Droplet Speed Research Articles

Study on the process of droplet formation when liquid flows out of a capillary

This article presents the theoretical background for the justification of the parameters of the rotating sprayer. Theoretical studies show that an increase in the rotation frequency of the disk at a constant air flow velocity leads to a minimum median mass diameter of the droplets. Therefore, when justifying the diameter of the sprayed droplets, it is necessary to consider the combination of the disk rotation speed and the axial velocity of the air flow. To obtain high-quality air-droplet flow, the initial speed of the main droplets discharged from the periphery of the spray disc should be less than the air velocity and rotational frequency Pavlovskyi spray is recommended to be applied with in ω=60… 200 c1.

The wetting properties of frosted glass

Frosted glass is a common, low cost material. Its roughness can be used to control how it is wet by water. In this paper, the wetting properties of silicone oil and water are investigated. For the oil, wetting is total since the oleophilic character of the glass is enhanced by its roughness. Due to the remarkable optical properties of frosted glass, the spreading of oil droplets on its surface was recorded over three months. Frosted glass is a parahydrophilic surface because of its large contact angle hysteresis (up to 80° ). The behaviour of oil and water droplets was compared on a long piece of inclined frosted glass. The trajectories (and the spreading) of the droplets were studied and phenomenological laws were deduced to describe the dependence of the droplet speed on the initial volume of the droplet and the angle of inclination. Such dependences of speed at long travel distances (100 times the capillary length) were deduced and rationalised with a simple model that takes into account the thickness of the wake. Moreover, we analysed the flow inside the wake of water droplets sliding on inclined frosted glass. Suggestions are given on how to exploit drainage of the water droplet wake and the high hysteresis of water within the framework of open microfluidics.

Dynamic Control of Speed and Trajectories of Active Droplets in a Nematic Environment by Electric Field and Focused Laser Beam

One objective of active matter science is to unveil principles by which chaotic microscale dynamics could be transformed into useful work. A nematic liquid crystal environment offers a number of possibilities, one of which is a directional motion of an active droplet filled with an aqueous dispersion of swimming bacteria. In this work, using the responsiveness of the nematic to the electric field and light, we demonstrate how to control the direction and speed of active droplets. The dielectric response of nematic to the electric field causes two effects: 1) reorientation of the overall director, and 2) changing the symmetry of the director configuration around the droplet. The first effect redirects the propulsion direction while the second one changes the speed. A laser beam pointed to the vicinity of the droplet can trigger the desired director symmetry around the droplet, by switching between dipolar and quadrupolar configurations, thus affecting the motility and polarity of propulsion. The dynamic tuning of the direction and speed of active droplets represents a step forward in the development of controllable microswimmers.

Flow dynamics of droplets expelled during sneezing.

Respiratory infections transmit through droplets and aerosols generated by the infected individual during respiratory emissions. It is essential to study the flow dynamics of these emissions to develop strategies for mitigating the risk of infection. In particular, the dynamics of droplets expelled during violent exhalations such as sneezing is crucial, but has received little attention to date. Here, for the first time, we present the results of droplet dynamics of 35 sneezes, obtained from four volunteers, using particle tracking velocimetry experiments. Our results reveal a mean droplet velocity of 2–5.4 m/s across the different subjects. These values are significantly lower than what is usually assumed in the studies simulating or replicating sneezes. Furthermore, the large variation in droplet speeds, flow direction, spread angle, and head movement is also quantified. These findings will enable the refinement of models and simulations of sneezes toward improving infection control guidelines.

On the physics of transient ejection from bubble bursting

Using a dynamical scaling analysis of the flow variables and their evolution due to bubble bursting, here we predict the size and speed of ejected droplets for the whole range of experimental Ohnesorge and Bond numbers where ejection occurs. The transient ejection, which requires the backfire of a vortex ring inside the liquid to preserve physical symmetry, shows a delicate balance between inertia, surface tension and viscous forces around a critical Ohnesorge number, akin to an apparent singularity. Like in other natural phenomena, this balance makes the process extremely sensitive to initial conditions. Our model generalizes or displaces other recently proposed ones, impacting on, for instance, the statistical description of sea spray.

