Influence of flexoelectricity on electrohydrodynamic instabilities in nematics

We have incorporated the flexoelectric terms in developing one-dimensional models of E.H.D. instabilities in nematics both under D.C. and A.C. excitations. It is shown that, using this model, we can account for the following experimental observations, which could not be adequately explained by earlier models: (i) The observation of oblique rolls whose wave vector q makes an angle α with the undistorted director n ₀, up to some frequency v ₀ in the conduction regime; (ii) the oblique rolls found in the dielectric regime; and (iii) the ‘longitudinal’ E.H.D. instabilities in some systems with negative conductivity anisotropy. We also present some experimental observations under D.C. excitation.     

Influence of surface topography on the interactions between nanostructured hydrophobic surfaces

Nanostructured particle coated surfaces, with hydrophobized particles arranged in close to hexagonal order and of specific diameters ranging from 30 nm up to 800 nm, were prepared by Langmuir-Blodgett deposition followed by silanization. These surfaces have been used to study interactions between hydrophobic surfaces and a hydrophobic probe using the AFM colloidal probe technique. The different particle coated surfaces exhibit similar water contact angles, independent of particle size, which facilitates studies of how the roughness length scale affects capillary forces (previously often referred to as "hydrophobic interactions") in aqueous solutions. For surfaces with smaller particles (diameter < 200 nm), an increase in roughness length scale is accompanied by a decrease in adhesion force and bubble rupture distance. It is suggested that this is caused by energy barriers that prevent the motion of the three-phase (vapor/liquid/solid) line over the surface features, which counteracts capillary growth. Some of the measured force curves display extremely long-range interaction behavior with rupture distances of several micrometers and capillary growth with an increase in volume during retraction. This is thought to be a consequence of nanobubbles resting on top of the surface features and an influx of air from the crevices between the particles on the surface.     

Influence of surface topography on osteoblast response to fibronectin coated calcium phosphate thin films

The ability to engineer biomaterial surfaces that are capable of a dynamic interaction with cells and tissues is central to the development of medical implants with improved functionality. An important consideration in this regard is the role played by the extracellular proteins that bind to an implant surface in vivo. Deliberate use of an ad-layer of such proteins on an implant surface has been observed to guide and direct cell response. However, the role that changes in surface topography might play in determining the nature of this cell–protein–surface interaction has not been investigated in detail. In this study, calcium phosphate (CaP) thin films have been deposited onto substrates with varying topography such that this is reflected in the (conformal) CaP surface features. A fibronectin (FN) ad-layer was then deposited from solution onto each surface and the response of MG63 osteoblast-like cells investigated. The results revealed that in all cases, the presence of the adsorbed FN layer on the CaP thin films improved MG63 cell adhesion, proliferation and promoted early onset differentiation. Moreover, the nature and scale of the response were shown to be influenced by the underlying CaP surface topography. Specifically, MG63 cell on FN-coated CaP thin films with regular topographical features in the nanometer range showed statistically significant differences in focal adhesion assembly, osteocalcin expression and alkaline phosphase activity compared to CaP thin films that lacked these topographical features. As such, these data indicate that surface topography can be used to further influence cell adhesion and downstream differentiation by enhancing the effects of a surface adsorbed FN layer.     

Effect of surface topography on the anodization of titanium

The influence of surface topography and electrolyte composition on the anodization of titanium was investigated. The growth of the anodic oxides layer and the impedance response of the electrode/electrolyte interface were studied by means of ac-voltammetry, and the surface topography before and after anodization was visualized by atomic force microscopy. It was observed that the final topographic features and the instability phenomenon during the anodization of titanium were dependent on the surface finishing and on the nature of the anions in the electrolyte. In particular, it appears that rougher surfaces lead to a more homogeneous anodic film, while polished ones form an irregular morphology. The presence of sulfates is enhancing the breakdown of the film.     

Electrical driving of the surface memorization in dimerized nematics with short range smectic order

Electrical control of the surface memorization of an oriented smectic C texture in the temperature range of dimerized nematics with short range smectic C order (4-n-alkyloxybenzoic acids) is presented. It is demonstrated that by suitable choice of the electric field parameters and surface conditions it is possible to separate the thermal and electrical components of the total erasure activation energy. The significant role of the double charge electric layer in the mechanism of the surface memorization is verified. By the electrical control of the memorization, we have confirmed the electrical part of the modified Rapini-Papoular anchoring energy in the effective surface energy.     

On a theoretical analysis on the influence of non-uniformity of the order parameter on the surface energy in nematics

Recently Pikin and Terent'ev have analysed the influence of the non-uniformity of the order parameter on the measured value of the surface cohesion energy in nematic liquid crystals (1988, Sov. Phys. Crystallogr., 33, 641). We argue that since the mathematical problem is ill-posed, the analysis has to be revised. In particular, quadratic terms in the second derivatives of the deformation angle and the order parameter should be taken into account.     

