Mass transfer within oscillating liquid droplets

All existing relations describing mass transfer within oscillating droplets were critically compared with experimental data in which resistance to transfer is predominantly inside the droplet. Several new relations are also developed to estimate oscillating droplet extraction efficiencies, one of which is recommended in view of its theoretical appeal, accuracy, and ease of application. The most accurate relation currently available is an empirical expression developed by Skelland and Wellek.     

Interface morphology and phase separation in polymer-dispersed liquid crystal composites

It is widely appreciated that electro-optic activity in polymer-dispersed liquid crystals (PDLCs) depends on separation of the polymer and liquid crystal (LC) phases. Since the phase structure develops in a non-equilibrium system, the morphology of the LC domains depends on the details of the chemical and physical processes active during domain formation. The nature of the interface between the polymer and liquid crystal phases is of particular interest. This work discusses the two-phase morphology in an acrylate-based system that develops during polymerization-induced phase separation (PIPS). Using small-angle X-ray scattering (SAXS) and ultra-small-angle X-ray scattering (USAXS), we find that interfaces in PDLCs developed from an acrylate-based recipe are more disordered than generally appreciated. Information gained from SAXS and USAXS is compared to data from scanning electron microscopy (SEM) and transmission electron microscopy (TEM). To elucidate the apparent discrepancies between imaging and scattering, we investigated the effects of SEM sample preparation. We observe significant alteration of the interface morphology due to the leaching of the LC phase.     

Molecular dynamics simulation of liquid crystal formation within semi-flexible main chain LCPs

Molecular dynamics simulations of a semi-flexible main chain LCP (liquid crystalline polymer) have been carried out using a newly developed model named solo-LJ-spring-GB model. The new model represents the molecular chain in the form of GB-spring-LJ-spring-…-LJ-spring-GB sections that simplifies the model and reduces the simulation computation by many times. The new model was evaluated by studying the phase behaviors of semi-flexible main chain LCPs through simulation. The results, such as the spontaneous phase transition from isotropic phase to nematic phase as the system temperature decreases and the odd-even effect of the number of flexible spacers on its thermodynamic properties agree well with other experimental results as well as simulations using the traditional GB/LJ model. The orientational and translational mobilities of mesogenic units in the new model have also been measured and compared with those in the traditional GB/LJ model with very little differences found.     

Synthesis of liquid crystalline polyesterimides by melt polymerization and its application to polymer-dispersed liquid crystal

N-(4-Carboxyphenyl)trimellitimide (DAI) was synthesized from trimellitic anhydride and p-aminobenzoic acid by a one-step reaction utilizing m-cresol as a solvent. DAI was reacted with hexanediol and nonanediol to give bishydroxyhexyltrimellitimide (BHHI) and bishydroxynonyltrimellitimide (BHNI), respectively. Thermotropic liquid crystalline polyesterimides with BHHI and BHNI as a mesogenic group and 6 and 9 methylene units as a spacer were synthesized by a direct melt-polymerization method similar to the one used in the preparation of commercial polyester. Polyesterimides were characterized by IR, ¹H-NMR, DSC, and a polarized optical microscope. Their application as a matrix (binder) of polymer-dispersed liquid crystal (PDLC) was investigated with 4-heptyloxybenzoic acid as a low molecular weight liquid crystal (LMWLC). From the PDLC experiment, polyesterimides were found to be useful as a matrix for the high-temperature PDLC above 100°C. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1517–1522, 1998     

Effect of the ionic conductivity of a polymer matrix on the electrooptical properties of polymer-dispersed liquid crystal films

Summary The influence of ionic conductivity on the electrooptical response was investigated. A new monomer of 2-(poly(ethylene glycol)oxycarbonyl)-bicyclo[2.2.1] hepta-2,5-diene (PEGOC-BCHD) was synthesized and polymerized. Polymer/liquid crystal composite films are composed of poly(PEGOC-BCHD), E8, and alkali metal salts. The ionic conductivities were measured as a function of the ratio of LiClO₄/ethylene oxide for various molecular weights of poly(ethylene glycol). The results indicate that the threshold field and response rise time can be reduced by increasing ionic conductivity of the matrix polymer. Liquid crystal droplets were observed in a continuous matrix phase in the form of ‘Swiss cheese’ morphology. Received: 13 March 1998/Accepted: 19 May 1998     

Influence of the multi-functional epoxy monomers structure on the electro-optical properties and morphology of polymer-dispersed liquid crystal films