Ultrafast self-propelled water droplet transport on a graphene-covered nanocone

Cacti, a common drought-tolerant plant, enable the directional transport of droplets from the tip of the spikes to the roots in order to collect moisture from the air, creating conditions for cactus survival in arid areas. Inspired by this interesting natural phenomenon, a novel design for a monolayer graphene-covered nanocone (GNC) is proposed to realize ultrafast water droplet transport from the tip to the end of the GNC. The results show that the self-driving speed of a water droplet can reach ∼80 m s−1 driven by the Young–Laplace pressure difference. The rule of energy change during the droplet self-driving process indicates that the potential energy of the droplet and the interaction energy between the droplet and the GNC undergo cooperation and competition successively, resulting in the droplet first speeding up and then slowing down to a steady moving state. A larger droplet can extend the high-speed stage, which is beneficial to long-distance self-driving of droplets in a water-harvesting process. Continuum theory in the self-driving of a droplet at a microscale is used to describe the steady moving process, in order to further understand the rule in GNC-based water transport. The findings will open a broad range of novel perspectives for spontaneous droplet transport and water harvesting by graphene-covered functional surfaces.

A Multi-Fidelity Model for Simulations and Sensitivity Analysis of Piezoelectric Inkjet Printheads.

The ink drop generation process in piezoelectric droplet-on-demand devices is a complex multiphysics process. A fully resolved simulation of such a system involves a coupled fluid–structure interaction approach employing both computational fluid dynamics (CFD) and computational structural mechanics (CSM) models; thus, it is computationally expensive for engineering design and analysis. In this work, a simplified lumped element model (LEM) is proposed for the simulation of piezoelectric inkjet printheads using the analogy of equivalent electrical circuits. The model’s parameters are computed from three-dimensional fluid and structural simulations, taking into account the detailed geometrical features of the inkjet printhead. Inherently, this multifidelity LEM approach is much faster in simulations of the whole inkjet printhead, while it ably captures fundamental electro-mechanical coupling effects. The approach is validated with experimental data for an existing commercial inkjet printhead with good agreement in droplet speed prediction and frequency responses. The sensitivity analysis of droplet generation conducted for the variation of ink channel geometrical parameters shows the importance of different design variables on the performance of inkjet printheads. It further illustrates the effectiveness of the proposed approach in practical engineering usage.

A study on the design parameters for water–solid triboelectric energy harvesting with a channel device

This study demonstrates a channel device to harvest energy from water flow in a channel, which can fix the water droplet’s shape and speed. By using this device configuration, design parameters like material thickness, water droplet’s speed and length, and electrode length were able to analyze independently. An electrical output is generated when different Teflon thicknesses coated on the top and bottom electrodes of the channel. The device with the overall thinnest Teflon layers can produce the highest short circuit current of 2.346 ± 0.018 nA and open-circuit voltage of 2.01 ± 0.01 V. Furthermore, the short circuit current increases with the water droplet speed in the channel. However, short circuit current output remains unchanged when the electrode and the water droplet’s length changes. For potential device application, the channel device can produce a direct current – like short circuit current output with a long duration (~1s) of constant current (Ipk = 2.358 nA). This was achieved simply by using a long electrode and water droplet length of 25 mm. The findings in this study not only provide information to understand the design parameters in water–solid triboelectric energy harvesting devices but also helpful for future device design to generate a constant current output.

Numerical simulation of oil droplets spreading on solid surface in water

Plate separators are widely used for oil-water separation in the petrochemical industry. The spreading process of oil droplets at the solid surface is the key to understanding the oil-water separation mechanism in the plate separator. This study uses VOF method to capture oil-water phase interface change and sets up a two-dimensional model of oil droplets spreading on solid surface in water. The influences of droplet diameter, oil-water interfacial tension and viscosity are studied. The numerical simulation results show that the maximum spreading length of oil droplets increases with the increase of droplet size and interfacial tension. The average spreading speed of oil droplets decreases with the increase of oil droplet size, and increases with the increase of surface tension. The maximum spreading length and average spreading speed of oil droplets decrease with the increase of the dynamic viscosity of the oil droplets.

