Uniform-sized silicone oil microemulsions: preparation, investigation of stability and deposition on hair surface

Emulsions are commonly used in foods, pharmaceuticals and home-personal-care products. For emulsion based products, it is highly desirable to control the droplet size distribution to improve storage stability, appearance and in-use property. We report preparation of uniform-sized silicone oil microemulsions with different droplets diameters (1.4-40.0 μm) using SPG membrane emulsification technique. These microemulsions were then added into model shampoos and conditioners to investigate the effects of size, uniformity, and storage stability on silicone oil deposition on hair surface. We observed much improved storage stability of uniform-sized microemulsions when the droplets diameter was ≤22.7 μm. The uniform-sized microemulsion of 40.0 μm was less stable but still more stable than non-uniform sized microemulsions prepared by conventional homogenizer. The results clearly indicated that uniform-sized droplets enhanced the deposition of silicone oil on hair and deposition increased with decreasing droplet size. Hair switches washed with small uniform-sized droplets had lower values of coefficient of friction compared with those washed with larger uniform and non-uniform droplets. Moreover the addition of alginate thickener in the shampoos and conditioners further enhanced the deposition of silicone oil on hair. The good correlation between silicone oil droplets stability, deposition on hair and resultant friction of hair support that droplet size and uniformity are important factors for controlling the stability and deposition property of emulsion based products such as shampoo and conditioner.     

Microfluidic droplet sorting with a high frequency ultrasound beam

This paper presents experimental results demonstrating the feasibility of high frequency ultrasonic sensing and sorting for screening single oleic acid (lipid or oil) droplets under continuous flow in a microfluidic channel. In these experiments, hydrodynamically focused lipid droplets of two different diameters (50 μm and 100 μm) are centered along the middle of the channel, which is filled with deionized (DI) water. A 30 MHz lithium niobate (LiNbO(3)) transducer, placed outside the channel, first transmits short sensing pulses to non-invasively determine the acoustic scattering properties of the individual droplets passing through the beam's focus. Integrated backscatter (IB) coefficients, utilized as a sorting criterion, are measured by analyzing the received echo signals from each droplet. When the IB values corresponding to 100 μm droplets are obtained, a custom-built LabVIEW panel commands the transducer to emit sinusoidal burst signals to commence the sorting operation. The number of droplets tested for the sorting is 139 for 50 μm droplets and 95 for 100 μm droplets. The sensing efficiencies are estimated to be 98.6% and 99.0%, respectively. The sorting is carried out by applying acoustic radiation forces to 100 μm droplets to direct them towards the upper sheath flow, thus separating them from the centered droplet flow. The sorting efficiencies are 99.3% for 50 μm droplets and 85.3% for 100 μm droplets. The results suggest that this proposed technique has the potential to be further developed into a cost-effective and efficient cell/microparticle sorting instrument.     

Emulsion Templating of Poly(lactic acid) Particles: Droplet Formation Behavior

Monodisperse poly(dl-lactic acid) (PLA) particles of diameters between 11 and 121 μm were fabricated in flow focusing glass microcapillary devices by evaporation of dichloromethane (DCM) from emulsion droplets at room temperature. The dispersed phase was 5% (w/w) PLA in DCM containing 0.1-2 mM Nile Red and the continuous phase was 5% (w/w) poly(vinyl alcohol) in reverse osmosis water. Particle diameter was 2.7 times smaller than the diameter of the emulsion droplet template, indicating very low particle porosity. Monodisperse droplets have only been produced under dripping regime using a wide range of dispersed phase flow rates (0.002-7.2 cm(3)·h(-1)), continuous phase flow rates (0.3-30 cm(3)·h(-1)), and orifice diameters (50-237 μm). In the dripping regime, the ratio of droplet diameter to orifice diameter was inversely proportional to the 0.39 power of the ratio of the continuous phase flow rate to dispersed phase flow rate. Highly uniform droplets with a coefficient of variation (CV) below 2% and a ratio of the droplet diameter to orifice diameter of 0.5-1 were obtained at flow rate ratios of 4-25. Under jetting regime, polydisperse droplets (CV > 6%) were formed by detachment from relatively long jets (between 4 and 10 times longer than droplet diameter) and a ratio of the droplet size to orifice size of 2-5.     

