Biomedical subjects

Alejandro D Rey

Publications and source records attributed to Alejandro D Rey.

At least 19 recordsLinked to original sources

PubMed2006

Optical modeling of liquid crystal biosensors.

Optical simulations of a liquid crystal biosensor device are performed using an integrated optical/textural model based on the equations of nematodynamics and two optical methods: the Berreman optical matrix method [J. Opt. Soc. Am. 62, 502 (1972)] and the discretization of the Maxwell equations based on the finite difference time domain (FDTD) method. Testing the two optical methods with liquid crystal films of different degrees of orientational heterogeneities demonstrates that only the FDTD method is suitable to model this device. Basic substrate-induced texturing process due to protein adsorption gives rise to an orientation correlation function that is nearly linear with the transmitted light intensity, providing a basis to calibrate the device. The sensitivity of transmitted light to film thickness, protein surface coverage, and wavelength is established. A crossover incident light wavelength close to lambda(co) approximately 500 nm is found, such that when lambda>lambda(co) thinner films are more sensitive to the amount of protein surface coverage, while for lambda<lambda(co) the reverse holds. In addition it is found that for all wavelengths the sensitivity increases with the amount of protein coverage. The integrated device model based on FDTD optical simulations in conjunction with the Landau-de Gennes nematodynamics model provides a rational basis for further progress in liquid crystal biosensor devices.

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PubMed2006

Microscopic observations and simulations of Bloch walls in nematic thin films.

We study Bloch wall defects formed by quenching nematic thin films from planar anchoring to homeotropic anchoring through a temperature-driven anchoring transition. The director profiles of the walls are directly visualized using fluorescence confocal polarizing microscopy, and shown to agree well with the simulation based on the Frank elasticity theory. A pure twist wall exists if the ratio of sample thickness to surface extrapolation length p is smaller than or close to 1; while a diffuse Bloch wall is obtained if p is much greater than 1.

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PubMed2006

Polar fluid model of viscoelastic membranes and interfaces.

The polar surface fluid model is used to derive the generalized dynamic shape equation and the interfacial rheological material functions for viscoelastic membranes and curved interfaces, taking viscous bending and torsion modes into full account. The materials modeling approach based on the polar surface fluid leads to the integration of bending and torsion dissipative modes with their elastic counterparts that appear in the dynamic shape equation and in the interfacial rheological functions. The covariant bending and torsion rates derived in this paper are shown to be related to the interfacial co-rotational derivative of the curvature tensor. The dynamic shape equation is used to analyze shape fluctuation in planar geometries, and to establish the role of bending dissipation in shape dynamics. The dynamic shape equation generalizes the static Helfrich shape equation by incorporating bending and torsion dissipation, and it generalizes the dynamic shape equation based on the Boussinesq-Scriven model by incorporating bending and torsion elasticity and dissipation.

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PubMed2006

Liquid crystal model of membrane flexoelectricity.

An interfacial liquid crystal model is formulated and used to derive a membrane shape equation that takes into account pressure, tension, bending, torsion, and flexoelectric forces. Flexoelectricity introduces electric field-induced curvature and is of relevance to the study and characterization of biological membranes. It is shown that flexoelectricity renormalizes the membrane mechanical tension, shear, and bending effects, and hence it offers diverse pathways to manipulate the membrane's shape. The derived electroelastic shape equation provides systematic guidance on how to use electric fields in membrane studies.

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PubMed2006

Interfacial nematodynamics of heterogeneous curved isotropic-nematic moving fronts.

The early stages of liquid crystal phase ordering upon thermal quenches of isotropic phases into unstable and metastable temperature ranges is studied using two-dimensional (2D) computational solutions of the governing Landau-de Gennes (L-dG) equations for low molar mass nematic liquid crystals and analysis based on the corresponding interfacial nematodynamic model. The early phase ordering stage, for both unstable and metastable quenches of the isotropic phase, is shown to lead to highly textured nematic spherulites through a mechanism of interfacial defect nucleation. The underlying mechanisms of interface-driven texturing are elucidated using complementary 2D computational parametric studies of the bulk L-dG equation and analysis of the IN model. It is shown that for highly curved nanodomains and realistic elastic anisotropy, sharp interfacial transitions between uniaxial and biaxial states arise and are resolved by interfacial defect nucleation, which upon subsequent migration into the spherulite's interior leads to strong texturing. This paper shows that texture formation in the early stages of phase ordering is interface driven, and due to low interface tension, elastic anisotropy, and large curvature. Interfacial defect shedding in highly curved, low tension, anisotropic interfaces is a significant defect nucleation mechanism that needs to be taken into account when considering texturing processes.

