Instability Of Droplets Research Articles

Electromagnetic effects on the solidification of a metallic alloy droplet impacting onto a surface

The effect of magnetic fields on the solidification of a droplet impacting a substrate, and its potential for controlling the topology and properties of the final solidified splat, has not been explored thus far. In this research, we conduct a computational study on the solidification of a metallic alloy droplet impacting onto a substrate under the influence of a magnetic field, for the first time. Our model undergoes rigorous assessments through a systematic procedure to avoid error-hiding of different sub-models. We investigate the effects of applying direct and alternating magnetic fields with different Strouhal numbers (or dimensionless frequencies). Our findings demonstrate that MagnetoHydroDynamics (MHD) can effectively control the shape of the frozen splat upon impact. Depending on the interplay between the inertia and capillary forces as well as the MHD braking and shaking effects (damping and oscillating Lorentz forces), we achieve a hemispherical, oblate, prolate, or cupcake solidified splat topology. Additionally, we identify a critical Strouhal number at which the solidified splat-shape parameters reach an extremum. Our analyses reveal two distinct causes underlying this phenomenon at low or moderate Weber numbers. At low Weber numbers, the resonance triggered by the proximity of the MHD actuation frequency and the capillary natural frequency of the splat is the determining factor. On the other hand, at the higher Weber numbers, the severe droplet regime changes and instability play a key role.

Magnetic hybrid micelles with controllable structures via interfacial instability-induced confined Co-assembly

Co-assembly of amphiphilic block copolymers (BCPs) and nanoparticles (NPs) is an encouraging route to produce shape-tunable functional hybrid micelles, which can integrate the excellent water stability of BCPs and the versatile functionalities of NPs. In this paper, we present an facile method to prepare magnetic hybrid micelles through confined co-assembly of polystyrene grafted Fe3O4@PS NPs and PS-b-poly(ethylene oxide) (PS-b-PEO) induced by interfacial instability of emulsion droplets. The morphology of hybrid micelles can be regulated by varying the surfactant concentration, weight fraction of Fe3O4 NPs, and block ratio of PS-b-PEO. The distribution of NPs inside the micelles is determined by the ratio (d/L) of the NP diameter (d) to the thickness of PS domain of the micelles (L). When 0.49 ≤ d/L < 0.76, the NPs locate within the cylindrical micelles. When d/L ≥ 0.76, the NPs selectively locate in the spherical micelles. More interestingly, the size-selective loading of NPs in the micelles can be achieved, even mixture of NPs different in size is applied. Moreover, these magnetic hybrid micelles show good biocompatibility and morphology-dependent magnetic resonance responsiveness, which may find potential applications in magnetic resonance imaging.

Modulational instability of a harmonically trapped quantum droplet

We investigate the modulational instability (MI) of a harmonically trapped quantum droplet. The MI is the key mechanism for the formation of soliton trains in diverse physical media, as a result of the interplay between the intrinsic nonlinearity and the kinetic-energy term. Here, we analytically get the time-dependent criterion for MI of a trapped quantum droplets with Lee–Huang–Yang (LHY) term. It is shown that the external harmonic potential can dramatically change the condition of modulational instability. Moreover, we find that there exists a critical time tc, that is the biggest time before instability of droplet will set in. We find the tc depends on the harmonic trapping frequency k. Meanwhile, the results show that the trapping frequency k also determine the solitons’ types, a single peak soliton or a soliton train can be excited by the different k. Our theoretical predicts are confirmed by the direct numerical calculation of the generalized Gross–Pitaevskii equation with LHY correction term.

