Biomedical subjects

Makoto Yoneya

Publications and source records attributed to Makoto Yoneya.

10 recordsLinked to original sources

PubMed2006

Numerical treatment of the dynamics of a conserved order parameter in the presence of walls.

We discuss how the diffusive dynamics of a conserved order parameter should be numerically treated when impenetrable wall surfaces are present and interact with the degrees of freedom characterized by the order parameter. We derive the discretization scheme for the dynamics, paying particular attention to the conservation of the order parameter in the strict numerical sense. The discretized chemical potential, or the functional derivative of the free energy, contains a surface contribution inversely proportional to the grid spacing Delta z, which was proposed heuristically in a recent paper of Henderson and Clarke [Macromol. Theory Simul. 14, 435 (2005)]. Although apparently that surface contribution diverges in the continuum limit Delta z --> 0, we can show, by an analytic argument and numerical calculations, that this divergence does not yield any anomalies, and that our discretization scheme is well defined in this limit. We also discuss the correspondence of our treatment to the model proposed by Puri and Binder [Phys. Rev. A 46, R4487 (1992)] extensively used for the present problem.

Journal Article↗

PubMed2006

Constant surface-tension molecular-dynamics simulation methods for anisotropic systems.

We propose a simulation method for liquid-liquid interface under constant surface tension and constant normal pressure. The method introduces an anisotropic factor in the cell dynamics which avoids artifacts such as continuous expansion or contraction of the cell lengths. This allows simulation of a full range of surface tensions including when the value is 0, i. e, hydrostatic pressure.

Journal Article↗

PubMed2004

Numerical investigation of liquid crystal colloids using a continuum description.

We investigate numerically the configuration of a nematic liquid crystal around two spherical particles. For the description of the orientational order of a nematic liquid crystal, we adopt a Landau-de Gennes continuum theory in terms of a second-rank tensor order parameter Q(ij) together with the use of bispherical coordinates to describe the geometry of the system with two spherical particles. Above but close to the nematic-isotropic transition point, we observe capillary condensation of a nematic liquid crystal between the two particles under appropriate conditions. Below the transition point where liquid crystals possess nematic order, a point-like defect called a hyperbolic hedgehog appears close to a particle when strong normal anchoring is imposed. With the aid of an adaptive mesh refinement scheme to achieve sufficient numerical resolution to describe topological defects, we present our numerical results showing how the orientation profile of a nematic liquid crystal is distorted when the distance between two particles is small enough.

Crystallization↗

PubMed2004

Spontaneous three-dimensional nanostructure formation of perfluoroalkyl terminated liquid crystal: a molecular dynamics simulation study.

Structure formation of a perfluoroalkyl terminated liquid crystal molecule was studied by molecular dynamics simulations. Two distinct structures with smectic-C-like layers and with bundles (blocks) of collapsed layers were spontaneously formed depending on the simulation temperatures. The bundles in the latter structure were somewhat positionally ordered (with respect to the small angle spots in its structure function) and orientationally isotropic overall even though the molecules making each bundle were well oriented. These characteristics of the simulated system well correspond to the cubic phase of the real system, and an even more precisely correspond to the proposed cubic structure model with respect to its hierarchical structure.

Journal Article↗

PubMed2004

Interaction between two spherical particles in a nematic liquid crystal.

We numerically investigate the interaction between two spherical particles in a nematic liquid crystal mediated by elastic distortions in the orientational order. We pay attention to the cases where two particles with equal radii R0 impose rigid normal anchoring on their surfaces and carry a pointlike topological defect referred to as a hyperbolic hedgehog. To describe the geometry of our system, we use bispherical coordinates, which prove useful in the implementation of boundary conditions at the particle surfaces and at infinity. We adopt the Landau-de Gennes continuum theory in terms of a second-rank tensor order parameter Q(ij) for the description of the orientational order of a nematic liquid crystal. We also utilize an adaptive mesh refinement scheme that has proven to be an efficient way of dealing with topological defects whose core size is much smaller than the particle size. When the two "dipoles," composed of a particle and a hyperbolic hedgehog, are in parallel directions, the two-particle interaction potential is attractive for large interparticle distances D and proportional to D-3 as expected from the form of the dipole-dipole interaction, until the well-defined potential minimum at D approximately 2.46 R0 is reached. For the antiparallel configuration with no hedgehogs between the two particles, the interaction potential is repulsive and behaves as D-2 for D less than or approximately equal 10R0, which is stronger than the dipole-dipole repulsion (approximately D-3 ) expected theoretically as an asymptotic behavior for large D.

Journal Article↗

PubMed2004

Molecular dynamic simulation methods for anisotropic liquids.

