Drop Detachment Research Articles

Analysis of the Flow Structure inside an Oscillating Sessile Drops in a Shear Flow Using Double-Helix 3D Particle Tracking Velocimetry and Adaptive Optics

The motion and particularly the detachment mechanism of sessile drops in an air-shear-flow is highly relevant for many technical applications. For instance, water drops in fuel cells can block the oxygen transport and thus lower the overall efficiency. Hence, a deeper understanding of the drop detachment mechanism could facilitate efficiency optimisations. Nonetheless, until now only simplified models exist for predicting the critical velocity that do not take into account the mutual interaction of the air and the liquid via the non-rigid interface. One reason for this is the lack of knowledge of the flow inside the oscillating drop. In this contribution, we present the first three-dimensional flow measurements in oscillating sessile drops in shear flow. Time-averaged flow measurements were conducted for different Reynolds numbers. Furthermore, the phase-averaged flow field was measured. The measurements were conducted with a novel 3D-PTV method, which offers scanless three-dimensional flow measurements with a single optical access for a measurement volume that covers about 90 % of the drop volume. Moreover, we present a novel technique for conducting flow measurements through fluctuating gas-liquid interfaces based on adaptive optics. We demonstrate that this technique enables the correction of a systematic error corresponding to the magnitude of the first drop eigenfrequency, which seems to play an important role in the detachment mechanism of drops. The results give new insights about the fluid mechanics of sessile drops and thus may help in the optimisation of drop removal in e. g. fuel cells.

Influence of surface and edge profile of a spinning disc on its atomization characteristics in direct drop mode

A spinning disc in direct drop regime is useful for contacting applications using one stream as drops. Here, we presenta systematic probe into the effect ofseveral edge and surface modification strategies on droplet characteristics. The characteristics of interest in this study are mean sizes of primary and secondary drops and their spread around respective mean values, and the evenness of circumferential distribution of drops. Exposure controlled static imaging is used to obtain blur free images in high resolution. Concentric grooving on surface offered stable film under limited flow condition, improves circumferential distribution, and significantly eliminates secondary drop formation. Serration at the disc edge controls the pitch of the drop detachment points, a feature naturally unique to only ligament mode. A new scale named maldistribution index (MI) is proposed to quantify circumferential distribution of released droplets.

Launching a Drop via Interplay of Buoyancy and Stick-Jump Dissolution.

According to Archimedes' principle, a submerged object with a density lower than that of aqueous acid solution is more buoyant than a smaller one. In this work, a remarkable phenomenon is reported wherein a dissolving drop on a substrate rises in the water only after it has diminished to a much smaller size, though the buoyancy is smaller. The drop consisting of a polymer solution reacts with the acid in the surrounding, yielding a water-soluble product. During drop dissolution, water-rich microdroplets form within the drop, merging with the external aqueous phase along the drop-substrate boundary. Two key elements determine the drop rise dynamics. The first is the stick-jump behavior during drop dissolution. The second is that buoyancy exerts a strong enough force on the drop at an Archimedean number greater than 1, while the stick-jump behavior is ongoing. The time of the drop rise is controlled by the initial size and the reaction rate of the drop. This novel mechanism for programmable drop rise may be beneficial for many future applications, such as microfluidics, microrobotics, and device engineering where the spontaneous drop detachment may be utilized to trigger a cascade of events in a densemedium.