Dynamics of acoustically-induced droplet instability

The advancement of the theory of droplet stability in the acoustic field is of significant value in improving ultrasonic atomization and ultrasonic levitation technology. In this work, in order to reveal the detailed mechanism of acoustic droplet instability and give the instability criterion for easy application, the dynamics of droplet instability in standing wave acoustic field (19.8 kHz) is studied by combining practical experiment, theoretical derivation and numerical calculation. The acoustic instability of the droplet occurring near the wave nodes is mainly manifested in two typical modes: disk instability and edge-sharpening instability. The appearance of these two instability modes depends on the relative magnitude of the standing wave field strength. Specifically, with the gradual enhancement of the intensity of the standing wave field, the instability mode of the droplet will gradually change from disc instability to edge-sharpened instability.The droplets show obvious self-accelerating expansion in the equatorial plane in the instability process. The positive feedback between the droplet aspect ratio and the negative pressure of acoustic radiation at the equator of the droplet is the reason for the above self-accelerating behavior. The theoretical results obtained through deduction indicate that the amplitude of the negative acoustic radiation pressure at the droplet equator is proportional to the square of the droplet aspect ratio. The surface tension of the droplet is the main factor hindering its deformation, while the acoustic radiation suction at the equator is the main factor driving the deformation of the droplet. Based on this, the force equilibrium equation of the droplet interface is established, and the dimensionless criterion of acoustic droplet instability, i.e. the acoustic Weber number &lt;i&gt;We&lt;/i&gt;&lt;sub&gt;a&lt;/sub&gt;, is derived. When &lt;i&gt;We&lt;/i&gt;&lt;sub&gt;a&lt;/sub&gt;≤1, the droplet interface stays in equilibrium, and when &lt;i&gt;We&lt;/i&gt;&lt;sub&gt;a&lt;/sub&gt; &gt;1, the equatorial acoustic suction is larger than the surface tension, and the droplet instability occurs, and the average error between the experimental results and the theoretical results is only 9%.

Organic/inorganic heterostructures templated by interfacial instability-driven BCP colloids in deformable emulsion droplets.

Hybrid heterostructure materials have received considerable attention due to the integration of each component and abundant functional applications in micromotors, catalysis, photothermal therapy, drug delivery, and bioimaging. However, the preparation of organic/inorganic heterostructure nanoparticles (HSNPs) with high quality still remains a remarkable challenge since thermodynamically metastable structures usually coexist, resulting in a lack of organic scaffolds with extreme uniformity both in shape and size distribution. Here, we prepared polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer (BCP) core-shell spherical colloids driven by interfacial instability of soft and deformable emulsion droplets. Ultra-low interfacial tension was achieved through the co-adsorption of BCP segments and sodium dodecyl sulfate (SDS) surfactant, which had a strong affinity with the P4VP segment at the interface of the emulsified droplets. The excellent and homogeneous BCP colloids were further utilized as organic scaffolds to selectively grow a functional SiO2 layer on the surface of the BCP spherical colloids, producing BCP/SiO2 HSNPs with highly uniform shape and size distribution originating from the PS-b-P4VP scaffolds, thus providing an efficient and general strategy to construct and design organic/inorganic HSNPs with diverse applications.

Flexible droplet transportation and coalescence via controllable thermal fields

Flexible droplet transportation and coalescence are significant for lots of applications such as material synthesis and analytical detection. Herein, we present an effective method for controllable droplet transportation and coalescence via thermal fields. The device used for droplet manipulation is composed of a glass substrate with indium tin oxide-made microheaers and a microchannel with two transport branches and a central chamber, and it's manipulated by sequentially powering the microheaters located at the bottom of microchannel. The fluid will be unevenly heated when the microheater is actuated, leading to the formation of thermal buoyancy convection and the decrease of interfacial tension of fluids. Subsequently, the microdroplets can be transported from the inlets of microchannel to the target position by the buoyancy flow-induced Stokes drag. And the droplet migration velocity can be flexibly adjusted by changing the voltage applied on the microheater. After being transported to the center of central chamber, the coalescence behaviors of microdroplets can be triggered if the microheater located at the bottom of central chamber is continuously actuated. The droplet coalescence is the combined effect of decreased fluid interfacial tension, the shortened droplet distance by buoyancy flow and the increased instability of droplet under the elevated temperature. The droplet coalescence efficiency is also related to the voltage of microheater, by increasing the voltage from 3.5 V to 7 V, the needed time for droplet coalescence dramatically decrease from 220s to 1.4 s. Finally, by the droplet coalescence-triggered calcium hydroxide precipitation reaction, we demonstrate the applicability of the droplet manipulation method on specific sample detection. Therefore, this approach used for droplet transportation and coalescence can be attractive for many droplet-based applications such as analytical detection.

Optimization of solvent evaporation method in liposomal nanocarriers loaded-garlic essential oil (Allium sativum): Based on the encapsulation efficiency, antioxidant capacity, and instability.

