Rupture Of Droplets Research Articles

Numerical simulation of droplet characterized by Rolie–Poly model with finite extensibility passing through cylinder obstacles

The deformation and rupture of viscoelastic droplet passing through cylinder obstacles in a microchannel are investigated using OpenFOAM. The constitute relationship of droplet is modeled by the Rolie–Poly model with finite extensibility, and the two-phase interface is tracked by the volume of fluid method. The effects of capillary number (Ca), the distance between cylinders (l1), relaxation time ratio (ξ), Weissenberg number (Wi), etc., on droplet deformation and rupture are mainly explored. When Ca decreases, the symmetry of droplet rupture changes and three behaviors of the droplet, i.e., symmetrical rupture, asymmetrical rupture, and non-rupture, can be captured. Further research shows that the stagnation area formed between cylinders is broken with the increase in l1, where the two sub-droplets merge again. Viscoelastic droplet with a smaller relaxation time ratio ξ is more likely to extend into thin and durable filament. Especially, when ξ=0.2, the filament will break many times during the stretching process. During above-mentioned two kinds of development, the normal stress difference develops obviously at the places, where the filament breaks or the sub-droplets combine together. This may imply that the normal stress difference facilitates the rupture and coalescence of droplets. In addition, with the increase in elasticity, the normal stress difference tends to develop at the phase interface.

Improvement of foaming and emulsifying properties of SPI via weak-base synchronized membrane separation: Insight from tuning the interfacial flexibility

Preparation of soy proteins with high functionalities is significant for enhancing their utilization. Here, we propose a green and mild weak-base synchronous membrane separation (MS) technique to enrich soy protein isolate (SPI), with two traditional extraction methods, alkali extraction-isoelectric point precipitation (AE) and salt extraction (SE), as the control. The interfacial properties of SPI were investigated by Cryo-SEM, interfacial rheology (dilatational and shear), and Langmuir-Blodgett films combined with AFM. Results showed that MS extraction significantly reduced the disulfide bond content of SPI, which induced partial depolymerization of the protein and exposed the internal hydrophobic groups, and thus leading to the reduction of particle size as well as the increase of surface hydrophobicity (H0). These structural properties conferred MS-SPI with a faster interfacial adsorption rate, followed by easier unfolding and rearrangement after anchoring to the interface. Finally, the flexible and denser interfacial layers with high mechanical properties were formed through strong in-plane interactions among SPI, which could effectively prevent the rupture of oil droplets and air bubbles. Therefore, the emulsification stability and foamability of MS-SPI could be maintained for up to ∼252 min and ∼232%, which was at least 2.4 times and 1.2 times than that of AE-SPI and SE-SPI, respectively. These findings demonstrated that MS extraction was an effective approach for broadening the applications of soy proteins in the food ingredient field.

Rapid preparation of Ni/Fe-LDH@ stainless steel mesh with interfacial emulsion breaking effect for efficient oil-in-water emulsion separation

Oil-water separation materials are particularly important for today's polluted water, especially the easy preparation of separation materials and the efficient separation of emulsions. The excellent properties of layered double hydroxide (LDH) have led to its successful application in the field of oil-water separation, but most of the existing studies have used high-temperature and long-time hydrothermal methods for the growth of LDH. Here, we present a stainless-steel mesh material for the preparation of LDH by electrochemical methods, which can be prepared in a few minutes, greatly improving the production efficiency. Adjusting the electrodeposition time of LDH can well regulate the aperture of the stainless-steel mesh to realize the sieving of emulsion droplets, in addition, the positive charge of LDH is helpful to promote the rupture of emulsion droplets and improve the emulsion separation efficiency. The separation efficiency of this rapidly prepared separation material for oil-in-water emulsions was stabilized at more than 99.3%. In addition, the preparation with low energy consumption is more in line with the concept of green development and more conducive to practical applications.

Dewetting regimes of condensation droplets in a microgroove

This paper describes a numerical investigation of the groove-embedded droplet dewetting process, namely the spontaneous transition from the Wenzel state to the Cassie state, using the multiphase lattice Boltzmann method. Numerical simulations are employed to reproduce the dynamic behaviors of extension, squeezing, rupture, and ejection of condensation droplets in a groove, allowing us to examine how the groove geometry and wettability affect the dewetting process. Our results identify three dewetting regimes, namely retention, partial dewetting, and complete dewetting. As the groove aspect ratio and hydrophilicity decrease, the dewetting regime changes from retention to partial dewetting, and then to complete dewetting. The partial dewetting and complete dewetting are two effective ways for droplet removing. In particular, a groove sidewall with enhanced hydrophobicity is desirable to stimulate the dewetting process.

Bubble-Induced Rupture of Droplets on Hydrophobic and Lubricant-Impregnated Surfaces.

