Viscosity Droplets Research Articles

Combustion Characteristics of Multicomponent Fuel Droplets

The present study investigates the combustion characteristics of the multicomponent diesel-water (D-W) emulsion fuel droplets. The stable blends obtained by using Surfactant, Span-80 (s) and three different fuels viz., WD1 (D-95%, W-3%, S-2%), WD2 (D-94%, W-4%, S-2%) and WD3 (D-96%, W-3%, S-1%) are subjected to the pendant mode of droplet combustion in the ambient conditions. The blend's stability depends on surfactant and water compositions. WD1 exhibited a higher viscosity W/s ≥ 1 at near-boiling point of the dispersed fluid. The higher viscosity droplets resulted in fewer secondary atomization. WD3 with lower viscosity (W/s >1.5) exhibited multiple cycles of the micro-explosion events. The cycles of micro-explosion depend on water to surfactant concentration. The blend with lesser surfactant exhibited the lesser cycles of explosion events. The regression rate of the droplets was fitted using d2-law. WD1 possesses the least burn time. The vapors of low volatile fluid, i.e., water vapor were trapped inside emulsion the droplets. Upon receiving the constant heat from flame surface, the surface of the droplets gets highly corrugated due to the capillary oscillations. Subsequently, the trapped bubbles of the less volatile fluid are ejected out as the daughter droplets. These ejections are more predominant in higher surfactant concentrations. The ejections frequency reduces with an increase in water %. This enabled the rapid burning of the droplet when compared to base fuel combustion.

Evaporation Characteristics of Viscosity Droplets on Stainless Steel Superhydrophobic Surface

Evaporation Characteristics of Viscosity Droplets on Stainless Steel Superhydrophobic Surface

Evaporation Characteristics of Viscosity Droplets on Stainless Steel Superhydrophobic Surface

Evaporation Characteristics of Viscosity Droplets on Stainless Steel Superhydrophobic Surface

Experimental design and evaluation of the influence of a glucomannan-based thickener on the characteristics of an oil/water emulsion

The thickening agents are an important part of the cosmetic formulation, offering an opportunity not only to regulate the product viscosity, but also to improve its stability. But, the way to use them and the impact of these natural ingredients on the qualities of the finished product are not always assessed. In this work, a new multifunctional glucomannan-based blend has been evaluated, the aim was to study its influence on the qualities of an oil/water emulsion. To get a rational understanding of this factor, an experimental design approach was adopted. An Optimal design optimized for a second-degree model (Quadratic) with only 12 runs and 1 repeated point was implemented. From the formulation, three ingredients concentrations varied as following; the thickener, a glucomannan-based blend, from 0.6% to 1.2%, one emollient from 3% to 7% and the emulsifier from 3% to 6%. The emulsion was prepared at 70°C and the thickener was added at 50°C. Viscosity measurement, droplets size and stability were analyzed 48 hours following the preparation. In the final emulsions, in terms of viscosity and stability to centrifugation, the glucomannan-based blend was is the parameter with the strongest impact. As for the droplets size, it was impacted the most, thus in a smaller extend, by the emulsifier concentration. Moreover, a synergistic effect between the emulsifier and thickener was monitored on the droplets size parameter while the concentration of squalene proved little influence. Finally, the optimal concentration of glucomannan-based blend to ensure system stability was determined at 1.2%. In conclusion, conducting an Optimal design allowed to model the effect of the thickener in formulation as well as the interactions between the monitored ingredients.

An experimental study of binary collisions of miscible droplets with non-identical viscosities

The dynamics of binary collisions of equi-diameter droplets with non-identical viscosities have been investigated experimentally and compared to previously generated data from identical droplet collisions (Al-Dirawi and Bayly in Phys Fluids 31(2):027105, 2019). Three hydroxypropyl methylcellulose (HPMC) aqueous solutions, 2%, 4%, and 8% HPMC, were used to generate the droplets of different viscosities, 2.8, 8.2, and 28.4 mPa s, respectively. High-speed imaging techniques were applied to observe and capture the collision outcomes. Collision outcomes were characterised and regime maps were generated. The non-identical viscosity droplet collisions produced regime maps with well-defined boundaries which are comparable in shape to the conventional regime maps of identical droplet collisions. The boundaries of the bouncing and reflexive separation regimes of the non-identical collisions show intermediate position between the identical cases of the low and the high viscosity droplets. However, the boundary of the stretching separation regimes of the non-identical collisions showed good agreement with the boundary of the identical case of the lower viscosity droplet. Moreover, the ability of models developed for predicting the regimes boundaries of collisions of identical viscosity droplets was assessed for the non-identical collisions. They proved capable in the non-identical cases, and the changes in adjustable parameters were consistent with the underlying physical basis of the models.Graphic abstract

