Biomedical subjects

Mitsumasa Iwamoto

Publications and source records attributed to Mitsumasa Iwamoto.

18 recordsLinked to original sources

PubMed2006

Shape and stability of two-dimensional lipid domains with dipole-dipole interactions.

We study the general energy and shape of the two-dimensional solid monolayer domains with the dipole-dipole interactions. Compared with the domain energy without tilted dipole moments [M. Iwamoto and Z. C. Ou-Yang, Phys. Rev. Lett. 93, 206101 (2004)], the general dipolar energy is not only shape and size but also boundary orientation dependent. The general shape equation derived by this energy using variational approach predicts a circular solution and an equilibrium shape grown from this circle. In particular, the latter is composed of two branches: a translation-induced growth of all odd harmonic modes and a pressure-induced cooperative deformation by all even harmonic modes. The good qualitative agreement between our prediction and the experimental observations shows the validity of the present theory.

Journal Article↗

PubMed2006

Compression induced chiral symmetry breaking of monolayers comprised of banana-shaped achiral molecules at an air-water interface: Williams-Bragg approach.

Chirality of monolayers comprised of banana-shaped achiral molecules at an air-water interface was investigated theoretically, and a forming mechanism of chiral structure as an assembly of achiral molecules was argued. A model of such monolayers was constructed taking into account the short-range repulsive interaction between constituent banana-shaped achiral molecules, and the free energy density functional of the model was derived as a generalization of Williams-Bragg approach. It was predicted that chiral symmetry breaking occurs by monolayer compression, where two-dimensional characteristics of monolayers at an interface plays an important role in the formation of chiral structure by banana-shaped achiral molecules.

Journal Article↗

PubMed2006

Surface freezing in normal alkanes: a statistical physics approach.

The present paper aims to understand the surface freezing occurring on the interface between liquid normal alkane and air. After proposing a simple microscopic model, it reveals that the model can describe the surface freezing of normal alkanes. Subsequently, surface freezing is immediately proved to be a first order phase transition, which has been illustrated by numerous experiments. Moreover, our calculation predicts a new first order phase transition on the interface. These two transitions correspond to the liquid to monolayer and monolayer to perfect solid transitions, respectively. A phase diagram is obtained directly from the calculations as well. The model indicates that both van der Waals interaction and the entropy influenced by the surface are essential for explaining the surface phase transition.

Journal Article↗

PubMed2006

Dynamic disorder in receptor-ligand forced dissociation experiments.

Recently experiments showed that some biological noncovalent bonds increase their lifetimes when they are stretched by an external force, and their lifetimes will decrease when the force increases further. Several specific quantitative models have been proposed to explain the intriguing transitions from the "catch bond" to the "slip bond." In this work we propose that the dynamic disorder of the force-dependent dissociation rate can account for the counterintuitive behaviors of the bonds. A Gaussian stochastic rate model is used to quantitatively describe the transitions observed recently in the single bond P-selctin glycoprotein ligand 1-P-selectin force rupture experiment [Marshall, Nature 423, 190 (2003)]. Our model agrees well with the experimental data. We conclude that the catch bonds could arise from the stronger positive correlation between the height of the intrinsic energy barrier and the distance from the bound state to the barrier; classical pathway scenario or a priori catch bond assumption is not essential.

Biophysics↗

PubMed2006

Monolayer alignment on azobenzene surfaces during UV light irradiation: analysis of optical polarized absorption measurement results and theoretical treatment.

The influence of the charge separation during the trans-cis conformational change on the surface of azobenzene 6Az10PVA monolayer on the polar liquid-crystal monolayer film, such as 4-n-pentyl-4'-cyanobiphenyl(5CB), is investigated. The effective anchoring energy (in the Rapini-Papolar form) is phenomenologically described in the framework of the molecular model, which takes into account the interaction between the surface polarization and surface electric field, for number of conformational states of the boundary surface. It is shown, using the experimental data for the voltage across the 6Az10PVA+5CB film, provided by the surface-potential technique, that the charge separation during the conformational changing, caused by the UV irradiation, may lead to changing of the surface alignment of liquid-crystalline molecules. The influence of the photoisomerization process on the orientational order parameter S2(t) using the optical polarized absorption measurement is also investigated.

Journal Article↗

PubMed2005

Flow-induced molecular orientation of amphiphile monolayers: incorporation of hexatic elasticity into Ericksen-Leslie theory.

By introducing the local lattice elasticity into the pure Ericksen-Leslie (EL) theory which only considers the deformation of the director, we developed a more general theory for understanding the behaviors of hexatic liquid crystal under flow. In the cases of amphiphilic monolayers in tilted phases (L2 and L'2), the exact solutions of the new EL equation in two types of flow, pure extension and simple shear, explain well most of the features of flow-induced tilt azimuth orientation observed by Fuller's group [Science, 274, 233 (1996)] and Schwartz's group [Nature (London) 410, 348 (2001)]. In particular, the "shear band" domain generated by flow discovered by the former is proved theoretically as the result of two-dimensional Wulf construction in L2 and L'2 phases.