Suppression of cross-infection in actual dental office using viscoelastic polyacrylate solution as a handpiece coolant

In the present work, aqueous solutions of NaPAA [poly (sodium acrylate)] or PAA [polyacrylic acid] are used as the coolants for a dental handpiece to evaluate their suppressive effect on the aerosolization and bacteria (Staphylococcus epidermidis) transmission in an actual dental environment. Both polymer solutions significantly suppressed the formation of aerosols (<50 μm) and droplets (50–100 μm). The suppression effect was stronger at higher concentrations. The 10 and 20 wt. % of viscous Newtonian solutions of low-molecular weight NaPAA were much less effective in disintegration suppression than the viscoelastic 1 and 2 wt. % PAA solutions. The latter was capable of complete suppression of disintegration, forming instead long liquid threads attached to the rotating bur and settling down underneath. The suppression efficiency of the 2 wt. % PAA solution stems from significant elastic forces in it which prevent drop detachment. In the case of water used as a coolant, the bacterial spread was observed through aerosol, droplets, and splatter. The bacterial spread by large splatters was inversely proportional to the distance from the rotating bur. The spread of aerosols significantly occurred in the direction that the handpiece was facing, and multiple airborne aerosols settled on the wall rather than on the floor. On the other hand, the viscoelastic aqueous 2 wt. % PAA solution suppressed bacterial spread, regardless of the distance or direction.

The detachment of a wall-bound pendant drop suspended in a sheared fluid and subjected to an external force field

The phase field approach is applied to numerically simulate the detachment of an isolated, wall-bound 2D pendant drop suspended in a fluid in a simple shear flow. The model has been previously employed to simulate several two-phase flow phenomena, assuming that the system consists of a regular, partially miscible mixture, with the drop and the continuous phase being in thermodynamic equilibrium with each other. In addition, it is assumed that the two phases are separated by an interfacial region having a non-zero characteristic thickness â, i.e., the interface is diffuse. In the creeping flow regime, the problem is described in terms of three non-dimensional numbers: the fluidity number Nα as the ratio between capillary and viscous fluxes, the Bond number NBo as the ratio between external and capillary forces, and the Peclet number NPe as a non-dimensional shear rate. We find that, at large fluidity numbers and for small droplets (i.e., for d̃drop=ddrop/â≤45), the onset of the drop detachment can be described in terms of a master curve, with the critical macroscopic Bond number NBo(M)=NBo·d̃drop2 decreasing monotonously with NPe·d̃drop1.5 for five drop sizes in the micrometer range.

Hovering spreading rebound on porous superhydrophobic surface with active air plastron for rapid drop detachment

Hovering spreading rebound on porous superhydrophobic surfaces with active air plastron achieves an unprecedentedly low contact time of ∼1.2 ms for impacting water drops, which is 68% smaller than that for pancake rebound.

Breakdown of dry aggregates by water drops after applications of poultry manure and spent mushroom wastes

Evaluation of soil properties that enable aggregates to resist breakdown from impacting water drops is useful in modeling soil erosion process and tillage of some tropical soils. The effect of applications of poultry manure and spent mushroom wastes on soil properties related to resistance of aggregates of a degraded ultisol to water drops detachment energy (D) in southern Nigeria was studied. The area was under continuous field trials for maize crop. The treatments were: control (PM0SM0), 5 and 10 t ha−1 poultry manure (PM5 and PM10, respectively), 5 and 10 t ha−1 spent mushroom wastes (SM5 and SM10, respectively), and 5 t ha−1 poultry manure plus 5 t ha−1 spent mushroom wastes (PM5 + SM5) applied in seven consecutive years. Experimental design was a randomized complete block design (RCBD) in five replications. Soil samples were analyzed for texture, organic matter, aggregate stability, water retention, bulk density, clay mineralogy and water drop test. Results revealed that seven-year applications of PM5, SM5, SM10, and PM5 + SM5 treatments had significant positive effects on soil structural and hydraulic properties than PM10. However, in comparison to PM0SM0, applications of 10 t ha−1 PM had significant positive effects on these properties. Saturated hydraulic conductivity was 46.6, 35.4, 32.6 and 25.5 cm h−1 in PM5 + SM5, SM10, SM5 and PM10 soils, respectively; compared to 9.6 cm h−1 in PM0SM0. A low aggregate ratio (AR) obtained for PM10 compared to higher values for SM10 and PM5 + SM5 suggest that PM10 in this case had a disaggregating effect on the soil aggregates. Sand content accounted for 81.2% of negative effect on water drop detachment energy (D), while clay content accounted for 86.8% of the positive correlation with D. Only water-stable aggregates (WSA) 0.5–0.25 mm had a significant positive correlation with D (r = 0.661, p < 0.05). Clay dispersion index (CDI) had a highly negative correlation with D (r = − 0.896, p < 0.01). Volumetric water content at FC (− 10 kPa) and PWP (− 1500 kPa) correlated positively with D (r = 0.618, P < 0.05 and r = 0.614, p < 0.05). There was a non-significant positive correlation of Muscovite and Berlinite with D. Significant positive correlations of calcite with D (r = 0.694, p < 0.05), and between mica and D (r = 0.731, p < 0.05), revealed their strong linkage with macro-aggregate stability. It was found that high rates of poultry manure alone could increased erodibility and decreased water retention at FC and PWP, while a combination of poultry manure and spent mushroom wastes could reduce erodibility. These properties could be used to make a quick inference on the resistance of soil aggregates to water drop impact.

