Microparticle detachment from surfaces exposed to turbulent air flow: controlled experiments and modeling

This work presents the results of experiments conducted to characterize the detachment of microparticles from surfaces exposed to turbulent air during accelerated free-stream flow. Smooth glass plates used as substrates are scanned with an atomic force microscope to determine their roughness-height distributions. Microparticles of different sizes, materials and shapes (mostly microspheres) are deposited as sparse monolayers onto the substrates under controlled clean and dry conditions. The microparticles attach to the substrate in a condition of static equilibrium due to adhesion and reside completely within the viscous sublayer as the flow is accelerated. Microvideographic observations of individual microparticle detachment show that detachment occurs primarily as rolling motion along the surface and not as lift-off. Detachment is not necessarily followed by entrainment in the flow. Results are presented as detachment fractions as function of time.The experimental results reveal that detachment is governed by a balance of the moments of aerodynamic drag and rough-surface pull-off forces. This is substantiated using a recently developed attachment theory that takes into account surface roughness to determine the pull-off force of microparticles. The sensitivity of the free-stream threshold velocity for detachment to five factors contained in the experiments and the model is analyzed. Results indicate that the surface energy of adhesion and the microsphere radius have the most influence on the threshold velocity for detachment.     

SURFACE ROUGHNESS EFFECTS ONMICROPARTICLE ADHESION

A new self-consistent model is developed to treat the static contact of a microparticle with a flat barrier in the presence of molecular adhesion and surface roughness. Separation between their mean datum planes is modeled considering the elastic deformation of the microparticle and surface. The contact pressure is computed from the Lennard-Jones law following the Derjaguin approximation. The elastic deflection of the mean datum plane is calculated from the effective pressure by the half-space elastic theory. Roughness is modeled by introducing a Gaussian distribution to the gap between the surfaces. An effective pressure is defined as the statistical average of the contact pressure over the roughness heights. A solution satisfying all of the above conditions gives a self-consistent method of modeling adhesion between the microparticle and the flat barrier. Using collocation methods the equations are discretized as a large system of nonlinear algebraic equations. A continuation method is used to find the multiple numerical solutions for the mean separation and the effective contact pressure. Finally, adhesive contacts of both smooth and rough surfaces are simulated in a comparative manner to elucidate the features of surface roughness in the presence of molecular adhesion. The standard deviation of the Gaussian distribution is used as a parameter to assess the effects of roughness on the pull-off force. It is shown that increasing surface roughness significantly reduces the pull-off force and decreases the tendency for the microsphere to snap-on and snap-off.     

Surface roughness effects onmicroparticle adhesion

A new self-consistent model is developed to treat the static contact of a microparticle with a flat barrier in the presence of molecular adhesion and surface roughness. Separation between their mean datum planes is modeled considering the elastic deformation of the microparticle and surface. The contact pressure is computed from the Lennard-Jones law following the Derjaguin approximation. The elastic deflection of the mean datum plane is calculated from the effective pressure by the half-space elastic theory. Roughness is modeled by introducing a Gaussian distribution to the gap between the surfaces. An effective pressure is defined as the statistical average of the contact pressure over the roughness heights. A solution satisfying all of the above conditions gives a self-consistent method of modeling adhesion between the microparticle and the flat barrier. Using collocation methods the equations are discretized as a large system of nonlinear algebraic equations. A continuation method is used to find the multiple numerical solutions for the mean separation and the effective contact pressure. Finally, adhesive contacts of both smooth and rough surfaces are simulated in a comparative manner to elucidate the features of surface roughness in the presence of molecular adhesion. The standard deviation of the Gaussian distribution is used as a parameter to assess the effects of roughness on the pull-off force. It is shown that increasing surface roughness significantly reduces the pull-off force and decreases the tendency for the microsphere to snap-on and snap-off.     