On the flexoelectric effect in nematics

Flexoelectricity is a general and fundamental phenomenon in liquid crystals. It describes the linear coupling between an applied electric field and gradients in the director field. Whereas flexoelectricity has for decades been regarded as only of academic interest, we think it is time to point out its considerable application potential, for instance in the case of the flexoelectrooptic effect, and to urge a revival of interest in the subject. As a result of long-time neglect, published data on flexoelectric coefficients are scarce and inconsistent, even with regard to the sign of the reported effect. In this paper we critically review the possible definitions of flexocoefficients in order to propose an international standard. We point out that the absence of such a standard obstructs the understanding of the physical basis, microscopically as well as macroscopically, of the effect, and leads to the introduction of nonsensical concepts like 'flexoelectric anisotropy'. Based on the only natural convention, we finally propose a simple method for measuring sign and magnitude of the effective flexoelectric coefficient which is the control parameter in electro-optic effects.     

Size and surface effects on phase transitions

Size effects on liquid crystal phase transitions are investigated by scanning calorimetry of samples absorbed in porous silica. For pore diameter of order 100 Å, the liquid crystal transition depressions are of order 0.1°C and the solid melting points of order 10°C.     

Research of protein adsorption on the different surface topography of the zinc oxide

The effect of the surface topography on the protein adsorption process is of great significance for designing biomaterial surfaces and the biocompatibility for specific biomedical applications. In this work, we have systematically investigated the mono‐protein adsorption kinetics of bovine serum albumin (BSA) and fibrinogen (Fg) adsorbed on the four different surface topographies (nanoparticles (NPs), nanorods (NRs), nanosheets (NSs) and nanobeams (NBs) of Zinc oxide (ZnO), respectively. The competition of multi‐protein adsorbed on them has been studied as well. Results showed that each protein had a singular process of adsorption that fitted well by Spreading Particle Model (SPM). It confirmed that ZnO NRs compared with other samples had more adsorption sites, which could provide more opportunities for the interaction between material and protein molecules. In addition, the Fg compared to the BSA could be more tightly adsorbed to the surface, both of which existed slight conformational changes by Fourier transform infrared (FTIR) and circular dichroism spectra (CD). Taken together, all these consequences well demonstrated that NRs may have wider applications in designing biomaterial surfaces and the biocompatibility for implanted biomaterials. Copyright © 2014 John Wiley & Sons, Ltd.     

Changes in asphaltene surface topography with thermal treatment

The impact of thermal cracking reaction on asphaltene structure and morphology has been investigated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The structural and morphological changes at a microscopic level were monitored by comparing the parent asphaltenes from different vacuum residues (VRs) to their corresponding thermally treated asphaltenes, obtained from the by-product pitch after thermal treatment. The SEM analysis indicated that the asphaltene aggregates extracted from atmospheric residues have smooth and rough surfaces with agglomerate particles and bright inclusions. The SEM images of asphaltene aggregates that are extracted from the pitch samples after mild cracking demonstrated cleavage fracture morphology with obvious reduction in inclusions sizes and intensities. The TEM analysis, on the other hand, indicated that the asphaltenes from residual oils have tangled structures, with edges similar to a cauliflower. The tangled structure is mainly credited to the alkyl side-chains that impede the aromatic sheets from stacking. At mild cracking (400 °C), the asphaltene began to exhibit well-ordered layer structures near the edges due to the rupture of the alkyl side-chains. However, the tangled structure has been preserved in the interior of the sample. As the reaction severity increases (415 °C), the stacking of aromatic sheets became more evident even in the sample interior. At the most severe cracking condition (430 °C), an obvious reduction in the cluster diameter has been observed, which mainly resulted from the reduction in the number of aromatic sheets per stack.     

Control of surface topography in biomimetic calcium phosphate coatings

The behavior of cells responsible for bone formation, osseointegration, and bone bonding in vivo are governed by both the surface chemistry and topography of scaffold matrices. Bone-like apatite coatings represent a promising method to improve the osteoconductivity and bonding of synthetic scaffold materials to mineralized tissues for regenerative procedures in orthopedics and dentistry. Polycaprolactone (PCL) films were coated with calcium phosphates (CaP) by incubation in simulated body fluid (SBF). We investigated the effect of SBF ion concentration and soaking time on the surface properties of the resulting apatite coatings. CaP coatings were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), and energy dispersive X-ray spectrometry (EDX). Young's modulus (E(s)) was determined by nanoindentation, and surface roughness was assessed by atomic force microscopy (AFM) and mechanical stylus profilometry. CaP such as carbonate-substituted apatite were deposited onto PCL films. SEM and AFM images of the apatite coatings revealed an increase in topographical complexity and surface roughness with increasing ion concentration of SBF solutions. Young's moduli (E(s)) of various CaP coatings were not significantly different, regardless of the CaP phase or surface roughness. Thus, SBF with high ion concentrations may be used to coat synthetic polymers with CaP layers of different surface topography and roughness to improve the osteoconductivity and bone-bonding ability of the scaffold.     