Anode-metal drop formation and detachment mechanisms in liquid metal batteries

We study numerically localised short circuits in Li||Bi liquid metal batteries. In the prototype of a classical, three liquid-layer system, we assume a perceptible local deformation of the Li-salt interface. We find that there always exists a critical current at which a Li-droplet is cut off from this hump, and transferred to the Bi-phase. In a second case, we assume that the molten Li is contained in a metal foam, and that a small Li-droplet emerges below this foam due to insufficient wetting. This droplet is deformed by Lorentz forces, until eventually being pinched off. Here, the critical current is slightly lower than in the three-layer system, and both, a droplet transfer and complete short circuits are observed. Finally, we discuss the relevance of our simulations for experimentally observed short circuits and non-faradaic Li-transfer.

A study of transition from periodic dripping to jetting in the presence of an electric field

A liquid is issued from the outlet of the capillary tube vertically, and drop formation may be in the form of periodic dripping, dripping faucet and jetting, which usually depends on flow rate or Weber number. We consider the critical Weber number (WeC=ρV02D/σ) at which the transition from periodic dripping to jetting occurs in the presence of an electric field, where high-resolution images are recorded. The necking time is mainly related to nozzle diameter and surface tension, and independent of electric potential and flow rate (in a reasonable range).The first threshold limit decreases with an increase in electric potential, where the dripping faucet is clearly affected and the point of drop detachment gradually increases. When the drop formation is in the form of dripping faucet, the transition from dripping faucet to jetting regime is enhanced by electric potential, where a sudden transition could be observed in an extremely limited range of liquid flow rate. The critical criterion (Weber number) is presented to determine the transition from dripping to jetting in the presence of an electric field. The critical Weber number is in a reasonable agreement with the experiment, although the predicted results are quantitatively divergent with experimental data.

A finite-volume method for simulating contact lines on unstructured meshes in a conservative level-set framework

Accurate numerical simulation of moving contact lines on complex boundaries with surface wettability effect remains a challenging problem. In this paper, we introduce a robust and accurate method to perform 3D contact line simulations on unstructured meshes with an imposed contact angle. The contact angle is imposed through a sub-grid scale curvature term and the contact line motion is enabled thanks to partial slip on the wall. Moreover, an original strategy has been designed to improve normal and curvature computation at contact line from the level-set field. The whole method is validated on 2D and 3D test cases and shows good mass conservation properties. The drop detachment from a horizontal fiber due to gravity or surface tension is then investigated. For this purpose, dynamic mesh adaptation is used to keep high resolution around the interface with moderate number of cells. These realistic cases demonstrate the ability of the numerical method to handle surface wettability effects on resolved complex geometries.

