Droplet Volume Fraction Research Articles

EFFECT OF Sn ON THE SOLIDIFICATION PROCESS AND MICROSTRUCTURE OF Al-Pb MONOTECTIC ALLOYS

When a homogeneous, single-phase liquid of monotectic alloy is cooled into the miscibility gap, the components are no longer miscible and two liquid phases develop. Generally, the liquid-liquid decomposition causes the formation of the microstructure with serious phase segregation. Many efforts have been made to use the demixing phenomenon for the production of well dispersed composite materials. It has been demonstrated that the rapid directional solidification technique is an effective method to prevent the formation of the phase segregated microstructure in immiscible alloys. Directional solidification experiments were carried out to study the influence of the addition of Sn on the solidification process of Al-Pb alloys. The experimental results show that the addition of a small amount of Sn causes a decrease in the interface energy between the matrix and the minority phase liquids and, thus, an increase in the nucleation rate of the minority phase droplets during the liquid-liquid phase transformation or a decrease in the average size of the minority phase particles. With the increase of the Sn content, both the volume fraction of the minority phase droplets and the temperature range of the liquid-liquid phase zone and the liquid-liquid-solid tertiary phase zone of the phase diagram increase. These are favorable for the coarsening of the minority phase droplets. The addition of Sn leads to the formation of a dendrite solid/liquid interface. This may promote the formation of a well dispersed microstructure and shows great effect on the distribution of the minority phase particles.

In Situ Observation on the Dynamic Process of Evaporation and Crystallization of Sodium Nitrate Droplets on a ZnSe Substrate by FTIR-ATR

Sodium nitrate is a main component of aging sea salt aerosol, and its phase behavior has been studied repeatedly with wide ranges observed in the efflorescence relative humidity (RH) in particular. Studies of the efflorescence dynamics of NaNO3 droplets deposited on a ZnSe substrate are reported, using an in situ Fourier transform infrared attenuated total reflection (FTIR-ATR) technique. The time-dependence of the infrared spectra of NaNO3 aerosols accompanying step changes in RH have been measured with high signal-to-noise ratio. From the IR difference spectra recorded, changes of the time-dependent absorption peak area of the O-H stretching band (ν-OH, ∼3400 cm(-1)) and the nitrate out-of-plane bending band (ν2-NO3(-), ∼836 cm(-1)) are obtained. From these measurements, changes in the IR signatures can be attributed to crystalline and solution phase nitrate ions, allowing the volume fraction of the solution droplets that have crystallized to be determined. Then, using these clear signatures of the volume fraction of droplets that have yet to crystallize, the homogeneous and heterogeneous nucleation kinetics can be studied from conventional measurements using a steady decline in RH. The nucleation rate measurements confirm that the rate of crystallization in sodium nitrate droplets is considerably less than in ammonium sulfate droplets at any particular degree of solute supersaturation, explaining the wide range of efflorescence RHs observed for sodium nitrate in previous studies. We demonstrate that studying nucleation kinetics using the FTIR-ATR approach has many advantages over brightfield imaging studies on smaller numbers of larger droplets or measurements made on single levitated particles.

Self-limiting droplet fusion in ionic emulsions.

We make an oil-in-water emulsion, which is initially stabilized using a first ionic surfactant, and mix it with a solution of a second ionic surfactant having the opposite charge, thereby inducing massively parallel droplet fusion. A transient disruption of the screened-charge repulsive barrier between interacting droplets, caused by the second ionic surfactant, arises from significant yet temporary charge neutralization of the first ionic surfactant on the surfaces of the oil droplets while mixing occurs. Interestingly, if a moderate molar excess of one surfactant exists, then the resulting emulsion re-stabilizes after limited droplet fusion. By adjusting the droplet volume fraction, concentrations of first and second surfactants, and volumes of the emulsion and the solution of the second surfactant, we control the degree of droplet coalescence and achieve a self-limiting droplet fusion process. Using optical microscopy, we observe that flat, thin, crystalline films can form between the two oil compartments after fusion of two or more immiscible microscale droplets. However, no such crystalline films are seen on the highly curved oil-oil interfaces inside nanoscale droplets that are composed of two or more immiscible oils and have been fused in the same manner, as revealed by cryogenic transmission electron microscopy.

