Behaviour of free-standing smectic A films upon heating

In the framework of a previously proposed microscopic mean-field model for thin smectic A liquid crystal films with two boundary surfaces, the behaviour of free-standing smectic A films upon heating is investigated theoretically. It is shown that the model accounts for both the film rupture and layer-thinning transitions with increasing temperature. A close correlation between the behaviour of the film upon heating and the surface tension is found. The model accounts also for essential features of the layer-thinning transitions for thick and thin free-standing smectic A films of various liquid crystal compounds.     

Behaviour of free-standing smectic A films upon heating

In the framework of a previously proposed microscopic mean-field model for thin smectic A liquid crystal films with two boundary surfaces, the behaviour of free-standing smectic A films upon heating is investigated theoretically. It is shown that the model accounts for both the film rupture and layer-thinning transitions with increasing temperature. A close correlation between the behaviour of the film upon heating and the surface tension is found. The model accounts also for essential features of the layer-thinning transitions for thick and thin free-standing smectic A films of various liquid crystal compounds.     

Facile fabrication of free-standing colloidal-crystal films by interfacial self-assembly

This paper reports a rapid and facile method of preparing free-standing colloidal crystals from monodisperse charged polystyrene (PS) microspheres. Mixed solvents (ethanol/water) were used as the dispersion medium in the self-assembly process of colloidal crystals. By a simple "floating self-assembly" method, PS microspheres floated on the surface of liquid and self-assembled into large area of three-dimensional (3D) ordered colloidal crystals within 15 min. Then epichlorohydrin was added in as a cross-linking agent to strengthen the colloidal-crystal film. After cross-linking reactions between the microspheres, the obtained colloidal-crystal film was free-standing and could be easily transferred to other substrates. Using tetrabutyl titanate as a titania precursor, 3D porous TiO(2) materials with rodlike skeletal structure were fabricated from the prepared free-standing colloidal crystal. This work provides a facile method to fabricate free-standing colloidal-crystal film, which can be used as an ideal template for the preparation of porous materials.     

Spontaneously created ring structures in free-standing liquid crystal films and their dependence on temperature and material parameters

A rotating electric field can induce a spontaneous formation of ring structures in free-standing ferroelectric liquid crystal films. A phase delay between the field rotation and the director movement on the smectic cone is the reason for the ring creation. The appearance of these structures depends on the field parameters, on the material parameters of the liquid crystal compound used, and on the temperature. The temperature dependence of the pattern formation and of material parameters (spontaneous polarization, rotational viscosity, pitch) have been investigated in more detail. It could be shown that the rotational viscosity of the liquid crystal is the most important parameter with respect to the influence of temperature. The experimental results are compared with the theoretical interpretation of the ring formation.     

Resonant X-ray scattering studies of the B2 phase formed by bent-core molecules

We have performed resonant X-ray scattering on two bent-core liquid crystal compounds exhibiting the B2 phase using three sample geometries: free-standing film and two others with free surface arrangements. The results conclusively demonstrate the two-layer orientational periodicity in this phase suggested by optical studies. We have performed the first resonant scattering experiments on liquid crystals at the chlorine K-edge, opening up a new class of compounds for resonant X-ray scattering studies. Furthermore, we have achieved an excellent alignment of the B2 phase with a free surface.     

Resonant X-ray scattering studies of the B2 phase formed by bent-core molecules

We have performed resonant X-ray scattering on two bent-core liquid crystal compounds exhibiting the B2 phase using three sample geometries: free-standing film and two others with free surface arrangements. The results conclusively demonstrate the two-layer orientational periodicity in this phase suggested by optical studies. We have performed the first resonant scattering experiments on liquid crystals at the chlorine K-edge, opening up a new class of compounds for resonant X-ray scattering studies. Furthermore, we have achieved an excellent alignment of the B2 phase with a free surface.     

Droplet size effects on film drainage between droplet and substrate

When a droplet approaches a solid surface, the thin liquid film between the droplet and the surface drains until an instability forms and then ruptures. In this study, we utilize microfluidics to investigate the effects of film thickness on the time to film rupture for water droplets in a flowing continuous phase of silicone oil deposited on solid poly(dimethylsiloxane) (PDMS) surfaces. The water droplets ranged in size from millimeters to micrometers, resulting in estimated values of the film thickness at rupture ranging from 600 nm down to 6 nm. The Stefan-Reynolds equation is used to model film drainage beneath both millimeter- and micrometer-scale droplets. For millimeter-scale droplets, the experimental and analytical film rupture times agree well, whereas large differences are observed for micrometer-scale droplets. We speculate that the differences in the micrometer-scale data result from the increases in the local thin film viscosity due to confinement-induced molecular structure changes in the silicone oil. A modified Stefan-Reynolds equation is used to account for the increased thin film viscosity of the micrometer-scale droplet drainage case.     

