Biomedical subjects

H R Brand

Publications and source records attributed to H R Brand.

13 recordsLinked to original sources

PubMed2006

Splay-bend textures involving tetrahedratic order.

We show that defect-free splay-bend textures are less energetic compared to uniform states in liquid-crystalline phases that possess both quadrupolar (nematic) and octupolar (tetrahedratic) order. This is because, in such systems, there is a symmetry-allowed linear gradient term in the energy. Another unusual feature of these splay-bend textures is the fact that they have a non-homogeneous, space-dependent free-energy density. These results may help clarify some mysterious features noted for the B7 liquid-crystal phase formed by achiral banana-shaped molecules.

Journal Article↗

PubMed2005

A phenomenological theory of the isotropic to chiral smectic-C phase transition.

In this paper we discuss the direct isotropic to chiral smectic-C phase transition on the basis of a phenomenological theory. The model free energy is written in terms of the coupled order parameters including the spontaneous polarization. We present a detailed analysis of the different phases that can occur and analyze the question under which conditions a direct isotropic to chiral smectic-C phase transition is possible when compared to other phase transitions. On the basis of this model the isotropic-smectic-C* transition is always of first order. The theoretical predictions are compared with the available experimental results.

Journal Article↗

PubMed2005

Undulation versus Frederiks instability in nematic elastomers in an external electric field.

We study the behaviour of a nematic side-chain elastomer under the influence of an external static electric field for a specific geometry. For this investigation, the nematic elastomer is considered to be a perfect insulator. On the basis of a macroscopic description we generalize the classical Frederiks transition in a low-molecular-weight (LMW) nematic liquid crystal to the elastomeric case. We predict, using a linear stability analysis, that the onset of the instability can be qualitatively different from the LMW case: in liquid crystalline elastomers an undulation instability can arise at onset. Whether the analogue of a Frederiks instability or an undulation instability occurs first depends on the sample thickness as well as on the material parameters. It turns out that the parameter which describes the coupling between the deformations of the elastomer and the relative rotations between the elastomer and the director field of the nematic phase is most important for the predicted response of the system. Furthermore, we find that the magnitude of the critical electric field is much higher in the elastomeric than in the low-molecular-weight case.

Journal Article↗

PubMed2004

Macroscopic dynamics of uniaxial magnetic gels.

We present the derivation of the macroscopic equations for uniaxial ferrogels. In addition to the usual hydrodynamic variables for gels we introduce the magnetization and the relative rotations between the magnetization and the network as macroscopic variables. The relative rotations introduced here for a system with magnetic degrees of freedom are the analog of the relative rotations introduced by de Gennes in nematic elastomers for rotations of the director with respect to the elastomeric network. These variables give rise to a large number of static as well as dynamic effects due to their coupling to the magnetization, the strain field, and the density of linear momentum. A few of them are discussed for specific geometries, for example, the case of a shear-induced magnetization perpendicular to the preferred direction.

Journal Article↗

PubMed2004

Mechanical properties of mono-domain side chain nematic elastomers.

We investigate the behavior of the shear rigidity modulus G = G' + iG" of three mono-domain side chain liquid-crystalline elastomers composed of side chain polysiloxanes cross-linked by either flexible or rigid cross-linkers. The measurements were taken in a frequency domain ranging from approximately 0.02 Hz to approximately 10(4) Hz applying the shear in a plane perpendicular to or containing the director. The measurements as a function of temperature show an anisotropy of G' which appears around T(NI), when decreasing the temperature, and which is due to the expected lowering of G'(//) coming from the coupling between the shear and the director. The measurements as a function of frequency show that G has two components for both geometries, in both the isotropic phase and in the nematic phase around the phase transition. One reflects the network behavior in its hydrodynamic regime ( G' is constant and G'' is approximately f, where f is the frequency), the other which appears at higher frequencies is characterized by a scaling law behavior (G' is approximately G" is approximately f(0.5)) of the Rouse type. We discuss the results in the framework of available theories and show that the three elastomers present a non-soft behavior, even for the elastomer for which the contrary was claimed, and that there is no separation of time scales between the director and the network. We also present data on a poly-domain sample and a non-mesomorphic one which complement these results.

Anisotropy↗

PubMed2003

Hydrodynamics of isotropic ferrogels.

We derive the complete set of macroscopic dynamic equations for ferrogels under an external magnetic field, including the magnetization as an independent dynamic degree of freedom. The magnetoelasticity comes in the form of magnetostriction and through the magnetic part of the Maxwell stress. Various dynamic couplings of the elastic degree of freedom with the magnetization and the magnetic field are found. We discuss static elongation, shear deformations, and the modified sound spectrum in the presence of an external magnetic field.

Journal Article↗

PubMed2003

Deformable tetrahedratic phases: the effects of external fields and flows.

We discuss changes in the symmetry and physical properties of an isotropic phase which has initially tetrahedral symmetry characterized by four unit vectors. In its undeformed state, these four vectors are at the tetrahedral angle (109.47 degrees) to each other. We find that this optically isotropic phase becomes uniaxial under the influence of an external electric field, E, resulting in a phase with C3v symmetry. For an applied simple shear flow, the system becomes biaxial and a time-dependent state with C1 symmetry arises. We discuss to what extent deformations induced by external forces and flows on this optically isotropic phase, which we call a "deformable tetrahedratic phase", are consistent with observations at the isotropic-B7 transition found recently in compounds composed of banana-shaped molecules and suggest a number of experiments to test the conclusions of this model.

Biophysics↗

PubMed2001

Macroscopic dynamics near the isotropic-smectic-A phase transition.

The hydrodynamic theory for the smectic-A phase and the isotropic phase is generalized to the macroscopic dynamics in the vicinity of the isotropic-smectic-A phase transition. The macroscopic dynamic equations are presented on the isotropic side as well as on the smectic-A side of the phase transition, incorporating the effect of an external electric field. Specific experiments to test some of the effects contained in the macroscopic dynamic equations are suggested.

Journal Article↗

PubMed2001