Coalescence and splashing threshold for head-on collisions of liquid metal nanodroplets

Head-on collisions of liquid metal nanodroplets in a vacuum are investigated through molecular dynamics simulations in order to determine the transition threshold between the coalescing and splashing regimes for six different materials (aluminum, calcium, cerium, gold, platinum, and tin). Droplets of various sizes and initial speeds are simulated, and it is found that the Reynolds and Ohnesorge numbers are able to predict the transition between the coalescing and splashing regimes. An energy balance for coalescing droplets shows that the initial energy is mainly converted to thermal energy increasing the temperature of the combined droplets by several hundred to several thousand kelvin depending on the material, and this result is confirmed in the simulations. When splashing occurs, the number of smaller droplets formed and the spreading rate are found to be dependent on the initial size and initial speed of the original droplets.

Simulation of jet printing of solder paste for surface mounted technology

Purpose The purpose of this study is to propose a novel simulation framework and show that it captures the main effects of the deposition process, such as droplet shape, volume and speed. Design/methodology/approach In the framework, the time-dependent flow and the fluid-structure interaction between the suspension, the moving piston and the deflection of the jetting head is simulated. The system is modelled as a two-phase system with the surrounding air being one phase and the dense suspension the other. The non-Newtonian suspension is modelled as a mixed single phase with properties determined from material testing. The simulations were performed with two coupled in-house solvers developed at Fraunhofer-Chalmers Centre; IBOFlow, a multiphase flow solver; and LaStFEM, a large strain FEM solver. Experimental deposition was performed with a commercial jet printer and quantitative measurements were made with optical profilometry. Findings Jetting behaviour was shown to be affected by not only piston motion, fluid rheology and head deformation but also the viscous energy loss in the jetting head nozzle. The simulation results were compared to experimental data obtained from an industrial jetting head and found to match characteristic lengths, speed and volume within ca 10%. Research limitations/implications The simulations are based on a rheological description using the Carreau model that does not include a time-dependent relaxation of the fluid. This modelling approach limits the descriptive nature of the deposit after impact on the substrate. The simulation also adopts a continuum approach to the suspension, which will not accurately model the break-off of the droplet filament under the characteristic diameter of the particles in the suspension. Practical implications The ability to accurately simulate the deposition of functional materials will enable the efficient development of novel product designs with a minimum of used resources and minimised product development duration. Social implications The ability to accurately simulate the deposition of functional materials will enable the efficient development of novel product designs with a minimum of used resources and therefore an improvement from a sustainability perspective. The ability to plan deposition strategies virtually will also enable a decrease in consumables at manufacturers which will in turn decrease their carbon foot print. Originality/value While basic fluid dynamic simulations have been performed to simulate flow through nozzles, the ability to include both fluid-structure interaction and multiphase capability together with a more accurate rheological description of the suspension and with a substrate for surface mount applications has not been published to the knowledge of the authors.

Droplet Mobility on Slippery Lubricant Impregnated and Superhydrophobic Surfaces under the Effect of Air Shear Flow.

The focus of this study is to investigate and compare the behavior of a droplet on superhydrophobic (SHS) and slippery lubricant impregnated (SLIPS) surfaces under the effect of air shear flow. In this regard, both experimental and numerical analyses have been conducted to compare their performance on droplet mobility under different air speeds. Two different lubricants have been utilized to scrutinize their effect on droplet movement. The numerical simulations have been performed based on the volume of fluid method coupled with the large eddy simulation turbulent model in conjunction with the dynamic contact angle method in addition to a model that can represent the effect of lubricants on slippery surfaces. The numerical simulations are compared with the experimental study in order to shed light on the underlying mechanisms. The results showed that under the same conditions, the critical velocity for droplet movement on the superhydrophobic surfaces is lower than that on the slippery lubricant impregnated surfaces due to the smaller droplet base diameter and the larger contact angle. The hydrodynamics of droplet mobility on superhydrophobic surfaces exhibits a rolling behavior while for the slippery lubricant impregnated surfaces a combination of rolling and sliding is observed. Beyond the critical airflow speed, a complete droplet shedding on all surfaces occurs. The wetting length and position of the droplet on superhydrophobic and slippery surfaces have been measured. On slippery surfaces, the speed of droplets is greatly affected by the lubricant properties while similar behavior in the wetting lengths is observed.