Theory of solvation in polar nematics

We develop a linear response theory of solvation of ionic and dipolar solutes in anisotropic, axially symmetric polar solvents. The theory is applied to solvation in polar nematic liquid crystals. The formal theory constructs the solvation response function from projections of the solvent dipolar susceptibility on rotational invariants. These projections are obtained from Monte Carlo simulations of a fluid of dipolar spherocylinders which can exist both in the isotropic and nematic phases. Based on the properties of the solvent susceptibility from simulations and the formal solution, we have obtained a formula for the solvation free energy which incorporates the experimentally available properties of nematics and the length of correlation between the dipoles in the liquid crystal. The theory provides a quantitative framework for analyzing the steady-state and time-resolved optical spectra and makes several experimentally testable predictions. The equilibrium free energy of solvation, anisotropic in the nematic phase, is given by a quadratic function of cosine of the angle between the solute dipole and the solvent nematic director. The sign of solvation anisotropy is determined by the sign of dielectric anisotropy of the solvent: solvation anisotropy is negative in solvents with positive dielectric anisotropy and vice versa. The solvation free energy is discontinuous at the point of isotropic-nematic phase transition. The amplitude of this discontinuity is strongly affected by the size of the solute becoming less pronounced for larger solutes. The discontinuity itself and the magnitude of the splitting of the solvation free energy in the nematic phase are mostly affected by microscopic dipolar correlations in the nematic solvent. Illustrative calculations are presented for the equilibrium Stokes shift and the Stokes shift time correlation function of coumarin-153 in 4-n-pentyl-4'-cyanobiphenyl and 4,4-n-heptyl-cyanopiphenyl solvents as a function of temperature in both the nematic and isotropic phases.     

On the conformational effects of intermolecular interactions in nematics

Abstract

A perturbation expansion of the pair correlation function is used to derive the molecular field self-consistency equations for non-rigid molecules. The order parameters and the thermodynamic functions are expressed directly in terms of the segmental interaction coupling constants. The values of these constants for the 4-n-alkyl-4'-cyanobiphenyis (NCB) are determined by analysing the orientational order parameters observed by N.M.R. in the nematic phase; they are in reasonable agreement with values obtained from calculations of the nematic-isotropic transition temperatures. It is found that contributions of the isotropic intermolecular interactions to the conformational energy of the alkyl chain are comparable in magnitude to the direct intramolecular contributions.     

Viscoelastic coefficients of glass-forming nematics

In this paper, we report measurements of the viscoelastic properties of nematic liquid crystals which exhibit a glass transition in the nematic phase. We have studied the Freedericksz transition in planar cells with a magneto-optical method. K₁ was determined from the critical field, and the rotational viscosity, γ₁, from the response time for the director orientation by the external field. We found a temperature dependence of γ₁ of the Vogel type, with absolute values ranging over several orders of magnitude, and K₁ values similar to those of conventional thermotropic low molar mass nematics.     

Surface energy of biaxial nematics

In this paper, a form of surface energy for biaxial nematics is derived. The methods follow those for deriving Landau elastic energy Frank elastic energy for bulk nematics. The surface energy can also be derived in rotation matrix expansion. The result shows that in the first order approximation, there are four independent coefficients in the surface energy. When each of the three orthogonal directors of biaxial nematics coincides with its corresponding easy axis, the surface energy is linearly proportional to the order parameters. An application of this surface energy is discussed and possible experimental measurements of three linear combinations of the four coefficients are explored.     

Phase diagram of Gaussian-core nematics

The authors study a simple model of a nematic liquid crystal made of parallel ellipsoidal particles interacting via a repulsive Gaussian law. After identifying the relevant solid phases of the system through a careful zero-temperature scrutiny of as many as eleven candidate crystal structures, they determine the melting temperature for various pressure values, also with the help of exact free energy calculations. Among the prominent features of this model are pressure-driven reentrant melting and the stabilization of a columnar phase for intermediate temperatures.     

Phase transitions in surface layers of polycrystalline solids

The fundamental equations of the thermodynamics of films were applied to describe the properties of polycrystalline solids, namely, first-order phase transitions of grain boundaries with the formation of two-dimensional liquids. The generalized equation obtained was used to calculate the temperature of this phase transition (the premelting temperature) for metals; it was found to lie between 0.55 and 0.86 of the temperature of fusion. The experimental nitrogen diffusion coefficient in steel over this temperature range was the same as in liquid steel.     

Dynamic characterization of surface topography of plastic films

It is well known that surface roughness plays an important role on the handling and winding of flexible polymer films such as PET which is widely used in various applications. In order to characterize the surface topography of such materials, an air layer is squeezed between a rigid smooth substrate and a film sample. For that purpose a novel experimental set-up has been built. Using an interferometric method and image processing, we have observed the evolution of the air layer thickness and measured its reduction for several configurations and squeezing pressures. It is found that the reduction of the central air layer thickness follows a linear law versus time allowing a parameter, called “dynamic roughness”, to be defined. This parameter, which characterizes the kinetics of the air layer being squeezed, represents the dynamic manifestation of the influence on the flow of more conventional “static” parameters representative of the film roughness. We have developed a theoretical model based on the hypothesis of perfectly flexible film samples and on the concept of equivalent smooth surfaces. The predictions are in good agreement with the experimental results and for each film tested the value of the characteristic parameter associated to its “dynamic roughness” is determined.