Journal Article↗

PubMed2005

Detection of flexoelectric effect from 4-heptyloxy-4'-cyanobiphenyl monolayers at an air-water interface by Maxwell displacement current and optical second harmonic generation.

The flexoelectric effect of 4-heptyloxy-4'-cyanobiphenyl (7OCB) monolayers at the air-water interface is studied by Maxwell displacement current (MDC) and optical second harmonic generation measurements. Though MDC was expected to increase during the compression of 7OCB monolayers in L2L2' phase from the MDC theory developed previously, decrease of MDC was detected in these phases. This abnormalous phenomenon is found to be due to the quench of flexoelectric effect by the flow orientation of monolayers.

Journal Article↗

PubMed2004

Shape deformation and circle instability in two-dimensional lipid domains by dipolar force: a shape- and size-dependent line tension model.

The dipolar energy of a solid monolayer domain surrounded by a fluid phase at an air-water interface is derived approximately as a sum of an additionally negative line tension and a curvature-elastic energy at the boundary. Variation of the domain energy yields an equilibrium domain shape equation. The obvious solutions of the domain shape equation clearly predict a circle, torus, D-form, S-form, and serpentine manner shape found experimentally, depending on the difference in the Gibbs free energy between the solid and fluid phases and the total line tension. Analysis of linear instability for a circle with a fixed area shows that, above a threshold size, the circle can be deformed into an m-sided quasipolygon. The good agreement with the observation and numerical calculation reported by Lee and McConnell [J. Phys. Chem. 91, 9532 (1993)]] shows the quantitative validity of the present theory.

Lipids↗

PubMed2004

Equilibrium and kinetics: water confined in carbon nanotubes as one-dimensional lattice gas.

We present a simple one-dimensional lattice gas model, which describes very well the equilibrium and kinetic behaviors of water confined in a thin carbon nanotube found in an atomistic molecular dynamics simulation [G. Hummer, J. C. Rasaiah, and J. P. Noworyta, Nature (London), 414, 188 (2001)]. The model parameters correspond to various physical interactions and can be calculated or estimated by using statistical mechanics. Then, the roles of all interactions in the water filling, emptying, and transporting processes are clearly understood. Our results indicate that the interaction from the water molecules outside the nanotube plays a key role in these processes and the interaction can be simply treated as an average effect of the bulk water.

Gases↗

PubMed2004

Relaxation processes in Langmuir films under lateral compression.

The orientational relaxation process of the director n to its equilibrium orientation n(eq), in the Langmuir film, during the lateral compression in absence of flow, is investigated. The relaxation time, during compression of 4-n-pentyl- 4(') -cyanobiphenyl monolayer (multilayer) films on the water surface, using the Ericksen-Leslie theory, has been calculated for a number of dynamic regimes. It is also shown that the viscous and electric forces exerted per unit volume of the monolayer Langmuir film may excite the solitary wave propagating along the air-water interface.

Journal Article↗

PubMed2004

Spectroscopic consideration of the surface potential built across phthalocyanine thin films on a metal electrode.

The nonlinear optical properties of tert-butyl phthalocyanine copper Langmuir-Blodgett (CuttbPc LB) films and vacuum-evaporated phthalocyanine copper (CuPc) films deposited on a metal surface were investigated by second-harmonic generation (SHG) spectroscopy. At the organic/metal interface, a space charge field is formed due to the presence of excess charge injected from a metal electrode to the organic layer. Since the Pc molecule has D4h symmetry, an inversion center is present and the optical SH process is not allowed under the electric-dipole approximation. However, the space charge field at the interface directly influences the symmetric structure of the electrons in the Pc molecule. We investigated the contributions of the surface potential to the SHG using Pc LB and vacuum-evaporated films deposited on aluminum (Al) and gold (Au) metal electrodes, where a distinctive difference in the spectrum for the Pc films on the Al and Au surfaces was observed. The contribution of the surface potential was revealed based on the resonant conditions of the SH process, taking into account the electric-quadrupole transition and dc-field-induced electric-dipole transition.

Journal Article↗

PubMed2004

Orientational orders of small anisotropic molecules confined in slit pores.