Determination of symmetry and molecular arrangements of free-standing liquid crystal films using null-transmission ellipsometry

A high resolution null-transmission ellipsometry system has been developed for measurement of the optical properties of liquid crystal free-standing films. The ellipsometric parameters are measured to within a resolution of 0.002° from samples held in a sealed, temperature-controlled environment. By measuring the ellipsometric parameters as a function of applied electric field orientation, the optical symmetry and molecular orientations of free-standing liquid crystal films can be determined in some cases. Three carefully selected applications of this technique are presented.     

Layer-thinning transitions in free-standing smectic A films

In the present paper a discrete mean-field model for thin smectic A liquid crystal films with two boundary surfaces is offered. The model accounts for the recently observed phenomenon of layer-thinning transitions in free-standing smectic A films upon heating. In particular, the model predicts the observed multiple layer jumps, as well as, for films thinner than 13 layers, the observed power law dependence of the layer-thinning transition temperatures on film thickness.     

Determination of symmetry and molecular arrangements of free-standing liquid crystal films using null-transmission ellipsometry

A high resolution null-transmission ellipsometry system has been developed for measurement of the optical properties of liquid crystal free-standing films. The ellipsometric parameters are measured to within a resolution of 0.002° from samples held in a sealed, temperature-controlled environment. By measuring the ellipsometric parameters as a function of applied electric field orientation, the optical symmetry and molecular orientations of free-standing liquid crystal films can be determined in some cases. Three carefully selected applications of this technique are presented.     

Humidity responsive asymmetric free-standing multilayered film

A humido-responsive free-standing film has been created using layer-by-layer assembly technique. Polyethylenimine (PEI) at high pH was assembled with poly(acrylic acid) (PAA) at low pH on a Teflon substrate to yield a micro-nanostructured surface that can be made superhydrophobic after being coated with a low surface energy compound. The resulting asymmetric free-standing film with one surface being superhydrophobic while the other is hydrophilic after detachment from the substrate can undergo reversible bending/unbending shape transitions when the environmental humidity is changed. The bending/unbending movement of the free-standing film can be ascribed to the different responses of these two surfaces to humidity.     

双网络增强自支撑多层膜的制备

通过石英晶体微天平(QCM)研究不同最外层对单体、交联剂、引发剂扩散入多层膜的结果显示,聚乙烯亚胺( PEI)为最外层的多层膜更有利于负电荷分子扩散.将(聚乙烯亚胺/聚丙烯酸)12-聚乙烯亚胺[( PEI/PAA)12-PEI]多层膜浸入带有单体、引发剂、交联剂的溶液后,经过光聚合交联与热交联后可形成自支撑膜.XPS数...     

Effect of Surfactants on the Film Drainage

Drainage of a partially mobile thin liquid film between two deformed and nondeformed gas bubbles with different radii is studied. The lubrication approximation is used to obtain the influence of soluble and insoluble surfactants on the velocity of film thinning in the case of quasi-steady state approach. The material properties of the interfaces (surface viscosity, Gibbs elasticity, surface diffusivity, and/or bulk diffusivity) are taken into account. In the case of deformed bubbles the influence of the meniscus is illustrated assuming simple approximated shape for the local film thickness. Simple analytical solutions for large and small values of the interfacial viscosity, and for deformed and nondeformed bubbles, are derived. The correctness of the boundary conditions used in the literature is discussed. The numerical analysis of the governing equation shows the region of transition from partially mobile to immobile interfaces. Quantitative explanation of the following effects is proposed: (i) increase of the mobility due to increasing bulk and surface diffusivities; (ii) role of the surface viscosity, comparable to that of the Gibbs elasticity; and (iii) significant influence of the meniscus on the film drainage due to the increased hydrodynamic resistance. Copyright 1999 Academic Press.     

Theory and application of a quartz resonator as a sensor for viscous liquids

A comprehensive analysis of the interaction between an AT-cut quartz crystal resonator and a viscous fluid is presented. The analysis, which includes peizoelectric effects, assumes a liquid of finite extent and therefore could also be used to study thin film of viscous liquids. A novel continuous flow cell system was designed and fabricated to monitor viscosity using an 11-MHz quartz crystal resonator. Measured data for frequency shifts of aqueous solutions of alcohols and sugars are in excellent agreement with theory.     

Nucleation of filaments of the TGBA phase in free-standing films

A model of filament nucleation in the twist grain boundary A (TGBA) phase in free-standing smectic films is proposed. It is based on a concept of finite blocks of parallel smectic layers forming a helical structure in the interior of a film. In our model, the blocks resemble twin-like domains in smectic-A liquid crystal. The blocks are mediated to surrounding smectic layers in film either continuously (coherent twin boundary) or discontinuously via dislocation loops wrapping up blocks. Edge components of dislocation loops form incoherent twin boundary. Screw components forming twist grain boundary connect neighbour blocks among themselves. Nucleation of TGBA filament in a film is treated using a simplified model based on isotropic smectic-A elastic theory. Discussion showed that filaments can be nucleated just below the transition temperature from the isotropic to the TGBA phase under slow temperature decrease. The nucleation of dislocation loops is possible due to low compression modulus of material which can be expected just below the transition temperature.     