High-speed, clinical-scale microfluidic generation of stable phase-change droplets for gas embolotherapy

In this study we report on a microfluidic device and droplet formation regime capable of generating clinical-scale quantities of droplet emulsions suitable in size and functionality for in vivo therapeutics. By increasing the capillary number-based on the flow rate of the continuous outer phase-in our flow-focusing device, we examine three modes of droplet breakup: geometry-controlled, dripping, and jetting. Operation of our device in the dripping regime results in the generation of highly monodisperse liquid perfluoropentane droplets in the appropriate 3-6 μm range at rates exceeding 10(5) droplets per second. Based on experimental results relating droplet diameter and the ratio of the continuous and dispersed phase flow rates, we derive a power series equation, valid in the dripping regime, to predict droplet size, D(d) approximately equal 27(Q(C)/Q(D))(-5/12). The volatile droplets in this study are stable for weeks at room temperature yet undergo rapid liquid-to-gas phase transition, and volume expansion, above a uniform thermal activation threshold. The opportunity exists to potentiate locoregional cancer therapies such as thermal ablation and percutaneous ethanol injection using thermal or acoustic vaporization of these monodisperse phase-change droplets to intentionally occlude the vessels of a cancer.     

Effects of formulation variables on liquid‐crystal droplet size distributions in ultraviolet‐cured polymer‐dispersed liquid‐crystal cells

The size distributions of liquid‐crystal droplets in ultraviolet‐cured polymer‐dispersed liquid‐crystal cells have been studied with optical microscopy. It has been observed that (1) the relative masses of the liquid crystal and crosslinking agent determine the droplet size distribution for submicrometer droplet diameters and (2) only the liquid‐crystal mass fraction affects the droplet size distribution for diameters ranging from 1 to 4 μm. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1842–1848, 2005     

Automated nanoliter solution deposition for total reflection X-ray fluorescence analysis of semiconductor samples

In this study, a BioDot BioJet dispensing system was investigated as a nanoliter sample deposition method for total reflection X-ray fluorescence (TXRF) analysis. The BioDot system was programmed to dispense arrays of 20 nL droplets of sample solution on Si wafers. Each 20 nL droplet was approximately 100 μm in diameter. A 10 × 10 array (100 droplets) was deposited and dried in less than 2 min at room temperature and pressure, demonstrating the efficiency of the automated deposition method. Solutions of various concentrations of Ni and Ni in different matrices were made from stock trace element standards to investigate of the effect of the matrix on the TXRF signal. The concentrations were such that the levels of TXRF signal saturation could be examined. Arrays were deposited to demonstrate the capability of drying 100 μL of vapor phase decomposition-like residue in the area of a typical TXRF detector.     

Synthesis of monodisperse polymer microspheres by photopolymerization of microdroplets

We have examined photopolymerization of highly monodisperse microdroplets of monomer solutions under UV-light radiation. Microdroplets were generated using a modified vibrating aerosol generator, and the diameter of the droplets can be tuned to any size between 5 to 100 m. Polymer particles derived from the droplets were characterized by optical microscopy and SEM. The results show that the polymer particles, under optimum conditions, can be highly spherical and monodisperse. The diameter and morphology of resulting microspheres depend on the diameter of the monomer solution droplets, monomer concentration, photopolymerization reaction temperature, residence time, and droplet dispersion.     

New Pickering emulsions stabilized by bacterial cellulose nanocrystals

We studied oil in water Pickering emulsions stabilized by cellulose nanocrystals obtained by hydrochloric acid hydrolysis of bacterial cellulose. The resulting solid particles, called bacterial cellulose nanocrystals (BCNs), present an elongated shape and low surface charge density, forming a colloidal suspension in water. The BCNs produced proved to stabilize the hexadecane/water interface, promoting monodispersed oil in water droplets around 4 μm in diameter stable for several months. We characterized the emulsion and visualized the particles at the surface of the droplets by scanning electron microscopy (SEM) and calculated the droplet coverage by varying the BCN concentration in the aqueous phase. A 60% coverage limit has been defined, above which very stable, deformable droplets are obtained. The high stability of the more covered droplets was attributed to the particle irreversible adsorption associated with the formation of a 2D network. Due to the sustainability and low environmental impact of cellulose, the BCN based emulsions open opportunities for the development of environmentally friendly new materials.     

Creaming Stability of Flocculated Monodisperse Oil-in-Water Emulsions

The influence of droplet flocculation on the creaming stability of monodisperse n-hexadecane oil-in-water emulsions was studied. The creaming velocity of emulsions with different droplet radii (0.43 and 0.86 μm), droplet concentrations (1-67 vol%), and sodium dodecyl sulfate (SDS) concentrations (7-80 mM) were measured. Depletion flocculation was observed in the emulsions when the aqueous phase SDS concentration exceeded a particular level ( approximately 40 mM for 0.43-μm droplets and approximately 15 mM for 0.86-μm droplets). Creaming was monitored by measuring the back-scattered light from an emulsion as a function of its height. The creaming velocity increased with increasing flocculation and decreased with increasing droplet concentration. These results have important implications for the formulation of emulsion-based materials. Copyright 2000 Academic Press.     

