A novel particle separation method based on induced‐charge electro‐osmotic flow and polarizability of dielectric particles

A new microfluidic method of particle separation was proposed and studied theoretically in this paper. This method is based on the induced charge electro‐osmotic flow (ICEOF) and polarizability of dielectric particles. In this method, a pair of metal plates is embedded on the side channel walls to create a region of circulating flows under applied electric field. When a dielectric particle enters this region, the vortices produced by ICEOF around the particle will interact with the circulating flows produced by the metal plates. Such hydrodynamic interaction influences the particle's trajectory, and may result in the particle being trapped in the flow circulating zone or passing through this flow circulating zone. Because the hydrodynamic interaction is sensitive to the applied electric field, and the polarizability and the size of the particles, separation of different particles can be realized by controlling these parameters. Comparing with electrophoresis and dielectrophoresis methods, this strategy presented in this paper is simple and sensitive.     

MMA/BA无皂乳液聚合机理研究--三阶段成粒机理

用Coulter LS230激光粒径分析仪研究MMA/BA无皂乳液共聚合中单分散粒子的成粒机理.根据理论和实验数据分析,其成核过程为均相成核机理.根据粒径分布和粒子数变化情况,把无皂乳液聚合过程分为3个阶段:第一阶段为成核-凝聚阶段,体系粒子数密度迅速增加,而粒径变化较小;第二阶段为成核-凝聚、增长-聚并共存阶段,当粒子数密度开始快速减少时,标志着第一阶段结束,第二阶段开始,当初级粒子开始消失时,标志着第二阶段结束;第三阶段为增长-聚并阶段.成核过程结束后,粒子迅速增长,较小粒子在增长过程中的优先聚并,导致粒径逐渐趋向均一,最终生成单分散性乳液.     

Continuous and precise particle separation by electroosmotic flow control in microfluidic devices

A new scheme has been described for continuous particle separation using EOF in microfluidic devices. We have previously reported a method for particle separation, called "pinched flow fractionation (PFF)", in which size-dependent and continuous particle separation can be achieved by introducing pressure-driven flows with and without particles into a pinched microchannel. In this study, EOF was employed to transport fluid flows inside a microchannel. By controlling the applied voltage to electrodes inserted in each inlet/outlet port, the flow rates from both inlets, and flow rates distributed to each outlet could be accurately tuned, thus enabling more effective separation compared to the pressure-driven scheme. In the experiment, the particle behaviors were compared between EOF and pressure-driven flow schemes. In addition, micrometer- and submicrometer-sized particles were accurately separated and individually collected using a microchannel with multiple outlet branch channels, demonstrating the high efficiency of the presented scheme.     

Lateral and cross-lateral focusing of spherical particles in a square microchannel

The inertial migration of particles in micro-scale flows has received much attention due to its promising applications, such as the membrane-free passive separation of particles or cells. The particles suspended in rectangular channels are known to be focused near the center of each channel face as the channel Reynolds number (R(C)) increases due to the lift force balance and the hydrodynamic interactions of the particles with the wall. In this study, the three-dimensional positions of neutrally buoyant spherical particles inside a square microchannel are measured using the digital holographic microscopy technique, and a transition from the lateral tubular pinch to the cross-lateral focusing with increasing R(C) is reported. The particles are found to migrate first in the lateral direction and then cross-laterally toward the four equilibrium positions. A general criterion that can be used to secure the fully developed state of particle focusing in Lab-on-a-Chip applications is also derived. This criterion could be helpful for the accurate estimation of the design parameters of inertial microfluidic devices, such as R(C), channel length and width, and particle diameter.     

Crystallization mechanisms in convective particle assembly

Colloidal particles are continuously assembled into crystalline particle coatings using convective fluid flows. Assembly takes place inside a meniscus on a wetting reservoir. The shape of the meniscus defines the profile of the convective flow and the motion of the particles. We use optical interference microscopy, particle image velocimetry, and particle tracking to analyze the particles' trajectory from the liquid reservoir to the film growth front and inside the deposited film as a function of temperature. Our results indicate a transition from assembly at a static film growth front at high deposition temperatures to assembly in a precursor film with high particle mobility at low deposition temperatures. A simple model that compares the convective drag on the particles to the thermal agitation explains this behavior. Convective assembly mechanisms exhibit a pronounced temperature dependency and require a temperature that provides sufficient evaporation. Capillary mechanisms are nearly temperature independent and govern assembly at lower temperatures. The model fits the experimental data with temperature and particle size as variable parameters and allows prediction of the transition temperatures. While the two mechanisms are markedly different, dried particle films from both assembly regimes exhibit hexagonal particle packings. We show that films assembled by convective mechanisms exhibit greater regularity than those assembled by capillary mechanisms.     