Flexible and efficient regulation of coalescence-induced droplet jumping on superhydrophobic surfaces with string

Coalescence-induced droplet jumping on superhydrophobic surfaces has good application potential in the fields of heat transfer enhancement, self-cleaning, water collection, and microfluidics. However, the direction of droplets jumping on the plane is uncontrollable and the energy conversion rate is low, which limits the application of droplets jumping. In this work, we report a flexible approach to efficiently regulate the direction and velocity of droplet jumping on superhydrophobic surfaces using superhydrophobic string. By changing the inclination angle of the string, the jumping direction of the coalescent droplet shows a maximum deviation of 68.1° from the surface normal. The maximum energy conversion rate is 32.1%, which is about 5.4 times higher than that of jumping on the plane. We analyzed the mechanism of the efficient regulation and found that the string affected the hydrodynamic characteristics of the jumping process, thus changing the jumping direction and the speed of the coalescent droplet.

The role of surfactants on the shape of confined droplets moving in circular microchannel

Droplet microfluidics is widespread in many chemical and biological applications where each droplet can be considered as a single, independent reactor unaffected by the presence of channel walls. This compartmentalization is facilitated by the addition of surfactants to increase the emulsion stability. However, the presence of surfactants is expected to strongly affect the dynamics and shape of flowing droplets. We report a systematic experimental study of the curvature of the front and the rear menisci of confined droplets flowing in a circular channel, with and without surfactants. In detail, the role played by surfactants on the droplet shape is investigated by dispersing them either in the droplet or in the continuous phases. The curvatures are evaluated by varying droplet speed, interfacial tension, and surfactant concentration. The curvature of the droplet front is found to scale with the capillary number (Ca) regardless of the presence or absence of surfactants. Differently, the curvature of the rear meniscus strongly depends on the surfactant concentration and whether surfactants are dispersed in the droplet or continuous phases. The surfactant accumulation at concentrations higher than the critical micelle concentration leads to an unexpected increase in the curvature in the former case and to droplet breakup in the latter.

Fabrication of slippery liquid-infused porous surfaces for anti-fouling applications

Lubricant-infused textured surfaces have gained much attention as liquid-repellent surfaces owing to high surface stability compared to their superhydrophobic counterparts. In this study, polydimethylsiloxane (PDMS) micro-structures (width: 20 μm, height: 80 μm), were fabricated using soft lithography. Effects of micro-structural geometry, including pillar-type (pillar, discrete ridge) and hole-type (hole, continuous ridge, circular rings with eight stripe supporters), on infiltration capability of the lubricants with different viscosity (Krytox-103; η: 0.131 Pa.s, Krytox-105; η: 0.737 Pa.s, paraffin oil; η: 0.112 Pa.s) were investigated. Partial infiltration was observed on the pillar-type micro-structures, while more completed infiltration was obtained on the hole-type micro-structures. More viscous Krytox-105 resulted in completed infiltration for all micro-structures, thus creating a slippery liquid-infused porous surface (SLIPS). The best SLIPS, signified by a low sliding angle and a fast speed of water droplet, prepared from the hole pattern infused with Krytox-105 exhibited good anti-fouling performance against chlorophyll and milk yogurt.

Evaluation of the standard deviation of droplet speed on grey-scale technique of DoD inkjet printer

The print quality of an inkjet printer is determined by the quality of the droplets. The droplet quality of a DoD inkjet printer is affected by speed variation between the droplets that generated from different nozzles. The effectiveness of a preliminary vibration using the W waveform design to generate spherical droplet without satellite and ligament has been denoted from previous studies. The waveform design considered to influence the speed variations of droplet. The standard deviation of droplet speed from different waveform design was investigate and evaluated. In this study, it is evidenced that by performing rear-wave adjustment on the W wave design can effectively reduce the standard deviation of droplet speed and produce the highest average speed.

