Designer hydrophilic regions regulate droplet shape for controlled surface patterning and 3D microgel synthesis

A simple technique is presented for controlling the shapes of micro- and nanodrops by patterning surfaces with special hydrophilic regions surrounded by hydrophobic boundaries. Finite element method simulations link the shape of the hydrophilic regions to that of the droplets. Shaped droplets are used to controllably pattern planar surfaces and microwell arrays with microparticles and cells at the micro- and macroscales. Droplets containing suspended sedimenting particles, initially at uniform concentration, deposit more particles under deeper regions than under shallow regions. The resulting surface concentration is thus proportional to the local fluid depth and agrees well with the measured and simulated droplet profiles. A second application is also highlighted in which shaped droplets of prepolymer solution are crosslinked to synthesize microgels with tailored 3D geometry.     

Laser manipulation in liquid crystals: an approach to microfluidics and micromachines

Laser trapping of particles in three dimensions can occur as a result of the refraction of strongly focused light through micrometre-sized particles. The use of this effect to produce laser tweezers is extremely common in fields such as biology, but it is only relatively recently that the technique has been applied to liquid crystals (LCs). The possibilities are exciting: droplets of LCs can be trapped, moved and rotated in an isotropic fluid medium, or both particles and defects can be trapped and manipulated within a liquid crystalline medium. This paper considers both the possibilities. The mechanism of transfer of optical angular momentum from circularly polarized light to small droplets of nematic LCs is described. Further, it is shown that droplets of chiral LCs can be made to rotate when illuminated with linearly polarized light and possible mechanisms are discussed. The trapping and manipulation of micrometre-sized particles in an aligned LC medium is used to provide a measure of local shear viscosity coefficients and a unique test of theory at low Ericksen number in LCs.     

Fabrication of advanced particles and particle-based materials assisted by droplet-based microfluidics

Recent advances in the fabrication of complex particles and particle-based materials assisted by droplet-based microfluidics are reviewed. Monodisperse particles with expected internal structures, morphologies, and sizes in the range of nanometers to hundreds of micrometers have received a good deal of attention in recent years. Due to the capability of generating monodisperse emulsions and of executing precise control and operations on the suspended droplets inside the microchannels, droplet-based microfluidic devices have become powerful tools for fabricating complex particles with desired properties. Emulsions and multiple-emulsions generated in the microfluidic devices can be composed of a variety of materials including aqueous solutions, gels, polymers and solutions containing functional nanoparticles. They are ideal microreactors or fine templates for synthesizing advanced particles, such as polymer particles, microcapsules, nanocrystals, and photonic crystal clusters or beads by further chemical or physical operations. These particles are promising materials that may be applicable for many fields, such as photonic materials, drug delivery systems, and bio-analysis. From simple to complex, from spherical to nonspherical, from polymerization and reaction crystallization to self-assembly, this review aims to help readers be aware of the many aspects of this field.     

含油污水陶瓷膜处理工艺的数值模拟

陶瓷膜污水处理技术近年来在油田含油污水领域得到了迅速发展,跨膜压差、处理温度以及油滴和悬浮颗粒的浓度都直接关系到膜处理时对离散相粒子直径的选择性和处理效果.运用Fluent计算软件在微观上对含油污水的处理条件(包括跨膜压差、温度以及含油污水的浓度)进行了数值模拟研究,结果表明,在适当的跨膜压差下,提高污水处理温度,减小含油污水浓度(进行污水预处理)可以减小膜污染的程度,从而明显地提高污水处理的效果.     