Measurement of the adhesion between single melamine-formaldehyde resin microparticles and a flat fabric surface using AFM

An understanding of the adhesion of microparticles, particularly microcapsules, containing a functional component to a fabric surface is crucial to an effective application of this component to the fibre. Fabric surface is very rough; hence, direct measurement of the adhesion of single microparticles to surfaces with a roughness greater than the particle diameter is difficult. In the study reported here, cotton films were generated by dissolving cotton powder in an organic solvent and their properties including surface roughness, thickness, contact angle and purity were characterised. The adhesive forces between single melamine-formaldehyde (MF) resin microparticles and a cotton film under ambient conditions with a relative humidity of above 40% were measured using atomic force microscopy; they are considered to be dominated by capillary forces. It was found that there was little adhesion between a MF microparticle and a cotton film in an aqueous solution of sodium dodecylbenzene sulphonate as surfactant. Repulsion between them was observed, but it reduced with increase in the surfactant concentration and decrease in the pH of the solution. The repulsion contributions are thought to originate mainly from electrostatic repulsion. It is believed that the studies on the adhesion between single MF microparticles and a cotton film under ambient conditions or dispersed in surfactant solutions, are beneficial to the attempts to enhance the adhesion of microcapsules to fabric surfaces via a modification of their surface composition and morphology.     

EHD强化相变换热同轴圆柱特征电极的理论分析

针对施加直流高压电场的电水动力学（EHD）强化管外沸腾换热和EHD强化管内凝结换热两种换热方式,提出了3个同轴圆柱特征电极参数,分别与最小场强电极、最小电场力电极、最小能耗电极对应.分析了3个电极参数的几何特性,结果表明,电极直径的大小,在EHD强化相变换热中起重要作用.     

Effects of conductive polyaniline (PANI) preparation and platinum electrodeposition on electroactivity of methanol oxidation

A modified galvanostatic method, termed the ‘pulse galvanostatic method’ (PGM) was used to synthesize nanofibular polyaniline (PANI). In contrast to granular PANI prepared by the conventional galvanostatic method (GM), nanofibular PANI has better conductivity and higher specific surface area. The nanofibular PANI electrode modified by Pt microparticles, at the same Pt loading, exhibits a considerably higher electrocatalytic activity on the methanol oxidation than that of the granular PANI electrode modified by Pt microparticles. Furthermore, the PGM method can be used as a good method for Pt microparticle electrodeposition. The composite electrode composed of PANI and Pt microparticles has the best electrocatalytic activity in the experimental range. The effects of Pt loading and methanol concentration, on the electrocatalytic activity for methanol oxidation have also been researched.     

不同电极间距下自生电场膜生物反应器中的膜污染行为分析

以自制铜纳米线（Cu-NWs）导电微滤膜为膜组件兼阴极,构建自生电场膜生物反应器（SEF-MBR）,研究不同电极间距下自生电场强度、跨膜压差（TMP）与膜污染行为的变化规律,可为MBR技术升级及进一步推广应用提供理论依据。结果表明,当电极间距从4cm减小到2cm时,自生电场强度从0.7mV/cm提高到1.2mV/cm,TMP的上升速度从0.15kPa/d降低到0.08kPa/d。缩短电极间距有利于缓解膜污染。随电极间距减小,静电斥力提高了40.5%,阴极产生的H₂O₂和·OH浓度也大大增加。电极间距对SEF-MBRs好氧活性污泥的胞外聚合物含量影响不大,但均低于对照MBR系统。共聚焦激光扫描电镜（CLSM）观察发现,与对照系统相比,导电微滤膜表面的污染层厚度减小了20.2%,膜表面污染物以微生物总细胞和β-D-glucopyranose多糖为主。缩短电极间距可提高系统自生电场强度,进而提高静电斥力及H₂O₂和·OH产生量,减缓膜污染。     

Influence of the geometric factors of the experimental device used in suspension polymerization on the properties of poly(styrene‐co‐divinylbenzene) microparticles

The prediction of the final particle size for reactive systems such as the reactions of suspension polymerization is a complex matter. Thus, the preparation of very small microparticles is specially challenging, probably because of the coalescence of the polymeric beads taking place during the later stages of the polymerization. In this work, very small gel‐type styrene‐co‐divinylbenzene beads were synthesized by using a previously determined set of experimental synthesis conditions in which the stabilization of the dispersion of the monomeric droplets was ensured, and, under these conditions, the factors related to the geometry of the experimental device were modified to determine their actual effect on the final size of the microparticles. From the experimental results, a very simple and useful model was obtained that was able to predict the final size of the microparticles as a function of the values of the geometric factors of the reactor. This model indicates that the most influential factors in the final size of the microparticles are the liquid depth inside the reactor and the stirrer diameter; thus, an increase in the liquid depth produces larger particles, and, conversely, the particle size decreases when using larger stirrer diameters. Additionally, the model permits the design of polymerization experiments aimed at obtaining microparticles with a diameter smaller than 50 μm. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