氧化-还原低温引发甲基丙烯酸甲酯/丙烯酸丁酯超浓乳液聚合研究

以乳化剂十二烷基硫酸钠 (SDS)和共乳化剂十六烷醇 (HD)作为复合乳化体系 ,过氧化二苯甲酰(BPO)和N ,N 二甲基苯胺 (DMA)作为氧化还原引发体系 ,甲基丙烯酸甲酯 丙烯酸丁酯 (MMA BA)作为混合单体 ,制备了分散相占 83 %以上的稳定的超浓乳液 ,然后在低温下引发聚合 .探讨了引发剂浓度、氧化剂与还原剂的摩尔比、乳化剂的浓度、液膜增强剂的种类、聚合温度等因素对聚合稳定性和聚合速率的影响 ,测定并计算得到了聚合速率的公式 ;用激光散射粒度分布仪测定了聚合物乳胶粒子的大小及粒径分布 ,用透射电子显微镜观察了聚合物乳胶粒的形态 ,讨论了乳化剂浓度、聚合温度等对乳胶粒形态、大小的影响     

氮和碳共掺杂TiO2纳米晶的制备及可见光催化性能

以钛酸四丁酯为钛源, 冰醋酸为抑制剂, 超细铵盐为固体载体, 采用新型溶胶-凝胶法制备了氮和碳共掺杂TiO2纳米晶(N-C-TiO2) 光催化剂. 透射电子显微镜(TEM)结果表明, N-C-TiO2样品颗粒均匀, 尺寸细小, 且分散性好; 热失重分析(TGA)、 X射线粉末衍射(XRD)和X射线光电子能谱(XPS)研究结果表明, 复合干凝胶经低温热处理, 使铵盐载体分解、 挥发去除, 样品为单一的锐钛矿相, N和C原子扩散进入晶格结点或间隙位置, 与TiO2化学键结合; 氮气等温吸附-脱附结果表明, 样品比表面积高达356 m2/g, 孔体积为0.27 mL/g. 以氙灯为可见光光源, 罗丹明B水溶液为模拟污染物, P25为参比催化剂, 在辐射强度为100 mW/cm2的可见光照射条件下, N-C-TiO2具有很高的光催化活性, 其可见光催化活性明显高于P25.     

Fast evaporation of spreading droplets of colloidal suspensions

When a coffee droplet dries on a countertop, a dark ring of coffee solute is left behind, a phenomenon often referred to as the coffee-ring effect. A closely related yet less-well-explored phenomenon is the formation of a layer of particles, or skin, at the surface of the droplet during drying. In this work, we explore the behavior of a mathematical model that can qualitatively describe both phenomena. We consider a thin axisymmetric droplet of a colloidal suspension on a horizontal substrate undergoing spreading and evaporation. In contrast to prior work, precursor films (rather than pinned contact lines) are present at the droplet edge, and evaporation is assumed to be limited by how quickly molecules can transfer out of the liquid phase (rather than by how quickly they can diffuse through the gas phase). The lubrication approximation is applied to simplify the mass and momentum conservation equations, and the colloidal particles are allowed to influence the droplet rheology through their effect on the viscosity. By describing the transport of the colloidal particles with the full convection-diffusion equation, we are able to capture depthwise gradients in particle concentration and thus describe skin formation, a feature neglected in prior models of droplet evaporation. The highly coupled model equations are solved for a range of problem parameters using a finite-difference scheme based on a moving overset grid. The presence of evaporation and a large particle Peclet number leads to the accumulation of particles at the liquid-air interface. Whereas capillarity creates a flow that drives particles to the droplet edge to produce a coffee ring, Marangoni flows can compete with this and promote skin formation. Increases in viscosity due to particle concentration slow down droplet dynamics and can lead to a reduction in the spreading rate.     