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PubMed2006

Dynamic interactions between nematic point defects in the spinning extrusion duct of spiders.

Spider silk fibers have remarkable mechanical properties as a result of an ultraoptimized spinning process. Silk fibers are spun from a lyotropic nematic liquid crystalline anisotropic fluid phase which undergoes significant structural changes throughout the spinning pathway. In the silk extrusion duct, those structural changes are expected to be driven by elastic-mediated interactions between point defects. In this work, the interaction between two point defects of opposite topological charges located on the axis of a cylindrical cavity is studied using a tensor order parameter formalism. Distinct regimes leading to defect annihilation and structural transitions are described in detail. The driving force setting the defects into motion is also examined. The different results suggest that the tensorial approach is primordial in describing the complicated physics of the problem. The phenomenon described is important to the understanding of the process-induced structuring of silk fibers and to defect physics in a more general context.

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PubMed2006

Anisotropic fluctuation model for surfactant-laden liquid-liquid crystal interfaces.

A model for thermal fluctuations on surfactant-laden liquid-liquid crystal interfaces is formulated and used to derive the expression of the mean square displacement as a function of four elastic moduli of the interface. The measurable liquid crystal contributions to thermal roughness include the average molecular orientation, the interfacial anchoring modulus, and the bulk elasticity modulus. Surfactant-driven interfacial orientation transitions provide an additional means to extract interfacial elastic moduli parameters in surfactant-laden liquid-liquid crystal interfaces in conjunction with thermal roughness measurements.

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PubMed2006

Mechanical model for anisotropic curved interfaces with applications to surfactant-laden liquid-liquid crystal interfaces.

A mechanical model for anisotropic curved interfaces, applicable to thermodynamically closed surfactant-laden liquid-liquid crystal interfaces is developed. The model takes into account the mechanical effects due to surface bending and surface tilting (anchoring) and incorporates liquid crystal anisotropy into classical fluid membrane mechanics. In the absence of the aligned liquid crystal, the model converges to the fluid membrane mechanical model, and in the absence of surfactant, it converges to the nematic interface mechanical model. Use of the well-known Helfrich-Rapini-Papoular surface energies leads to the Laplace equation for anisotropic curved interfaces, whose material limits are the vesicle shape equation and the liquid crystal Herring equation. Applications of the model to shape selection in liquid drops embedded in aligned nematic liquid crystals illustrates the competition between anchoring and bending and shows how anisotropic surface tension distorts the droplet and how bending tends to restore the spherical shape. This theory presented in this article shows that the interaction of interfacial anchoring and bending creates new regimes in classical fluid membrane mechanics.

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PubMed2006

Computational studies of optical textures of twist disclination loops in liquid-crystal films by using the finite-difference time-domain method.

Optical images of textured liquid-crystal films containing various types of twist disclination loops are computed using an approximate matrix method and a direct numerical simulation based on the finite-difference time-domain (FDTD) method. The selected defects introduce large multidirectional spatial gradients in the optic axis, mimicking the orientation textures that arise in the construction and use of biosensors based on liquid-crystal vision. It is shown that under these experimentally relevant conditions, the matrix method fails to capture important signatures in the transmitted light intensity under crossed polarizers. The differences between the predictions by the two methods are analyzed with respect to gradients in the optic axis. We show that the FDTD method is a useful tool to perform computational optics of textured liquid-crystal films.

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PubMed2005

Texture rules for concentrated filled nematics.