Nanoemulsions containing Vitamin E acetate prepared by PIC(phase inversion composition) methods: Factors affecting droplet sizes

We have investigated the influence of system composition and preparation conditions on the particle size of vitamin E acetate (VE)-loaded nanoemulsions prepared by PIC, phase inversion composition emulsification. This method relies on the formation of very fine oil droplets when water is added to oil/surfactant mixture. The oil-to-emulsion ratio content was kept constant (5 wt.%) while the surfactant-to-oil ratio (%SOR) was varied from 50 to 200 %. Oil phase composition (vitamin E to medium chain ester ratio, %VOR) had an effect on particle size, with the smallest droplets being formed below 60 % of VOR. Food-grade non-ionic surfactants (Tween 80 and Span 80) were used as an emulsifier. The effect of f on the droplet size distribution has been studied. In our system, the droplet volume fraction, given by the oil volume fraction plus the surfactant volume fraction, was varied from 0.1 to 0.3. The droplet diameter remains less than 350 nm when O/S is fixed at 1:1. The droplet size increases gradually as the increasing the volume fraction. Particle size could also be reduced by increasing the temperature when water was added to oil/ surfactant mixture. By optimizing system composition and homogenization conditions we were able to form VE-loaded nanoemulsions with small mean droplet diameters (d < 50 nm). The PIC emulsification method therefore has great potential for forming nanoemulsion-based delivery systems for food, personal care, and pharmaceutical applications.

Linear and nonlinear rheology of dense emulsions across the glass and the jamming regimes

We discuss the linear and nonlinear rheology of concentrated microscale emulsions, amorphous disordered solids composed of repulsive and deformable soft colloidal spheres. Based on recent results from simulation and theory, we derive quantitative predictions for the dependences of the elastic shear modulus and the yield stress on the droplet volume fraction. The remarkable agreement with experiments we observe supports the scenario that the repulsive glass and the jammed state can be clearly identified in the rheology of soft spheres at finite temperature while crossing continuously from a liquid to a highly compressed yet disordered solid.

Micellar interactions in water-AOT based droplet microemulsions containing hydrophilic and amphiphilic polymers

We investigate the influence of addition of hydrophilic and amphiphilic polymer on percolation behavior and micellar interactions in AOT-based water-in-oil droplet microemulsions. We focus on two series of samples having constant molar water to surfactant ratio W = 20 and constant droplet volume fraction Φ = 30%, respectively. From dielectric spectroscopy experiments, we extract the bending rigidity of the surfactant shell by percolation temperature measurements. Depending on droplet size, we find stabilization and destabilization of the surfactant shell upon addition of hydrophilic poly(ethylene glycol) (PEG) (Mn = 3100 g mol(-1)) and amphiphilic poly(styrene)-b-poly(ethylene glycol) copolymer with comparable length of the hydrophilic block. Complementary small angle X-ray scattering experiments corroborate the finding of stabilization for smaller droplets and destabilization of larger droplets. Subsequent analysis of dielectric spectra enables us to extract detailed information about micellar interactions and clustering by evaluating the dielectric high frequency shell relaxation. We interpret the observed results as a possible modification of the inter-droplet charge transfer efficiency by addition of PEG polymer, while the amphiphilic polymer shows a comparable, but dampened effect.

Influence of the inner droplet fraction on the release rate profiles from multiple W/O/W emulsions

Water-in-oil-in-water (W/O/W) double emulsions were prepared and the kinetics of release of magnesium ions from the internal to the external water phase was investigated as a function of the inner droplet volume fraction, at constant temperature (T=25°C). All the emulsions were formulated using Miglyol as the oil phase and the same surface-active species, namely polyglycerol polyricinoleate (oil-soluble) and sodium caseinate (water-soluble). Also, the internal droplet and oil globule diameters were almost identical for all systems under study. The external aqueous phase was a lactose solution at approximately the same osmotic pressure as that of the inner droplets, in order to avoid osmotic water transfer phenomena. We probed two different initial configurations in terms of inner droplet composition: (i) “one-component” systems where all droplets were composed of the same MgCl2 solution, and (ii) a “two-component” systems, where droplets containing MgCl2 and lactose coexisted. In all cases, coalescence of the inner droplets on the oil globule surface was marginal so that magnesium leakage was mainly due to diffusion phenomena. The rate of magnesium release was lowered by increasing the inner droplet fraction, regardless of the initial configuration. Assuming that the transfer of the encapsulated species mainly occurs via thin liquid films, such evolution could reflect the increasing tortuosity of the diffusion pathways with the inner droplet fraction.

