface area of a millimeter-sized droplet in a predefined sinusoidal waveform. Here, we demonstrated the capacity of this AWG, in conjunction with constrained drop surfactometry (CDS), in studying the surface dilational rheology of adsorbed surfactant and protein films. It is found that the surface dilational modulus determined for a dilute surfactant (C12DMPO) and two protein solutions (bovine serum albumin and β-casein) revealed their adsorption mechanisms. Our methods hold promise in studying the interfacial rheology of various thin-film materials, biomembranes, foams, and emulsions.

Nanometric Surface Oscillation Spectroscopy of Water-Poor Microemulsions

Selectively exchanging metal complexes between emulsified water-poor microemulsions and concentrated solutions of mixed electrolytes is the core technology for strategic metal recycling. Nanostructuration triggered by solutes present in the organic phase is understood, but little is known about fluctuations of the microemulsion-water interface. We use here a modified version of an optoelectric device initially designed for air bubbles, in order to evidence resonant electrically induced surface waves of an oily droplet suspended in an aqueous phase. Resonant waves of nanometer amplitude of a millimeter-sized microemulsion droplet containing a common ion-specific extractant diluted by dodecane and suspended in a solution of rare earth nitrate are evidenced for the first time with low excitation fields (5 V/cm). From variation of the surface wave spectrum with rare earth concentration, we evidence uptake of rare-earth ions at the interface and at higher concentration the formation of a thin "crust" of liquid crystal forming at unusually low concentration, indicative of a surface induced phase transition. The effect of the liquid crystal structure on the resonance spectrum is backed up by a model, which is used to estimate crust thickness.