Modeling flows of confined nematic liquid crystals

The flow of nematic liquid crystals in tightly confined systems was simulated using a molecular theory and an unsymmetric radial basis function collocation approach. When a nematic liquid crystal is subjected to a cavity flow, we find that moderate flows facilitate the relaxation of the system to the stable defect configuration observed in the absence of flow. Under more extreme flow conditions, e.g., an Ericksen number Er=20, flows can alter the steady-state defect structure observed in the cavity. The proposed numerical method was also used to examine defect annihilation in a thin liquid crystal film. The flows that arise from shear stresses within the system result in a higher velocity for s = +1∕2 defect than for the defect of opposing charge. This higher velocity can be attributed to reactive stresses within the deformed liquid crystal, which result in a net flow that favors the motion of one defect. These two examples serve to illustrate the usefulness of radial basis functions methods in the context of liquid crystal dynamics both at and beyond equilibrium.     

The hydrodynamics of the amperometric detector flow cell with a rotating disk electrode

Series of photographs of the sample flow pattern in the flow cell with a stationary as well as a rotating disk electrode (RDE) were taken with a motor-driven camera. With the stationary electrode, the flow pattern in the cell was mushroom-like. Rotating the electrode generated a secondary fluid motion in the flow cell which manifested itself as vertical circulation of the solution present in the flow cell. A qualitative hydrodynamic explanation of the observed flow patterns is given. Peak broadening effects induced by the RDE in the flow cell were observed only at very fast rotation speeds and high nozzle heights. The response surface of the amperometric detector flow cell with the RDE as a function of the rotation speed and the nozzle height was measured by applying the detector in combination with high-performance liquid chromatography, flow injection analysis and continuous flow analysis. Model curve-fitting calculations indicate that the flow pattern in the flow cell can be laminar or turbulent, depending on the exact cell geometry, rotation speed and nozzle height.     

Orientation of a dispersion of kaolinite flowing in a jet

Orientational alignment in a dilute dispersion of kaolinite particles has been investigated in a flow pattern that combines both shear and elongational stress, namely flow at a jet created by a 2 mm diameter nozzle inserted in a 6 mm diameter pipe. Spatially-resolved X-ray diffraction with synchrotron radiation permits detailed maps of the alignment to be deduced and compared with fluid mechanics calculations of the flow. The angular distribution of diffracted intensity from a given position in the pipe provides information about the orientation distribution of the particles. This is quantified and presented in terms of order parameters. The cone-shaped nozzle provides a jet of liquid giving a high degree of alignment of the particles that is uniform along lines across the conical section and constant in the small straight-sided region at the exit of the nozzle. The vortex motion that arises from the flow with a modest Reynolds number could be determined as well as the tendency for some particles to align with their large faces perpendicular to the overall flow direction at the flat surface of the nozzle outlet.     

Air-facilitated three-phase contact formation at hydrophobic solid surfaces under dynamic conditions

The paper presents results documenting the mechanism of facilitation of the three-phase contact (TPC) formation due to gas entrapped during immersion of hydrophobic (Teflon) plates into distilled water and n-octanol solutions. Collisions, bouncing, the time scale of the TPC formation, and bubble attachment to Teflon plates of different surface roughness were studied using a high-speed camera. Processes occurring during the microscopic wetting film formation at the Teflon plates were monitored using the microinterferometric method (Scheludko-Exerowa cell). A strong relation between the time necessary to form a stable TPC and the roughness of the Teflon was observed. The higher the Teflon roughness was the shorter the time for the TPC formation. This effect can be attributed to two factors: (i) local differences in the thickness of the thinning intervening liquid layer (quicker attainment of rupture thickness at pillars of rough surface) and/or (ii) the presence of gas at the hydrophobic surface. Experimental findings, that (i) prolongation of the plate immersion time resulted in quicker TPC formation, (ii) white irregular and disappearing spots (air pockets) were recorded during the wetting film formation, and (iii) high n-octanol concentration caused prolongation of the time of the TPC formation, show that attachment (TPC formation) of the colliding bubble to hydrophobic surfaces was facilitated by air entrapped at the Teflon plates (and re-distributed) during their immersion into water phase. Thus, on collision instead of solid/gas wetting liquid film a thin gas/liquid/gas foam film was formed which facilitated the TPC formation.     