Near-wall velocities of particles suspended in shear flow and a streamwise electric field

Particles with a diameter of ∼0.5 µm in a dilute (volume fractions φ_∞ < 4 × 10⁻³) suspension assemble into highly elongated structures called “bands” under certain conditions in combined Poiseuille and electroosmotic flows in opposite directions through microchannels at particle-based Reynolds numbers Re_p < < 1. The particles are first concentrated near, then form “bands” within ∼6 µm of, the channel wall. The experiments described here examine the near-wall dynamics of individual “tracer” particles during the initial concentration, or accumulation, of particles, and the steady-state stage when the particles have formed relatively stable bands at different near-wall shear rates and electric field magnitudes. Surprisingly, the near-wall upstream particle velocities are found to be consistently greater in magnitude than the expected values based on the particles being convected by the superposition of both flows and subject to electrophoresis, which is in the same direction as the Poiseuille flow. However, the particle velocities scale linearly with the change in electric field magnitude, suggesting that the particle dynamics are dominated by linear electrokinetic phenomena. If this discrepancy with theory is only due to changes in particle electrophoresis, electrophoresis is significantly reduced to values as small as 20%–50% of the Smoluchowski relation, or well below previous model predictions, even for high particle potentials.     

Temperature-sensitive hairy particles prepared by living radical graft polymerization

Four types of temperature-sensitive hairy particles were prepared by living radical graft polymerization using a photoiniferter. The hairs were poly(N-isopropylacrylamide) (N), poly(N-isopropylacrylamide)ran-poly(acrylic acid) (NA), and diblock copolymers composed of N and NA. The block copolymer was attached to the particle in different modes, that is, one has a N-block inner and a NA-block outer but the other has the inverse arrangement. The acrylic acid content in NA was adjusted to be only 1%, but NA had a higher transition temperature by 5 degrees C than N in a neutral aqueous solution. The sequence of blocks attached onto the particle was the key factor to control the temperature responsiveness of the particle. The hairy particles exhibited a two-step transition with increasing temperature under certain conditions. The hairy particle also responded to the pH and ionic strength. Some unique behaviors of the hairy particles were studied in detail in terms of electrophoretic mobility and adsorption of dye molecules as well as swelling/deswelling.     

Phase-transfer behavior of cross-linked poly(acrylic acid) particles prepared by dispersion polymerization from ionic liquid to water

The phase-transfer behavior of poly(acrylic acid) (PAA) particles from the hydrophobic ionic liquid N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)amide phase to the water phase in the particle state, which we reported previously, was examined in more detail. PAA particles were prepared in the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([Bmim][TFSA]) and the organic solvent chloroform and were extracted. The transfer of PAA particles to water in the particle state was also observed in [Bmim][TFSA] systems. In contrast, the transfer phenomenon was not observed in the chloroform system. It was clarified that water/oil interfacial tension γ(wo) is an important parameter in the extraction of PAA in the particle state from the viewpoint of free energy. When the cationic surfactant tetradecyltrimethylammonium bromide, aqueous solution was used as the extraction medium, the PAA particles were extracted in the particle state from chloroform to water, in which γ(wo) became as low as that of the ionic liquid. This suggests that the phase-transfer phenomenon of PAA particles in the particle state was induced by the ionic liquid's unique property of low interfacial tension with water despite its high hydrophobic character.     

范德华作用和静电作用下的二元粒子自组装

采用简单粗粒化粒子模型,通过郎之万动力学模拟研究了具有范德华作用和静电作用的二元粒子自组装．研究发现,通过改变粒子尺寸和粒子间作用强度,二元粒子能够自发形成各种聚集结构,如球形、堆叠层状与管状结构．利用两亲性分子或两嵌段聚合物自组装理论,解释了二元粒子聚集结构的形成规律．当向溶液中加入反电荷离子时,模拟表明粒了聚集结构在相图中的分布出现了明显偏移．     

Assembly of colloidal particles by evaporation on surfaces with patterned hydrophobicity

Drops containing suspended particles are placed on surfaces of patterned wettability created using soft lithography; the drop diameter is large compared to the dimensions of the patterns on the substrate. As the three-phase contact line of the drop recedes, spontaneous dewetting of the hydrophobic domains and flow into the hydrophilic domains create discrete fluid elements with peripheries that can mimic the underlying surface topography. Suspended particles are carried with the fluid into the wetted regions and deposit there as the discrete fluid domains evaporate. If particle volume fractions are sufficiently high, the entire wetted domain can be covered with colloidal crystals. At lower volume fractions, flow within the evaporating fluid element can direct the deposition of colloidal particles at the peripheries of the domains. High-resolution arrays of particles were obtained with a variety of features depending upon the relative size of the wetting regions to the particles. When the wetting region is larger than the particles, three-dimensional and two-dimensional arrays of ordered particles mimicking the shape of the wetting pattern form, depending on the particle volume fraction. For lower volume fractions, one-dimensional (1-D) arrays along the wet/non-wet boundaries form. When the particle size is similar to the height of fluid on the wetted domain, zero-dimensional distributions of single particles centered in the wet regions can form for wetted squares or 1-D distributions (stripes) form along the axis of striped domains. Finally, when the wetting region is smaller than the particle size, the particles do not deposit within the features but are drawn backward with the receding drop. These results indicate that evaporation on surfaces of patterned wetting provides a highly parallelizable means of tailoring the geometry of particle distributions to create patterned media.     