Cellulose‐Based Microparticles for Magnetically Controlled Optical Modulation and Sensing

Responsive materials with birefringent optical properties have been exploited for the manipulation of light in several modern electronic devices. While electrical fields are often utilized to achieve optical modulation, magnetic stimuli may offer an enticing complementary approach for controlling and manipulating light remotely. Here, the synthesis and characterization of magnetically responsive birefringent microparticles with unusual magneto‐optical properties are reported. These functional microparticles are prepared via a microfluidic emulsification process, in which water‐based droplets are generated in a flow‐focusing device and stretched into anisotropic shapes before conversion into particles via photopolymerization. Birefringence properties are achieved by aligning cellulose nanocrystals within the microparticles during droplet stretching, whereas magnetic responsiveness results from the addition of superparamagnetic nanoparticles to the initial droplet template. When suspended in a fluid, the microparticles can be controllably manipulated via an external magnetic field to result in unique magneto‐optical coupling effects. Using a remotely actuated magnetic field coupled to a polarized optical microscope, these microparticles can be employed to convert magnetic into optical signals or to estimate the viscosity of the suspending fluid through magnetically driven microrheology.     

Characteristics of spray flames and the effect of group combustion on the morphology of flame-made nanoparticles

Characteristics of burning and non-burning sprays generated by a coaxial air-assist nozzle, previously used for the synthesis of ceramic nanoparticles by flame spray pyrolysis (FSP), are studied using phase Doppler anemometry. Also, the effect of droplet interaction on the overall combustion behavior of the spray (group combustion) and, consequently, on the characteristics of flame-made ceramic particles is investigated. A physical model is proposed which correlates the formation of inhomogeneous mixtures of micron-sized hollow particles and solid nanoparticles to the combustion mode: the precursor droplets which entirely evaporate in the hot flame are responsible for the formation of nanoparticles. The vapor species react, forming intermediate and product molecules and clusters that quickly grow to nanosized ceramic particles. On the other hand, under certain conditions, a small number of the droplets, particularly with large initial sizes, escape from the spray boundaries and become extinguished, producing large hollow ceramic particles. It is also possible that some of the large droplets, which lie within the spray core, do not entirely evaporate. These surviving droplets then form large particles which are usually hollow but can collapse to solid particles at sufficiently high temperatures. Also, a criterion for the formation of homogeneous ceramic nanoparticles is presented.     

Quantification and characterization of micrometer and submicrometer subvisible particles in protein therapeutics by use of a suspended microchannel resonator

The ability to characterize micrometer and submicrometer particles in solution is of fundamental importance to understanding the relationship between protein particles in biotherapeutics and concerns raised regarding immunogenicity. While a number of characterization methods are available for analyzing subvisible particle content in protein pharmaceuticals, counting and characterizing particles within the entire subvisible size range remains a significant challenge due to the properties of the proteinaceous particles themselves and to the limitations of the available techniques. Additionally, as silicone oil-lubricated prefilled syringes become a favored primary packaging for biotherapeutic products, proteinaceous subvisible particle characterization is further complicated by the presence of silicone oil droplets in solution. Here, we critically evaluate and apply a novel method for particle characterization that relies on differences in particle buoyant mass to characterize particle content in the range of ca. 0.5-5 μm. A model particle system was specifically designed to evaluate the ability of the suspended microchannel resonator (SMR) to distinguish between buoyant particles (e.g., silicone oil) and dense particles (e.g., protein particles) in aqueous solution. In addition, this emerging technique was successfully applied to high-concentration monoclonal antibody solutions stored in prefilled syringes in stressed stability studies. It is shown that the SMR system can potentially distinguish between silicone oil droplets and protein particles in a size range that is challenging for many subvisible particle characterization methods. Limitations of the SMR method are also discussed.     

Topological considerations of parametric electromechanical devices and their parametric analysis

One of the methods for attaining free suspension of objects in magnetic fields is magnetic levitation by tuned circuits. Tuned-circuit levitators exhibit typical dynamic instability. However, the inherent tendency of the suspended object to oscillate may serve as a basis for the construction of relatively new types of machines. An attempt is made to treat, all these devices from a generalized point of view. A method similar to that used for the analysis of parametric electronic networks is suggested here to tackle parametric electromechanical systems. The paper concludes with two recent examples which illustrate how unexpected are the phenomena, where parametric electromechanical effects are of relevance. A peculiar mechanical instability which occurred in the VHF resonators used in particles accelerators, has been described recently. It has also been found that both a rotation of a suspended object as well as its levitation can be achieved by using a levitator with only one electromagnet.     

