Production of uniform droplets using membrane, microchannel and microfluidic emulsification devices

This review provides an overview of major microengineering emulsification techniques for production of monodispersed droplets. The main emphasis has been put on membrane emulsification using Shirasu Porous Glass and microsieve membrane, microchannel emulsification using grooved-type and straight-through microchannel plates, microfluidic junctions and flow focusing microfluidic devices. Microfabrication methods for production of planar and 3D poly(dimethylsiloxane) devices, glass capillary microfluidic devices and single-crystal silicon microchannel array devices have been described including soft lithography, glass capillary pulling and microforging, hot embossing, anisotropic wet etching and deep reactive ion etching. In addition, fabrication methods for SPG and microseive membranes have been outlined, such as spinodal decomposition, reactive ion etching and ultraviolet LIGA (Lithography, Electroplating, and Moulding) process. The most widespread application of micromachined emulsification devices is in the synthesis of monodispersed particles and vesicles, such as polymeric particles, microgels, solid lipid particles, Janus particles, and functional vesicles (liposomes, polymersomes and colloidosomes). Glass capillary microfluidic devices are very suitable for production of core/shell drops of controllable shell thickness and multiple emulsions containing a controlled number of inner droplets and/or inner droplets of two or more distinct phases. Microchannel emulsification is a very promising technique for production of monodispersed droplets with droplet throughputs of up to 100?l?h^?1.     

Preparation of Protein-Stabilized β-Carotene Nanodispersions by Emulsification–Evaporation Method

This work was initiated to prepare protein-stabilized β-carotene nanodispersions using emulsification–evaporation. A pre-mix of the aqueous phase composed of a protein and hexane containing β-carotene was subjected to high-pressure homogenization using a microfluidizer. Hexane in the resulting emulsion was evaporated under reduced pressures, causing crystallization and precipitation of β-carotene inside the droplets and formation of β-carotene nanoparticles. Sodium caseinate (SC) was the most effective emulsifier among selected proteins in preparing the nanodispersion, with a monomodal β-carotene particle-size distribution and a 17-nm mean particle size. The results were confirmed by transmission-electron microscopy analysis. SC-stabilized nanodispersion also had considerably high ζ-potential (−27 mV at pH 7), suggesting that the nanodispersion was stable against particle aggregation. Increasing the SC concentration decreased the mean particle size and improved the polydispersity of the nanodispersions. Nanodispersions prepared with higher β-carotene concentrations and higher organic-phase ratios resulted in larger β-carotene particles. Although increased microfluidization pressure did not decrease particle size, it did improve the polydispersity of the nanodispersions. Repeating the microfluidization process at 140 MPa caused the nanodispersions to become polydisperse, indicating the loss of emulsifying capacity of SC due to protein denaturation.     

Production of monodisperse water-in-oil emulsions consisting of highly uniform droplets using asymmetric straight-through microchannel arrays

This paper reports the production of monodisperse water-in-oil (W/O) emulsions using new microchannel emulsification (MCE) devices, asymmetric straight-through MC arrays that were hydrophobically modified. The silicon asymmetric straight-through MC arrays consisted of numerous pairs of microslots and circular microholes whose cross-sectional sizes were 10 μm. This paper primarily focused on investigating the effect of the osmotic pressure of a dispersed phase (Π_d) on MCE. This paper also investigated the effects of the type of continuous-phase oils and the dispersed-phase flux (J _d) on MCE. The dispersed phases were Milli-Q water and Milli-Q water solutions containing sodium chloride. The continuous phases were decane (as control), hexane, medium chain triacylglyceride (MCT), and refined soybean oil (RSO) solutions containing tetraglycerin monolaurate condensed ricinoleic acid ester (TGCR) as a surfactant. At Π_d of exceeding threshold, highly uniform aqueous droplets with coefficients of variation of less than 3% were stably generated via hydrophobic asymmetric straight-through MCs. Monodisperse W/O emulsions with average droplet diameters between 32 and 45 μm were produced using the alkane–oil and triglyceride–oil solutions as the continuous phase. This work also demonstrated that the hydrophobic asymmetric straight-through MC array had remarkable ability to produce highly uniform aqueous droplets at very high J _d of up to 1,200 L m⁻² h⁻¹.     

Generation of uniform drops via through-hole arrays micromachined in stainless-steel plates

This study investigated the generation of oil drops using new symmetric and asymmetric through-hole-array devices made of stainless steel. The through-hole-array devices were built by piling up six stainless-steel plates, each having circular micro-holes with a diameter of 300 or 500 μm or micro-slots with a shorter line of 300 or 500 μm. Drops were generated by injecting a dispersed phase (refined soybean oil) via the through-hole array into a compartment filled with a continuous phase (Milli-Q water solution containing one of two emulsifiers). The drop detachment from symmetric and asymmetric through holes was observed in real time and analyzed. Uniform oil drops with average diameters of 1.0–4.1 mm and coefficients of variation of typically less than 6% were generated using symmetric and asymmetric through-hole-array devices. The resultant drop diameters for asymmetric through-hole arrays were significantly smaller than those for symmetric through-hole arrays. This paper also discusses experimental results regarding the effects of the microstructure, the dimensions of the through holes, and the type of emulsifier on drop generation and the resultant drop diameter.     