Based on a constant-pressure Monte Carlo molecular simulation, we have studied orientationally ordered transitions of small anisotropic molecules confined in two parallel hard walls. These molecules are modeled by the hard Gaussian overlap model. The molecular elongations of the chosen molecules are so small that the molecules cannot form stable liquid-crystal (LC) phases in the bulk. But in the slit pores, we found, while the distance between two walls of the pores decreases to the molecular scale, an orientationally ordered phase can form. It shows that even hard confining surfaces favor the alignment of the small anisotropic molecules. Thus we conclude that the required molecular elongation for forming LC phases will decrease in confinement. Our results indicate that some non-LC small molecules may form stable LC phases due to the inducement of confining surfaces.

Journal Article↗

PubMed2004

Orientational order in binary mixtures of hard Gaussian overlap molecules.

Based on a standard constant-pressure Monte Carlo molecular simulation, we have studied liquid crystal phases of binary mixtures of nonspherical molecules. The components of the mixtures are two types of hard Gaussian overlap (HGO) molecules. The first type of molecule has a small molecularelongation parameter (short HGO molecules) and cannot form stable liquid crystal phase in the bulk by themselves. The second type of molecule has a large elongation parameter (long HGO molecules) and can form a liquid crystal phase easily. In the mixtures, the short HGO molecules can form an orientationally ordered phase because the long HGO molecules form confining surfaces to induce the alignment of the short molecules. We also study the isotropic-nematic phase transition in different mixtures composed of short and long HGO molecules with different elongations and concentrations. The obtained result implies that small anisotropic molecules can show liquid crystal behavior.

Journal Article↗

PubMed2003

Orientational relaxation phenomena in Langmuir-Blodgett films at the air-water interface.

The nature of the orientational relaxation process of the director n to its equilibrium orientation n(eq), in the mono-(multi)layer(s) Langmuir-Blodgett film, during the lateral compression in absence of flow, is investigated. The relaxation time, during compression of 4-n-pentyl-4(')-cyanobiphenyl mono-(multi)layer(s) film on the water surface, using the Ericksen-Leslie theory, has been calculated for the number of dynamic regimes.

Journal Article↗

PubMed2003

Compression-shear-induced tilt azimuthal orientation of amphiphilic monolayers at the air-water interface: a C(infinity)-->C2v transition in the flow of a two-dimensional hexatic structure.

Compression-shear-induced tilt azimuthal orientation of amphiphilic monolayer in tilting phases (L2 and L'2) at the air-water interface is analyzed as dynamical equilibrium of the elastic distortion of orthogonally hexagonal structure of the molecules under compression-induced shear flow. It is shown theoretically that the compression can induce molecular tilts lying along and/or against the flow direction. All these tilts makeup the initial random tilt azimuth of the molecular tails along a uniform direction. At a threshold compression speed, it causes a C(infinity)-->C(2v)-symmetry transition at the air-water interface. With Maxwell displacement current and optical second-harmonic generation measurements, the above theoretical results are verified experimentally in a monolayer of 4-heptyloxy-4(')-cyanobiphenyl.

Journal Article↗

PubMed2002

Monolayers at the air-water interface: Maxwell displacement current and optical second-harmonic generation studies and theoretical treatment.

The structural properties and the surface pressure pi-A isothermal diagram have been investigated during compression of the 4-n-pentyl-4'-cyanobiphenyl (5CB) monolayer film on the water surface, both experimentally, using both the Maxwell displacement current (MDC) and optical second-harmonic generation (SHG) techniques, and theoretically, in the framework of the molecular model, assuming that the randomly tilted polar 5CB molecules on the water surface are replaced by the collective tilted array of unit vectors. The average angle is evaluated based on the both MDC and SHG techniques. The reasonable agreement between the calculated and experimental values of pi(A) was obtained.

Journal Article↗

PubMed2002

Nonunique steady states in the disordered harmonic chain.

The heat transport in disordered harmonic chains (DHCs) with arbitrary heat baths is studied, based on a general formulation developed by Dhar [Phys. Rev. Lett. 86, 5882 (2001)]. The obtained temperature profile of a steady state is very unusual for any heat bath: (i) it is not unique, but dependent on the initial condition; (ii) it may be highly nonlinear, even though the temperature difference of the two ends of the system is in zero limit, and the temperature gradient inverted Delta T is not inversely proportional to the system size; and (iii) when a DHC is coupled to two thermostats with the same temperature, the temperature of the system is still not uniform. The localized higher frequency normal modes induced by the mass disorders are responsible for these strange properties.

Journal Article↗

PubMed2002

Weak boundary anchoring, twisted nematic effect, and homeotropic to twisted-planar transition.

Expansion analysis shows that in second order, the weak boundary coupling of nematic liquid crystals should be depicted by two anchoring coefficients and an orthonormal vector triplet. Using this binomial anchoring energy, we have derived the analytical expression of the threshold and saturation properties of the twisted nematic effect and the homeotropic to twisted-planar transition. Our results prove clearly that these two quite different transitions are reverse effects of each other.

Journal Article↗