Additive Manufacturing by laser-assisted drop deposition from a metal wire

The subject of Additive Manufacturing includes numerous techniques, some of which have reached very high levels of development and are now used industrially. Other techniques such as Micro Droplet Deposition Manufacture are under development and present different manufacturing possibilities, but are employed only for low melting temperature metals. In this paper, the possibility of using a laser-based drop deposition technique for stainless-steel wire is investigated. This technique is expected to be a more flexible alternative to Laser Metal Wire Deposition. Laser Droplet Generation experiments were carried out in an attempt to accurately detach steel drops towards a desired position. High-speed imaging was used to observe drop generation and measure the direction of detachment of the drops. Two drop detachment techniques were investigated and the physical phenomena leading to the drop detachment are explained, wherein the drop weight, the surface tension and the recoil pressure play a major role. Optimised parameters for accurate single drop detachment were identified and then used to build multi-drop tracks. Tracks with an even geometry were produced, where the microstructure was influenced by the numerous drop depositions. The tracks showed a considerably higher hardness than the base wire, exhibiting a relatively homogeneous macro-hardness with a localised softening effect at the interfaces between drops.

Study of arc welding stability in flux cored arc welding process and pulsed continuous current

ABSTRACT In the execution of a welding process, there are a large number of variables that need to be adjusted by the operator/welder to improve stability of the process. The quality of the welding process is closely linked to the stability that has been carried out. In this work, the coating welding process in pulsed direct current with Gas-Shielded Flux Cored Arc Welding (FCAW-G) was used, having SAE 1020 steel as base metal and martensitic stainless steel 410NiMo with 1.2 mm in diameter as additional metal. Current and voltage data were analysed to verify, using computational tools and the Vilarinho Regularity Index (IVcc ), the performed process. It was observed that the effectiveness of the tools was used as a way of detecting the phenomena of voltage signal variation, which serve as indications of drop detachment. Five samples were extracted from the test, where the occurrence of voltage peaks was noted, indicating the occurrence of drop detachment in a stable manner, according to the result observed by the Vilarinho Regularity Index (IVcc ).

Hydrodynamics of detachment, free falling and impact of drops

The results of optical measurements of drop dynamics are presented. Oscillations and waves on its surface were traced. The pattern of the secondary droplets falls onto an immersed drop and the discrete distribution of the substance of a uniformly colored drop in the targeted liquid was visualized. The important role of energy transport processes, both fast – local atomic-molecular – and slow that are translational and dissipative, in the formation of flow patterns is highlighted. A system of fundamental equations of fluid mechanics with equations of state for the Gibbs potential and density is applied. Ligaments – thin trickles with scales from atomic-molecular and macroscopic sizes – are investigated. Their images in the family of intrinsic solutions of the fundamental system and drop impact phenomena are shown.

The dynamics of droplet detachment in reversed electrowetting (REW)

Droplet actuation by electrowetting (EW) has drawn significant interest due to the potential applications in micro- and nano-fluidics, and the droplet departure is crucial for separation of the drop phase from the solid surface. However, the operating condition for droplet detachment, induced by the traditional electrowetting is quite strict, making it inconvenient for practical application. Recently, we considered the reversed electrowetting (REW) phenomenon, where a non-conductive droplet, settled on an adhesive surface, is dewetted continuously by applying the potential different between the substrate and surrounding fluid (Wang et al. 2020). Detachment of the oil drop is induced naturally and the detaching process is controllable. We have investigated the physical process of REW in the previous experiments. However, the dynamics of droplet detachment and the underlying mechanism are not well explained by the macroscopic experimental approach. For complementation, we build a numerical scheme in this study and examine the transient dynamics of droplet motion in the REW. The interface of the liquid-liquid system is captured by a phase-field method, and a correlation for the dynamic contact angle is incorporated in the numerical model. The simulated results are validated with the experimental cases. The process of droplet detachment and the critical condition are investigated by analyzing the energy transition during the droplet motion.

