Physical approach to averaging theorems on phase interfaces in a dispersed multiphase medium

Within the framework of a procedure for scale-changing by averaging a representative elementary volume, theorems are developed to relate the averages of derivatives to the derivatives of averages over a surface, using elementary differential calculus. These theorems form the basis of a general macroscopic balance equation for a given quantity over interfaces of a dispersed multiphase medium. The equations of phase interfaces complement equations related to bulk phases describing transport in dispersed multiphase media.     

Turbulent forced convective heat transfer in two-phase evaporating droplet flow through a vertical pipe

A physical model was developed to study heat transfer in turbulent dispersed flow at very high vapor quality in a vertical pipe by numerically solving the coupling governing differential equations for both phases. Major heat transfer mechanisms included in the model were the thermal nonequilibrium effects, droplet vaporization, droplet deposition on the duct wall and thermal radiative transfer. The predicted results indicated that vapor superheating is dominant for the cases with high wall superheat, otherwise droplet vaporization dominates the energy transport processes. Heat transfer during the droplet-wall interaction only exists at low wall superheat but in small amounts.     

基于聚合物支撑形貌液晶/聚合物光栅的低阈值分布反馈式激光器

利用低光强曝光进行全息液晶/聚合物光栅(HPDLC)的制备,获得了内部无液晶微滴具有聚合物支撑形貌的低散射透射光栅结构.分别研究了染料PM567,DCM,DCJTI的放大的自发辐射(ASE)阈值及相对出射光强等性质,得到三种染料中DCJIT的ASE阈值最低,相对出射光强最高,表明DCJTI更适于制备低阈值、高转化效率激光器.分别采用这三种染料制备基于聚合物支撑形貌光栅的分布反馈式激光器,通过改变光栅周期得到不同出射波长的激光.其中,以DCJTI为工作物质,得到中心波长为648 nm,阈值为0.65μJ/pulse,转化效率为1.6%,线宽为0.3 nm的高性能激光.与国内外同类激光器相比,在阈值、转化效率、线宽三个方面均有不同程度的提高.     

Electro-optical and dielectric study of multi-walled carbon nanotube doped polymer dispersed liquid crystal films

The self-organising property of nematic liquid crystals (LCs) was used to align multi-walled carbon nanotubes (MWCNT) dispersed in them. MWCNT not only well integrate in the matrix but also, even at very low concentration, have a detectable effect on the LC properties that can be very attractive for display applications. In the present work, MWCNT were doped (0–0.5% wt/wt) in two different types of LCs. These MWCNT doped polymer dispersed LC (CPDLC) films were studied comprehensively using fundamental techniques. Polarising optical microscope (POM) and scanning electron microscope (SEM) techniques used for morphological study reveal that the LC droplet size remains unchanged with increase in MWCNT concentration. The electro-optical (EO) study performed by increasing voltage in steps of 10 V up to 100 V at an optimised frequency of 200 Hz and at temperature 25°C shows that the low MWCNT concentration films show good optical response than the higher one. The dielectric behaviour of CPDLC films in the frequency range 20 Hz to 20 MHz was investigated using precision impedance analyser. The obtained data were modelled with Debye and Cole-Cole methods to calculate relaxation time and distribution parameter (α). The zero value of α indicates Debye type relaxation processes.     

Ferroelectric liquid crystal dopants with a chiral (R,R)‐2,3‐difluorooctyloxy side‐chain: host dependence of the polarization power

The polarization powers δ_p of four chiral dopants with (R,R)‐2,3‐difluorooctyloxy side‐chains were measured in four liquid crystal hosts with isotropic (I)–nematic (N)–smectic A (SmA)–smectic C (SmC) phase sequences. The four chiral dopants differ in terms of their core structures: 2‐phenylpyridine (MDW950), biphenyl (5), 2‐phenylpyrimidine (6) and 2‐(3‐nitrophenyl)pyrimidine (7). In each case, δ_p varies with the structure of the liquid crystal host, which is consistent with the behaviour of so‐called Type II dopants that normally feature a chiral core structure. The δ_p(host) profile was found to depend on the degree of biaxiality of the dopant core structure, and on the degree of steric coupling between the chiral 2,3‐difluorooctyloxy side‐chain and the core. Conformational analyses at the B3LYP/6‐31G* level suggest that the 2,3‐difluorooctyloxy side‐chain is conformationally more rigid than conventional chiral side‐chains due to the added electrostatic repulsion of the two adjacent fluoro groups combined with the hyperconjugative ‘gauche effect’, and may therefore have a higher degree of biaxiality on the time average. This biaxial character should make the chiral side‐chain more sensitive to variations in quadrupolar ordering imposed by the SmC phase of the liquid crystal host, and may therefore explain the dependence of δ_p on the host structure reported herein.     