Defect textures in concentrated fiber-filled polygonal networks in nematic liquid crystals are analyzed using differential geometry and computational modeling based on Landau--de Gennes theory. Micron fibers exhibit singular cores of strength -1/2 for odd polygons and escaped cores of strength -(N-2)/2 for even polygons (N: number of sides), in agreement with experiments while simulations predict singular cores of strength -1/2 in submicron fibers. The computed textures satisfy physical and topological stability rules, and the total charge inside each polygon obeys the Poincaré-Brouwer theorem.

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PubMed2005

Mechanics of soft-solid-liquid-crystal interfaces.

The interfacial mechanics of soft elastic solids and nematic liquid crystals is presented. The theory can be applied to interfaces involving gels, elastomers, biomaterials, and thermotropic nematic liquid crystals. A model of anisotropic elastic interfaces is formulated and used to derive two fundamental capillary quantities: (i) interfacial torques on the nematic orientation, and (ii) capillary pressure. The couplings between soft-solid deformation and liquid-crystal anisotropic interfacial tension is shown to lead to strain-induced anchoring transitions, and strain-induced morphological instabilities.

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PubMed2005

Computational modeling of the propagation of light through liquid crystals containing twist disclinations based on the finite-difference time-domain method.

The finite-difference time-domain (FDTD) method is used to compute propagation of light through textured uniaxial nematic-liquid crystal (NLC) films containing various types of twist disclination (defect) lines. Computational modeling by the FDTD method provides an accurate prediction of the optical response in multidimensional and multiscale heterogeneities in NLC films in which significant spatial optic axis gradients are present. The computations based on the FDTD method are compared with those of the classic Berreman matrix-type method. As expected, significant deviations between predictions from the two methods are observed near the twist disclination line defects because lateral optic axis gradients are ignored in the matrix Berreman method. It is shown that the failure of Berreman's method to take into account lateral optic axis gradient effects leads to significant deviations in optical output. In addition, it is shown that the FDTD method is able to distinguish clearly different types of twist disclination lines. The FDTD optical simulation method can be used for understanding fundamental relationships between optical response and complex NLC defect textures in new liquid-crystal applications including liquid-crystal-based biosensors and rheo-optical characterization of flowing liquid crystals.

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PubMed2005

Optical and structural modeling of disclination lattices in carbonaceous mesophases.

An integrated microstructural and optical model for carbonaceous mesophases is developed and used to explain the principles that govern the formation and stability of experimentally observed disclination lattices. The model is able to capture the orientation features of disclination lattices, including the type and location of disclination lines, and the orientation field in the mesophase matrix. The optical model based on reflection polarized optical microscopy is able to replicate all the details observed in actual observations. The typical brush figures have the proper distribution, orientation, and intensity. The computational predictions offer science-based routes to create and control desirable material architectures based on carbonaceous mesophase-carbon fiber composites.

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PubMed2004

Thermodynamic model of surfactant adsorption on soft liquid crystal interfaces.

The Gibbs adsorption isotherm for planar liquid crystal/fluid interfaces is derived using the anisotropic Gibbs-Duhem equation. The Gibbs adsorption isotherm for planar interfaces is used to analyze the adsorption-driven orientation transition in aqueous solutions of anionic surfactants in contact with rodlike uniaxial nematic liquid crystal films. In qualitative agreement with experiments, the model predicts that, as the surfactant concentration increases, the tangential (planar) average molecular orientation of the liquid crystal with respect to the interface undergoes a transition to a normal (homeotropic) orientation. The anchoring coefficient or strength of anisotropic component of the interfacial tension is shown to depend on the surfactant's concentration. Analyzing the response to addition of a co-cation, the model reveals that, as the fractional coverage of the surfactant's chains increases, the interpenetration of liquid crystal molecules between the adsorbed surfactant tails promotes the orientation transition; at even higher surfactant chain concentrations, interpenetration is hindered because of lack of available space and a random surface orientation emerges. Thus, for aqueous surfactant solutions in contact with nematic liquid crystals, increasing the surfactant concentration leads to the following interfacial liquid crystal orientation transition cascade, planar orientation --> homeotropic orientation --> random orientation, which can lead to new sensor capabilities and surface structuring processes.

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PubMed2004

Texture formation under phase ordering and phase separation in polymer-liquid crystal mixtures.