Straight-through microchannel devices for generating monodisperse emulsion droplets several microns in size

The authors recently proposed a promising technique for producing monodisperse emulsions using a straight-through microchannel (MC) device composed of an array of microfabricated oblong holes. This research developed new straight-through MC devices with tens of thousands of oblong channels of several microns in size on a silicon-on-insulator plate, and investigated the emulsification characteristics using the microfabricated straight-through MC devices. Monodisperse oil-in-water (O/W) and W/O emulsions with average droplet diameters of 4.4–9.8 μm and coefficients of variation of less than 6% were stably produced using surface-treated straight-through MC devices that included uniformly sized oblong channels with equivalent diameters of 1.7–5.4 μm. The droplet size of the resultant emulsions depended greatly on the size of the preceding oblong channels. The emulsification process using the straight-through MC devices developed in this research had very high apparent energy efficiencies of 47–60%, defined as (actual energy input applied to droplet generation/theoretical minimum energy input necessary for making droplets) × 100. Straight-through MC devices with numerous oblong microfluidic channels also have great potential for increasing the productivity of monodisperse fine emulsions.     

Thermogravimetric behavior of perfluoropolyether

In this article, two kinds of perfluoropolyether (PFPE) with different terminal groups were investigated using thermogravimetric analysis (TGA), infrared (IR) spectroscopy, gel permeation chromatography (GPC), and gas chromatography/gas mass spectrum (GC/MS) analysis. PFPEs with a hydroxyl or carboxylic acid terminal group were more heat stable than was PFPE with carboxylic methyl ester. Perfluoropropylene oxide-type PFPE with a perfluoroethyl terminal group at one end tends to lose weight more rapidly than does copolymer-type PFPE with dihydroxyl or dicarboxyl methyl ester terminal groups at both ends. The residual weight fraction of PFPE with a perfluoroethyl terminal group was dependent on the average molecular weight. The number-average molecular weight of PFPE can be calculated from the peak intensity ratio between the polar group and C F stretching by measuring the IR spectrum of PFPE. The number-average molecular weight of PFPE increased because of the evaporation loss of its low molecular weight fraction and the crosslinking reaction of PFPE with increase in temperature. GC/MS analysis showed that the main product of the pyrolysis of PFPE was hexafluoropylene. We speculated on the PFPE degradation mechanism and the optimum PFPE chemical structure in terms of heat stability. © 1996 John Wiley & Sons, Inc.     

Stability of protein‐stabilised β‐carotene nanodispersions against heating,salts and pH

BACKGROUND: Milk proteins are used in a wide range of formulated food emulsions. The stability of food emulsions depends on their ingredients and processing conditions. In this work, β‐carotene nanodispersions were prepared with selected milk‐protein products using solvent‐displacement method. The objective of this work was to evaluate the stability of these nanodispersions against heating, salts and pH. RESULTS: Sodium caseinate (SC)‐stabilised nanodispersions possessed the smallest mean particle size of 17 nm, while those prepared with whey‐protein products resulted in larger mean particle sizes (45–127 nm). Formation of large particles (mean particle size of 300 nm) started after 1 h of heating at 60 °C in nanodispersions prepared with SC. More drastic particle size changes were observed in nanodispersions prepared with whey protein concentrate and whey protein isolate. The SC‐stabilised nanodispersions were fairly stable against Na⁺ ions at concentrations below 100 mmol L^?1, but drastic aggregation occurred in ≥ 50 mmol L^?1 CaCl₂ solutions. Aggregation was also observed in whey protein‐stabilised nanodispersions after the addition of NaCl and CaCl₂ solutions. All sample exhibited the smallest mean particle size at neutral pH, but large aggregates were formed at both ends of extreme pH and at pH around the isoelectric point of the proteins. CONCLUSION: The nanodispersions prepared with SC were generally more stable against thermal processing, ionic strength and pH, compared to those prepared with whey proteins. The stable β‐carotene nanodispersions showed a good potential for industrial applications. Copyright © 2008 Society of Chemical Industry     

Security by facility design for sabotage protection

Facility design of nuclear power plant (NPP) for a sabotage protection is investigated and an effect of the design change for damage control on reduction of sabotage risk is shown using the vital area identification methodology. In a sabotage incident, it is not straightforward to identify the most credible scenario for NPP. However, the loss of offsite power leading to the station blackout is assumed to be a typical example for further evaluation. In this study, the vulnerability of vital area is considered in terms of the accessibility, the distribution of vital equipment, and the adversary's interference. As seen in the past report, the built-in measures for damage control are important in case of the existence of adversary's interference until neutralization. It is confirmed that not only the physical protection system, but also the facility design on structures, systems, and components play an important role in the effective and efficient sabotage protection. To reduce any vulnerability in the design of NPP, it is very important to introduce a security by design approach in an initial stage of the NPP construction while considering the interface between safety and security.     

cis/trans-Isomerisation of triolein,trilinolein and trilinolenin induced by heat treatment

To estimate the trans-fatty acid production of edible oils during the frying process, 1.0 g of triolein, trilinolein and trilinolenin, as representative oils, were heated at 180 °C for a defined period. The amounts of trans-fatty acids in heated triacylglycerols were quantitatively determined by gas chromatography after methylation. It was revealed that heating induced cis to trans-isomerisation of unsaturated triacylglycerols, and that trans-fatty acid amounts increased gradually, depending on the heating period. For example, trans-isomer amounts in triolein, trilinolein and trilinolenin (per gram) were 5.8 mg, 3.1 mg and 6.5 mg, respectively, after 8 h incubation at 180 °C. At that time, the contents of polar compounds contained in the heated triolein, trilinolein and trilinolenin were 22%, 27% and 31%, respectively. When triolein was heated under a N₂ stream, neither trans-isomerisation nor polar compounds were detected. The addition of α-tocopherol (1.0%) to triolein significantly prevented not only lipid oxidation but also trans-isomerisation during heating. A commercially available vegetable oil was also heated under the same conditions as these model oils. Compared with the trans-isomerisation in model oils, the degree of trans-isomerisation in the edible oil was relatively low. Tocopherols in the oil would prevent not only lipid oxidation but also isomerisation. These results suggest that the geometric isomerisation of unsaturated fatty acids during heating accompanies lipid oxidation.