匀强电场作用下分散相液滴的变形和破裂

基于Cahn-Hilliard方程的相场方法,建立了在匀强电场作用下液滴的变形和破裂行为模型。从微观角度研究分散相液滴变形过程中电荷密度、电场强度和电场力的分布规律以及流场和电场分布,探讨了微观液滴变形机理;采用数值模拟方法研究了电场强度、液滴直径和界面张力对液滴变形的影响,结果表明电场强度越强,液滴直径越大,界面张力越小,液滴变形量越大;分析了液滴的两种主要破裂方式,其破裂主要取决于连续相和分散相物性条件,为电破乳技术提供了理论基础。     

Rapid identification of bacteria utilizing amplified dielectrophoretic force-assisted nanoparticle-induced surface-enhanced Raman spectroscopy

Dielectrophoresis (DEP) has been widely used to manipulate, separate, and concentrate microscale particles. Unfortunately, DEP force is difficult to be used in regard to the manipulation of nanoscale molecules/particles. For manipulation of 50- to 100-nm particles, the electrical field strength must be higher than 3 × 10⁶ V/m, and with a low applied voltage of 10 V_p-p, the electrode gap needs to be reduced to submicrons. Our research consists of a novel and simple approach, using a several tens micrometers scale electrode (low cost and easy to fabricate) to generate a dielectrophoretic microparticle assembly to form nanogaps with a locally amplified alternating current (AC) electric field gradient, which is used to rapidly trap nanocolloids. The results show that the amplified DEP force could effectively trap 20-nm colloids in the nanogaps between the 5-μm particle aggregates. The concentration factor at the local detection region was shown to be approximately 5 orders of magnitude higher than the bulk solution. This approach was also successfully used in bead-based surface-enhanced Raman spectroscopy (SERS) for the rapid identification of bacteria from diluted blood.     

非均匀电场下乳化油中液滴变形动力学行为

外加电场下液滴的变形动力学行为是乳化液电脱水机理研究的重要内容。基于Cahn-Hilliard方程的相场方法,建立了液滴在非均匀电场下的仿真模型,研究了电场作用下乳化液中液滴在形变、移动和聚结过程中电荷密度和电场力的分布规律,以及流场和电场的耦合作用。仿真分析了液滴粒径、电场强度以及电场非均匀系数对液滴运动行为的影响。利用实验室小型脱水系统开展了乳化液脱水实验,并通过高速摄像机对乳化液中液滴的运动行为进行了观测与分析。研究结果表明,在非均匀电场中液滴表面的极化电荷分布不均,由液滴中部向两端逐渐增大,在靠近电场集中方向处的电荷密度和Maxwell应力值最大;在一定范围内增大电场强度、电场非均匀系数或液滴粒径,可使液滴形变量增大,液滴向电场集中区域的移动速度以及液滴间的聚结速度增加。     

熔体静电纺丝中电极结构对电场和纤维的影响

采用Ansys数值模拟方法,建立了电场分析的有限元模型,研究了电极结构不同时的电场分布及其对纤维直径的影响,以及电压大小对纤维直径的影响。并进行了实验对照分析,发现电极结构包括圆板电极和圆环电极影响熔体静电纺丝的电场分布,但场强最大都出现在喷嘴处,并随接收距离的增大成不同趋势减小,但中空电极能集聚电场,稳定场强,获得更细的纤维。     

Aerosol and Microparticle Generation Using a Commercial Inkjet Printer

Simple modifications to widely available commercial-off-the-shelf (COTS) inkjet printers and cartridges enabled production of aerosols that were tunable in number, size, and size-distribution width. Aerosols generated from various solutions using an inkjet printer with varying control software settings were dehydrated into solid microparticles for characterization using an aerodynamic particle sizer (APS) to determine particle number and size. The duration of microdroplet and microparticle production using this method was adjustable by varying the size of the print job image sent from the control computer. The microparticles created using the inkjet printer were predominantly 1–5 μm in aerodynamic diameter. Tuning of the size of the created microparticle size-distribution was achieved by changing the solute concentration in the printed solution. The size of the dried particles increased monotonically with solute concentration as expected. The aerosol size-distribution width was customizable by varying the COTS inkjet printer control software settings. The highest quality programmed print modes produced a narrower size-distribution compared to the rapid, low quality print options available in the software. The morphology of the dehydrated microparticles was determined to be spherical using scanning electron microscopy (SEM). These experiments demonstrated the successful use of commonly accessible commercial inkjet printers and software to produce tunable and well-characterized aerosol size-distributions in an easily controlled manner. The customizable aerosols and microparticles produced using this method displayed diverse properties that may be tailored to a wide range of experimental applications.