Droplet size based separation by deterministic lateral displacement-separating droplets by cell--induced shrinking

We present a novel method for passive separation of microfluidic droplets by size at high throughput using deterministic lateral displacement (DLD). We also show that droplets containing Saccharomyces cerevisiae shrink significantly during incubation while droplets containing only yeast media retain or slightly increase their size. We demonstrate the DLD device by sorting out shrunken yeast-cell containing droplets from 31% larger diameter droplets which were generated at the same time containing only media, present at a >40-fold excess. This demonstrates the resolving power of droplet separation by DLD and establishes that droplets can be separated for a biological property of the droplet contents discriminated by a change of the physical properties of the droplet. Thus suggesting that this technique may be used for e.g. clonal selection. The same device also separates 11 μm from 30 μm droplets at a rate of 12,000 droplets per second, more than twofold faster than previously demonstrated passive hydrodynamic separation devices.     

Infrared Laser Ablation Sample Transfer for MALDI and Electrospray

We have used an infrared laser to ablate materials under ambient conditions that were captured in solvent droplets. The droplets were either deposited on a MALDI target for off-line analysis by MALDI time-of-flight mass spectrometry or flow-injected into a nanoelectrospray source of an ion trap mass spectrometer. An infrared optical parametric oscillator (OPO) laser system at 2.94 μm wavelength and approximately 1 mJ pulse energy was focused onto samples for ablation at atmospheric pressure. The ablated material was captured in a solvent droplet 1–2 mm in diameter that was suspended from a silica capillary a few millimeters above the sample target. Once the sample was transferred to the droplet by ablation, the droplet was deposited on a MALDI target. A saturated matrix solution was added to the deposited sample, or in some cases, the suspended capture droplet contained the matrix. Peptide and protein standards were used to assess the effects of the number of IR laser ablation shots, sample to droplet distance, capture droplet size, droplet solvent, and laser pulse energy. Droplet collected samples were also injected into a nanoelectrospray source of an ion trap mass spectrometer with a 500 nL injection loop. It is estimated that pmol quantities of material were transferred to the droplet with an efficiency of approximately 1%. The direct analysis of biological fluids for off-line MALDI and electrospray was demonstrated with blood, milk, and egg. The implications of this IR ablation sample transfer approach for ambient imaging are discussed.     

Microstructuring of Polystyrene Surfaces with Nonsolvent Sessile Droplets

Herein, we study the microstructuring of toluene‐vapor‐softened polystyrene surfaces with nonsolvent sessile droplets. Arrays of microvessels are obtained by depositing non‐evaporating droplets of ethylene glycol/water on the original polystyrene surfaces and subsequently exposing them to saturated toluene vapor. The droplets act as a mask on the polymer, thereby impeding the toluene vapor to diffuse and soften the polystyrene surface below them. Alternatively, the formation of microcraters at random positions—with an average depth‐to‐width aspect ratio of 0.5 and a diameter as small as 1.5 μm—is achieved by condensing water droplets on a softened polystyrene surface. The cross‐sections of the microvessels and the contact angle of the sessile water droplets suggest that the structures are formed by the combined action of the Laplace pressure at the bottom of the droplet and the surface tension acting at the three‐phase contact line of the droplets. As a support, the rim height and the depth of the microvessels are fitted with an elastic theory to provide Young’s modulus of the softened polystyrene surface.     

Tensile film stress of parylene deposited on liquid

We found that liquid droplets encapsulated by Parylene deposited directly on a liquid surface deformed toward spherical shapes during Parylene deposition. This deformation suggested that the film stress was tensile. We calculated the film stress of such Parylene films by studying the surface mean curvature of the droplet shape and found the film stress measured about 0.7-0.9 MPa tensile. This film stress is of opposite type to that of as-deposited Parylene films deposited on solid substrates, which was compressive. This difference might indicate a profound change of the Parylene polymer due to the use of liquid surface as deposition substrate. The tensile film stress and its effect on the droplet shape also have implications in the fabrication and operation of Parylene microdevices that have encapsulated liquid structures such as microlens or micropumps.     

