Droplet Size Determination Research Articles

Droplet Size Scaling of Water-in-Oil Emulsions under Turbulent Flow

The size of droplets in emulsions is important in many industrial, biological, and environmental systems, as it determines the stability, rheology, and area available in the emulsion for physical or chemical processes that occur at the interface. While the balance of fluid inertia and surface tension in determining droplet size under turbulent mixing in the inertial subrange has been well established, the classical scaling prediction by Shinnar half a century ago of the dependence of droplet size on the viscosity of the continuous phase in the viscous subrange has not been clearly validated in experiment. By employing extremely stable suspensions of highly viscous oils as the continuous phase and using a particle video microscope (PVM) probe and a focused beam reflectance method (FBRM) probe, we report measurements spanning 2 orders of magnitude in the continuous phase viscosity for the size of droplets in water-in-oil emulsions. The wide range in measurements allowed identification of a scaling regime of droplet size proportional to the inverse square root of the viscosity, consistent with the viscous subrange theory of Shinnar. A single curve for droplet size based on the Reynolds and Weber numbers is shown to accurately predict droplet size for a range of shear rates, mixing geometries, interfacial tensions, and viscosities. Viscous subrange control of droplet size is shown to be important for high viscous shear stresses, i.e., very high shear rates, as is desirable or found in many industrial or natural processes, or very high viscosities, as is the case in the present study.

Droplet Tracking for Single Molecule Confinement

We report on a robust system for tracking femtoliter and smaller aqueous droplets in oil over tens of minutes with a few nanometers accuracy in a 150 Hz bandwidth. Tracking permits the droplet position to be maintained in the detection volume of a confocal microscope without need for physical or optical trapping, permitting detailed optical analysis of droplet contents. In addition, droplet tracks can be used to determine droplet size. As an alternative to surface attachment or liposomal confinement, we use droplets to confine individual biomolecules for study using single-molecule fluorescence trajectories. Back focal plane imaging onto a position sensitive detector (PSD), similar to the configuration frequently used to determine the position of a particle in an optical tweezer, is used to determine droplet position with respect to the confocal detection volume. Our tracking system, inspired in part by the work in Ref. (1), utilizes a dedicated microprocessor to implement a feedback loop between the PSD and a fast piezoelectric flexure stage. Motion of the stage keeps the particle in the confocal detection volume. Here we present data characterizing the system and also report on progress in using tracked droplets in single-molecule studies of RNA. 1. Desai, K. V., T. G. Bishop, L. Vicci, E. T. O'Brien, R. M. Taylor, and R. Superfine. 2008. Agnostic particle tracking for three-dimensional motion of cellular granules and membrane-tethered bead dynamics. Biophys. J. 94:2374–2384.

Probing Microscopic Wetting Properties of Superhydrophobic Surfaces by Vibrated Micrometer-Sized Droplets

We determine contact angles of micrometer-sized NaCl-water droplets on superhydrophobic surfaces by analyzing their lowest-order axisymmetric vibrational resonances driven by vertical oscillations of the surface. Fluorescence spectra of the dye-doped droplets excited by laser light feature whispering-gallery modes (WGMs) whose spectral widths depend on the droplet vibration amplitude, thus enabling precise measurements of the droplet mechanical resonant frequency. Following droplet size determination by WGM mode-matching, we calculate the contact angles from the dependence of the measured mechanical resonant frequency on the droplet size for two surfaces with different superhydrophobicity levels, and find a good correlation with the values measured by direct imaging of millimeter-sized droplets.

Oil droplet size determination in complex flavor delivery systems by diffusion NMR spectroscopy

Droplet size distribution of flavor oils in two different solid flavor delivery systems were determined with pulsed field gradient NMR spectroscopy: yeast encapsulation system, a spray dried flavor encapsulation system based on empty yeast cells, and glassy encapsulation system, an extruded solid water soluble carbohydrate delivery system. The oil droplet sizes are limited by the yeast cell walls in the yeast encapsulation system and the size distribution is unimodal according to images from transmission electron microscopy. The droplet size determination with diffusion NMR is based on the Murday and Cotts theory of restricted diffusion of liquids in geometrical confinements. Good fits of the diffusion data could be obtained by applying a unimodal, log-normal size distribution model and average droplet sizes of about 2 μm were found that correspond approximately to the inner diameter of the yeast cells. Scanning electron microscopy images showed a multimodal droplet size distribution in the glassy extruded delivery systems. To fit the NMR data a bimodal log-normal distribution function with five independent fitting parameters was implemented that yielded consistent and robust results. The two size populations were found in the micron and sub-micron range, respectively. The method was sufficiently accurate to depict variation of droplet size distributions in glassy encapsulation systems of different formulation.