Polymer dispersed liquid crystal (PDLC) films were prepared by polymerization-induced phase separation method with nematic LC content as low as 40 wt%, and the electro-optical properties were carefully investigated. To accomplish this, the structure of multi-functional curable epoxy monomers with different composition feed ratios and the weight percentages of the two groups were examined in this study. The combined effects of heat-curable monomers’ structure on the conspicuous morphology of polymer network of PDLC films formed small holes and suitably distributed coin-like networks in both groups A and B, respectively. The detailed characteristics and morphology of polymer network of PDLC films were analyzed by employing liquid crystal device parameter tester, UV-Vis-NIR spectrophotometer and scanning electron microscope. Meanwhile, the enhanced curing temperature effects on the alkyl chain length, short flexible chain length, and rigid chain segment containing epoxy monomers structure on the increasing morphology of polymer network as well as electro-optical properties of PDLC films were also studied. It was found that the LC domain size of the polymer network could be regulated by adjusting the structure and composition ratio of curable epoxy monomers, and then the electro-optics of the PDLC films could be optimized, which is beneficial for decreasing the total LC content in PDLC devices.     

电场对液滴界面张力及动力学特征影响的研究进展

电场是改变液滴界面张力的重要因素,施加不同类型的电场对驱动微流体运动、变形、分裂及合并等行为起着至关重要的作用,该技术广泛用于微液滴操控、电子显示和原油脱水等领域,并具有潜在的应用前景。文中综述了电场作用下微液滴气-液、气-固与液-液交界面张力的变化,分析了与界面张力对微液滴运动学行为影响相关的实验现象及模拟成果;指出电场作用下气-液交界面处表面张力的变化趋势,因实验及模拟方法不同目前仍存在较大争议;探讨了直流电和交流电对电润湿过程液滴铺展的不同影响和液滴形态振荡特征;分析了直流电、交流电及电脉冲对电破乳过程中液滴运动行为的重要作用。总结了电场对液滴界面张力影响研究中存在的问题,并提出了值得进一步深入研究的可能方向。     

Effect of temperature on the optical and electro-optical properties of poly(methyl methacrylate)/E7 polymer-dispersed liquid crystal composites

Experiments on the effect of temperature on the optical and electro-optical behaviors of polymer-dispersed liquid crystals (PDLCs) are considered. Composite films composed of poly(methyl methacrylate) (PMMA) and the nematic-type liquid crystal (LC) E7 were prepared by solvent casting in chloroform. The PDLC film contained droplets of E7 from 10 to 80 wt % in a PMMA matrix. Morphological studies illustrated the formation of isolated droplets of LC due to phase separation, and their homogeneous distribution increased with increasing E7 content. Thermo-optical studies showed an increase in the nematic–isotropic transition temperature of composites, which indicated preferential solvation during the phase-separation process. The electro-optical characteristics were studied under the conditions of an externally applied square wave electric field with a He–Ne laser (λ = 632.8 nm) as a light source. The responses improved as the E7 content in PMMA increased. Semipermanent memory effects were noticed in composites at higher temperatures. Changes in the transmittance due to thermal variations provided the possibility of using such a device as a temperature sensor. The results obtained indicate that under these experimental conditions, the output can be controlled to the desired level by the selection of a suitable loading of LC to prepare PDLC electro-optically active composite films with a response time on the order of only a few milliseconds. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Thermophysical and thermomechanical properties of Norland Optical Adhesives and liquid crystal composites

Thermophysical and thermomechanical properties of UV-cured Norland Optical Adhesives (NOA65) and low molecular weight Liquid Crystal E7 systems were investigated. Thin films of Polymer Dispersed Liquid Crystal were prepared by Polymerization Induced Phase Separation and UV-curing. Phase diagrams were established using Polarized Optical Microscopy and DSC to determine the lines of nematic to isotropic phase transition as well as the glass transition temperature changes with the system composition. Mechanical measurements were performed in the static and dynamic modes to determine the moduli in terms of the curing time, the temperature and the amount of low molecular weight liquid crystal molecules. Thermomechanical measurements were found to overestimate the glass transition temperature as compared to calorimetric data. Variations of the Young's modulus and the molecular weight defining the mesh size of the network were analyzed in terms of the curing time and liquid crystal concentration. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Dielectric relaxation dynamics in liquid crystal – dye composites

Dielectric relaxation spectroscopic studies were undertaken to investigate the role of the resistivity of a host liquid crystal in the relaxation dynamics of dyed systems. Two different types of liquid crystal mixture were used, namely, one that was characterized by high ion contamination and low resistivity and the other which was of high resistivity and virtual ion-free; both types were doped using an azo or anthraquinone dye. Dielectric spectra of both the pristine liquid crystals and their composites at different external fields were measured in homogenously-aligned cells. The use of both dyes on an ion-enriched liquid crystal not only enhanced ion concentration but also slowed down the relaxation process. In contrast, the relaxation process for composites with a high-resistivity liquid crystal host was faster than that of the pristine liquid crystal.     

