Microdroplet Formation Research Articles

Influence of chemical and plasma treatments on the adhesive properties of PTFE with an epoxy resin

Poly(tetrafluoroethylene) (PTFE) films and fibres were surface modified by different methods (chemical and plasma treatments) to improve their wetting and adhesion properties. Both chemical and, to a lesser extent, ammonia and hydrogen plasma treatments have been shown to greatly enhance the adhesion between PTFE and an epoxy resin. In the last case (ammonia and hydrogen plasma treatments), the adhesion increase has been reiated to the degree of defluorination rather than to the presence of polar functions at the surface. In the case of the chemical treatment, in addition to the defluorination, the incorporation of a large amount of oxygen moieties can play an important role in adhesive properties. In all cases the failure zones, observed by X-ray photoelectron spectroscopy after pull-off tests, are characteristic of a cohesive failure. The differences between the chemical and the plasma treatments have been attributed to the modified depth, which is more important in the former case. Fibre wettability is enhanced by a previous ammonia plasma treatment making easier the formation of an epoxy microdroplet for microbond tests. For ammonia-plasma-treated PTFE fibres the ultimate load increases; this cannot be attributed to a debonding process since either the droplet or the fibre breaks.

Characterization of Polymer-Dispersed Liquid Crystal Systems by FT-IR Microspectroscopy

Infrared microspectroscopy was used to generate functional group images of liquid crystal (E7) droplets dispersed in poly( n-butyl methacrylate) (PBMA). The spatial concentration fluctuations that occur within the system were studied as a function of time. This approach is possible because spectral information can be obtained by focusing on regions on the order of tens of micrometers. The peak intensities were used as a measure of concentration of the components. The amount of liquid crystal dissolved in the polymer matrix determines the extent to which the polymer is plasticized, which in turn affects the shape and size of the droplets. The growth of the domains at any temperature is also determined by whether the system is maintained above or below the glass transition temperature of the matrix. It is observed that the growth of the droplets follows temporal power laws. The spatially resolved spectroscopic images provide valuable insight into the phase separation process and the formation of microdroplets of E7 in PBMA.

Electrothermal graphite furnace atomic absorption signal for gold in organic matrices

The electrothermal atomic absorption signal for gold deposited after pyrolysis of organic matrices was investigated. The signal shape was altered by pyrolysis of a volatile organic matrix (methanol, propan-1-ol and pentane-1,5-diol) and also a nonvolatile matrix (ascorbic acid, glucose and sucrose), with a low-temperature shift of signal and the formation of a shoulder (peak) on the tail. Pyrolysis of the organic matrix produced two types of carbon residue (active carbon and a thermally stable carbon residue). The early shift occurred as a result of the formation of smaller microdroplets of Au by adsorption of the analyte on the active carbon. The latter shift resulted from the formation of larger sized microdroplets by admission, diffusion or transport of the analyte into the inside of the thermally stable carbon residue. These mechanisms were supported by kinetic investigations.

Laser ablation of absorbing liquids

We have studied experimentally the formation of microdroplets upon the ablation of an aqueous CuCl2 solution by pulsed laser radiation. Laser fluence dependences are obtained from the number of droplets formed and from their statistical size distribution. The experimental data obtained can be explained within the framework of our acoustical microfragmentation model providing for excitation and interference in the field of laser acoustic waves of random phases. The ablation mechanism suggested is of universal character and manifests itself particularly in the laser ablation of biotissues.

Formation and growth of water microdroplets in nonpolar oil—fiber—aqueous surfactant systems

In this paper are reported the effects of prolonged soaking on a model system: nonpolar oil drops on fibrous polymer substrates immersed in aqueous solutions of various surfactants. In certain of these systems a layer of aqueous microdroplets grew on the surface of the fiber, beneath the oil drops. Addition of sodium chloride to the bulk aqueous phase reversed this growth. The rate of growth increased with increasing solubility of water in the oil. It also increased when an oil-soluble surfactant was added to the system. Addition of a surfactant that was not oil-soluble (SDS) produced no such effect. An explanation of these phenomena is advanced in terms of the Kelvin equation. Secondary oil droplets, surrounding the main oil drop, appeared in some systems. These can be attributed to an effect of capillarity, oil spreading out from the central oil drop in microgrooves on the fiber surface. Naturally occurring oils and fibers and certain osmotic effects which may operate in such systems are also discussed.