This study is aimed to optimise the preparation factors, such as sonication time (5-20min), cholesterol to lecetin ratio (CHLR) (0.2-0.8), and essential oil content (0.1-0.3g/100g) in solvent evaporation method for formulation of liposomal nanocarriers containing garlic essential oil (GEO) in order to find the highest encapsulation efficiency and stability with strongest antioxidant and antimicrobial activity. The droplet size, zeta potential, encapsulation efficiency, turbidity, changes in turbidity after storage (as a measure of instability), antioxidant capacity, and antimicrobial activity were measured for all prepared samples of nanoliposome. The sonication time is recognised as the most effective factor on the droplet size, zeta potential, encapsulation efficiency, turbidity, and instability while CHLR was the most effective factor on zeta potential and instability. The content of GEO significantly affected the antioxidant and antimicrobial activity in particular against gram-negative bacteria (Escherichia coli). The results of FTIR based on the identification of functional groups confirmed the presence of GEO in the spectra of the prepared nanoliposome and also it was not observed the interaction between the components of the nanoliposome. The overall optimum conditions were determined by response surface methodology (RSM) as the predicted values of the studied factors (sonication time: 18.99min, CHLR: 0.59 and content of GEO: 0.3g/100g) based on obtaining the highest stability and efficiency as well as strongest antioxidant and antimicrobial activity.

Quaternization-Assisted Assembly of Polymer-Tethered Gold Nanoparticles into Superlattices with a Tunable Structure

Programmable organization of functional inorganic nanoparticles (NPs) into well-defined hierarchical superstructures has raised considerable concern due to the enhanced magnetic, electronic, and optical properties arising from the strong plasmonic coupling effect. However, the spontaneous self-assembly of inorganic NPs into uniform superlattices with the desired or tunable shape remains a formidable challenge. Herein, we report the quaternization-assisted hierarchical assembly of gold nanoparticles (AuNPs) tethered with polystyrene (PS) and quaternized poly(2-vinylpyridine) (P2VP) ligands into controllable superstructures in emulsion droplets. The quaternization interaction between bromoalkyl molecular additives (e.g., CnH2n+1Br) and P2VP ligands results in the increase of the hydrophilicity degree of quaternized P2VP ligands and triggers the interfacial instability of emulsion droplets, achieving the continuous transformation of generated superstructures from solid assemblies to hollow colloidosomes and then to the multilayer sheet-like superlattices. Furthermore, the degree of quaternization can be readily tuned by the added content of additives and the length of the alkyl chain, providing a unique and promising approach to prepare contrivable superstructures and functional materials.

Impact of Minor Alloy Components on the Electrocapillarity and Electrochemistry of Liquid Metal Fractals

AbstractExploring and controlling surface tension‐driven phenomena in liquid metals may lead to unprecedented possibilities for next‐generation microfluidics, electronics, catalysis, and materials synthesis. In pursuit of these goals, the impact of minor constituents within liquid alloys is largely overlooked. Herein, it is showed that the presence of a fraction of solute metals such as tin, bismuth, and zinc in liquid gallium can significantly influence their electrocapillarity and electrochemistry. The instability‐driven fractal formation of liquid alloy droplets is investigated with different solutes and reveals the formation of distinctive non‐branched droplets, unstable fractals, and stable fractal modes under controlled voltage and alkaline solution conditions. In their individually unique fractal morphology diagrams, different liquid alloys demonstrate significantly shifted voltage thresholds in transition between the three fractal modes, depending on the choice of the solute metal. Surface tension measurements, cycle voltammetry and surface compositional characterizations provide strong evidence that the minor alloy components drastically alter the surface tension, surface electrochemical oxidation, and oxide dissolution processes that govern the droplet deformation and instability dynamics. The findings that minor components are able to regulate liquid alloys’ surface tensions, surface element distributions and electrochemical activities offer great promises for harnessing the tunability and functionality of liquid metals.