Ionspray microchip

An ionspray microchip is introduced. The chip is based on the earlier presented nebulizer microchip that consists of glass and silicon plates bonded together. A liquid inlet channel, nebulizer gas inlet, and nozzle are etched on the silicon plate and a platinum heater is integrated on the glass plate. The nebulizer microchip has been previously used in atmospheric pressure chemical ionization, atmospheric pressure photoionization, sonic spray ionization, and thermospray ionization modes. In this work we show that the microchip can be operated also in ionspray mode by introducing high voltage to the silicon plate of the microchip. The effects of operation parameters (voltage, nebulizer gas pressure, sample solution flow rate, solvent composition, and analyte concentration) on the performance of the ion spray microchip were studied. Under optimized conditions the microchip provides efficient ionization of small and large compounds and good quantitative performance. The feasibility of the ion spray microchip in liquid chromatography/mass spectrometry (LC/MS) was demonstrated by the analysis of tryptic peptides of bovine serum albumin. Copyright © 2010 John Wiley & Sons, Ltd.     

Complex study of reorientational dynamics of the liquid crystal in PDLC films

Morphological, electro-optical and switching properties of polyester resin/nematic liquid crystal composite films have been studied for varying composition (10-40 wt% of LC), temperature (20-50 C), film thickness (10-75 mum) and UV curing time of the matrix (0.25-12 min). The PDLC films were formed by LC separation in a UV polymerization process of the thin layer of oligoester resin (liquid crystal mixture) between ITO coated glass plates. The electro-optical and response behaviour based on the electric field controlled light scattering of the composite films was observed. The results were interpreted in terms of effective anchoring strength at the interface of the polymer and liquid crystal depending either on the area fraction of the interface in the composite film (dependent on the size and shape of the liquid crystal droplets) or the stiffness and resistivity of the polyester resin changing in the course of the crosslinking polymerization.     

Polymeric film formation in spin coating

The results of numerical modeling of polymeric film formation under mass force action are presented. The instability of non‐Newtonian liquid front edge at the initial stage of flow over a disk is studied. The quasi‐stationary shape of the liquid film (in front edge vicinity) speading over the surface is determined for certain rheology laws. At the modeling of the second stage of coating flow, the special attention was paid to the effects connected with the two‐dimensional character of the flow. The impact of rheological properties of a liquid on the free surface shape was studied using codes that calculate the two‐dimensional non‐stationary flow of non‐Newtonian liquid. The factors that determine the final shape of polymeric coating free surface are discussed.     

A comparative study of jet formation in nozzle‐ and nozzle‐less centrifugal spinning systems

Centrifugal spinning, a recently developed approach for ultra‐fine fiber production, has attracted much attention as compared with the electrospinning, due to its high yield, no solution polarity and high‐voltage electrostatic field requirements, etc. In this study, the jet formation process and spinning parameters on jet path are explored and compared in nozzle‐ and nozzle‐less centrifugal spinning systems. For nozzle‐less centrifugal spinning, fingers are formed at the front of thin liquid film due to the theory of Rayleigh–Taylor instability. We find that the lower solution concentration and higher rotational speed favor the formation of thinner and longer fingers. Then, the critical angular velocity and initial jet velocity for nozzle‐/nozzle‐less centrifugal spinning are obtained in accordance with the balance of centrifugal force, viscous force, and surface tension. When jet leaves the spinneret, it will undergo a series of motions including necking and whipping processes, and then, a steady spiral jet path is formed with its radius getting tighter. Finally, we experimentally study the effect of rotational speed and solution concentration on jet path, which shows that the higher rotational speed results in a larger radius of jet path while the solution concentration has little effect on it. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1547–1559     