Oil-continuous microemulsions mixed with an amphiphilic graft copolymer or with the parent homopolymer Polymer–droplet interactions as revealed by phase behavior and light scattering

Polymer–droplet interactions have been studied in AOT/water/isooctane oil-continuous microemulsions mixed with an amphiphilic graft copolymer, or with the parent homopolymer (AOT = sodium bis(2-ethylhexyl) sulfosuccinate). The graft copolymer has an oil-soluble poly(dodecyl methacrylate) backbone and water-soluble poly(ethylene glycol) side chains. Pseudo-ternary polymer/droplet/isooctane phase diagrams have been established for both the parent homopolymer and the graft copolymer, and the two types of mixture display entirely different phase behavior. The homopolymer–droplet interaction is repulsive, and a segregative phase separation occurs at high droplet concentrations. By contrast, the graft copolymer–droplet interaction is attractive: the polymer is insoluble in the pure oil, but dissolves in the microemulsion. A comparatively high concentration of droplets is required to solubilize even small amounts of polymer. Static and dynamic light scattering has been performed in order to obtain information on structure and dynamics in the two types of mixture. For optically matched microemulsions, with a vanishing excess polarizability of the droplets, the polymer dominates the intensity of scattered light. The absolute intensity of scattered light increases as phase separation is approached owing to large-scale concentration fluctuations. Dynamic light scattering shows two populations of diffusion coefficients; one population originates from “free” microemulsion droplets and the other from the polymer (for homopolymer mixtures) or from polymer–droplet aggregates (for mixtures with the graft copolymer). The graft copolymer forms large polymer–droplet aggregates with a broad size distribution, which coexist with a significant fraction of free droplets.     

Emulsion of aqueous-based nonspherical droplets in aqueous solutions by single-chain surfactants: templated assembly by nonamphiphilic lyotropic liquid crystals in water

Single-chain surfactants usually emulsify and stabilize oily substances into droplets in an aqueous solution. Here, we report a coassembly system, in which single types of anionic or non-ionic surfactants emulsify a class of water-soluble nonamphiphilic organic salts with fused aromatic rings in aqueous solutions. The nonamphiphilic organic salts are in turn promoted to form droplets of water-based liquid crystals (chromonic liquid crystals) encapsulated by single-chain surfactants. The droplets, stabilized against coalescence by encapsulated in a layer (or layers) of single chain surfactants, are of both nonspherical tactoid (elongated ellipsoid with pointy ends) and spherical shapes. The tactoids have an average long axis of ～9 μm and a short axis of ～3.5 μm with the liquid crystal aligning parallel to the droplet surface. The spherical droplets are 5-10 μm in diameter and have the liquid crystal aligning perpendicular to the droplet surface and a point defect in the center. Cationic and zwitterionic surfactants studied in this work did not promote the organic salt to form droplets. These results illustrate the complex interplay of self-association and thermodynamic incompatibility of molecules in water, which can cause new assembly behavior, including potential formation of vesicles or other assemblies, from surfactants that usually form only micelles. These unprecedented tactoidal shaped droplets also provide potential for the fabrication of new soft organic microcapsules.     

Influence of interfacial characteristics on Ostwald ripening in hydrocarbon oil-in-water emulsions

The influence of the nature of the interfacial membrane on the kinetics of droplet growth in hydrocarbon oil-in-water emulsions was investigated. Droplet growth rates were determined by measuring changes in the droplet size distribution of 1 wt % n-tetradecane or n-octadecane oil-in-water emulsions using laser diffraction. The interfacial properties of the droplets were manipulated by coating them with either an SDS layer or with an SDS-chitosan layer using an electrostatic deposition method. The emulsion containing SDS-coated octadecane droplets did not exhibit droplet growth during storage for 400 h, which showed that it was stable to Ostwald ripening because of this oils extremely low water-solubility. The emulsion containing SDS-coated n-tetradecane droplets showed a considerable increase in mean droplet size with time, which was attributed to Ostwald ripening associated with this oils appreciable water-solubility. On the other hand, an emulsion containing SDS-chitosan coated n-tetradecane droplets was stable to droplet growth, which was attributed to the ability of the interfacial membrane to resist deformation because of its elastic modulus and thickness. This study shows that the stability of emulsion droplets to Ostwald ripening can be improved by using an electrostatic deposition method to form thick elastic membranes around the droplets.     