Role of operating conditions in determining droplet size and viscosity of tackifier emulsions formed via phase inversion

AbstractWater‐based pressure‐sensitive adhesives are formulated by combining a polymer latex with a tackifier emulsion. The latter is an oil‐in‐water emulsion made by the process of phase inversion. The phase inversion itself is carried out in a stirred tank fitted with a heating jacket by progressively adding water to a water‐in‐oil emulsion. The point of onset of phase inversion and the characteristics of the emulsion that is formed depend on process conditions; these include temperature, rate of water addition, and agitation speed. The role of these operating conditions is elucidated here. Increasing temperature delayed the onset of phase inversion slightly, but it did not affect emulsion particle size, provided it remained below a critical value. Agitation speed had to be increased upon increasing the water flow rate to prevent increasing the particle size. Finally, the point of onset of phase inversion could be predicted reasonably well using a model available in the literature. © 2010 American Institute of Chemical Engineers AIChE J, 57: 96–106, 2011

CFD Simulation of a Solvent Extraction Pump Mixer Unit: Evaluating Large Eddy Simulation and RANS Based Models

Mixer-settler equipment is widely used for solvent extraction (SX) operations. The pump mixer is the heart of an SX process. Any improvement in understanding of hydrodynamics and flow instabilities within a SX pump mixer unit would enable effective design of the mixer -settler equipment. In this direction, the present work investigates the predictive performance of the Large Eddy Simulation (LES) model vis-à-vis the PIV experimental results and RANS based model. Comparisons have been made initially for single phase operation of a Mixer unit, and then for the multiphase operation. The ANSYS/CFX modelling package has been used to set-up a transient three-dimensional CFD model using the sliding mesh approach for impeller motion and Eulerian-Eulerian approach for multi-phase flows. The present paper compares the flow patterns predicted by the LES model and compares them to RANS model prediction and PIV data. The prediction of flow structures and turbulence intensities will eventually pave the way for determination of droplet size and mass transfer rates, which are required in designing these systems.

In situ droplet size and speed determination in a fluid-bed granulator

The droplet size affects the final product in fluid-bed granulation and coating. In the present study, spray characteristics of aqueous granulation liquid (purified water) were determined in situ in a fluid-bed granulator. Droplets were produced by a pneumatic nozzle. Diode laser stroboscopy (DLS) was used for droplet detection and particle tracking velocimetry (PTV) was used for determination of droplet size and speed. Increased atomization pressure decreased the droplet size and the effect was most strongly visible in the 90% size fractile. The droplets seemed to undergo coalescence after which only slight evaporation occurred. Furthermore, the droplets were subjected to a strong turbulence at the event of atomization, after which the turbulence reached a minimum value in the lower halve of the chamber. The turbulence increased as speed and droplet size decreased due to the effects of the fluidizing air. The DLS and PTV system used was found to be a useful and rapid tool in determining spray characteristics and in monitoring and predicting nozzle performance.

Preparation of W1/O/W2 emulsions and droplet size distribution measurements by pulsed‐field gradient nuclear magnetic resonance (PFG‐NMR) technique

AbstractMultiple emulsions of W1/O/W2 type are of major interest in life sciences, offering possibilities for the encapsulation of water‐soluble active agents. In food science, they are also applied for fat reduction. The droplet size distributions of the inner and outer emulsions are of main importance as they influence the rheological and sensorial properties, the release kinetics, as well as the structural and microbial stability. However, the determination of the inner and outer droplet size distributions is a major challenge, as conventional measurement techniques cannot be applied. Pulsed‐field gradient nuclear magnetic resonance (PFG‐NMR) is well known as a non‐destructive tool for droplet size determination, especially in simple emulsions. In this work, double emulsions of the W1/O/W2 type were prepared with polyglycerol‐polyricinoleate (PGPR) and polyoxyethylen‐20‐sorbitan monolaurate (Tween 20) as emulsifiers by means of rotor‐stator emulsification machines. PFG‐NMR was applied for measurements of the inner phase (W1) droplet size distribution as well as for the characterization of the O phase. The W1 values were compared with results from laser light diffraction of simple emulsions (W1/O type) and were found to be consistent within the experimental errors, if restricted diffusion in the outer water phase (W2) and additional effects are considered.