Motion tracking and analysis of spray water droplets studied by high-speed photography using an iPhone X

Smartphones are not only becoming an inseparable part of our daily lives, but also a low-cost, powerful optical imaging tool for more and more scientific research applications. In this work, we hypothesized that some smartphones could be used as a low-cost, high-speed, photographic alternative to expensive equipment, such as those typically found in scientific research labs, to accurately perform motion tracking and analysis of fast-moving objects. This demonstrative experiment was done at home at very low cost by using a low-power green laser (<0.1 W) purchased from Amazon, an iPhone X, and a water spray bottle. The sprayed water droplets were illuminated by a divergent green laser beam and recorded with a smartphone using the Slow-Motion Video function (240 frames per second), which allows us to record high-speed events. The Slow-Motion Video was converted to a series of consecutive images taken every 1/240 second (or 4.16 ms), enabling tracking and motion analysis of the sprayed water droplets. By analyzing these consecutive images, the speed and flight trajectory of water droplets in the air were obtained, thereby enabling us to estimate the area of the water droplets landing on the ground.

Interfacially-adsorbed particles enhance the self-propulsion of oil droplets in aqueous surfactant.

Understanding the chemo-mechanical mechanisms that direct the motion of self-propulsive colloids is important for the development of active materials and exploration of dynamic, collective phenomena. Here, we demonstrate that the adsorption of solid particles on the surface of solubilizing oil droplets can significantly enhance the droplets' self-propulsion speeds. We investigate the relationship between the self-propulsion of bromodecane oil droplets containing silica particles of varying concentration in Triton X-100 surfactant, noting up to order of magnitude increases in propulsion speeds. Using fluorescently labeled silica, we observe packing of the particles at the oil-water interfaces of the rear pole of the moving droplets. For bromodecane oil droplets in Triton X-100, the highest droplet speeds were achieved at approximately 40% particle surface coverage of the droplet interface. We find particle-assisted propulsion enhancement in ionic surfactants and different oil droplet compositions as well, demonstrating the breadth of this effect. While a precise mechanism for the propulsion enhancement remains unclear, the simple addition of silica particles to droplet oil-water interfaces provides a straightforward route to tune active droplet dynamics.

Tetragonal Superlattice of Elongated Rhombic Dodecahedra for Sensitive SERS Determination of Pesticide Residues in Fruit.

The self-assembly of plasmonic nanoparticles into highly ordered superlattices could pave the way toward novel nanomaterials for surface-enhanced Raman scattering (SERS). Here, we propose the formation of large-area superlattices of elongated rhombic dodecahedra in a vertical orientation via a controlled droplet evaporation process. Expectedly, the constant humidity of the experimental condition could control the evaporation speed of droplets and this procedure promotes the balance between driven depletion attraction and electrostatic repulsion in the system, leading to the generation of well-organized three-dimensional (3D) superlattices. The unique geometry of elongated rhombic dodecahedra could establish the tetragonal superlattices, which breaks the conventional hexagonal symmetry of gold nanorods. Specifically, the influence of the type and concentration of surfactants, the concentration of nanoparticles, and the amount of droplets on the preparation results were systematically investigated to find the optimal assembly parameters. Remarkably, such close-packed tetragonal arrays of vertically aligned elongated rhombic dodecahedra exhibit more excellent SERS performance compared with the traditional hexagonal superstructure of gold nanorods. Benefiting from the high sensitivity and reproducibility of elongated rhombic dodecahedron superlattices, their applications in the determination of pesticide residues in apple and grape peels were successfully demonstrated. As a result, this study may advance the production of innovative plasmonic nanomaterials for a broad range of fields.

A versatile droplet on demand generator based on active pressure control.

The development of a novel on-demand droplet generator is reported in this paper based on off-the-shelf electromechanical components. With the ability to produce a broad range of droplet sizes, this device was designed to have minimum components without compromising reliability. The generator performance was assessed by producing water droplets for fire dynamics and suppression studies. Droplet formation at the tip of the nozzle was visualized using high-speed imaging. Stable and unstable regions can be identified easily through the images. Interchangeable nozzles can be used, allowing different droplet sizes. The setting parameters of the device allow accurate control of droplet size and initial speed. The device controls the liquid pressure actively and precisely. The active control of pressure and the ability to have specific droplet ejection parameters ensured the repeatability of droplet conditions. Characterization of the device is presented along with the calibration data, which can be collected in an efficient methodology. These data enable the selection of droplet size and speed when required. This system could easily be implemented in other research areas. Hence, the intention is to share and highlight the generator features for its use in a range of other scientific applications.