Bursting of bubbles is ubiquitous with numerous applications ranging from spraying of pesticides, drug delivery, and inkjet printing to forming emulsions. Understanding the parameters that influence the dynamics of bubble rupture is crucial to design systems with improved performance. Here, we experimentally investigate the behavior of air-bubble-induced rupture of a sessile droplet placed on hydrophobic and lubricant-impregnated surfaces (LIS). We demonstrate that the bubble dynamics inside a sessile droplet and subsequent rupture of the thinning film is dependent on the nature of the underlying substrate and the thermodynamic state of the droplet on the substrate. The growth of the bubble is shown to be dependent on the contact angle hysteresis of water and air on the substrate and the presence of a cloaking oil film around the water droplet. On a plain and textured surface with a high contact angle hysteresis, bubble-induced rupture initiates at the apex of the droplet. In the case of LIS that offers negligible pinning forces, the bubble-induced rupture of liquid shell initiates near the triple contact line. The dynamics of rupture of droplets placed on LIS is shown to be dependent on the viscosity of the impregnating and cloaking lubricant.

Demulsification and breaking mechanism of variable quartz concentrates obtained from sand

This study reports the efficacy of quartz particles extracted from sand for the destabilization of crude oil emulsions and efficient water removal. Demulsification performance of quartz concentrates obtained from stacked sieves with openings: 75, 125, 250 and 300 μm and quartz quantity varying from 1% to 7% were investigated using the bottle test technique at a temperature of 90 °C. The breaking mechanism of emulsion driven by quartz was analyzed by rheology, Fourier Transform Infrared (FTIR) spectroscopy, X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM). Experimental results showed that the quartz concentrates generated from stacked sieves: 75, 125, 250, and 300 μm had optimal demulsification efficiency (DE) of 98%, 98%, 99%, and 99%, respectively. Moreover, the quartz concentration needed to attain these DEs levels ranges from 3 to 5%. In addition, the performance of quartz as a demulsifier was compared with a commercial demulsifier (ethyl cellulose). Viscosity versus time measurements demonstrated the change in emulsion's viscosity before and after quartz addition. A drastic reduction in the emulsion's viscosity was observed after quartz addition, indicating rupture and coalescence of water droplets. The FTIR and XRD analysis buttressed the functional groups and structural pattern while optical microscope and SEM analysis exhibited the morphology of quartz particles before and after the demulsification processes. A mechanism elucidating the breaking of crude oil emulsion by quartz is proposed. The results indicated that quartz particles processed from the sand will be useful as demulsifiers in the petroleum industry.

Evolution of the Size Distribution of an Emulsion under a Simple Shear Flow

Understanding the rheology of immiscible liquids mixtures, as well as the role played by its micro-structures are important criteria for the production of new materials and processes in industry. Here, we study changes over time of the droplet size distributions of emulsions induced by slow shearing flows. We observe that the initial heterogeneous microstructure may evolve toward more complex structures (such as bimodal distribution) as a result of coalescence and rupture of droplets. These dynamic structures were produced using a flow cell made up of two parallel disks, separated by a gap of 100 µm. The steady rotation of the lower disk generates a simple shear flow of γ˙=0.75 s−1, during ~400 s. After a brief rest time, this procedure was repeated by applying a step ramp until the maximum shear rate of 4.5 s−1 was reached, using step increments of 0.75 s−1. During the last portion of the flow and during the rest time in between flows, structures of emulsions were characterized. Initially, a broad single-peak distribution of drops was observed, which evolved toward a rather narrower bimodal distribution, at first due to the coalescence of the smaller droplets and subsequently of the larger drops. The rupture of drops at higher shear rates was also observed. The observed evolutions also presented global structures such as “pearl necklaces” or “bands of particles”, the latter characterized by alternating bands of a high density of particles and regions of the continuous phase with only a few droplets. These changes may indicate complex, time-dependent rheological properties of these mixtures.

Role of Acetone in the Formation of Highly Dispersed Cationic Polystyrene Nanoparticles

Abstract A modified emulsion polymerisation synthesis route for preparing highly dispersed cationic polystyrene (PS) nanoparticles is reported. The combined use of 2,2′-azobis[2-(2-imidazolin- 2-yl)propane] di-hydrochloride (VA-044) as the initiator and acetone/water as the solvent medium afforded successful synthesis of cationic PS particles as small as 31 nm in diameter. A formation mechanism for the preparation of PS nanoparticles was proposed, whereby the occurrence of rapid acetone diffusion caused spontaneous rupture of emulsion droplets into smaller droplets. Additionally, acetone helped to reduce the surface tension and increase the solubility of styrene, thus inhibiting aggregation and coagulation among the particles. In contrast, VA-044 initiator could effectively regulate the stability of the PS nanoparticles including both the surface charge and size. Other reaction parameters i.e. VA-044 concentration and reaction time were examined to establish the optimum polymerisation conditions.