Printed two-dimensional micro-zone plates for chemical analysis and ELISA

In this study we report a new concept of printing low-cost two-dimensional micro-zone plates for chemical and biochemical assays and ELISA. It is different from the concept of forming multi-zone micro-zones on paper by hydrophobic barriers first reported by Yagoda. Instead, the desired multi-zone micro-zone pattern was first printed using a UV-curable varnish onto a polymer film; then fine powders of cellulose or other materials were applied onto the uncured varnish, allowing the powder to stick to the varnish layer. After UV-curing, the powder particles were fixed by the cured varnish, leading to the formation of porous, water absorbing micro-zones on the polymer film. This type of micro-zone plate has the required liquid handling capacity for a variety of low-volume and portable two-dimensional plates for chemical, biochemical assays and ELISA. By suitably spacing porous micro-zones on a non-porous substrate, this type of plate eliminates inter-zone sample leaking. Rapid colorimetric analysis of the results can be performed using a portable battery-powered colour densitometer or a desktop scanner. By introducing a refractive-index-matching liquid into the micro-zones, the plate can be easily analyzed using transmission instruments. A major advantage of this plate fabrication method is that it enables the printing of different powders or functionalized powders to form micro-zones on a same plate, potentially allowing certain more difficult functionalization of materials to be performed before printing. We demonstrate the assaying performance of the plates using an analytical system that aims to clarify the analytical interference between NO(2)(-) and uric acid (UA) in the analysis of samples where both chemical species are present. We also show that ELISA assays can be performed using this type of plates.     

Micro-fabricated metal nozzle plates used for water-in-oil and oil-in-water emulsification

In this paper the fabrication and use of micro-structured metal nozzle plates as emulsification devices is investigated and discussed. These structured metal nozzle plates were fabricated via two distinct routes. Laser ablation, performed with a femtosecond laser, was used to drill micrometer-sized holes into stainless steel and aluminum foils. Also a conventional steel mesh with an average pore size of 2.4 μm fabricated by weaving and roll compaction of micrometer-sized steel wires was investigated. The perforated metal nozzle plates were used for oil-in-water and after hydrophobization with alkylchlorosilanes for water-in-oil emulsification as well. In both cases, two types of drop formation processes were observed. The first one is the shear-induced drop formation well known for cross-flow membrane emulsification. The second is the spontaneous drop formation known from microchannel emulsification.     

循环气流床反应器甲醇合成研究

1975年美国Chem Systems Inc公司提出液相法甲醇合成工艺的概念,采用导热性能优良的液体作为热载体,床层温度均匀易控,从而可提高原料气的单程转化率,出口甲醇质量分数可达到15％。传统浆态床反应器相对固定床虽然具有良好的传热性能,温度分布比较均匀,但是浆态床由于液体溶剂的存在以及气体分布不均匀,给传质过程带来了负面影响,增加了体系的复杂性。尽管优化气体分布器结构,可以起到改善传质的效果,但是还是无法明显改善传统浆态床内气液传质性能,反应器内固体沉积团聚严重。循环气流床通过循环泵的强制循环和喷嘴的雾化作用,相比如传统浆态床反应器,具有良好的传质能力、固体悬浮,并且能有效地减少轴向返混,具有相间接触充分、气液比（气固比）调节灵活、催化剂利用率高等特点。本实验研究了空速、循环量、喷嘴个数、催化剂浓度对甲醇合成的影响,为以后反应器的放大优化提供基础数据。     

Hydrodynamics of nonlinear viscoplastic fluid in cylindrical hydrocyclone

A flow of non-linear viscoplastic fluid film in a cylindrical hydrocyclone was simulated. The distribution of the velocity component and also a dependence of a fluid film thickness on an axial coordinate were determined by numerical solution of a set of rheodynamic model equations for various rheological properties of the liquid and dimensionless quantities. An influence of plastic properties of the liquid on damping of the circumferential component of the velocity was examined. The results were physically substantiated.     