Innovations in Non-Linear Oscillations of a Pendent Drop From a Capillary Tip During Formation and Detachment - An LBM Simulation

Individual drops are suitable tools to study the liquid-fluid interfacial properties. In this work, forcedisplacement equation and non-linear oscillations of a pendent drop are numerically investigated. The presented novel force-displacement function allows following the dynamics of a pendent drop and realizing its elastic behavior. The growth and detachment of drop, which is pending due to gravity from a capillary tip, is considered (assuming high density and high viscosity ratios and immiscible two-phase flows). Twodimensional multi-relaxation time lattice Boltzmann method (MRT-LBM) was used to simulate growth, detachment, and oscillations of the drop using a conservative model for high-density ratio. The forcedisplacement function of a pendent drop (FDFPD), which is non-linear, was introduced. Using FDFPD, the non-linear elastic specifications of the pendent drop were determined. It was realized that the drop shows three different elastic behaviors simultaneously (hardening, linear, and softening). The drop superharmonic and subharmonic frequencies were calculated, using the natural frequency of the linear portion of FDFPD. Besides, the drop would grow as long as its displacement is between the extrema of FDFPD. In addition, a dynamic criterion for the onset of detachment was established. Also, increasing the Bond number from 0.11 to 1.96, while keeping Reynolds number equal to 0.023, accelerates the drop detachment and increases the linear portion of FDFPD. It was shown that increasing Capillary number from 1.8E-5 to 7.3E-4, while keeping Reynolds number equal to 0.023, accelerates the drop detachment and increases the non-linear portions of FDFPD.

A new approach to controlling metal transfer by dynamic modification in gas composition of arc atmosphere: studies on pulsed gas MAG welding*

ABSTRACT It has been well known that, in GMA welding, various welding phenomena are largely affected by the type of shielding gas used. However, up to the present time, almost no attempt has been made to utilize ‘gas composition’ for controlling welding phenomena. This study focused on this point and, thus, an investigation was made to understand how dynamic gas compositional modification affected various welding phenomena. In this study, pulsed Ar gas was injected periodically into CO2 shielding gas so that the gas composition of arc atmosphere largely modified in a very short period of time. It was found that this quick gas compositional change from CO2 to Ar led to a large decrease in arc voltage and an arc shape change from a constricted shape to a flare shape and that a molten droplet formed at the electrode tip during the CO2 period was released due to a sudden change in force balance between the molten droplet and the arc. The increase in current stimulated by the voltage drop may have assisted the metal drop detachment due to an increase in electromagnetic pinch force. If the cycle of this periodical Ar gas injection was selected to be shorter than that of repelled metal transfer in CO2 atmosphere, the repelled transfer was avoided and stable drop transfer occurred instead with far less spatter. Appropriate selections of the duration time of pulsed Ar gas injections avoided spray transfer which might cause undesirable finger-like penetration profiles. This new conceptual welding method, namely ‘Pulsed Gas MAG Welding’, required only a small amount of Ar gas to obtain the effects mentioned above.

Multi-scale three-domain approach for coupling free flow and flow in porous media including droplet-related interface processes

Drops on a free-flow/porous-medium-flow interface have a strong influence on the exchange of mass, momentum and energy between the two macroscopic flow regimes. Modeling droplet-related pore-scale processes in a macro-scale context is challenging due to the scale gap, but might be rewarding due to relatively low computational costs. We develop a three-domain approach to model drop formation, growth, detachment and film flow in a lower-dimensional interface domain. A simple upscaling technique allows to compute the drop-covered interface area fraction which affects the coupling fluxes. In a first scenario, only drop formation, growth and detachment are taken into account. Then, spreading and merging due to lateral fluxes are considered as well. The simulation results show that the impact of these droplet-related processes can be captured. However, extensions are necessary to represent the influence on the free flow more precisely.