Wetting of a particle in a thin film

When a particle is placed in a thin liquid film on a planar substrate, the liquid either climbs or descends the particle surface to satisfy its wetting boundary condition. Analytical solutions for the film shape, the degree of particle immersion, and the downward force exerted by the wetting meniscus on the particle are presented in the limit of small Bond number. When line tension is significant, multiple solutions for the equilibrium meniscus position emerge. When the substrate is unyielding, a dewetting transition is predicted; that is, it is energetically favorable for the particle to rest on top of the film rather than remain immersed in it. If the substrate can bend, the energy to drive this bending is found in the limits of slow or rapid solid deflection. These results are significant in a wide array of disciplines, including controlled delivery of drugs to pulmonary airways, the probing of liquid film/particle interface properties using particles affixed to AFM tips and the positioning of small particles in thin films to create patterned media.     

Synthesis of raspberry‐like organic–inorganic hybrid nanocapsules via pickering miniemulsion polymerization: Colloidal stability and morphology

Raspberry‐like hybrid nanocapsules with a hydrophobic liquid core were successfully prepared via the copolymerization of styrene, divinylbenzene (DVB), and 4‐vinyl pyridine (4‐VP) in Pickering‐stabilized miniemulsions by using silica particles as the sole emulsifier and hexadecane (HD) as liquid template. When compared with conventional Pickering miniemulsions and Pickering suspensions, the colloidal stability of the current systems is much more sensitive to the variation of reaction parameters such as pH, size, amount of silica particles, and content of 4‐VP. The systems without coagulum were only obtained in a narrow pH range at around 9.5 and by using 12 nm silica particles as emulsifier. The formation of well‐defined raspberry‐like capsules was confirmed by transmission electron microscopy (TEM) and high‐resolution scanning electron microscopy (HRSEM). The stable attachment of silica particles on the surface of hybrid particles was verified by centrifugation and subsequent characterizations, such as Fourier transform infrared spectroscopy, TEM, and HRSEM. The influence of pH and weight content of HD, DVB, and 4‐VP on the particle morphology was extensively investigated. Interestingly, the particle morphology strongly depends on the particle size. When compared with the organic surface‐active surfactant, the formation of capsule morphology could be promoted by the application of silica particles taking advantage of their surface inactivity. The formation mechanisms of capsules/solid particles are discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Electrically driven flow near a colloidal particle close to an electrode with a Faradaic current

To elucidate the nature of processes involved in electrically driven particle aggregation in steady fields, flows near a charged spherical colloidal particle next to an electrode were studied. Electrical body forces in diffuse layers near the electrode and the particle surface drive an axisymmetric flow with two components. One is electroosmotic flow (EOF) driven by the action of the applied field on the equilibrium diffuse charge layer near the particle. The other is electrohydrodynamic (EHD) flow arising from the action of the applied field on charge induced in the electrode polarization layer. The EOF component is proportional to the current density and the particle surface (zeta) potential, whereas our scaling analysis shows that the EHD component scales as the product of the current density and applied potential. Under certain conditions, both flows are directed toward the particle, and a superposition of flows from two nearby particles provides a mechanism for aggregation. Analytical calculations of the two flow fields in the limits of infinitesimal double layers and slowly varying current indicate that the EOF and EHD flow are of comparable magnitude near the particle whereas in the far field the EHD flow along the electrode is predominant. Moreover, the dependence of EHD flow on the applied potential provides a possible explanation for the increased variability in aggregation velocities observed at higher field strengths.     

Preparation of sulphonate-containing core/shell latex particles via seeded emulsion copolymerization

<正>In this study,P(St-MAA) seed latex particles were first prepared via soap-free emulsion polymerization of styrene(St) and methacrylic acid(MAA),then the seed particles were allowed to swell with St at room temperature,and the P(St-MAA)/P(StNaSS) core/shell latex particles were then synthesized via seeded emulsion copolymerization of St and sodium styrene sulphonate (NaSS) using AIBN as initiator in the presence of N,N'-methylenebisacrylamide(BAA,water-soluble crosslinker).Results showed that the polymerization could be carried out smoothly when the ratio of BAA to total monomers was less than 3 mol%,the narrow dispersed P(St-MAA) seed particles with the diameter of 150 nm and the P(St-MAA)/P(St-NaSS) core/shell latexes with the particle size of about 200 nm were synthesized.When the 25/75 mole ratio of NaSS/(St + MAA) and 2 mol%of BAA were used in the seeded emulsion polymerization,the resulted P(St-MAA)/P(St-NaSS) latex product showed a low weight loss after water extraction,and the NaSS unit content in the whole particle and in the shell reached 11.7 mol%and 34.6 mol%,respectively.     