On the role of the surface rheology in film drainage between fluid particles

Coalescence in the presence of low surfactant concentrations is investigated via a film drainage model, where the interface is a Boussinesq surface fluid with surfactant concentration dependent physical properties. Three cases are considered representing the systems where water is the continuous phase and the dispersed phase is either high viscosity droplets, droplets of comparable viscosity to water or gas bubbles. In the former, the immobilization of the interface is due to high dispersed phase viscosity and surface viscosities, whereas in the latter two, the Marangoni flow plays an important role, too. When droplets of comparable viscosity to water or gas bubbles are considered, it is seen that both the Marangoni flow and the surface viscosities can change the coalescence time significantly for the experimentally encountered values of the initial surfactant concentration, and the Boussinesq and surface Péclet numbers. In all cases, the impact of the surface phenomena amplifies with the approach velocity, especially for the dimpled interfaces. A complete immobilization criterion that is independent of the dispersed phase viscosity is proposed as a function of the continuous phase and surface viscosities, and the particle radii.

Spontaneous Displacement of High Viscosity Micrometer Size Oil Droplets from a Curved Solid in Aqueous Solutions.

Spontaneous displacement of high viscosity (∼103 Pa·s) micrometer size oil droplets from a curved solid in aqueous solutions was investigated. For high viscosity oils, the dynamic droplet shape was found to deviate significantly from a spherical cap shape due to the considerable viscous force in the oil phase. The displacement dynamics of high viscosity droplets were analyzed using molecular kinetic and hydrodynamic models. The molecular kinetic model was found to describe the dynamic displacement well for the droplets of small departure from the spherical cap shape, while the hydrodynamic model is more applicable to the droplets of higher three-phase contact line displacement velocities and hence larger deviation of the droplets from the spherical cap shape.

Simulations of splashing high and low viscosity droplets

In this work, simulations are presented for low viscosity ethanol and high viscosity silicone oil droplets impacting on a dry solid surface at atmospheric and reduced ambient pressure. The simulations are able to capture both the effect of the ambient gas pressure and liquid viscosity on the droplet impact and breakup. The results suggest that at early times droplet impact and gas film behavior for both low and high viscosity liquids share the same physics. However, at later times, during liquid sheet formation and breakup, high and low viscosity liquids behave differently. These results explain why for both kinds of liquids the pressure effect can be observed, while at the same time different high and low viscosity splashing regimes have been identified experimentally.

Momentum coupling at laser ablation of liquids in a porous matrix

The main problems at laser impact on liquids are energy dissipation and splashing, leading to poor performance even after application of special tricks like viscosity increase, thin films, cavities or droplets formation. We studied the laser ablation of liquids, including energetic ones, confined by porous matrix made of sintered titanium powder. We have compared the liquid-filled targets (ethanol, kerosene, nitromethane) momentum coupling coefficient dependency on laser fluence with the same unfilled target. Filling a porous target with a liquid may have both positive and negative effects depending on impact regime. Porous matrix prevents heat and mass losses characteristic to bulk solid and liquid targets. Momentum coupling coefficient doubled at porous target density increase by 6% only with ethanol filling, and at least tripled at ca. 22% density decrease as compared to a bulk target. Filling of porous matrices with energetic liquids leads to improvement of laser thrust generation due to combination of heat and splashing losses substantial reduction and chemical energy release.

Fabricating High-viscosity Droplets using Microfluidic Capillary Device with Phase-inversion Co-flow Structure.