Numerical study of droplet breakup and merging in a microfluidic channel

Droplet breakup and merging in a microfluidic channel, which are applied to lab-on-a-chip devices for biomedical testing and synthesis, are simulated numerically by solving the conservation equations of mass and momentum. The droplet surface is computed using the volume-of-fluid method of the commercial code FLUENT. The numerical simulation demonstrates that the variation of obstacle geometry in a microchannel determines the droplet breakup pattern and the volume fraction of split droplets. The computation also shows that droplet merging depends on the channel-chamber width ratio. The effect of microchannel and obstacle configuration on the droplet motion is investigated to find the optimal conditions for droplet breakup and merging.

Concentration induced ordering of microemulsion droplets in bulk and near the liquid–air interface

The structure of the air interface of water–AOT (sodium bis(2-ethyl hexyl) sulfosuccinate)–decane microemulsions in the droplet phase with ω = 20, 30, 40 is studied in a range of compositions with droplet volume fractions of ϕ = 0.2 to 0.7. A combination of small angle X-ray scattering (SAXS) and X-ray reflectometry (XR) reveals a detailed image of the morphology in the bulk and at the microemulsion–air interface. The structure of the volume is described in terms of effective structure factors to obtain the bulk droplet structure and correlations. With increasing droplet density ϕ the droplets remain unchanged in their size and show an increase in the local order. The reflectivity behaviour of the samples can be consistently explained by bulk small angle scattering and specular reflectivity at the surface. The microemulsion–air interface consists of droplets with the same ϕ-dependent local order as in the bulk, e.g. we found a droplet volume fraction induced ordering of droplets.

Slow dynamics and intermittent quakes in soft glassy systems

We present measurements of the thermal fluctuations of the free surface of oil-in-water emulsions which exhibit a glassy behavior. The Surface Fluctuation Specular Reflection (SFSR) technique was applied to measure the free surface fluctuations. When the volume fraction of the oil droplets is close to or larger than the disordered packing volume fraction, the emulsion ages and its free surface exhibits abnormal fluctuations, consisting of rare but large amplitude quakes. From a statistical analysis of the signal, we observe that the relative importance of such intermittent quakes grows as the system ages. More precisely, we observe for the first time that the kurtosis of the fluctuations increases with aging time with an exponent of about 1.3. These quakes correspond to large changes in the local slope of the free surface over a few tenths of a second. We conjecture that such quakes reflect the dynamics peculiar to glassy systems driven by the relaxations of internal stress.

AOT microemulsions: droplet size and clustering in the temperature range between the supercooled state and the upper phase boundary

The main focus of this work is the investigation of the mechanism of droplet aggregation and the droplet structure of water–AOT (sodium bis(2-ethyl hexyl) sulfosuccinate)–decane microemulsions between the supercooled state and the upper phase boundary by means of SAXS. We study the temperature dependent droplet structure and size in detail at compositions of droplet volume fractions ϕ = 0.1–0.4 and water/AOT molar ratios of ω = 5–40. We found a change in droplet size at varying temperatures and explain our results by the change of the effective droplet surface covered by an AOT molecule (between the two phase boundaries) and by macro-phase separated water ice below a certain supercooling temperature. We monitor the continuous change of droplet interaction by means of a phenomenological structure factor combining hard sphere interactions with a Lorentzian function accounting for droplet density fluctuations in the system. We analyse the droplet aggregates in dependency on ϕ and ω leading to a better understanding of droplet aggregation with increasing temperature.

Vorticity Banding in Biphasic Polymer Blends

Pattern formation under the action of flow is a subject of considerable scientific interest with applications going from microfluidics to granular materials. Here, we present a systematic investigation of shear-induced banding in confined biphasic liquid-liquid systems, i.e., formation of alternating regions of high and low volume fraction of droplets in a continuous phase (shear bands). This phenomenon is investigated in immiscible polymer blends sheared in a sliding parallel plate flow cell. Starting from a spatially uniform distribution of droplets, the formation of bands aligned along the flow direction is observed, eventually leading to an almost complete separation between droplet-rich and continuous phase regions. The initial band size is related to the gap dimension; the merging of bands and consequent spacing reduction has also been observed for long times. Shear banding is only observed when the viscosity of the dispersed phase is lower as compared to the continuous phase and in a limited range of the applied shear rate. Rheological measurements show that band formation is associated with a viscosity decrease with respect to the homogeneous case, thus implying that system microstructure is somehow evolving toward reduced viscous dissipation under flow.