Cure Parameters and Phase Behavior of An Ultraviolet-Cured Polymer-Dispersed Liquid Crystal

Abstract Using calorimetry and scanning electron microscopy we have investigated the formation energetics and kinetics as well as the phase behavior of an ultraviolet-cured polymer-dispersed liquid crystal (PDLC) system. The purpose of this work was to determine those cure conditions which lead to liquid crystal microdroplet formation. We have found that a range of cure temperatures exists in which the degree of polymer cure is highest. In this range phase separation of liquid crystal from the polymer is maximized, and the best droplet microstructure occurs. Samples cured in the optimum temperature range also have the highest nematic-isotropic transition temperature. Results of calorimetric and electron microscopic studies are presented.

Study of Formation, Phase Behavior, and Microdroplet Size of a Polyurethane-based Polymer-dispersed Liquid Crystal

Using calorimetry and scanning electron microscopy, we have investigated the formation process and phase behavior of a polyurethane-based polymer-dispersed liquid crystal system. We have measured the kinetics and energetics of the cure process during which liquid crystal microdroplets form by phase separation from the matrix as it cross-links. The greatest degree of cure occurs for sample cured at 375 K. For a given cure temperature, the heat of cure decreases more or less linerly with increasing liquid crystal concentration due to a dilution effect. The time constant for the cure process decreases rapidly with increasing temperature but is much less sensitive to liquid crystal content. Samples cured below 375 K are apparently not fully phase separated, but subsequent treatment at higher temperatures evidently increases the degree of cure. The highest nematic-isotropic transition temperatures were achieved for liquid crystal concentrations above 40 volume percent. The nematic-isotropic transition enthalpy, △H NI , is a measure of the amount of liquid crystal contained in the microdroplets. A model has been developed which explains the linear increase of △H NI with increasing liquid crystal concentration. Optimum microdroplet formation occurs at 375 K, but only for liquid crystal concentrations below about 53 volume percent. At higher concentrations a reversed phase ( polymer ball ) morphology is seen. For the lower concentration droplet size increases linearly with LC content. Droplet number density decreases with increasing droplet size in rough agreement with a simple model

Comparison of the energetics of desorption of solution and vapour phase deposited analytes in graphite furnace atomic absorption spectrometry

The energetics of atomisation of Pb, Cu, Ag and Au in pyrolytic graphite coated, uncoated electrographite and glassy carbon tubes was studied following deposition of the analyte as a solution (primary site) and as an aerosol vapour (secondary site). Arrhenius activation energies (Ea) for Pb and Cu were independent of tube type and mode of analyte deposition. An apparent first-order rate of release occurs from all tube types and from both primary-and secondary-deposition sites, suggesting that both elements desorb from the surface as individual atoms. Values of Ea for desorption of Ag and Au were independent of furnace tube type but dependent on the deposition mode, with fractional orders of desorption obtained throughout. Data for these elements suggest the formation of micro-droplets or caps (primary site) and islands (secondary site) with desorption occurring from the droplet surface or at the metal-graphite interface.

MODELING OF LEAKAGE IN LIQUID SURFACTANT MEMBRANE SYSTEMS

Abstract The stability of double emulsions or liquid surfactant membranes, an important topic in liquid membrane extraction processes, was investigated. The percentage of liquid membrane leakage which reflects the stability of the liquid surfactant membranes was measured as a function of time using sodium hydroitide as a tracer. Water-in-oil emulsions were prepared with SOLTROL 220, an isoparaffinic solvent, and solvent extracted neutral oils—S100N and S500N. A stochastic model for droplet formation under the conditions of isotropic turbulence was developed. It assumes the presence of complete randomness in the process of emulsification. The mean growth rate obtained from this model is in excellent agreement with microdroplet formation data from the literature. A model for leakage of internal reagent into the bulk phase for liquid membrane systems was derived from the droplet formation model, assuming macrodroplet breakup to be the main mechanism responsible for microdroplet release. The internal reagent ...