The generation of monodisperse droplets with high viscosity has always been a challenge in droplet microfluidics. Here, we demonstrate a phase-inversion co-flow device to generate uniform high-viscosity droplets in a low-viscosity fluid. The microfluidic capillary device has a common co-flow structure with its exit connecting to a wider tube. Elongated droplets of the low-viscosity fluid are first encapsulated by the high-viscosity fluid in the co-flow structure. As the elongated low-viscosity droplets flow through the exit, which is treated to be wetted by the low-viscosity fluid, phase inversion is then induced by the adhesion of the low viscosity droplets to the tip of the exit, which results in the subsequent inverse encapsulation of the high-viscosity fluid. The size of the resultant high-viscosity droplets can be adjusted by changing the flow rate ratio of the low-viscosity fluid to the high-viscosity fluid. We demonstrate several typical examples of the generation of high-viscosity droplets with a viscosity up to 11.9 Pas, such as glycerol, honey, starch, and polymer solution. The method provides a simple and straightforward approach to generate monodisperse high-viscosity droplets, which may be used in a variety of droplet-based applications, such as materials synthesis, drug delivery, cell assay, bioengineering, and food engineering.

Phase inversion of slug flow on step surface to form high viscosity droplets in microchannel

Slug flow is a typical two phase flow in which droplets are lubricated by an immiscible continuous phase in a microchannel. The step surface is found to break the lubrication film and induce a phase inversion of the slug flow. The reduction in the film thickness by the step is quantitatively calculated and measured, and the critical droplet length for the breakup of the film is also revealed. This step-induced phase inversion is demonstrated to form high-in-low-viscosity emulsion droplets from a low-in-high-viscosity slug flow.

Effect of interfacial slip on the deformation of a viscoelastic drop in uniaxial extensional flow field

The effect of interfacial slip on the deformation of a viscoelastic droplet, suspended in another viscoelastic medium, in the presence of a uniaxial extensional flow, is studied analytically. Using the Oldroyd-B constitutive relation, the Stokes flow problem is solved in the limit of a small capillary number and small Deborah number. Experimentally observed interfacial velocity slip is incorporated using a Navier slip boundary condition. The interfacial slip significantly reduces the magnitude of droplet deformation when the droplet has larger viscosity as compared with the suspending phase. The droplet shape becomes less ellipsoidal in the presence of slip. The effect of slip diminishes for low viscosity droplets. Slip effectively weakens the dependence of the droplet deformation on the droplet to medium viscosity ratio. The viscoelasticity of the suspending phase plays a dominant role on the droplet deformation as compared with the viscoelasticity of the droplet phase when there is velocity slip at the droplet surface. The interfacial slip aids the viscoelasticity of the suspending phase in deforming the droplet, while the effect of viscoelasticity of the droplet phase is suppressed by the interfacial slip.

Dynamic measurements and simulations of airborne picolitre-droplet coalescence in holographic optical tweezers.

We report studies of the coalescence of pairs of picolitre aerosol droplets manipulated with holographic optical tweezers, probing the shape relaxation dynamics following coalescence by simultaneously monitoring the intensity of elastic backscattered light (EBL) from the trapping laser beam (time resolution on the order of 100 ns) while recording high frame rate camera images (time resolution <10 μs). The goals of this work are to: resolve the dynamics of droplet coalescence in holographic optical traps; assign the origin of key features in the time-dependent EBL intensity; and validate the use of the EBL alone to precisely determine droplet surface tension and viscosity. For low viscosity droplets, two sequential processes are evident: binary coalescence first results from the overlap of the optical traps on the time scale of microseconds followed by the recapture of the composite droplet in an optical trap on the time scale of milliseconds. As droplet viscosity increases, the relaxation in droplet shape eventually occurs on the same time scale as recapture, resulting in a convoluted evolution of the EBL intensity that inhibits quantitative determination of the relaxation time scale. Droplet coalescence was simulated using a computational framework to validate both experimental approaches. The results indicate that time-dependent monitoring of droplet shape from the EBL intensity allows for robust determination of properties such as surface tension and viscosity. Finally, the potential of high frame rate imaging to examine the coalescence of dissimilar viscosity droplets is discussed.