Highly Stable Concentrated Nanoemulsions by the Phase Inversion Composition Method at Elevated Temperature

Oil-in-water nanoemulsions were produced in the system water/Span 80-Tween 80/paraffin oil via the phase inversion composition (PIC) method at elevated temperature. With the increase of preparation temperature from 20 to 70 °C, we found that the emulsion droplet diameter decreases from 10.3 μm to 51 nm, proving the formation of nanoemulsions. The viscosity of nanoemulsions clearly increases with droplet volume fraction, φ, but the droplet size changes less. Significantly, at φ ≤ 0.5, the size distribution of nanoemulsions can be kept unchangeable more than 5 months. These results proved that the highly viscous paraffin oil can hardly be dispersed by the PIC method at 25 °C, but the increase in preparation temperature makes it possible for producing monodisperse nanoemulsions. Once the nanoemulsion is produced, the stability against Ostwald ripening is outstanding due to the extremely low solubility of the paraffin oil in the continuous phase. The highly stable nanoemulsions are of great importance in practical applications.

The effect of polymeric surfactants on the rheological properties of nanoemulsions

We have investigated the rheological properties of submicron emulsions and how they are affected by the structure of polymeric surfactants. We have prepared oil-in-water emulsions stabilized with five steric surfactants, two of them belonging to the Myrj family and three belonging to the Pluronic family, with key differences on their structures. Droplet size and volume fraction have been kept constant to analyze only the influence of the surfactant. The viscoelasticity has been characterized by dynamic oscillatory shear experiments, while the shear viscosity was measured during steady shear flow tests. The results show a qualitatively similar gel-like behavior for all the emulsions, but with remarkable quantitative differences. Surfactants with longer hydrophilic tails produced emulsions with higher viscoelasticity. Pluronics, having a central hydrophobic part between two hydrophilic tails, produced emulsions with notably higher viscoelasticity and yield stress than Myrjs with comparable hydrophilic tails. The reason for this seems to be a more efficient steric barrier at the interface, induced by this central hydrophobic part.

Sharp acoustic multipolar-resonances in highly monodisperse emulsions

We report the achievement of highly monodisperse emulsions exhibiting about ten acoustic Mie resonances. Thanks to robotics, the effective acoustic properties of such strongly scattering media can be precisely targeted by means of the production of calibrated (random) liquid-droplets. Ultrasonic experiments are compared, with an excellent quantitative agreement, to theoretical predictions derived within the framework of the independent scattering approximation. The dependence of the sound speed and of the acoustic attenuation on both the size and the volume fraction of droplets is quantitatively examined for dilute and more concentrated emulsions, and is presented in a dimensionless way.

Direct numerical simulation of surfactant-stabilized emulsions

A 3D lattice Boltzmann model for two-phase flow with amphiphilic surfactant was used to investigate the evolution of emulsion morphology and shear stress in starting shear flow. The interfacial contributions were analyzed for low and high volume fractions and varying surfactant activity. A transient viscoelastic contribution to the emulsion rheology under constant strain rate conditions was attributed to the interfacial stress. For droplet volume fractions below 0.3 and an average capillary number of about 0.25, highly elliptical droplets formed. Consistent with affine deformation models, gradual elongation of the droplets increased the shear stress at early times and reduced it at later times. Lower interfacial tension with increased surfactant activity counterbalanced the effect of increased interfacial area, and the net shear stress did not change significantly. For higher volume fractions, co-continuous phases with a complex topology were formed. The surfactant decreased the interfacial shear stress due mainly to advection of surfactant to higher curvature areas. Our results are in qualitative agreement with experimental data for polymer blends in terms of transient interfacial stresses and limited enhancement of the emulsion viscosity at larger volume fractions where the phases are co-continuous.