Porous epoxy microparticles prepared by an advanced aqueous method

This paper reports the synthesis of emulsions of epoxy compounds with various ratios of epoxy, polyamide and calcium carbonate by homogenization in water at room temperature. The emulsion is cured at 80 °C, and during curing the small epoxy droplets in the emulsion coalesce. The cured coalesced epoxy droplets contain holes formed due to water trapped inside them and also due to the difference in curing rates between epoxy molecules on the outside versus inside of the coalesced droplets. The porous epoxy particles are obtained when the cured coalesced epoxy droplets are treated with hydrochloric acid to remove the calcium carbonate. The size of the porous epoxy particles and the number of holes formed depends on the curing rate of the epoxy compound.     

ON THE STABILITY AND SEGREGATION OF SUSPENDED PARTICLES IN FLOW AND POLARIZATION FIELDS

The stability of an assembly of particles which are suspended in a flow field of a gas, under the action of force fields, is considered. It is shown that the stability of the particle dispersion depends on the composite energy density due to the flow and the force fields. The stability can be maintained by balancing the change occurring in the flow and force fields, so that the total energy density after the change remains fixed. This capability is well established in Magnetically Stabilized Fluidized Beds (MSFBs), where the magnetic field can be used to lower the energy of the system against the rise in the kinetic energy of the fluidizing gas. A general criterion for stability is formulated in terms of the balance between energy densities that can be assigned to the dispersion as a composite entity. The sedimentation behavior of particles fluidized by gas discloses the factors that affect the intensity of instability of these systems. Simulation of batch sedimentation of concentrated, i.e. low voidage, polydisperse particle mixtures shows that the evolution of voidage disturbances is enhanced at higher particle concentrations and narrower size distributions. Under these conditions, relaxation time of the system becomes too high so that the evolution of the disturbance cannot be suppressed. This behaviour is the result of fluid particle interaction and the high sensitivity of the system to relatively small voidage perturbation. The application of polarization fields can eliminate this high sensitivity and render the system more stable. The extra stability is achieved by formation of ordered, pearl chain structures of the polarized particles. These structures render the system higher permeability levels to the flow of gas, thus allowing higher velocities at the same pressure drop. Alternatively, the formation of pearl chains changes the fluid particles interactions, decreasing the fluid drag and increasing the sedimentation velocity of the particles.     

The influence of electro-freezing on ice formation on high-voltage DC transmission lines

Supercooled droplets ranging from 15 to 38 μm diameter are accumulated on an aluminium conductor of 1 cm in diameter installed in the working section of an open wind tunnel. Density and adhesive strength of the ice are measured as a function of the applied field strength at the surface of the conductor. All experiments are conducted in a cold chamber under constant atmospheric conditions.The results obtained with ice formed from droplets of 20 μm mean diameter show that DC negative and positive electric fields above ?10 and +15 kV/cm respectively decrease considerably the density, and consequently the adhesive strength of ice. Alternating fields show weaker effects upon ice density when compared with DC fields.The effect of the electric field upon ice density decreases with increasing size of the supercooled droplets. Alternating fields have practically no effect upon the density of ice formed from 38 μm droplets. An increase in the conductivity of water by the addition of a small quantity of NaCl results in an increase in ice density even in the absence of electric field.Ice accretions under DC electric fields contain many heavily branched ice trees. These ice branches present a converging structure under a positive field and a diverging one under a negative field. From the similarity of the shapes of ice branches and corona streamers, it appears that supercooled droplets freeze along the corona streamers due to the corona wind, which is caused by the evacuation of ionised gas particles.     