Jet formation and breakup inside highly deformed bubbles

Bubble deformation and its outcome play a central role in natural and industrial processes. In this paper, we investigate experimentally the formation and breakup of an inertial jet arising from the relaxation of a highly deformed bubble produced by coaxial coalescence. It is found that various jets can emerge inside the bubbles with given shapes and velocities. We develop a robust scaling law to describe the jet velocity and unravel its dependence on initial bubble shape, capillary, gravity, viscosity and liquid properties. With such a model, we demonstrate a capillary-driven mechanism for the jet formation. Then, the evolution of the jet-pinching process is analyzed in detail, and we confirm two different mechanisms at play in jet breakup, namely end-pinching and root-pinching. It appears that jet breakup can be predicted well by considering the bubble Ohnesorge and Bond numbers. Moreover, in this parameter space, a phase diagram for jet breakup and no breakup is established. Following jet breakup, an interesting “Liquid in Gas” structure: one or multiple droplets encapsulated in a large bubble, is observed. We establish two different scaling laws for top jet drop radius from the perspective of bubble collapse and jet dynamics, respectively. These scaling behaviors corresponding to different parameter spaces shed light on the effects of viscous, capillary and gravitational forces on drop detachment clearly. Especially, a simple linear relationship between the drop Ohnesorge and jet Capillary numbers is demonstrated. Furthermore, we systematically establish a complete functional relationship from bubble collapse to jet formation to droplet detachment, which is helpful to control the bubble-collapsing jet.

Experimental study on the metal transfer control by using pulsed Ar addition in CO2 arc welding process: studies on pulsed gas MAG welding

ABSTRACT CO2 gas-shielded arc welding, which is widely used in the industrial world, has been recognized as an efficient and economical GMA welding process. However, its characteristic repelled transfer produces a large amount of spatter which results in costly cleanup and, thus, practical methods to overcome this weakness have been highly demanded. In the series of this study, ‘Pulsed Gas MAG’ welding was proposed as a new approach to reducing spatter, in which Ar gas pulses were periodically injected into CO2 shielding gas so that gas composition of arc atmosphere was largely modified in a very short period of time and a metal droplet formed at the electrode tip during CO2 atmosphere was released before repelled transfer had occurred. In this report, the influence of parameters of Ar gas injections, such as frequencies, average gas flow rates and gas injection periods, on the metal transfer was investigated. It was found that synchronous metal transfer stimulated by pulsed Ar gas injections was achieved at any frequencies from 35 to 65 Hz under the conditions of gas injection periods longer than 4 ms and of Ar gas volumes larger than approximately 1mL/pulse. This is because Ar gas injections below these critical levels resulted in lack of time for arc shape changes and metal drop detachment. From these experimental results, a model to explain the metal transfer phenomena was developed. The amount of spatter was reduced to the levels as low as that of the conventional MAG welding with 80 %Ar +20 %CO2 shielding gas mix and penetration profiles were round shape like those obtained in CO2 arc welding.

Experimental investigation of the effects of surfactant on the dynamics of formation process of liquid drops

In the present study, the formation of surfactant-laden viscous drops in ambient air is experimentally investigated using a high-speed digital camera. The mixtures of 90%, 92.5%, and 95% glycerol by weight in water containing various concentrations of sodium dodecyl sulfate are chosen as the drop phase fluid. The focus of this work is to investigate the effects of concentration of surfactant dissolved in solutions with different viscosities, on physical and geometrical parameters related to drop formation process, such as the drop elongation, minimum neck thickness, formation time, and the drop volume. The formation of satellite drops and the influence of surfactant concentration on their size are also studied. The obtained results indicate that increasing the surfactant concentration and the viscosity of drop phase fluid, slows down the necking process and causes the drop detachment length to increase. The entire process of drop formation is divided into two stages, and it was found that adding surfactant to the solutions has inverse effects on the duration of these two stages. It was also shown that surfactant addition contributes to an increase in the size of the satellite drop due to generation of Marangoni stresses on the surface of the drop's neck.