Computational modeling of texture formation in coupled phase separation-phase ordering processes in polymer/liquid crystal mixtures is performed using a unified model based on the nematic tensor order parameter and gradient orientation elasticity. The computational methods are able to resolve defect nucleation, defect-defect interactions, and defect-particle interactions, as well as global and local morphological features in the concentration and order parameter spatiotemporal behavior. Biphasic structures corresponding to polymer dispersed liquid crystals (PDLCs), crystalline filled nematic (CFNs), and random filled nematics (RFNs) are captured and analyzed using liquid crystal defect physics and structure factors. Under spinodal decomposition due to concentration fluctuations, the PDLC structure emerges, and the nucleation and repulsive interaction of defects within nematic droplets leads to bipolar nematic droplets. Under spinodal decomposition due to ordering fluctuations, the CFNs structure emerges, and the stable polymer droplet crystal is pinned by a lattice of topological defects. For intermediate cases, where the mixture is unstable to both concentration and nematic order fluctuations, the RFN structure emerges, and polymer droplets and fibrils are pinned by a defect network, whose density increases with the curvature of the polymer-liquid crystal interface. The simulations provide an information of the role of topological defects on phase separation-phase ordering processes in polymer-liquid crystal mixtures.

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PubMed2004

Thermodynamics of soft anisotropic contact lines.

Contact lines arising from the intersection of interfaces between liquids and nematic liquid crystals are representative models of soft anisotropic contact lines. This paper presents the thermodynamics of soft anisotropic contact lines and the derivation of the one dimensional (1D) Gibbs-Duhem adsorption equation. Consistency between the 1D Gibbs-Duhem equation and the classical equations of lineal nematostatics is shown. Using a phase space that takes into account thermodynamics, liquid crystalline order, and geometric variables, the generalized nematic line Gibbs-Duhem equation reveals the presence of couplings between curvature, torsion, adsorption, temperature, and average molecular orientation. Merging the thermodynamic analysis with nematostatics results in a model for contact line shape and orientation selection. The ability of an adsorbed solute to orient the director and to bend and twist the contact line is predicted. The thermodynamic origin of preferred orientation at a straight contact line is established.

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PubMed2004

Line tension vector thermodynamics of anisotropic contact lines.

Multiphase materials with intersecting diving surfaces give rise to contact lines. A line tension vector thermodynamics formalism is developed and used to analyze contact line problems in the presence of anisotropy, taking into account two elastic modes: change in contact line length and change in contact line orientation. Using this formalism, the contact line-shape equation is derived, and the renormalization of the line tension due to anisotropy is characterized. The correspondence and analogies between the shape equation for anisotropic surfaces (Herring equation) and the shape equation for contact lines is established. Line energies for nematic liquid crystals, representative of generic anisotropic contact lines, are used to derive a shape equation that takes into account ambient orientation effects. It is found that anisotropic line tension may promote bending and chiral modes to avoid unfavorable orientations of the contact line with respect to the ambient nematic ordering.

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PubMed2004

Chiral front propagation in liquid-crystalline materials: Formation of the planar monodomain twisted plywood architecture of biological fibrous composites.

Biological fibrous composites commonly exhibit an architecture known as twisted plywood, which is similar to that of the cholesteric liquid-crystalline mesophases. The explanation for the structural similarity is that biological fibrous composites adopt a lyotropic cholesteric liquid-crystalline phase during their formation process. In this work, a mathematical model based on the Landau-de Gennes theory of liquid crystals has been developed to reproduce the process by which long chiral fibrous molecules form the twisted plywood structures observed in biological composites. The dynamics of the process was then further investigated by analytically solving a simplified version of the governing equations. Results obtained from the model are in good qualitative agreement with the theory of Neville [Biology of Fibrous Composites (Cambridge University Press, Cambridge, England, 1993)] who hypothesized the necessity of a constraining layer to lock the direction of the helical axis of the plywood in order to create a monodomain structure. Computational results indicate that the plywood architecture is obtained by a chiral front propagation process with a fully relaxed wake. The effects of chirality and concentration on the formation process kinetics are characterized.

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