Fingering Instability of Binary Droplets on Oil Pool

The interfacial instability of a complex fluid in a multiphase flow system is ubiquitous in both nature and industry. We experimentally investigated the spreading and interfacial instability dynamics of a binary droplet (a water and 2-propanol (IPA) mixture) on an immiscible (sunflower oil) pool. For droplets of 40 wt% IPA solution on sunflower oil, fingering instability occurred at the spreading liquid front. To reveal the interfacial characteristics of the spreading and fingering processes, we analyzed the interplay among the speed, diameter, and number of fingers on the spreading front. Based on our observations, the finger length, wavelength between the fingers, head length, and neck length were quantified. Our experimental results clearly demonstrate that fingering instability can be driven by the capillary effect for a liquid–liquid system as well as the Plateau–Rayleigh instability. We hope that our results will inspire further experimental and numerical investigations to provide deeper insights into the interfacial dynamics of multicomponent droplets in a liquid pool.

On ghost fluid method-based sharp interface level set method on a co-located grid and its comparison with balanced force-based diffuse interface method

This paper, on Computational multi-Fluid Dynamics (CmFD) development, presents an extension of the Ghost Fluid Method (GFM)-based Sharp Interface Level Set Method (SI-LSM) (originally proposed on a staggered grid) for a co-located grid system. Further, this paper presents a comparative study for the GFM-based SI-LSM and a balanced force method (BFM)-based Diffuse Interface Level Set Method (DI-LSM). The BFM-based DI-LSM avoids a pressure-interfacial force decoupling, during the implementation of the MIM, by a proper-discretization of the surface tension term; not needed for the single-phase flow. Whereas, the GFM-based SI-LSM implicitly couples two immiscible, incompressible fluids via interface jump condition, which makes the Momentum Interpolation Method (MIM) for the two-fluid flow same as that for a single-fluid flow. A unified, for both SI-LSM and DI-LSM, mathematical and numerical formulations are presented. Order of accuracy is demonstrated in-between first-order and second-order; similar to that reported earlier for the various CmFD methods. Finally, validation and relative-performance study are presented for four test cases: static droplet, dam break, drop coalescence, and Rayleigh-Taylor instability. For the static droplet, the relative-performance of the present SI-LSM as compared to the DI-LSM is found similar to that in the literature for SI-VOF (Volume of Fluid) method as compared to DI-VOF method. For the various test cases, superior performance is demonstrated for the SI-LSM as compared to DI-LSM. This paper presents a broader perspective to the MIM for a single-versus-two fluid-flow and diffuse-versus-sharp approaches; and a novel GFM-based SI-LSM on a co-located grid.

The Microfluidic Ice Nuclei Counter Zürich (MINCZ): a platform for homogeneous and heterogeneous ice nucleation

Abstract. Ice nucleation in the atmosphere is the precursor to important processes that determine cloud properties and lifetime. Computational models that are used to predict weather and project future climate changes require parameterizations of both homogeneous nucleation (i.e. in pure water) and heterogeneous nucleation (i.e. catalysed by ice-nucleating particles, INPs). Microfluidic systems have gained momentum as a tool for obtaining such parameterizations and gaining insight into the stochastic and deterministic contributions to ice nucleation. To overcome the shortcomings of polydimethylsiloxane (PDMS) microfluidic devices with regard to temperature uncertainty and droplet instability due to continuous water adsorption by PDMS, we have developed a new instrument: the Microfluidic Ice Nuclei Counter Zürich (MINCZ). In MINCZ, droplets with a diameter of 75 µm are generated using a PDMS chip, and hundreds of these droplets are then stored in fluoropolymer tubing that is relatively impermeable to water and solvents. Droplets within the tubing are cooled in an ethanol bath. We validate MINCZ by measuring the homogeneous freezing temperatures of water droplets and the heterogeneous freezing temperatures of aqueous suspensions containing microcline, a common and effective INP in the atmosphere. We obtain results with a high accuracy of 0.2 K in measured droplet temperature. Pure water droplets with a diameter of 75 µm freeze at a median temperature of 237.3 K with a standard deviation of 0.1 K. Additionally, we perform several freeze–thaw cycles. In the future, MINCZ will be used to investigate the freezing behaviour of INPs, motivated by a need for better-constrained parameterizations of ice nucleation in weather and climate models, wherein the presence or absence of ice influences cloud optical properties and precipitation formation.