Methods of molecular dynamics simulations for anisotropic molecules are presented. The new methods, with an anisotropic factor in the cell dynamics, dramatically reduce the artifacts related to cell shapes and overcome the difficulties of simulating anisotropic molecules under constant hydrostatic pressure or constant volume. The methods are especially effective for anisotropic liquids, such as smectic liquid crystals and membranes, of which the stacks of layers are compressible (elastic in direction perpendicular to the layers) while the layer itself is liquid and only elastic under uniform compressive force. The methods can also be used for crystals and isotropic liquids as well.

Journal Article↗

PubMed2004

Searching for nanostructures of the cubic mesophase of liquid crystal molecules, BABH8.

Nanostructures of a thermotropic cubic phase forming liquid crystal compound, 1,2-bis-[4-n-octyloxy-benzoyl]-hydrazine was studied by molecular dynamics (MD) simulations. A model for its cubic phase structure was proposed, which was constructed from the building unit of a locally orientational ordered "bundle" consistent with the cell parameter and the space group (Ia3d) from the recent x-ray results. Stability of the model structure was studied by multinanosecond MD simulations. A periodic nanostructure with 2.6 nm periodicity [coincides with the Ia3d (211) reflection] was kept up to 60 ns in the reduced pressure simulation which realizes the experimental value of density. However, the calculated fourth-rank order parameter shows that the simulated final state above does not have cubic orientational symmetry but rather isotropic symmetry.

Journal Article↗

PubMed2004

Nematic liquid crystal around a spherical particle: Investigation of the defect structure and its stability using adaptive mesh refinement.

We investigate the orientation profile and the structure of topological defects of a nematic liquid crystal around a spherical particle using an adaptive mesh refinement scheme developed by us previously. The previous work [J. Fukuda et al., Phys. Rev. E 65, 041709 (2002)] was devoted to the investigation of the fine structure of a hyperbolic hedgehog defect that the particle accompanies and in this paper we present the equilibrium profile of the Saturn ring configuration. The radius of the Saturn ring r(d) in units of the particle radius R(0) increases weakly with the increase of Epsilon, the ratio of the nematic coherence length to R(0). Next we discuss the energetic stability of a hedgehog and a Saturn ring. The use of adaptive mesh refinement scheme together with a tensor orientational order parameter Q (alpha, beta) allows us to calculate the elastic energy of a nematic liquid crystal without any assumption of the structure and the energy of the defect core as in the previous similar studies. The reduced free energy of a nematic liquid crystal, F= F/L1RO, with L(1) being the elastic constant, is almost independent of Epsilon in the hedgehog configuration, while it shows a logarithmic dependence in the Saturn ring configuration. This result clearly indicates that the energetic stability of a hedgehog to a Saturn ring for a large particle is definitely attributed to the large defect energy of the Saturn ring with a large radius.

Colloids↗

PubMed2002

Tristable nematic liquid-crystal device using micropatterned surface alignment.

It has long been appreciated that liquid-crystal (LC) devices in which the LC molecules adopt multiple stable orientations could drastically reduce the power consumption required for high-information-content displays. But for the commonly used nematic LCs, which are intrinsically uniaxial in symmetry, no industrially feasible multi-stable LC device has been realized. Recently we demonstrated how bistability can be robustly engineered into a nematic LC device, by patterning a substrate with an orientational chequerboard pattern that enforces orthogonal LC alignment in neighbouring square domains. As a result of the four-fold symmetry of the pattern, the two diagonal axes of the chequerboard become equally stable macroscopic orientations. Here we extend this symmetry approach to obtain a tristable surface-aligned nematic LC. A microscopic pattern exhibiting six-fold symmetry is inscribed on a polyimide surface using the stylus of an atomic force microscope. The hexagonal symmetry of the microscopic orientational domains in turn gives rise to three stable macroscopic LC orientations, which are mutually switchable by an in-plane electric field. The resulting switching mode is surface driven, and hence should be compatible with demanding flexible display applications.

Journal Article↗

PubMed2002

Defect structure of a nematic liquid crystal around a spherical particle: adaptive mesh refinement approach.

We investigate numerically the structure of topological defects close to a spherical particle immersed in a uniformly aligned nematic liquid crystal. To this end we have implemented an adaptive mesh refinement scheme in an axi-symmetric three-dimensional system, which makes it feasible to take into account properly the large length scale difference between the particle and the topological defects. The adaptive mesh refinement scheme proves to be quite efficient and useful in the investigation of not only the macroscopic properties such as the defect position but also the fine structure of defects. It can be shown that a hyperbolic hedgehog that accompanies a particle with strong homeotropic anchoring takes the structure of a ring.

Journal Article↗