Effect of droplet size on the dielectric properties of PDLC films

Polymer dispersed liquid crystal (PDLC) films (5CB/PMMA, 60/40) of different droplet size were prepared by a solvent-induced phase separation method under different N₂ flow speeds. The effects of droplet size on the thermal transitions of the LC and various dielectric properties such as dielectric constant, conductance, dielectric loss, and the electric field induced in a droplet were examined. The configuration of the LC in the film with smaller droplets can be identified by comparing the dielectric constant of the film with the one predicted by Boettcher's mixture formula. In addition, the effect of droplet size on the electro-optical response of the PDLC film was investigated. Variations of the conductance and the dielectric constant of the film were analyzed under various AC frequencies, with the purpose of elucidating the polarization mechanism of the LC molecules in the droplet. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1373–1381, 1997     

Kinetic Spectrofluorimetric Determination of Nitrite in Water Samples and Nitrogen Dioxide in the Atmosphere Sampled by the Liquid Droplet Method

 A simple, sensitive and selective kinetic spectrofluorimetric determination of NO₂ sampled by the droplet method in the atmosphere was proposed on the basis of the reaction of safranin O with nitrite. By this reaction, a diazonium salt is formed, which causes a fluorescent reddish-orange dye color of the solution to change into a non fluorescent blue color. The reaction was monitored fluorimetrically by measuring the decrease in fluorescence intensity of safranin O at (λ_ex/λ_em = 536 nm/579 nm) by a fixed-time method. The experimental conditions were optimized. Under the optimum conditions in the concentration range of 7.5–400 ng/ml, a linear calibration curve (r² = 0.9978) was obtained with a detection limit of 7.5 ng/ml. The method was applied successful ly to the determination of nitrite in spiked water and NO₂ in the atmosphere, as sampled by a liquid droplet method. Received August 23, 1999. Revision February 29, 2000.     

轻组份燃料对乳化油蒸发与着火影响的研究

本文研究了较组份添加剂对单滴乳化油着火规律的影响。实验采用挂滴法。实验中,测量了体积比为十二烷：正庚烷：水6：0：4,5：1：4,4：2：4,3：3：4的着火延迟时间。实验结果和数值计算表明,在乳化油中加入易挥发添加剂能有效缩短乳化油的着火延迟时间,着火延迟时间随着添加剂加入量的增加而缩短,当易挥发添加剂含量较大时,着火延迟随着添加剂加入量的增加,变化越来越缓慢。这一结果对解决在掺水量大的条件下保证柴油机中乳化燃料的正常点火与启动具有实用价值。     

材料介观有序结构小角光散射图案的FFT算法

用快速傅里叶变换计算方法(FastFourierTransformation,FFT)直接获得高分子材料中介观有序结构小角激光光散射(SALS)图案,从而对实验获得的球晶和液晶液滴的SALS图案进行结构预测.作为实例,计算了几种典型液晶液滴构型的SALS图案.     

Crystallization of interphase droplets in emulsion explosive matrices

The instability of emulsion explosive matrices is mainly due to the crystallization of interphases as oversaturated aqueous solutions of nitrate salts. The principal features of crystallization for this type of emulsion have been previously studied; however, there is no consensus regarding the mechanism of crystallization for an emulsion explosive matrix. This study is devoted to the investigation of the crystallization behavior of interphase droplets. By monitoring the mass change of emulsions during their aging process, it was found that the mass of the emulsions remains almost constant and that water still completely existed in the emulsion system after crystallization of the interphase droplets. The ammonium nitrate (NH₄NO₃) crystals in the emulsion explosive matrices were then separated successfully using a simple method. The thermal behavior of pure NH₄NO₃ and crystals in the emulsion explosive matrices was studied by differential scanning calorimetry-thermogravimetric analysis (DSC-TG) at a heating rate of 10?K/min. The experimental results show that the thermal behavior of the crystals in the emulsions was exactly the same as for pure NH₄NO₃, meaning that only NH₄NO₃ crystallized from the emulsion explosive matrices with no water crystals. Thus, it could be concluded that after crystallization of the dispersed drops in the emulsion explosive matrices, pure NH₄NO₃ crystals and new smaller droplets were produced.     

Possible symmetrization of Fisher's droplet picture nearT c : A bubble-droplet formula

An expression for the pressure is proposed which leads to a symmetric equation of state for liquid and gas near the critical point. Our bubble-droplet formula is similar to Fisher's cluster expansion but contains an additional term due to the density dependence of the surface tension. Also, it assumes the density difference between a droplet (or bubble) and the surrounding medium to be proportional tol ^{– 1/} and not to be independent of the droplet sizel. Then, the scaling homogeneity assumption and some scaling laws, includingd=2–, can be derived (d is dimensionality). The additional assumption of spherical droplets and bubbles leads to a new scaling law 1+=(d–1), which is only slightly violated in the lattice gas ford=2, 3, 4.Work partially supported by Research Corporation, a Frederick Gardner Cottrell grant aid.     