Evaluation of the synergistic effect of plant-based components on the stability of curcuminoid emulsion

In this study, the effect of matrix compounds from natural curcuminoid resources on the stability of curcuminoids and emulsions thereof was evaluated. Curcuminoid emulsions were prepared curcuminoid rich sources (curcuminoid extract, an aqueous turmeric concentrate and turmeric powder) with medium-chain triglyceride oil as lipid phase, lecithin, and pectin as emulsifiers. The curcuminoid emulsions were exposed to light in the visible wavelength range (300 nm–800 nm) at the specific energy input of 0.47 kW/m2 for 7 days and to the temperature of 4 °C, 25 °C, 40 °C for 49 days. The total curcuminoid retention (TC), droplet size (DS) change, instability index (InI), and yellowness reduction (YR) was observed during the storage time. The half-life of curcuminoids in emulsions was increased to 21 h, while the half-life of free curcuminoids was 1.3 h in the light exposure test. The co-compounds from the curcuminoid sources contributed to the emulsion stability by increasing the viscosity. In the thermal exposure test, the matrix compound system retained more than 93% curcuminoids after 49 days of storage at 40 °C, whereas the phase separation increased significantly. However, the TC reduction was independent of the InI change and droplet agglomeration. The YR depended on the TC and the amount of co-components in the emulsion.

Spinor-induced instability of kinks, holes and quantum droplets

We address the existence and stability of one-dimensional (1D) holes and kinks and two-dimensional (2D) vortex-holes nested in extended binary Bose mixtures, which emerge in the presence of Lee–Huang–Yang (LHY) quantum corrections to the mean-field energy, along with self-bound quantum droplets. We consider both the symmetric system with equal intra-species scattering lengths and atomic masses, modeled by a single (scalar) LHY-corrected Gross–Pitaevskii equation (GPE), and the general asymmetric case with different intra-species scattering lengths, described by two coupled (spinor) GPEs. We found that in the symmetric setting, 1D and 2D holes can exist in a stable form within a range of chemical potentials that overlaps with that of self-bound quantum droplets, but that extends far beyond it. In this case, holes are found to be always stable in 1D and they transform into pairs of stable out-of-phase kinks at the critical chemical potential at which localized droplets turn into flat-top states, thereby revealing the connection between localized and extended nonlinear states. In contrast, we found that the spinor nature of the asymmetric systems may lead to instability of 1D holes, which tend to break into two gray states moving in the opposite directions. Remarkably, such instability arises due to spinor nature of the system and it affects only holes nested in extended modulationally-stable backgrounds, while localized quantum droplet families remain completely stable, even in the asymmetric case, while 1D holes remain stable only close to the point where they transform into pairs of kinks. We also found that symmetric systems allow fully stable 2D vortex-carrying single-charge states at moderate amplitudes, while unconventional instabilities appear also at high amplitudes. Symmetry also strongly inhibits instabilities for double-charge vortex-holes, which thus exhibit unexpectedly robust evolutions at low amplitudes.

Influence of added dye on Marangoni-driven droplet instability

When studying multiphase flows, it is customary to add a dye to one of the phases to enhance the contrast between different phases. However, many dyes have surface-active effects which, while minute in stationary measurements, can have a significant influence on interfacial phenomena out of equilibrium. In this study, we quantify the consequences of dye addition for the Marangoni Bursting phenomenon, and we offer some insights on the dramatic effects caused by the dye. Furthermore, we show two different, straightforward approaches to quantify the contrast enhancement gained through dye addition.

Instability and self-propulsion of active droplets along a wall

In experiments, chemically active drops most often swim along a rigid wall, the impact of which on self-propulsion is in general completely overlooked in models. Using linear stability analysis, we demonstrate here that the proximity to a rigid surface promotes droplet propulsion as a result of the localization of the strongest interfacial flows within the thin lubrication gap separating the droplet from the wall.

Towards efficient solar demulsification (I): A solar electrical role on interfacial film of emulsions