Multicomponent Analysis of Liquids by Test Methods

Test strips were proposed for the simultaneous determination of the sum of heavy metals, iron(III), copper, zinc, total alkalinity, hardness, chlorides, sulfates, nitrates, nitrites, and fluorides, and composite test strips were proposed for the determination of copper, iron(III), and cobalt. Determination is based on the dependence of the length of colored zones of indicator papers sealed in a polymer film and with one edge in contact with the test solution on the concentrations of the analytes. The analytical range is 0.1–1000 mg/L. Procedures were developed for the determination of the above components in blood serum, hair, and solutions. The relative standard deviation is no higher than 40%, and the time of analysis is 15–20 min. Two-layer and composite test plates were proposed for the simultaneous determination of mercury(II), copper, iron(III), zinc, cadmium, fluorides, and chlorides by the diameter of colored zones after the impregnation of plates with the test liquid through a hole in the film with which the indicator paper was sealed. Two-layer test plates are squares of indicator paper, which do not contact with each other and are sealed with three layers of a transparent polymer film. Composite test plates are 3 × 3-cm squares made of indicator paper triangles and sealed with a polymer film from the two sides. Analytical ranges of different components are 0.5–50 mg/L. Procedures were developed for the determination of the above elements in galvanic slimes. The relative standard deviation is no higher than 40%; the time of analysis is 2–5 min.     

Detachment force of particles from air-liquid interfaces of films and bubbles

The detachment force required to pull a microparticle from an air-liquid interface is measured using atomic force microscopy (AFM) and the colloidal probe technique. Water, solutions of sodium dodecyl sulfate (SDS), and silicone oils are tested in order to study the effects of surface tension and viscosity. Two different liquid geometries are considered: the air-liquid interface of a bubble and a liquid film on a solid substrate. It was shown that detaching particles from liquid films is fundamentally different than from bubbles or drops due to the restricted flow of the liquid phase. Additional force is required to detach a particle from a film, and the maximum force during detachment is not necessarily at the position where the particle breaks away from the interface (as seen in bubble or drop systems). This is due to the dynamics of meniscus formation and viscous effects, which must be considered if the liquid is constrained in a film. The magnitude of these effects is related to the liquid viscosity, film thickness, and detachment speed.     

Effect of the flow profile on separation efficiency in pressure‐assisted reversed‐polarity capillary zone electrophoresis of anions: Simulation and experimental evaluation

Capillary electrophoresis connected to electrospray ionization mass spectrometry is a promising combination to analyze complex biological samples. The use of sheathless electrospray ionization interfaces, such as a porous nanoelectrospray capillary emitter, requires the application of forward flow (either by pressure or electroosmosis) to maintain the electrospray process. The analysis of solute molecules with strong negative charges (e.g., aminopyrenetrisulfonate labeled glycans) necessitates a reversed‐polarity capillary electrophoresis separation mode, in which case the electroosmotic flow is counter current, thus pressure assistance is necessary. In this study, we compared the effect of forced convection with and without counter electroosmotic flow on the resulting separation efficiency in capillary electrophoresis based on flow profile simulations by computational fluid dynamics technique and by actual experiments. The efficiencies of the detected peaks were calculated from the resulting electropherograms and found approximately 950 000 plates/m for electrophoresis with counter electroosmotic flow, 20 000 plates/m with pressure only (such as would be in open tubular liquid chromatography), and 480 000 plates/m for electrophoresis with simultaneous counter electroosmotic flow and forward pressure assistance, which validates the simulation data.     

Molecular dynamics study of thermal phenomena in an ultrathin liquid film sheared between solid surfaces: the influence of the crystal plane on energy and momentum transfer at solid-liquid interfaces

A molecular dynamics study has been performed on a liquid film sheared between moving solid walls. Thermal phenomena that occur in the Couette-like flow were examined, including energy conversion from macroscopic flow energy to thermal energy, i.e., viscous heating in the macroscopic sense, and heat conduction from the liquid film to the solid wall via liquid-solid interfaces. Four types of crystal planes of fcc lattice were assumed for the surface of the solid wall. The jumps in velocity and temperature at the interface resulting from deteriorated transfer characteristics of thermal energy and momentum at the interface were observed. It was found that the transfer characteristics of thermal energy and momentum at the interfaces are greatly influenced by the types of crystal plane of the solid wall surface which contacts the liquid film. The mechanism by which such a molecular scale structure influences the energy transfer at the interface was examined by analyzing the molecular motion and its contribution to energy transfer at the solid-liquid interface.     