Online deposition of nano‐aerosol for matrix‐assisted laser desorption/ionization mass spectrometry

An online nano‐aerosol sample deposition method for matrix‐assisted laser desorption/ionization (MALDI) mass spectrometry is described in which matrix and analyte particles between 50 and 500 nm are aerodynamically focused onto a tight spot, ca. 200 µm in diameter, on the target plate under vacuum. MALDI analysis of the target is performed without additional sample preparation. The method is evaluated with insulin as the analyte and alpha‐cyano‐4‐hydroxycinnamic acid (CHCA) as the matrix. Two preparation modes are compared with conventional dried‐droplet deposition: mixture deposition where a single layer is deposited consisting of particles that contain both matrix and analyte, and layered deposition where an underlayer of matrix particles and an overlayer of analyte particles are deposited separately. Desalting is performed by adding ammonium sulfate to the solution used to generate the matrix aerosol. With mixture deposition, the optimum matrix‐to‐analyte mole ratio is about 500:1 compared with 5000:1 for the conventional dried‐droplet method. With layered deposition, the thicknesses of the matrix and analyte layers are more important determinants of the analyte signal intensity than the matrix‐to‐analyte mole ratio. Analyte signal intensities are independent of matrix layer thickness above 200 nm, and the optimum analyte signal is obtained with an analyte layer thickness of about 100 nm. Copyright © 2009 John Wiley & Sons, Ltd.     

Spreading and infiltration of inkjet-printed polymer solution droplets on a porous substrate

Simultaneous spreading and infiltration of inkjet-printed droplets has been studied. Small (54- and 63-microm diameter) droplets of an aqueous polymer solution (2.4 vol% polyacrylic acid, PAA, MW 60,000) were deposited on high green density porous ceramic beds, and the wetting-induced spreading and infiltration of the droplets were characterized. The time scales for spreading and infiltration were comparable (approximately milliseconds), resulting in interruption of the spreading prior to completion by infiltration of the liquid into the powder bed, a situation that has received little treatment in the literature. The infiltration time was varied by changing the pore size (via particle size) in the powder bed, and it was confirmed that slower infiltration resulted in greater spreading of the liquid. The spreading and infiltration of the droplet were modeled to examine the coupling between the two processes and allow prediction of the maximum extension of the droplet as a function of the powder bed particle size. The liquid spreading was found to follow r(t)=a(b+t)(n) behavior, and the effect of particle size on infiltration time was used to predict the point at which spreading ceases due to infiltration for various particle sizes.     

Highly productive droplet formation by anisotropic elongation of a thread flow in a microchannel

We developed a microfluidic device to form monodisperse droplets with high productivity by anisotropic elongation of a thread flow, defined as a threadlike flow of a dispersed liquid phase in a flow of an immiscible, continuous liquid phase. The thread flow was anisotropically elongated in the depth direction in a straight microchannel with a step, where the microchannel depth changed. Consequently, the elongated thread flow was given capillary instability (Rayleigh-Plateau instability) and was continuously transformed into monodisperse droplets at the downstream area of the step in the microchannel. We examined the effects of the flow rates of the dispersed phase and the continuous phase on the droplet formation behavior, including the droplet diameter and droplet formation frequency. The droplet diameter increased as the fraction of the dispersed-phase flow rate relative to the total flow rate increased and was independent of the total flow rate. The droplet formation frequency proportionally increased with the total flow rate at a constant dispersed-phase flow rate fraction. These results are explained in terms of a mechanism similar to that of droplet formation from a cylindrical liquid thread flow by Rayleigh-Plateau instability. The microfluidic device described was capable of forming monodisperse droplets with a 160-microm average diameter and 3-microm standard deviation at a droplet formation frequency of 350 droplets per second from a single thread flow. The highest total flow rate achieved was 6 mL/h using the present device composed of a straight microchannel with a step. We also demonstrated parallel droplet formation by anisotropic elongation of multiple thread flows; the process was applied to form W/O and O/W droplets. The highly productive droplet formation process presented in this study is expected to be useful for future industrial applications.     

A method for investigating size distributions of aqueous droplets in the range 0.5–10 μm produced by pneumatic nebulizers

A novel method is described for the measurement of the droplet size distributions produced by nebulizers commonly employed in analytical atomic spectroscopy. It is shown theoretically that, at sufficiently high concentrations of dissolved sodium chloride, the evaporation of water from droplets as small as 0.5 μm in diameter may be reduced to a negligible level. When evaporation is reduced by the presence of a dissolved salt, a conventional cascade impactor may be used to elucidate the droplet size distribution. Empirical observations confirm that, at a sodium concentration of 10,000 μg ml^?1, evaporation is negligible: the method may be used to study particle size distributions over the size range 0.5–10 μm.