Copyright 2013 American Association for Aerosol Research     

The separation of different conducting multi-walled carbon nanotubes by AC dielectrophoresis

A microfluidic device was fabricated to separate different conducting parts of multi-walled carbon nanotubes (MWCNTs). The device consists of a curing PDMS fluidic-flow chamber covered on electrode coated glass. The electrode was designed to generate non-uniform electric field by patterning via lithography. A range of frequencies with low applied voltage was utilized to induce different sign of dielectrophoresis force. Two different separation schemes based on positive and negative dielectrophoresis were employed to separate different conducting parts in unsorted MWCNTs. From Raman spectroscopy, conducting MWCNTs collected by positive dielectrophoresis showed little variation in the intensities of D-band and G-band ratio while the less conducting MWCNTs collected by negative dielectrophoresis showed decreasing intensities in these positions. The ID/IG ratio in the samples collected by both separation schemes is decreasing compared to the unsorted samples. The electric properties of the samples were characterized by a dielectrophoresis frequency spectra method. The conductance in positive dielectrophoresis collected sample is the greatest while the conductance in negative dielectrophoresis collected sample is the smallest. The trend in the conductance in unsorted and sorted samples is confirmed by current-voltage measurements.     

A Novel Pool of Microparticle Cholesterol Is Elevated in Rheumatoid Arthritis but Not in Systemic Lupus Erythematosus Patients

Microparticles are sub-micron, membrane-bound particles released from virtually all cells and which are present in the circulation. In several autoimmune disorders their amount and composition in the circulation is altered. Microparticle surface protein expression has been explored as a differentiating tool in autoimmune disorders where the clinical pictures can overlap. Here, we examine the utility of a novel lipid-based marker—microparticle cholesterol, present in all microparticles regardless of cellular origin—to distinguish between rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). We first isolated a series of microparticle containing lipoprotein deficient fractions from patient and control plasma. There were no significant differences in the size, structure or protein content of microparticles isolated from each group. Compared to controls, both patient groups contained significantly greater amounts of platelet and endothelial cell-derived microparticles. The cholesterol content of microparticle fractions isolated from RA patients was significantly greater than those from either SLE patients or healthy controls. Our data indicate that circulating non-lipoprotein microparticle cholesterol, which may account for 1–2% of measured cholesterol in patient samples, may represent a novel differentiator of disease, which is independent of cellular origin.     

Non-coalescence behavior of neutral droplets suspended in oil under a direct current electric field

The excessive electric field gives rise to droplet non-coalescence and droplet chains in the electric dehydrator, which severely deteriorates oil–water separation efficiency and even leads to short circuit. To reveal the underlying mechanism of droplet non-coalescence, dynamic behavior of two neutral droplets in silicone oil under a direct current electric field is investigated by using high-speed photography. The experimental results show that there exists a critical electric field strength above which two droplets will bounce off after the contact. The critical electric field strength of droplet non-coalescence is affected by the initial separation distance between droplets, the radius of droplet, and the surfactant concentration. Whether the non-coalescence behavior occurs in the electric field is determined by the competition of electric force and capillary force, which dominates the evolution of tiny connection channel.     