A Light Control Film Composed of Liquid Crystal Droplets Dispersed in a UV-Curable Polymer

Abstract This paper describes a new class of light control films consisting of submicron liquid crystal droplets dispersed in ultraviolct-cured polymer matrices. These films, which can respond optically to both applied electric fields and temperature changes, are potentially useful for displays and light shutters. The optical performance of these films depends on a variety of structural, electro-optical and thermal properties. This report describes scanning electron microscope studies of film structure, measurements of voltage dependent film transmittance and light scattering, and calorimetric studies which indicate that microdroplet formation in the films occurs as a result of phase separation which takes place during the cure process.

A Light Control Film Composed of Liquid Crystal Droplets Dispersed in an Epoxy Matrix

Abstract This paper describes light control films consisting of submicron liquid crystal droplets dispersed in epoxy matrices. These films, which can respond optically to both applied electric fields and temperature changes, are potentially useful for displays and light shutters. The optical performance of these films depends on a variety of structural, electrooptical and thermal properties. This paper describes scanning electron microscope studies of film structure, measurements of voltage dependent film transmittance, contrast ratio and light scattering, and calorimetric studies of the cure process which governs microdroplet formation in the films.

Enzymic Milk Coagulation: Role of Equations Involving Coagulation Time and Curd Firmness in Describing Coagulation

Digitized curd firmness data were fitted to an exponential equation by a nonlinear, least squares regression computer program. Solutions were obtained for actual coagulation time, curd firming rate, and theoretical maximum firmness. Actual coagulation time occurred before an increase in firmness was detectable and coincided with the inflection point of apparent absorbance changes of coagulating milk.Only the initial portion of the curve of firmness on time could be fitted to the exponential equation. Beyond this region, experimentally observed curd firmness lagged behind predicted firmness. Apparent absorbance measurements indicated this was from effects of “microsyneresis”, the consolidation of casein micelle chains into strands and the concomitant formation of micro-droplets of milk serum between strands of the gel network.Actual coagulation time corresponds to the instant when an extended space network of casein particles is formed. For this network to be formed, aggregating micelle clusters must reach a critical volume subsequent to which any further diffusion results in immediate collision with another cluster. Dilution of milk inhibits attainment of this critical point, and a curd is not formed. Determination of coagulation time visually or rheologically is an estimate of actual time, and close correspondence of coagulation time to actual depends on the methods. In general, visual time is more than actual because a finite amount of rigidity must be produced before coagulation can be detected.

Reflections on IAGLR's Silver Anniversary

The high-throughput two-phase partition ability of microdroplet technology attracts attentions from many researchers in many fields. This paper reviews the main methods currently developed for microdroplet formation, including hydrodynamic method, pneumatic pressure method, optic-driven method, and the microfluidic techniques for microdroplet manipulation, such as splitting, coalescence, mixing, sorting, trapping and droplet position control.

A novel device for the accurate dispensing of small volumes of liquid samples

A sample dispenser with electronic control of delivered volume has been designed and evaluated. With this device, liquid samples are dispensed in the form of uniform droplets less than 4 nl in volume; total volumes as small as 40 nl can be delivered with 1.5% precision. Important features of the microdroplet formation process have been studied and the effect of sample solution concentration and viscosity have been examined. The dispenser is expected to find application in several areas, including electrothermal atomization atomic absorption spectrometry, automated titrimetry, and high-precision, low-volume solution sampling.

Electron-stimulated oxidation of InP (100) surfaces studied by Auger electron spectroscopy and by low-energy-loss spectroscopy

Stimulated oxidation of InP (100) surfaces by primary electron beams was studied by Auger electron spectroscopy. At the point of impact of the incident beam, we concluded that the formation of indium oxide and of a volatile phosphorus oxide occur, which explains the phosphorus depletion of the first monolayers. The low-energy electron-diffraction patterns for successive heat treatments and electron-stimulated oxidations of InP surfaces, and the corresponding characteristic-energy-loss spectra were performed. For heat treatments close to the decomposition temperature, we report here on how low-energy-loss spectroscopy was used to supply the deficiency of Auger electron spectroscopy, the latter method being unable to detect the formation of indium microdroplets.