Oil/water separation has become a significant issue since emulsions are detected in many industrial wastewater and severely threaten the ecological environment. Developing separation technique driven by solar energy with high separation efficiency is quite meaningful and challenging. In this work, solar electrical role on an interfacial film of emulsions was developed for efficient oil/water separation. Emulsified oil/water droplets from the ASP flooding were chosen to characterize the influence of solar electrical role on demulsification. Droplet size distribution, zeta potential, interfacial tension, viscosity, microscope image, and rheological properties were systematically studied to explore the mechanism of electrical role on the instability of oil droplets. Experimental results indicate solar electrical role on the oil-water interface can change the properties of the interface, help the oil-water emulsion instability, and promote oil-water separation. The instability mechanism of the emulsion can be summarized as the following. Firstly, the charged oil/water droplets interfere with the original electric field, which generates a force along with the droplet's tangential direction. Secondly, the force drives charged groups to radially migrate away from the electrode, which causes the entangled polymer to loosen, expand, and rearrange interface charges, resulting in decreasing in the rigidity and zeta potential. Thirdly, the force also promotes the electrohydrodynamic flow of oil/water droplets. The adjacent oil/water droplets mutually entrained in electrohydrodynamic fluid flow, resulting in coalescence. Furthermore, the interface-active components can be electrochemically oxidized to small molecular. The in-depth research of solar electrical role on interfacial film provides theoretical support for coupling of solar photo effect and thermal effect, to obtain highly efficient solar demulsification.

Interfacial Instability as Shaping Mechanism for Polystyrene Particles with Tunable Surface Texture

AbstractSurfactant‐driven interfacial instability of emulsion droplets has recently emerged as a means for shaping polymeric particles with controllable surface texture. This paper presents a suspension polymerization‐based method to produce surface‐textured polystyrene particles by inducing an instability at the interface of prepolymerized emulsion droplets followed by rapid cooling. The interaction of two surface‐active components, i.e., arachidic acid and cetyltrimethylammonium bromide (CTAB) at the phase boundary triggers the interfacial deformation at a specific point in time. Rapid cooling freezes the deformed emulsion droplets in a nonequilibrium state. Viscosity is proposed as the key parameter influencing the particle morphology, which can be controlled by easily adjustable factors such as initiator concentration, temperature, time, and cooling rate. Contact angle hysteresis measurements of particle thin layers spread on a flat substrate reveal a strong influence of the particle surface texture on the wetting behavior. The presented particle synthesis method requires no specialized equipment and has a high potential for upscaling, making it promising for various applications including hydrophobic coatings, catalyst supports, separation technology, tissue engineering, or drug delivery.

Effect of process parameters on emulsion stability and droplet size of pomegranate oil-in-water

The development of efficient emulsion is essential and requires a good understanding of the parameters that govern the formation and stability of the emulsion. The droplet size significantly affects the stability of the emulsion. In this study, the stability of pomegranate oil-in-water emulsions (0.5 to 7.0% v/v) was investigated using various emulsifiers in terms of droplet size and instability index during 16 days of storage. The Mastersizer and Lumisizer were used to measure the droplet size and instability index. It was observed that the minimum droplet size was achieved by using 0.3% carboxy methyl cellulose (5.37 μm) and maximum with 1.0/2.5% whey protein/maltodextrin (24.26 μm). The Lumisizer results during storage revealed the higher emulsion stability of carboxy methyl cellulose due to smaller droplet size and high thickness as compared to other emulsions studied. The findings of the present study would be useful for food applications to obtain fine and stable microcapsules.

Simulation of binary droplet collision with different angles based on a pseudopotential multiple-relaxation-time lattice Boltzmann model

Understanding the physics of droplet collisions is of crucial importance in many industrial processes such as inkjet printing, spray cooling, and spray combustion. In this paper, a modified multiple-relaxation-time pseudopotential multiphase lattice Boltzmann model is proposed to investigate binary droplet collision with different collision angles. The surface tension can be adjusted independently under the condition of high Reynolds number and large density ratio using this model. Then the model validation is performed by simulating three benchmark cases including stationary droplet, oscillating droplet, and capillary wave instability, respectively. Finally, different regimes of binary droplet collision are presented, and collision features such as liquid filament and satellite droplet are investigated at collision angles of 30°, 60°, 90°, 120°, and 150°. The results show that there is no liquid filament or satellite droplet at the collision angle of 30°, yet both of them appear as the angle increases. When the collision angle is 60°, the percentage of satellite droplet to liquid phase reaches 30%, while the proportions are less than 10% at collision angles of 90°, 120°, and 150°. Besides, two break-up mechanisms of “end-pinching” and capillary wave instability are inspected. It is found out that the back-flow phenomena at the joints of droplets and the liquid filament are caused by the “end-pinching” mechanism, which further leads to the capillary wave instability of the liquid filament.