丙烯酸甲酯与醋酸乙烯酯的种子乳液聚合

以过硫酸铵（ＡＰＳ） 为引发剂,合成了粒径分布较均匀的聚醋酸乙烯酯种子乳液（ＰＶＡｃ） ,然后以丙烯酸甲酯（ ＭＡ） 为第二单体和以油溶性偶氮二异丁腈（ＡＩＢＡ） 为引发剂,分别进行不溶胀与溶胀条件下的无皂种子乳液聚合,并用透射电子显微镜（ＴＥＭ） 表征了胶粒形态．表明在不溶胀条件下,胶粒形态随ＰＶＡｃ／ ＭＡ 重量比的不同而变化,当ＰＶＡｃ／ ＭＡ 为１／２ 时,形成以ＰＭＡ 为核,ＰＶＡｃ 为壳的胶粒．在溶胀条件下则得到类似互穿网络型乳胶粒．     

Hydrodynamics of interaction of particles (including cells) with surfaces

The study of the phenomena related to the motion of particles flowing in the proximity of the wall is pursued for purely cognitive reason as well as for some important practical purposes in various fields of technology, biology and medicine.When small spherical rigid particles move in the direction parallel to the surface their velocity is smaller than that of the fluid and depends on the ratio of the distance from the wall to the particle radius. The velocity of a particle falling down in a vertical cylinder is maximal in an eccentric position. A sphere in contact with the wall remains stationary. Translational velocity of spherical rigid particles the dimension of which are comparable to that of the tube is only slightly dependent of their lateral position. The differences in the flow parameters of deformable particles in comparison with rigid ones depend on the particle and fluid viscosity coefficient. When the particles move perpendicularly toward the wall, their velocity decreases as the particle approaches the surface. The change of particle velocity is inversely proportional to the gap.There are several theories explaining the influence of the channel diameter on the suspension viscosity (sigma phenomenon); a modern approach is based on the analysis of rheological properties of suspensions. The explanations of the Fahraeus effect (i.e. the fact that the concentration of particles flowing in a tube linking two containers are smaller than that in the containers) are based on non-uniform particle distribution in a transverse cross section and on the differences of velocities of particles and medium. The deviation of the velocity profile of a suspension of rigid particles flowing through a tube from the parabolic shape (blunting) does not depend on the flow velocity; as concerns deformable particles, however, this effect is the smaller the greater is the flow velocity.When the Reynolds number for particles is greater than 10^-3, there appears a component of particle velocity perpendicular to the streamline direction.This phenomenon is the cause of the lateral migration of particles. Neutrally buoyant rigid particles migrate to a certain concentrical region situated between the tube axis and the wall (tubular pinch region). Deformable neutrally buoyant particles migrate towards the tube axis, and deformable non-neutrally buoyant particles may move either toward the tube axis or toward the wall.In the research on the influence of the flow delimiting surface on the motion of particles in suspension a considerable progress has recently been made.However, the phenomena in this field are extremely complex. At present, two main types of approach may be distinguished. On a microscopic level direct interactions between particles and surfaces are analyzed. A macroscopic approach consists in treating particle suspension as fluid, and overall influence of the surface on its properties are studied. A comprehensive theory linking these two levels has not yet emerged.     

Cross-linked polyvinylpyrrolidone nanoparticles: a potential carrier for hydrophilic drugs

Injectable hydrogel polymeric nanoparticles of polyvinylpyrrolidone cross-linked with N,N'-methylene bis-acrylamide and encapsulating water-soluble macromolecules such as FITC-dextran (FITC-Dx) have been prepared in the aqueous cores of reverse micellar droplets. These particles are 100 nm and below in diameter with a narrow size distribution. When dispersed in aqueous buffer these particles appear to be transparent and give an optically clear solution. Lyophilized powder of these nanoparticles is redispersable in aqueous buffer without any change in the size and morphology of the particles. The efficiency of FITC-Dx entrapment by these nanoparticles is high (>70%) and depends on the amount of cross-linking agent present in the polymeric material. The release of the entrapped molecules from these nanoparticles depends on the degree of cross-linking of the polymer, particle size, pH of the medium, and extent of loading, as well as temperature.