Temperature-Sensitive Microemulsion Gel: An Effective Topical Delivery System for Simultaneous Delivery of Vitamins C and E

Microemulsions (ME)--nanostructured systems composed of water, oil, and surfactants--have frequently been used in attempts to increase cutaneous drug delivery. The primary objective addressed in this work has been the development of temperature-sensitive microemulsion gel (called gel-like ME), as an effective and safe delivery system suitable for simultaneous topical application of a hydrophilic vitamin C and a lipophilic vitamin E. By changing water content of liquid o/w ME (o/w ME), a gel-like ME with temperature-sensitive rheological properties was formed. The temperature-driven changes in its microstructure were confirmed by rotational rheometry, viscosity measurements, and droplet size determination. The release studies have shown that the vitamins' release at skin temperature from gel-like ME were comparable to those from o/w ME and were much faster and more complete than from o/w ME conventionally thickened with polymer (o/w ME carbomer). According to effectiveness in skin delivery of both vitamins, o/w ME was found the most appropriate, followed by gel-like ME and by o/w ME carbomer, indicating that no simple correlation between vitamins release and skin absorption could be found. The cytotoxicity studies revealed good cell viability after exposure to ME and confirmed all tested microemulsions as nonirritant.

Preparation and evaluation of self-microemulsifying drug delivery system of oridonin

The objective of this study was to develop self-microemulsifying drug delivery system (SMEDDS) to enhance the oral bioavailability of the poorly water-soluble drug, oridonin. The influence of the oil, surfactant and co-surfactant types on the drug solubility and their ratios on forming efficient and stable SMEDDS were investigated in detail. The SMEDDS were characterized by morphological observation, droplet size and zeta-potential determination, cloud point measurement and in vitro release study. The optimum formulation consisted of 30% mixture of Maisine 35-1 and Labrafac CC (1:1), 46.7% Cremopher EL, and 23.3% Transcutol P. In vitro release test showed a complete release of oridonin from SMEDDS in an approximately 12 h. The absorption of oridonin from SMEDDS showed a 2.2-fold increase in relative bioavailability compared with that of the suspension. Our studies demonstrated the promising use of SMEDDS for the delivery of oridonin by the oral route.

Model Supported Morphology Control of Electrospray Deposited Poly(vinylidene fluoride) Film

AbstractPreparation of polymer films with electrospray deposition has been investigated. Morphologies of poly(vinylidene fluoride), prepared under a range of different conditions, have been imaged with AFM. To generalize the conditions under which a particular morphology appears, a model has been used. The model incorporates the parameters that determine droplet size and droplet evaporation in the electrospray deposition process. With the model, film morphology has been shown to depend on the droplet volume at impact, available surface area on the substrate and rate of solvent evaporation from the film. With this information, the model can predict the film morphology under many different spray conditions. This has been demonstrated by determining and predicting the conditions under which PVDF displays the smoothest films.

Spectroscopic studies of the size and composition of single aerosol droplets

The characterization of aerosol properties and processes, non-intrusively and directly, poses a severe analytical challenge. In order to understand the role of aerosols in often complex environments, it is necessary to probe the particles in situ and without perturbation. Sampling followed by end-of-line analysis can lead to perturbations in particle composition, morphology and size, particularly when analysing liquid aerosol droplets containing volatile components. Optical spectroscopy can provide a strategy for the direct assessment of particle size, composition and phase. We review here the application of linear and non-linear Raman spectroscopies in the characterization of liquid aerosol droplets. Spontaneous Raman scattering can allow the unambiguous identification of chemical components and the determination of droplet composition. Stimulated Raman spectroscopy can allow the determination of droplet size with nanometre accuracy and can allow the characterization of near-surface composition. When combined, the mixing state and homogeneity in droplet composition can be investigated. We highlight some applications of these spectroscopic techniques in studies of the kinetics of particle transformation, the equilibrium composition of aqueous aerosol droplets, and the coagulation and mixing state of organic and aqueous aerosol components. Specifically, we examine the heat and mass transfer accompanying the evaporation of volatile components from liquid droplets, the equilibrium size of aqueous/sodium chloride droplets with varying relative humidity, and the mixing of the immiscible decane and water components during droplet coagulation. We conclude by considering the potential of these techniques for improving our understanding of aerosol properties and processes.