Nanoscale electrical property studies of individual GeSi quantum rings by conductive scanning probe microscopy

The nanoscale electrical properties of individual self-assembled GeSi quantum rings (QRs) were studied by scanning probe microscopy-based techniques. The surface potential distributions of individual GeSi QRs are obtained by scanning Kelvin microscopy (SKM). Ring-shaped work function distributions are observed, presenting that the QRs'' rim has a larger work function than the QRs'' central hole. By combining the SKM results with those obtained by conductive atomic force microscopy and scanning capacitance microscopy, the correlations between the surface potential, conductance, and carrier density distributions are revealed, and a possible interpretation for the QRs'' conductance distributions is suggested.     

A modified Gay–Berne model for liquid crystal molecular dynamics simulation

We present a modified Gay–Berne (GB) potential, in which the attraction force can be tuned by adjusting one parameter only. An attractive parameter Ps is introduced for describing the strength of attractive force relative to its repulsive counterpart between large particles. Using the proposed model, the phase transition phenomena of liquid crystal molecules as the density varies from 0.32 to 0.24, have been investigated. Simulation results are compared with the results of using traditional GB potential. The phases and phase transitions observed by using different attractive parameters show the importance of studying the variation of attractive forces, which resulted in dramatic different phase transition phenomena.     

Interdroplet heterogeneous nucleation of supercooled liquid droplets by solid droplets in oil-in-water emulsions

The crystallization of oil droplets inn-hexadecane oil-inwater emulsions was monitored by pulsed nuclear magnetic resonance (NMR). The emulsions initially contained an equal mixture of solid droplets and supercooled liquid droplets at either 6 or 8°C. The degree of crystallization in the droplets was determined by measuring the NMR signal 30 μs after a 90^o radio frequency pulse was applied to the sample. The signal from the solid droplets decayed rapidly after the radio frequency pulse, allowing the measured signal to be related to the fraction of liquid droplets. No crystallization was observed in a sample that contained only supercooled liquid droplets, but crystallization was observed when solid droplets of the same material were present. This indicated that crystallization was induced in the liquid droplets by the solid droplets and was most likely caused by interactions between solid and liquid droplets-that is, by interdroplet heterogeneous nucleation. The rate of induced crystallization decreased as the viscosity of the continuous phase was increased and the size of the droplets was increased, but was independent of droplet concentration (20–40%).     

Ignition characteristics of liquid hydrocarbon fuels as single droplets

An investigation into the effects of fuel boiling point and chemical type on the autoignition of single droplets of a number of pure hydrocarbons is reported. Ignition delay times were measured using the suspended droplet technique, and a numerical model was developed to extract reaction rate constants for the fuels. The results show that both fuel boiling point and chemical kinetics are important contributors to the ignition delay.     

Spread and recoiling of liquid droplets impacting solid surfaces

The impact of water droplets on solid surfaces is studied experimentally and theoretically. Theoretical equations based on energy conservation are developed. In our theoretical study, the droplet is modeled as a ring‐like shape, which matches the dynamic shape of droplets observed from our experimental tests. In the analysis of energy conservation, the nonuniform distribution of pressure inside the deformed droplet is taken into account by introducing a flow potential energy term in the theoretical equations. To derive viscous dissipation for recoiling, a viscous layer coefficient is introduced. Its values for the tests using smooth surfaces are found to be within a small range. Both theoretical predictions and experimental data show significant influence of surface wettability on maximum spread and recoiling process. With the increase of advancing contact angle, surface energy shows a decreasing trend, whereas flow potential energy shows an increasing trend and becomes significant for hydrophobic surfaces. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2683–2691, 2014     

The focal length method of measuring deposited liquid droplets

An extension to the work of May is presented which is used for the sizing of liquid droplets by the focal length method. Tables are given from which the spread term and the contact angle θ can be read directly. The effects of various surface preparations on contact angle are given for di-(2-ethylhexyl-sebacate droplets deposited on glass surfaces.     

Explosive boiling of liquid droplets at their superheat limits

The maximum temperature limit at which liquid boils explosively is called the superheat limit of liquids. Using the droplet explosion technique, the superheat limit of several hydrocarbons and their mixtures was measured. The dynamic behavior of the bubble formed from the fully evaporated droplet at its superheat limit was investigated by measuring the far-field pressure signal from the bubble and by visualizing its behavior. The evaporation process of a droplet and the evolution of the bubble from the evaporated droplet were also studied theoretically by using an appropriate bubble nucleation and dynamics model. It was found that the superheat limit of liquids could be accurately predicted when correct vapor-liquid equilibrium data for the liquids were provided. The calculated far-field pressure signals, which indicate thermal damping, are in good agreement with observed one until the bubble disintegrates.