Metastability and analyticity in a dropletlike model

We consider an urn model closely related to the Fisher-Felderhof droplet model for the purpose of studying the relation between metastability and analytic continuation. For this model both the statics and dynamics can be solved and we confirm the relation between the metastable decay rate and the imaginary part of the analytically continued free energy (actually, pressure, in this model). We also find that eigenvalue degeneracy, an old theme for static aspects of phase transitions, appears in the dynamics as well. When approaching the phase transition from the stable side it is a degeneracy in the eigenvalues of the linear operator appearing in the master equation that causes the system to lock into a particular phase.Supported in part by the US-Israel Binational Science Foundation and the Technion Fund for the Encouragement of Research.     

Nucleation and metastability in three-dimensional Ising models

We present Monte Carlo experiments on nucleation theory in the nearest-neighbor three-dimensional Ising model and in Ising models with long-range interactions. For the nearest-neighbor model, our results are compatible with the classical nucleation theory (CNT) for low temperatures, while for the long-range model a breakdown of the CNT was observed near the mean-field spinodal. A new droplet model and a zeroth-order theory of droplet growth are also presented.Supported in part by grants from ARO, ONR, and NSF.     

Experimental and Theoretical Study of the Impact of Alumina Droplets on Cold and Hot Substrates

A complex experimental set-up was built to study the impact of liquid alumina droplets on different substrates (stainless steel 304L, sintered alumina, carbon–carbon) kept at temperatures up to 2100 K. The impact behavior: rebound, deposition, splashing, spattering was systematically studied as well as the resulting splat shapes. The set-up consists in a controlled atmosphere chamber where molten alumina particles with diameters between 10 and 90 m, are produced by a d.c. plasma torch, substrates being heated by a second d.c. plasma torch. In such conditions, it was possible to achieve particle temperatures between 2300 and 4200 K with velocities in the range 50 to 300 m/s. The particle behavior at impact was characterized by the Sommerfeld parameter K (K=We^1/2 Re^1/4 We and Re being respectively the Weber and Reynolds numbers of impacting particles). It was possible to vary K between 3 and 1300. Low K values were obtained by tilting the substrate up to 60°. The parameters of a single particle at impact were measured: its velocity v_p and diameter d_p by Phase Doppler Anemometry (v_p=5%, d_p=10%) and its temperature T_p by fast (100 ns) two color pyrometry (T_p=15%). The particle impact was visualized by a fast camera coupled to a microscope (exposure delay time 50 ns . . .100 ms) with complex synchronization and light intensity problems. To solve the latter, the impacting particle had to be illuminated with a 2 W c.w. Ar⁺ laser at 488 nm. Unfortunately, the controlled atmosphere chamber did not allow to change the substrate after each particle impact. Starting from a smooth surface for the first impact, due to the successively deposited splats, rapidly droplets impacted on a rough surface (Ra5 m). For splats collected on a hot alumina substrate (2100 K), where flattening is completed before solidification starts (case similar to that of ethanol droplets on cold copper) deposition occurs for K between 4 and 90 while splashing occurs for K as low as 30. These results are slightly different from those related to the ethanol droplet for which deposition occurs for 357.7. This could be due to the precision of measured values and the rough surface. For splats collected in spraying conditions splashing is always the rule K values up to 1400) especially on rough surfaces. However the particle impact velocity and temperature, the substrate temperature and tilting plays an important role on the resulting splat diameters, distortion and elongation rates. The question which is still pending is which quantity of splashed material is incorporated within the constructing coating and how does it affect its thermophysical properties.     

Mobility‐dependent bifurcations in capillarity‐driven two‐phase fluid systems by using a lattice Boltzmann phase‐field model

Bifurcations in capillarity‐driven two‐phase fluid systems, due to different mobilities in phase‐field models for such systems, are studied by using a lattice Boltzmann method (LBM). Specifically, two‐dimensional (2D) and three‐dimensional (3D) droplets on a flat wall with given wettability variations are investigated. It is found that the mobility controls the rate of diffusive relaxation of the phase field from non‐equilibrium toward equilibrium, and similar to previous findings on mechanically driven two‐phase systems, the mobility is closely related to the contact line velocity. For the cases investigated, different mobilities across a critical value result in fundamentally different system evolution routes and final stable equilibrium states. These results may provide some implications for phase‐field study of droplet manipulations by surface wettability adjustments in microfluidics. Copyright © 2008 John Wiley & Sons, Ltd.