Evaporation of Ethanol/Water Droplets: Examining the Temporal Evolution of Droplet Size, Composition and Temperature

The evolving size, composition, and temperature of evaporating ethanol/water aerosol droplets 25-57 microm in radius are probed by cavity enhanced Raman scattering (CERS) and laser induced fluorescence. This represents the first study in which the evolving composition of volatile droplets has been probed with spatial selectivity on the millisecond time scale, providing a new strategy for exploring mass and heat transfer in aerosols. The Raman scattering intensity is shown to depend exponentially on species concentration due to the stimulated nature of the CERS technique, providing a sensitive measure of the concentration of the volatile ethanol component. The accuracy with which we can determine droplet size, composition, and temperature is discussed. We demonstrate that the CERS measurements of evolving size and composition of droplets falling in a train can be used to characterize, and thus avoid, droplet coagulation. By varying the surrounding gas pressure (7-77 kPa), we investigate the dependence of the rate of evaporation on the rate of gas diffusion, and behavior consistent with gas diffusion-limited evaporation is observed. We suggest that such measurements can allow the determination of the vapor pressures of components within the droplet and can allow the determination of activity coefficients of volatile species.

Coalescence and flocculation in o/w emulsions of native and denatured whey soy proteins in comparison with soy protein isolates

Comparison of coalescence and flocculation stability of o/w emulsions prepared with whey soy protein and soy protein isolates, native and denatured (NWSP, NSI; DWSP, DSI, respectively) was performed. Sodium caseinate (SC) was used as emulsifier control. Emulsions, without and with NaCl (500 mM in aqueous phase) were analysed by backscattering during 24 h in quiescent storage. In the absence of salt, the cream phase global destabilization percentage ( D%) at 24 h of NWSP emulsions was the highest, whereas in the presence of NaCl not only NWSP but also NSI emulsions were destabilised. Cream phase stability, expressed as coalescence ( C%) and coalescence plus flocculation percentages ( C+ F%) was calculated from mean droplet size ( D 43) without and with 1% SDS. C% and C+ F% had a polynomial correlation with D%, which indicate that this parameter could reflect the particle size increase due to both coalescence and flocculation processes. The higher stability of denatured soy protein (DWSP, DSI) emulsions respect to those prepared with native proteins could be related with the presence of hydrated flocs formed during quiescent storage. In these emulsions, high elastic modulus ( G′) and low oil volume fraction ( ϕ) values indicated an hydrated and gel-like structure of cream phase. Thermal denatured whey soy proteins, without antitryptic activity, allowed to obtain as coalescence stable emulsions as those prepared with SC. Although backscattering method detected changes in particle size with a less sensibility than droplet size determination, it allowed the continue evaluation of global stability in non-diluted emulsions.

Laboratory Studies and Numerical Simulations of Cloud Droplet Formation under Realistic Supersaturation Conditions

Abstract In this paper, a new device is introduced to study the formation and growth of cloud droplets under near-atmospheric supersaturations. The new device, called the Leipzig Aerosol Cloud Interaction Simulator (LACIS), is based on a laminar flow tube. It has been designed to reproduce the thermodynamic conditions of atmospheric clouds as realistically as possible. A series of experiments have been conducted that prove the definition and stability of the flow field inside the LACIS as well as the stability and reproducibility of the generated droplet size distributions as a function of the applied thermodynamic conditions. Measured droplet size distributions are in good agreement with those determined by a newly developed Eulerian particle–droplet dynamical model. Further investigations will focus on the influences of latent heat release during vapor condensation on the tube walls and the development of a more suitable optical particle counter for droplet size determination.