Study of electro-optical properties of liquid crystal/reactive mesogen-coated liquid crystal display fabricated by slit coater

We have recently proposed a slit coater method for printable liquid crystal (LC) devices. In this method, the LC/reactive mesogen (RM) mixture was directly coated uniaxially onto the substrate surface by shear flow between the slit nozzle and the substrate, and the liquid crystalline molecules were anchored by in situ photopolymerisation of RM monomers. In this study, the dependence of electro-optical properties and device parameters of LC sample cells fabricated by the slit coater on ultraviolet (UV) exposure dosage was investigated. Moreover, effects of UV exposure dosage and thermal annealing on the azimuthal anchoring energy of LC sample cells fabricated by the slit coater are presented.     

两种填料片表面润湿性能及其对CO2吸收传质的影响

为提高CO2吸收填料塔的传质效率,研究了填料片表面润湿性能变化对传质过程的影响．设计了一种能够进行CO2吸收的气液接触传质实验装置,并对两种具有不同液固接触角的平板进行了吸收传质对比实验,实验采用15wt％的MEA溶液为吸收剂,原料气CO2与空气比例为1：3．通过实验给出了润湿性能对吸收传质效率的影响．为进一步了解不同润湿条件下的流动和传质行为细节,建立了基于VOF方法的三维计算流体力学模型,模拟了与传质实验对应的不同液固接触角时液体降膜流动与传质行为,得到了不同润湿性能时的降膜流动速度分布及浓度分布图像,模拟结果与实验值吻合较好,定量解释了接触角变化导致液膜流动结构和吸收传质效果变化的原因．     

Determination of the permittivity of nematic liquid crystals in the microwave region

Two 3 mm thick microscope glass plates, having one face plus their two long edges coated by a thick metallic film, are spaced 75 μm apart by mylar spacers. Because of the metallic coatings on the inner faces the structure acts as a single metallic slit. The space between the two coated plates is filled with aligned nematic liquid crystal (E7, Merck/BDH) and the cell is inserted in an absorber aperture. This single metallic slit geometry supports resonant modes when microwaves are incident with their polarization (E-field) perpendicular to the slit. The structure gives a set of Fabry-Perot-like resonant transmission frequencies. These frequencies move when a voltage is applied between the two plates, the liquid crystal being first aligned homogeneously, then realigning homeotropically with the applied field. By minotoring these changes a fast and easy to use procedure for determining the permittivity and its anisotropy for nematic liquid crystals in the microwave region has been developed. The parameters determined for E7 are ε_e = 3.17 (n_e = 1.78 ± 0.01) and ε_o = 2.72 (n_o = 1.65 ± 0.01), (Δn ≈ 0.13) in the 40.0–60.0 GHz region.     

Hole‐growth instability in the dewetting of evaporating polymer solution films

We investigate the dewetting of aqueous, evaporating polymer [poly(acrylic acid)] solutions cast on glassy hydrophobic (polystyrene) substrates. As in ordinary dewetting, the evaporating films initially break up through the nucleation of holes that perforate the film, but the rapidly growing holes become unstable and form nonequilibrium patterns resembling fingering patterns that arise when injecting air into a liquid between two closely spaced plates (Hele–Shaw patterns). This is natural because the formation of holes in thin films is similar to air injection into a polymer film where the thermodynamic driving force of dewetting is the analogue of the applied pressure in the flow measurement. The patterns formed in the rapidly dewetting and evaporating polymer films become frozen into a stable glassy state after most of the solvent (water) has evaporated, leaving stationary patterns that can be examined by atomic force microscopy and optical microscopy. Similar patterns have been observed in water films evaporating from mica substrates, block copolymer films, and modest hole fingering has also been found in the dewetting of dry polymer films. From these varied observations, we expect this dewetting‐induced fingering instability to occur generally when the dewetting rate and film viscosity are sufficiently large. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2825–2832, 2002