An aerosol-based route to nanostructured powders synthesis in liquids

An alternative technique for synthesizing nanostructured powders in liquid solutions has been developed. The technique combines generation of charged aerosols via electrospray with reductive precipitation reactions in liquids. Electrospray of liquids is carried out to produce micrometric, nearly mono-dispersed airborne droplets from a precursor solution. The droplets, which are spatially separated due to electrostatic repulsion, are collected in a bath containing a reductive solution. The effect of some process parameters on the resulting material texture has been studied. Tin particles produced from tin chloride solutions are regarded as a model here, but it is stressed that this approach can be considered as a general method to synthesize many other metallic-like materials, such as alloys and intermetallics. Hence, the large variety of materials that can be produced in this way could find several relevant applications in different technological fields.     

Synthesis of submicron sized silver powder for metal deposition via laser sintered inkjet printing

Submicron sized silver powder was prepared from AgNO₃ using a chemical-reduction method. A spherical silver power exhibiting an average particle size distribution of 0.2–0.4 μm and an excellent dispersibility was achieved and applied to the inkjet printing process. A drop-on-demand (DOD) inkjetting system was used to print the silver particles suspended in a terpineol solvent. Through sintering at 300 °C, the size of the particles adjacent to the borderline of droplets were gradually increased and necking was observed between the droplets. Alternatively, the substrate for the particles could be heated to a lower temperature, and the sintering process of the conducting line was completed by the application of a laser beam. Increase in the laser power reduces the resistivity of the line. Through microstructure analysis, the necks between droplets were sintered at a specific energy density (ψ = 0.0398 J mm⁻³). The conducting lines were soldered and of a larger aggregation, between which a discontinuous micro-crack was observed. This was attributed to the surface tension effect and shrinkage during solidification. Influence of the densification parameters on resistivity was significant.     

Accurate treatment of lubrication forces between rigid spheres in viscous fluids using a traction-corrected boundary element method

Traditional boundary element methods cannot accurately resolve lubrication forces in the interstitial regions between nearly touching particles in viscous flows. In many cases, the interstitial tractions are underestimated and the relative particle velocities are overestimated resulting in significant errors in predicting particle trajectories. In order to accurately treat the lubrication forces between nearly touching particles, a traction-corrected boundary element method (TC-BEM) for multiple particles is developed by combining the analytical asymptotic solution for the tractions in the interstitial regions with the boundary element method. An adaptive meshing algorithm is developed to provide appropriate meshes on surfaces of particles with close interactions. The numerical method also employs an efficient parallelization scheme to make possible prediction of long-time behavior of particles suspended in viscous flow fields. The results of the TC-BEM are benchmarked by comparisons to analytical results for two particles in a linear shear flow and by considering the reversibility of three particles in a circular Couette flow. It is shown that the TC-BEM is able to correctly resolve the lubrication forces between nearly touching particles, thus enabling the accurate analysis of particles suspended in nonlinear shear flows.     

Measurement of the Viscosity of Dilute Magnetic Fluids

The capillary tube viscometer is used to measure the viscosity of aqueous magnetic fluids under the influence of parallel and perpendicular magnetic fields. The effects of the volume fraction of the suspended magnetic particles, the concentration of surfactants, and the external magnetic field strength, as well as the orientation, on the viscosity of the magnetic fluid are analyzed. The experimental results show that the viscosity of the sample magnetic fluids increases with increases in the concentrations of suspended magnetic particles and surfactants. The external magnetic field is also an important factor that affects the viscosity of the magnetic fluid. The viscosity first increases with the magnetic field and finally approaches a constant as the magnetization attains a saturation state. For the same magnetic fluid, the viscosity in a perpendicular magnetic field is larger than that in a parallel magnetic field for the same magnetic field.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China     

Template-assisted spray-drying method for the fabrication of porous particles with tunable structures

