Biomedical subjects

Alejandro D Rey

Publications and source records attributed to Alejandro D Rey.

23 records · Page 2Linked to original sources

PubMed2004

Thermodynamics of soft anisotropic interfaces.

The Gibbs-Duhem equation for interfaces between nematic liquid crystals and isotropic fluids is formulated and shown to be a generic equation for soft anisotropic surfaces. The one-to-one correspondence between the nematic and crystalline surface Gibbs-Duhem equations is established. Consistency between the surface Gibbs-Duhem equation and the classical equations of interfacial nematostatics is shown. Using a phase space that takes into account thermodynamics, liquid crystalline order, and geometric variables, the generalized nematic surface Gibbs-Duhem equation reveals the presence of couplings between shape, adsorption, temperature, and average molecular orientation. Merging the thermodynamic analysis with nematostatics results in a model for morphactancy, that is, adsorption-induced interfacial shape selection. The specific roles of gradient bulk Frank elasticity, interfacial tension, and anchoring energy are elucidated by analyzing particular paths in the thermodynamic-geometric phase space.

Journal Article↗

PubMed2003

Shear-induced textural transitions in flow-aligning liquid crystal polymers.

The equations of nematodynamics are formulated, solved, and used to model textural transformations in sheared thermotropic flow-aligning nematic polymers. The solutions are classified and characterized using analytical, scaling, and numerical methods. It is found that as the shear rate increases, the pathway between an oriented nonplanar state and an oriented planar state is through texture formation and coarsening. The two shear-rate dependent dimensionless numbers that control the texture formation and coarsening process are Ericksen Er and Deborah De numbers. The emergence of texture is independent of the Deborah number, and occurs at Er=10(4). As the shear rate increases and Er>10(4) the first texture that arises is a defect lattice. Further increases of the shear rate bring De close to 1, ignite the coarsening processes, and replace the defect lattice with a defect gas. The smallest texture length scale l(t) occurs at the defect lattice-defect gas transition. In the defect lattice regime the texture length scale decreases with increasing shear rate as l(t) proportional to (gamma-a)(-1/2), while in the defect gas regime it increases as l(t) proportional to (gamma-b sqrt[(gamma-a)]-c)(-1). Finally when De>2, an oriented monodomain state emerges, and the texture vanishes since coarsening overpowers defect nucleation. It is found that the texture transition cascade unoriented monodomain=>defect lattice=>defect gas=>oriented monodomain is remarkably consistent with the experimentally observed textural transitions of sheared lyotropic nematic polymers.

Biophysical Phenomena↗

PubMed2003

Nematostatics of triple lines.

The Landau-de Gennes model for nematic liquid crystal bulk and interfaces has been extended to nematic triple lines involving the intersection of two isotropic fluids and one nematic liquid crystalline phase. A complete set of bulk, interface, and triple line force and torque balance equations has been formulated. The triple line force and torque balance equations have linear, interfacial, and bulk contributions. The bulk contributions appear as junction integrals, the surface contributions as junctions sums, and the line contributions as gradients of stresses. Reduction of dimensionality from three to one dimensional creates the following effects: (a) bulk terms enter interfacial balances as surface jumps and line balances as junction integrals, and (b) surface terms enter linear balances as junction sums. Line stress and torque equations are derived using classical liquid crystal models. The correspondence between line stress and line torque and their surface and bulk analogs is established. The triple line force and torque balance equations are use to analyze the contact angle in a nematic lens lying at the interface between two isotropic fluids, when the preferred surface orientation is tangential. The effect of anisotropy and long range elasticity on triple line phases is established. Under weak anchoring the contact angle is shown to be a function of the anchoring energy at the nematic-isotropic interface, while under strong anchoring conditions the contact angle is a function of the Peach-Koehler force that originates from bulk long range elasticity and acts on the triple line. The use of the complete set of balance equations removes the classical inconsistency in force balances at a contact line by properly taking into account long range (bulk gradient elasticity) and anisotropic (interfacial anchoring elasticity) effects.

Journal Article↗

PubMed2002

Cahn-Hoffman capillarity vector thermodynamics for liquid crystal interfaces.

The classical Cahn-Hoffman capillarity vector formalism for anisotropic interfaces, widely used to analyze capillary and surface patterning processes in metallurgical systems, is applied to nematic liquid crystalline interfaces. The nematic capillarity vector is derived and expressed in terms of nematic surface energies. Expressions for surface tension forces on surface line elements are derived and shown to include the usual tangential forces as well as normal forces driven by surface tension anisotropy. The connections between interfacial rotational effects, surface tension anisotropy, and bending stresses are established. The vector formalism is shown to be a tractable and simple method to analyze capillarity processes in nematic liquid crystals. The application of the formalism to a straight nematic triple line shows that the interface configuration should be such that the projection of the sum of the three capillarity vectors on a plane normal to the contact line vanishes.

Journal Article↗

PubMed2002

Generalized cholesteric permeation flows.

The permeation flow equations of cholesteric liquid crystals are derived using a decoupled formulation of the Leslie-Ericksen equations. The formulation sheds light on the role of Ericksen elastic stresses in permeation flows. The Darcy flow regime is shown to emerge in the absence of velocity gradients. The permeation flow equations are generalized to gravity driven flow and used to analyze a free-boundary film flow over an inclined plane.

Journal Article↗