Free surface-induced bilayer smectic A phase in polar liquid crystals

Here the influence of the free surface on both a thick (semi-infinite) layer and a thin freely suspended film of a polar liquid crystal is investigated. It is shown that within the temperature range of the monolayer smectic A phase (S_A1) the interaction between polar molecules and the free surface of the liquid crystal gives rise to a bilayer smectic A, a structure with long range antiferroelectric order (S_A2) in the surface region of the semi-infinite layer. The dependence of the bilayer smectic order parameter on the strength of the interaction between the constituent molecules and the free surface as well as temperature and the distance to the free surface are determined. Sufficiently far from the S_A1-S_A2 transition the latter dependence has an exponential character and the depth of the S_A2 phase penetration into bulk liquid crystal is equal to the longitudinal correlation length for the bilayer smectic A structure fluctuations in the S_A1 phase. However, near the S_A1-S_A2 transition the bilayer smectic order parameter decays non-exponentially and more rapidly with increasing distance to the free surface. In addition, it is found that the bilayer S_A2 phase can form several smectic layers at the free surface of a semi-infinite polar liquid crystal layer with the S_A1 phase. Finally, it is shown that in a freely suspended film the free surface-induced S_A2 phase can completely occupy the volume of the sample. Hence in a freely suspended polar liquid crystal film the S_A1-S_A2 transition occurs with decreasing film thickness.     

Free-standing films of twist grain boundary TGBA and UTGBC* liquid crystals studied by fluorescence confocal polarizing microscopy

The director structures, meniscus profile, and defects in free-standing films of the twist grain boundary TGB_A and UTGB_C* liquid crystals were studied. The films were characterized by a combination of polarizing microscopy and fluorescence confocal polarizing microscopy. Five principal regions of meniscus were distinguished. When film thickness in the meniscus is much smaller then the TGB pitch, there is no difference between the free-standing films of TGB and ordinary smectic A liquid crystals. When the film thickness is larger than the TGB pitch, filamentary texture is observed. The 3D director pattern of the filaments are similar to the ground state director fields of TGB_A and UTGB_C* liquid crystals. In the intermediate thickness region of the meniscus, when the film thickness and TGB pitch are commensurate, a unique radial pattern is observed. Based on the fluorescence confocal polarizing microscopy studies of the director field, we propose a model for the 3D director structure in this part of the meniscus.     

Simultaneous determination of fundamental parameters for ferroelectric liquid crystals

An analysis of an experimental technique which allows for a simultaneous determination of several parameters with special interest for ferroelectric liquid crystals is presented. These parameters are the spontaneous polarization, the rotational viscosity and the switching time. In addition, the susceptibility associated with the soft mode free of contributions related to the helicoidal structure can also be obtained. The experimental results for these parameters for the ferroelectric liquid crystal HDOBACEEC are reported. A comparison between the switching time values deduced from the rotational viscosity and those obtained by optical measurements is performed.     

An Insoluble Surfactant Model for a Vertical Draining Free Film

A mathematical model is constructed to study the evolution of a vertically oriented thin liquid film draining under gravity when there is an insoluble surfactant with finite surface viscosity on its free surface. Lubrication theory for this free film results in three coupled nonlinear partial differential equations describing the free surface shape, the surface velocity, and the surfactant transport at leading order. We will show that in the limit of large surface viscosity, the evolution of the free surface is that obtained for the tangentially immobile case. For mobile films with small surface viscosity, transition from a mobile to an essentially immobile film is observed for large Marangoni effects. It is verified that increasing surface viscosity and the Marangoni effect retard drainage, thereby enhancing film stability. The theoretical results are compared with experiment; the purpose of both is to act as a model problem to evaluate the effectiveness of surfactants for potential use in foam-fabrication processes. Copyright 2000 Academic Press.     

The effects of some factors on the rheological properties of the lyotropic liquid crystals formed in Brij97/NaDC/IPM/water system: compositions,temperature and polyphenols

In this paper, interaction between Brij97 and NaDC in mixed micelles was studied by surface tension method. Phase behavior for Brij97/NaDC/IPM/H₂O system was investigated at the desire molar ratio of α_NaDC?=?0.42 and the human body temperature of 37?°C. Hexagonal and cubic liquid crystal were found in this system analyzed by small angle x-ray scattering (SAXS). some factors were changed to affect the rheological properties of liquid crystal. Rheological investigations showed: The higher content of water and lower experimental temperature favored the greater values of shear viscosity and viscoelastic modulus; The sample was closer to the plastic fluid and was more likely to recover from disturbed state to equilibrium state at lower ratio of NaDC; The curcumin and TP can influence the shear viscosity, viscoelastic properties and phase transition temperature, even change the structure of liquid crystal. This supplies a way to adjust rheological properties, structure and even phase transition by altering those factors.