Dynamic behaviour of buoyant high viscosity droplets rising in a quiescent liquid

The present paper initiates a systematic computational analysis of the rise dynamics of high viscosity droplets in a viscous ambient liquid. This represents a relevant intermediate case between free rigid particles and bubbles since their shape adjusts to outer forces while almost no inner circulation is present. As a prototype system, we study corn oil droplets rising in pure water with diameters ranging from 0.5 to 16 mm. Since we are interested in the droplet dynamics from the viewpoint of a bifurcation scenario with increasingly complex droplet behaviour, we perform fully three-dimensional numerical simulations, employing the in-house volume-of-fluid (VOF)-code FS3D. The smallest droplets (0.5–2 mm) rise in steady vertical paths, where for the smallest droplet (0.5 mm) the flow field, as well as the terminal velocity, can be described by the Taylor and Acrivos approximate solution, despite the Reynolds number being well above one. Larger droplets (3.2 mm) rise in an oblique path and display a bifid wake, and those with diameters in the range (3.7–8 mm) rise in intermittently oblique paths, showing an intermittent bifid wake of alternating vorticity. The droplets’ shapes in this range change from spherical into oblate ellipsoids of increasing eccentricity, followed by bi-ellipsoidal shapes with higher curvature on the downstream side. Even larger droplets (10–16 mm) rise in oscillatory, essentially vertical paths with drastically different wake structures, including deadzones and aperiodic or periodic vortex shedding. The largest considered droplets (diameter of 14 and 16 mm) display significant shape oscillations and vortex shedding is accompanied by a complex evolution of coherent vortex structures. Their rise paths are best described as zigzagging, but the bifurcation scenario seems to be substantially different from that leading to the zigzagging of air bubbles. In contrast to the rise behaviour of bubbles, helical paths are not observed in the present study.

Effect of chitosan molecular weight on the antimicrobial activity and release rate of carvacrol-enriched films

Edible films based on three types of chitosan which differ on their molecular weight were developed as carriers of carvacrol. Stable film forming dispersions were obtained and the one developed from chitosan with higher molecular weight presented higher viscosity and larger carvacrol droplets. Olive oil and 96% ethanol were used as food-simulating solvents to determine the carvacrol release rate from the chitosan matrices. The release of carvacrol was much faster when it was submerged in ethanol. Besides, the higher the chitosan molecular weight employed in the films formulation, the faster the release rate towards both solvents. The films showed antimicrobial effectiveness against the gram-negative bacteria Pseudomona fragi, Shewanella putrefaciens and Aeromonas hydrophila, common spoilers of fish and seafood products. Thus, in order to slowdown the carvacrol release rate, the use of chitosan with a low molecular weight and technical applications targeted for low-moderate fat content products is suggested.

Measurement and analysis of bimodal drop size distribution in a rotor–stator homogenizer

Rotor–stator homogenizers are widely used to manufacture liquid–liquid emulsions, but little emphasis has been placed on understanding the processing principles of these devices. In this study, a systematic series of experiments have been performed to investigate and quantify the effects of process parameters and formulation variables on the drop size distribution (DSD) and relative sizes of oil-in-water emulsions manufactured in a Rotor–stator homogenizer. The effects of rotor speed, the dispersed phase viscosity and the dispersed phase volume fraction were studied. The DSDs by volume were predominantly bimodal, and the smaller drop size peak appeared at 1μm for most of the oil-in-water emulsions in our experiment. Furthermore, the relationship between the Sauter mean diameter and the maximum stable drop diameter was found to be linear in emulsions with a bimodal volume distribution, and the proportionality constant somewhat decreases as the viscosity of the dispersed phase increases. Compared with the normal and log-normal function, the Frechet function, which is usually proposed to describe the extreme value problem, can describe the monomodal number DSD very well. The experiments also show that the low-viscosity droplets are stabilized by a combination of interfacial tension and viscous forces, while viscous forces primarily prevent breakage in high viscosity droplets. A modified version of Calabrese's model has been developed for combining the effect of the dispersed phase viscosity and volume fraction on drop size, which represents the experimental data well.