Further Studies on the Effect of Degassing on the Dispersion and Stability of Surfactant-Free Emulsions

Recently reported results indicate that the formation of surfactant-free, oil-in-water emulsions can be significantly enhanced by the almost complete removal of dissolved gases and that the reintroduction of dissolved gases does not immediately destabilize the already-formed emulsions. These initial experiments have been repeated and extended to include a wider range of organic liquids and the application of light scattering to determine droplet size and distribution. The earlier observations have been confirmed. In addition, a systematic trend was found between the solubility of the oil in water and the stability (lifetime) of the degassed oil droplets in water. The lower the solubility, the more stable the emulsion, and for oils that are sparingly soluble in water such as squalane, the small droplets remain stable for several weeks, with buoyancy separation being the main cause of instability of the large droplets with time. The addition of electrolytes, up to molar concentrations, substantially reduces the enhancement of the dispersions on degassing but appears to have little effect on the stability of the already-formed emulsions. The reduction of pH to about 2 significantly reduces both the enhancement of the dispersions on degassing and the stability of the already-formed emulsions. In contrast, the increase of pH to about 11 hardly affects the enhancement of the dispersions on degassing or the stability of the already-formed emulsions. We have confirmed the importance of dissolved gas and its association with the electrostatic effects, but we still cannot provide a complete explanation for the effect of degassing on the hydrophobic dispersions.

Cavity enhanced droplet spectroscopy: Principles, perspectives and prospects

Micron-sized liquid droplets exhibit unique optical properties, including lower threshold energies for non-linear optical processes than are observed in the bulk liquid phase. Whispering gallery modes (WGMs) and cavity quantum electrodynamics lead to cavity enhancement of fluorescence and Raman scattering at wavelengths commensurate with WGMs. The principles of cavity enhanced droplet spectroscopy will be reviewed, along with the major developments in the field over two decades. The prospect of applying cavity enhanced techniques in the simultaneous determination of droplet size and composition is discussed. A systematic experimental examination of the cavity enhanced Raman spectroscopy of water droplets is presented. In particular, the influences of laser power and illumination geometry, ionic strength and the presence of a second Raman active scatterer, nitrate, on the shape of the water Raman band are explored. Prospects for applying cavity enhanced techniques to studies of aerosol dynamics are also discussed.

Scope of droplet size measurements in food emulsions by pulsed field gradient NMR at low field

AbstractLow‐field NMR instrumentation offers a user‐friendly and cost‐effective means for assessment of water and oil droplet size measurements in food emulsions. Examples are given of major food application areas, and these are supported with validation data in terms of precision and equivalence to other methods. The NMR method gives the same results as other techniques, such as microscopy, laser diffraction and electric sensing, but with similar or better precision. NMR distinguishes itself by a simple and non‐perturbing sample preparation procedure (in addition to the aforementioned advantages). For reliable results, the NMR parameters should be properly chosen and adjusted. A survey is given of experimental NMR parameters than can have an impact on the performance of water and oil droplet size determination. Copyright © 2002 John Wiley & Sons, Ltd.

An optical method for determining size distributions of water droplets

A method for determining aerosol size distributions by single laser-shot single droplet cavity enhanced Raman scattering (CERS) is presented. Droplets are illuminated with the tripled output from a Nd:YAG laser at 355 nm and the CERS fingerprint acquired with a spectrograph and CCD. Droplets with radii in the range 10–50 μm are probed. The extension of this to the determination of a distribution of droplet sizes is illustrated. We suggest that the CERS signature from water could be used to determine droplet size while the observation of Raman scattering from other constituents could be used to identify trace chemical constituents within water droplets.

Experimental determination of droplet size and density field in condensing flows

We report a detailed experimental characterization of the process of homogeneous condensation in supersonic expanding flow. In our experiments, the supersaturated mixture expands in a Laval nozzle, where, depending on the initial conditions, a steady or periodically oscillating flow may evolve due to the non-linear interaction of nucleation and droplet growth rate with the flow field. Two experimental techniques are utilized: holographic interferometry for the determination of the density field and a time-resolved white-light extinction method. The latter is employed to derive the evolution in time of the droplet cloud (i.e., modal radius, number density, and relative width) and to measure the frequency of oscillations. In combination with the wide-field density data, droplet size measurements provide additional physical insights in the mechanism of interaction in condensing flows and serve as an excellent test case for the critical assessment of nucleation and droplet growth theories. To this purpose, the accuracy of the measurements is carefully reviewed due to the difficulties of characterizing dense sub-micron droplet clouds by means of light-scattering techniques. An important byproduct of this analysis is an evaluation of the applicability of single-scattering approximations, i.e., Lambert-Beer law, for a variety of experimental configurations.