Models of fragmentation with power law log-normal distributions

Two models of binary fragmentation are introduced in which a time-dependent transition size produces two regions of fragment sizes above and below the transition size. In the models, we consider a fixed rate of fragmentation for the largest fragment and two different rates of fragmentation for the two regions of sizes above and below the transition size. The models are solved exactly in the long time limit to reveal stable time-invariant solutions for the fragment size distributions. A rate of fragmentation proportional to the inverse of fragment size in the smaller size region produces a power-law distribution in that region. A rate of fragmentation combined of two terms, one proportional to the inverse of the fragment size and the other proportional to a logarithmic function of the fragment size, in the larger size region produces a log-normal distribution in that region. Special cases of the models with no fragmentation for the smaller fragments are also considered. The similarities between the stable distributions in our models and power-law log-normal distributions from experimental work on shock fragmentation of long thin glass rods and rupture of mercury droplets are investigated.

Emulsion multiple de type L/H/L: étude de l'obtention et du mécanisme de libération

Synopsis An O/W/O multiple emulsion was formulated using almond oil as the inner phase and liquid paraffin as the outer phase. The properties of the formulation ingredients were explored: effect of the type of primary emulsifier, and the nature and concentration of the secondary emulsifier. The volume fraction of the internal phase in the multiple emulsion was also optimized with regard to the stability. The multiple systems were assessed by evaluating several parameters such as the macroscopic aspect, the droplet size, the direction of the dispersion and the accelerated stability under centrifugation or elevated temperature. The effect of some additives (ozokerite, benzyl alcohol and glycerol) was examined. Whatever additive was used, a decrease in the formulation stability, which could be attributed to different phenomena, was observed. Furthermore, an analytical procedure was developed for studying the release of an active principle, alpha-tocopherol (vitamin E). This release could be a result of the instability of the multiple droplets or a diffusion through the water layer membrane. This procedure was based on the difference of chemical type between the two oil phases: mineral outer phase and fatty inner phase. Two chromatographic methods were used: a gas chromatographic assay of oleic acid which is the main component of the fatty phase, and a liquid chromatographic assay for the alpha-tocopherol, encapsulated in the inner phase. The presence of oleic acid in the paraffin phase originated from a rupture of the multiple droplets, whereas the presence of alpha-tocopherol in the outer phase could be due both to a diffusion phenomenon and to the instability of the formulation. The results obtained showed that only the rupture of the droplets occurred. Moreover, a part of the alpha-tocopherol was retained by Hypermer A 60 used as an emulsifier in the formulation.

Dynamics and rheology of complex interfaces. I

When a concentrated mixture of two immiscible fluid is sheared, a rather complex interface is formed due to the coagulation, rupture, and deformation of droplets. The dynamics and rheological properties of such system is discussed. A semiphenomenological kinetic equation is proposed which describes the time evolution of droplet size and orientation, and also the macroscopic stress in a flow field. The rheological properties are shown to be quite unusual: for example, the steady-state viscosity is independent of the shear rate, while the normal stress difference is nonzero and proportional to the magnitude of the shear rate.

Droplet Size Characteristics and Energy Input Requirements of Emulsions Formed Using High-Intensity-Pulsed Electric Fields

Experimental methods have been developed to measure droplet size characteristics and energy inputs associated with the rupture of aqueous droplets by high-intensity-pulsed electric fields. The combination of in situ microscope optics and high-speed video cameras allows reliable observation of liquid droplets down to 0.5 ..mu..m in size. Videotapes of electric-field-created emulsions reveal that average droplet sizes of less than 5 ..mu..m are easily obtained in such systems. Analysis of the energy inputs into the fluids indicates that the electric field method requires less than 1% of the energy required from mechanical agitation to create comparable droplet sizes.

Anomalous rheological characteristics of a high-internal-phase-ratio emulsion

High-internal-phase-ratio emulsions containing 97–98% liquid fuel as the dispersed phase, are shown to exhibit complex rheological behavior including reversible work hardening and apparent slip-flow anomalies. However, when proper precautions are taken to prevent contamination by demulsified fuel, steady flow in glass and steel tubes is characterized by the Bingham model up to a critical wall shear stress. At higher stresses a discontinuity in the flow curve is observed. Experiments in different diameter tubes are used to establish that the critical stress is associated with the rupture of emulsion droplets and the formation of a thin film of fuel at the wall of the tube. The apparent absence of a yield value and critical stress with PTFE tubes indicates that the film of fuel forms when the emulsion contacts a hydrophobic surface.

On the instability and rupture of droplets and bubbles in strong electric fields

An approximate quantitative theory of the equilibrium of a bubble or droplet in a uniform electric field is developed and applied to earlier experimental results. An explanation of the oscillation of bubbles in strong fields and the difference in behavior of positive and negative bubbles is explained on the basis of the discharge of electricity from points.