Collision between high and low viscosity droplets: Direct Numerical Simulations and experiments

Binary droplet collisions are of importance in a variety of practical applications comprising dispersed two-phase flows. In the present work we focus on the collision of miscible droplets, where one droplet is composed of a high viscous liquid and the other one is of lower viscosity. This kind of collisions take place in, for instance, spray drying processes when droplets with different solid content collide in recirculation zones. The aim of this paper is to investigate the details of the flow inside the colliding droplets. For this purpose, two prototype cases are considered, namely the collision of equal sized droplets and the collision between a small and highly viscous droplet and a bigger low viscous droplet. A new experimental method has been developed in order to visualize the penetration and mixing process of two colliding droplets, where a fluorescence marker is added to one liquid and the droplets are excited by a laser. The results show a delay in the coalescence which takes place during the initial stage of a collision of droplets with different viscosities. Direct Numerical Simulations based on the Volume-of-Fluid method are used to study these collisions and to allow for a more detailed inspection of the mixing process. The method is extended to consider a second liquid with a different viscosity. In order to reproduce the delay of coalescence, an algorithm for the temporal suppression of the coalescence is applied. A predictive simulation of the delay is not possible, because the extremely thin air gap separating the droplets cannot be resolved by the numerics. This approach is validated by comparison with experimental data. The results provide local field data of the flow inside the collision complex, showing in particular a pressure jump at the liquid–liquid interface although no surface tension is present. The detailed analysis of the terms in the momentum balance show that the pressure jump results from the viscosity jump at the liquid–liquid interface.

Hydrodynamics of liquid–liquid Taylor flow in microchannels

An understanding of the hydrodynamics of the flow of long liquid droplets in another liquid phase is necessary for droplet-based microfluidics and other Lab-on-a-Chip devices. In this work, we use high speed photography/brightfield microscopy and CFD simulations to study the hydrodynamics of vertically upward flow of Taylor droplets of water dispersed in a continuous hexadecane phase in a channel of diameter 1.06mm. The volume-of-fluid (VOF) method is used to model two-phase flow and the shapes and velocities of the droplets measured experimentally are in excellent agreement with those obtained from CFD simulations. It is observed that the film thicknesses and the dynamic pressure drop caused by the presence of the droplets for these low viscosity droplets (viscosity ratio=0.3) can be predicted by the Bretherton's expressions for an inviscid bubble. For a sufficiently long droplet, the flow field in the uniform thickness film region can be predicted by an ideal annular flow solution. A model to predict the overall pressure drop in a unit cell, consisting of a Taylor droplet surrounded by the two halves of continuous phase slugs is also presented by summing up the pressure drop contributions from the continuous phase liquid slugs, uniform thickness film region and interfacial pressure drops at the droplet front and rear. The model was accurate for long droplets and slugs but was not reliable for shorter droplets and slugs, for which the slug and film flows may not be fully-developed and the shapes of the front and rear of the droplets interact.

Spreading and atomization of droplets on a vibrating surface in a standing pressure field

We report the first observation and analytical model of deformation and spreading of droplets on a vibrating surface under the influence of an ultrasonic standing pressure field. The standing wave allows the droplet to spread, and the spreading rate varies inversely with viscosity. In low viscosity droplets, the synergistic effect of radial acoustic force and the transducer surface acceleration also leads to capillary waves. These unstable capillary modes grow to cause ultimate disintegration into daughter droplets. We find that using nanosuspensions, spreading and disintegration can be prevented by suppressing the development of capillary modes and subsequent break-up.

Droplet break-up by in-line Silverson rotor–stator mixer

Silverson high shear in-line rotor–stator mixers are widely applied in industry for the manufacture of emulsion-based products but the current understanding of droplet breakage and coalescence in these devices is limited. The aim of this paper is to increase the understanding of droplet break-up mechanisms and to identify appropriate literature correlations for in-line rotor–stator mixers. Silicone oils with viscosities ranging from 9.4 to 969 mPa s were emulsified with surfactant in an in-line Silverson at rotor speeds up to 11,000 rpm and flow rates up to 5 tonnes/h. The effect of rotor speed, flow rate, dispersed phase fraction up to 50 wt%, inlet drop size and viscosity ratio on droplet size was investigated. It was found that rotor speed and dispersed phase viscosity have a significant effect on the droplet size, while flow rate, inlet droplet size, viscosity ratio and dispersed phase volume have a lesser effect. The results indicate that low viscosity droplets are broken by turbulent inertial stresses, while droplets smaller than the Kolmogorov length scale are broken by a combination of inertial and viscous stresses. It also appears that the weak dependence of drop size on flow rate enables the energy efficiency of an in-line high shear Silverson to be significantly improved by operating at as high a flow rate as possible.