Improvements in morphological and electro‐optical properties of polymer‐dispersed liquid crystal grating using a highly fluorine‐substituted acrylate monomer

To improve the morphological and electro‐optical properties of a polymer‐dispersed liquid crystal (PDLC) grating, a type of highly fluorine‐substituted acrylate monomer was added to the prepolymer mixture. The morphologies of the PDLC gratings were investigated using atom force microscopy and scanning electron microscopy. The grating had a very clear polymer/LC interface after addition of 3.9 wt % of fluorine‐substituted monomer. The LC droplets in this case were much larger than the sample without fluorinated monomer. This phenomenon indicated that an almost complete phase separation had occurred. However, as the content of fluorine‐substituted monomer increased, the morphologies of gratings became less defined and the volumes of LC droplets were smaller. The diffraction efficiency (DE) decreased with increasing of fluoride content and the V ₉₀ increased simultaneously, which may be ascribed to the blurry interface and the small LC droplets. The highest DE (90%) and lowest V ₉₀ (70 V) were obtained simultaneously under the condition of 3.9 wt % fluoride added in the prepolymer. In addition, it was also found that the fluorine‐substituted monomer may disorder the alignment of LCs in the grating.     

Three‐dimensional structure in holographic polymer‐dispersed liquid crystals

Polymer‐dispersed liquid crystals, heterogeneous structures consisting of a liquid crystal and a polymer, are promising materials for generating holograms with switchable diffraction efficiency. The experiments presented here reveal that the generation of a face‐centered cubic structure can be achieved by exposing a liquid crystal/reactive monomer mixture to the interference pattern of four coherent laser beams under appropriate geometrical conditions. Copyright © 2012 John Wiley & Sons, Ltd.     

Switchable photonic crystal for polymer dispersed liquid crystal

Single-prism systems are used to fabricate electrically switchable Photonic Crystal (PhC) structures in Polymer Dispersed Liquid Crystal (PDLC) films. The optical configuration is simple, stable and repeatable. A good agreement between theoretical and experimental results is obtained for the PhC structures. Hexagonal far-field diffraction patterns and electrical switching characteristics are also investigated. The smallest droplets of liquid crystals are 10 nm in diameter. The switching voltage can be decreased to 13.3 V/μm.     

Temperature dependence of the phase separation in ethyl cyanoethyl cellulose/poly(acrylic acid)/4′‐n‐pentyl‐4‐cyano‐biphenyl composite films

A novel polymer‐dispersed liquid‐crystal film consisting of micrometer‐scale liquid‐crystal droplets in ultraviolet‐cured polymer composite matrices with cholesteric order was prepared and the influence of cure temperature on the phase separation was studied. The existence and pitch of the ethyl cyanoethyl cellulose cholesteric liquid‐crystalline phase were influenced by the existence of low molecular weight liquid crystals. The macromolecular cholesteric phase disappeared when the 4′‐n‐pentyl‐4‐cyano‐biphenyl concentration was over 40 wt %, and 4′‐n‐pentyl‐4‐cyano‐biphenyl domains were dispersed in the isotropic matrix of the polymer composite. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1334–1341, 2002     

Effect of electric field on phase separation of polymer dispersed liquid crystal

The kinetics of the polymerization induced phase separation of liquid crystal (LC)/monomer mixture has been investigated by means of depolarized light intensity technique and polarized light microscope (PLM). To examine the effect of the electric field, a DC electric field was applied across the mixtures during the phase separation process. The kinetic study indicates that the phase separation process is accelerated when the electric field is applied. The morphologies of the formed polymer dispersed liquid crystal (PDLC) films were observed by PLM. The electric field applied during the phase separation process yields the PDLC with small LC domains and fine morphologies. The clearing temperature (T_NI) of the formed PDLC films was measured by the PLM and it is found that the T_NI increases with the applied electric field intensity.