Gelation behaviour and rheological properties of acid-induced soy protein-stabilized emulsion gels

The protein concentration dependence on the rheological properties of acid-induced gels formed with unheated and heated soy protein-stabilized emulsions (UHSPE and HSPE) was investigated at different acidification temperatures. Pre-heat treatment on soy protein solutions resulted in a higher storage modulus (G′) and a shorter gelation time (tgel) of acid-induced emulsion gels. A maximum in tan δ was observed in the UHSPE gels but no maximum was detected in the HSPE gels. Increasing the acidification temperature decreased the G′ and tgel. The dependence of the G′ on the protein concentration (c) can be scaled with a power law: G′ ∼ cA. The exponent (A) increased with pre-heat treatment and acidification temperature. The experimental data.fitted the fractal scaling model (G′ ∼ φA) and the simple time- scaling model above very well for the acid-induced soy protein-stabilized HSPE gels with varying oil volume fraction. The large deformation and fracture properties were significantly affected by soy protein concentration, pre-heat treatment, acidification temperature and volume fraction of oil droplets (p < 0.05).

Water–AOT–alkylbenzene microemulsions: Influence of alkyl chain length on structure and percolation behavior

We study the percolation behavior of the water-in-oil (w/o) droplet phase of AOT (sodium bis[2-ethylhexyl] sulfosuccinate)-based microemulsions with different alkylbenzenes (toluene, ethylbenzene, butylbenzene or octylbenzene) as oil phase. We use microemulsions of varying composition with molar water to surfactant ratios 0⩽W⩽ 50 and droplet (water plus surfactant) volume fractions 10%⩽ϕ⩽50%. Using dielectric spectroscopy, a percolation transition is observed in w/o microemulsions with butylbenzene or octylbenzene. With increasing molecular weight of the alkylbenzene, the percolation temperature TP decreases. The structure of the microemulsions is determined by small angle X-ray scattering (SAXS). With increasing molar weight of the alkylbenzene, the stability range of the L2 droplet phase extends to higher W. The larger amount of solubilizable water can be related to variable oil penetration of the AOT monolayer, which affects the spontaneous curvature of the surfactant shell.

Investigating the evolution of the phase behavior of AOT-based w/o microemulsions in dodecane as a function of droplet volume fraction

AOT-based water in oil (w/o) microemulsions are one of the most extensively studied reverse micellar systems because of their rich phase behavior and their ability to form in the absence of any co-surfactant. The aggregation characteristics and interaction of the microemulsion droplets in these systems are known to be governed by AOT–oil compatibility and water to AOT molar ratio (w). In this manuscript by using Dynamic Light Scattering (DLS) and viscometry techniques, we show that droplet volume fraction too plays an important role in shaping the phase behavior of these microemulsions in dodecane. The phase separation characteristics and the evolution of the viscosity and the hydrodynamic radius of the microemulsion droplets on approaching the cloud points have thus been found to undergo complete transformation as one goes from low to high droplet volume fraction even at a fixed ‘w’. Modeling of the DLS data attributes this to the weakening of inter droplet attractive interaction caused by the growing dominance of the excluded volume effect with increase in droplet volume fraction. In the literature, the inter droplet attractive interaction driven phase separation in these microemulsions is explained based on gas–liquid type phase transition, conceptualized in the framework of Baxter adhesive hard sphere theory. The modeling of our viscosity data, however, does not support such proposition as the characteristic stickiness parameter (τ−1) of the microemulsion droplets in this system remains much lower than the critical value (τc-1≈10.25) required to enforce such phase transition.

Optically probing nanoemulsion compositions

Many types of colloids, including nanoemulsions, which contain sub-100 nm droplets, are dispersed in molecular and micellar solutions, especially surfactant solutions that confer stability. Since it would be desirable to measure the droplet volume fraction ϕ and surfactant concentration C of a nanoemulsion non-destructively, and since the droplet and surfactant structures are significantly smaller than the shortest wavelengths of visible light, optical refractometry could provide a simple and potentially useful approach. By diluting a silicone oil-in-water nanoemulsion having an unknown ϕ and C with pure water, measuring its refractive index n(ϕ,C) using an Abbé refractometer, and fitting the result using a prediction for n that treats the nanoemulsion as an effective medium, we show that ϕ and C can be deduced accurately over a relatively wide range of compositions. Moreover, we generalize this approach to other types of nanoemulsions in which a molecular constituent partitions in varying degrees between the dispersed and the continuous phases.