Electrohydrodynamic Precipitator Flow with a Barbed Plate Discharge Electrode

The nonuniform corona discharge in the wire-plate electrostatic precipitator results in a rotational electric body force which is a source of large-scale secondary flows and turbulence within the flow channel. The electrically induced flow causes large increases in diffusivities detrimental to the particle collection process. Since the electrode geometry and the structure of the corona discharge define the magnitude and character of the electric body force, it is theoretically possible to design a discharge electrode which minimizes electrohydrodynamic flow disturbances. As a first step in this direction, a novel planar electrode design in which electrical discharges are configured to reduce the inhomogeneities of the electric body force is experimentally studied in a negative polarity laboratory electrostatic precipitator. Hot-film anemometer measurements of the electrohydrodynamic turbulent velocity field downstream of the plate electrode are compared to those of a conventional wire-plate precipitator. Results confirm that electrode geometry has a significant role in turbulence production. Although there is some evidence that secondary flows are reduced in the planar geometry, spectral analysis of the flow downstream of the electrodes indicate that the barbed plate design increases turbulence intensity as much as 50% without reducing eddy size. Continued experimentation is necessary to fully assess the possible benefits of such a design.     

Study of the nanoparticle/microparticle powder systems by dry dispersion

The study of a nanoparticle/microparticle powder systems by dispersion of different type of nanoparticles over microparticles, as support or substrate, has been afforded by a soft mechanical and solvent-less procedure and its effect on the electrostatic charge was studied. The dispersion degree can be evaluated by using diffuse reflectance measurements due to the non-linear change of the reflectance as a function of the nanoparticles amount in the powder composite. The maximum slope variation of the non-linear curve revealed the point where the nanodispersion attained the highest efficiency. The interaction between the OH groups of the dissimilar surfaces provides the anchoring mechanism of the nanoparticles onto the microparticles and the net charge compensation. Furthermore, the dispersed and anchored nanoparticles provide a roughness increasing of the microparticles surface that is responsible for a formation of nanoparticle/microparticle composite powders in which tribo-charge maximized due to the enhancement of composite packaging.     

An improved method for preparing glutaraldehyde cross-linked chitosan–poly(vinyl alcohol) microparticles

The features of microparticles, as size, surface structure, and morphology, depend, mainly, on the methodology used for their preparation. Emulsion polymerization techniques are undoubtedly among the most widespread. However, the use of toxic, volatile organic solvents represents a major disadvantage, namely, because of environmental issues. In this study, we prepared glutaraldehyde cross-linked chitosan–poly(vinyl alcohol) microparticles by an improved water-in-oil emulsion technique using corn oil as organic phase. The application of this polymeric blend as microparticle is scarcely investigated. As resulting of the procedure here presented, spherical and smooth surface microparticles were obtained, with mean diameter of 16 μm. The cross-linking reaction between the aldehyde and the amino or the hydroxyl groups formed either an imine (Schiff’s base) or an acetal bond, respectively, as analyzed by infrared spectroscopy. The microparticles here described did not present cytotoxic potential. Accordingly, this study can find promising and successful application in biotechnology.     

Facile fabrication of polyaniline/polypyrrole copolymer nanofibers with a rough surface and their electrorheological activities

Hierarchical polyaniline/polypyrrole (PANI/PPy) copolymer nanofiber was prepared via a two‐step method and adopted as dispersing materials for electrorheological (ER) fluids. The first step was used to synthesize PANI nanofibers by a rapid mixing method. Subsequently, the PANI/PPy copolymer nanofibers with a rough surface were obtained using an in situ polymerization method continuously. The morphology of the resultant PANI/PPy copolymer nanofibers can be controlled by varying the amount of Py monomer in the secondary in situ polymerization method. The rough surface of PANI/PPy copolymer nanofibers were confirmed by scanning electron microscopy and transmission electron microscopy. The diameter of PANI/PPy nanofiber is within the range 100–200 nm. The obtained PANI/PPy copolymer particles all exhibit amorphous structure through X‐ray diffraction measurement. We also demonstrated that the hierarchical PANI/PPy copolymer nanofibers exhibited characteristic ER behaviors, which were investigated using a Haake rotational rheometer at various electric field strengths. The ER efficiency e for PANI‐1mLPPy and PANI‐2mLPPy ER fluids at shear rate 0.1 s⁻¹ is 36.6 and 28.5 under electric field strength E = 3 kV/mm, respectively. Low leaking current density is observed even at high electric field strength and wide plateau region appeared, which show a strong ER activity for the PANI/PPy composite nanofibers. The results also indicate that the PANI/PPy composite particles have distinctly enhanced ER effect compared with the pure PANI and PPy particles under electric stimuli. The significantly improved ER property of PANI/PPy‐based ER fluid is ascribed to the enhanced interfacial polarization. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46289.