Developing strategies for the production of porous particles with controllable structures using a spray-drying method has attracted attention of researchers for decades. Although many papers have reported their successful production of porous particles using this method, information on how to create and control the porous structures as well as what parameters involving and what formation mechanism occurring during the synthesis process are still not clear. To meet these demands, the present review covers strategies in the spray-drying developments for the fabrication of porous particles with controllable structure. This information is important for optimizing the production of porous particles with desirable properties. Regulation of process conditions and precursor formulations are also explained, including composition, type, and physicochemical properties of droplet and raw components used (i.e., host component, template, and solvent). The electrostatic interactions between the individual components and the droplets are also presented, while this information tends to be neglected in the conventional spray-drying process. To clarify how the porous particles are designed, current experimental results completed with illustrations for the proposal particle formation mechanism are presented. The review also completed with the opportunities and potential roles of the changing porous structures in practical uses. This review would provide information on how to produce porous particles that can be used for advanced functional materials, such as catalysts, adsorbents, and sensors.     

Non-equilibrium solidification of undercooled droplets during atomization process

Thermal history of droplets associated with gas atomization of melt has been investigated. A mathematical model, based on classical theory of heterogeneous nucleation and volume separation of nucleants among droplets size distribution, is described to predict undercooling of droplets. Newtonian heat flow condition coupled with velocity dependent heat transfer coefficient is used to obtain cooling rate before and after nucleation of droplets. The results indicate that temperature profile of droplets in the spray during recalescence, segregated and eutectic solidification regimes is dependent on their size and related undercooling. The interface temperature during solidification of undercooled droplets rapidly approaches the liquidus temperature of the alloy with a subsequent decrease in solid-liquid interface velocity. A comparison in cooling rates of atomized powder particles estimated from secondary dendrite arm spacing measurements are observed to be closer to those predicted from the model during segregated solidification regime of large size droplets.     

Development and evaluation of spherical molecularly imprinted polymer beads

The majority of studies on molecularly imprinted polymers has until now been carried out on irregularly shaped particles prepared by grinding of polymer monoliths. The preparation procedures are time- and labor-consuming and produce particles of wide size distributions. To answer the need for fast and straightforward routes to spherical molecularly imprinted polymer beads, we have developed a method comprising the formation of droplets of pre-polymerization solution directly in mineral oil by vigorous mixing followed by transformation of the droplets into solid spherical beads by photoinduced free-radical polymerization. No detergents or stabilizers were required for the droplet formation. Factors influencing the bead synthesis have been investigated and are detailed here. The beads were evaluated in parallel with corresponding irregularly shaped particles prepared from polymer monoliths. Conditions for the synthesis of propranolol-imprinted poly(methacrylic acid-co-trimethylolpropane trimethacrylate) beads in the size range of 1-100 microm in almost quantitative yield are described. The beads were applied as the recognition element in a 96-well plate format radioligand assay of propranolol in human serum.     

The Generation of Nanoparticles in Miniemulsions

A versatile method for the generation of nanoparticulate metals, ceramics, and polymers based on synthesis in miniemulsions—highly stable small droplets in a continuous phase—is presented here. It is revealed that in addition to nanoparticles, encapsulated materials, polymer capsules (see Figure), and hollow particles can also be obtained by careful selection of the starting materials.     

Experimental and Theoretical Investigation on Rapid Evaporation of Ethanol Droplets and Kerosene Droplets During Depressurization

This paper reports an experimental and theoretical study of rapid evaporation of ethanol droplets and kerosene droplets during depressurization. For experimental method, an ethanol droplet or a kerosene droplet was suspended on a thermocouple, which was also used to measure the droplet center temperature transition. And the droplet shape variation was recorded by a high speed camera. A theoretical analysis was developed based on the heat balance to estimate the droplet center temperature transition, and the evaporation model proposed by Abramzon and Sirignano was used to describe the droplet vaporization. According to the experimental data and theoretical analysis, both of the environmental pressure and the initial droplet diameter have a prominent influence on the droplet temperature transition. Comparing the evaporation processes of ethanol droplets and kerosene droplets with water droplets, the ethanol droplets have the fastest evaporation rate, followed by water, and the evaporation rates of kerosene droplets are the slowest. Also it was found that a bubble can easily emerge within kerosene droplet, and its lifetime is more than 1 s.