講師：Toshiki Mima （美馬 俊喜氏）(The University of Tokyo)
題目：Molecular dynamics simulation of fluids in nanopores
要旨：
Fluids in nanopores are of importance for a wide range of industrial applications because their mass and heat transfer characteristics are different from those of the bulk fluids. The major reason of the anomalous characteristics is that the number of molecules near the solid surface is comparable with that in the bulk region. The experimental approach is still a great challenge to observe molecules near the solid surface at an atomistic scale, therefore, the molecular dynamics simulation is one of the effective scheme. In this presentation, the properties of nano-confined liquid crystal and water are mainly investigated.
There are two topics of liquid crystals confined in nano-slit. First, the self-diffusion coefficient and the rotational coefficient depending on the distance from the solid surface are investigated. The results are analyzed through the Fourier transform of the correlation function corresponding to each individual transport coefficient. Second, the system size dependence of the order parameter is discussed. The result indicates that liquid crystals in the slit with the gap of several molecules length exhibit the Kosterlitz-Thouless transition. In the talk of water in nanocylinder, the wettability and pore size dependences of saturation pressure are investigated and the validity of the Kelvin equation is examined.
日時：2014年11月18日（火）10:30
場所：理学部5号館413号室
講師：Jack F. Douglas (National Institute of Standards and Technology)
題目：Cooperative Motion and Structural Relaxation in Glass-Forming Materials
要旨：
Collective motion and relaxation in coarse-grained polymeric liquids are investigated by complementary molecular dynamics (MD) simulations and analytic modeling. MD simulation indicates that nanoparticle additives, which can effectively modify molecular packing, can cause significant changes in both the strength of the temperature dependence of structural relaxation time $\tau$ (as measured by the fragility of glass formation) and the spatial extent of cooperative particle motion in the form of string-like dynamic structures that spontaneously emerge at equilibrium in glass-forming liquids. Physical arguments of Adam-Gibbs (AG) relating the rate of structural relaxation to the extent (average mass) of cooperative motion in glass-forming liquids are summarized and we test whether this model can rationalize the large nanoparticle additive-induced changes in polymer melt dynamics found in our simulations where the string excitations are directly identified with the abstract cooperatively rearranging regions of AG (see figure below and Refs. 1 and 2 for discussion). This comparison leads to a remarkable reduction of all our relaxation data and provides a new means of testing the AG model. These observations are consistent with the implicit AG picture that changes in the fragility of glass-forming liquids with the addition of additives, finite size confinement, and changes of molecular structure derive from changes in the strength of the temperature dependence of the extent of cooperative molecular motion. The classical entropy theory of glass-formation, combining a thermodynamic calculation of the configurational entropy sc and the AG model of structural relaxation relating $\tau$ to sc, is considered as a complementary tool to gain analytic insights into these additive-induced effects on the temperature dependence on structural relaxation in glass-forming liquids.
日時：2014年11月11日（火）13:00
場所：理学部5号館401号室
講師：Hiroaki Yoshida (Toyota Central R&D Labs.)
題目：Analysis of electrokinetic transports of nano-confined electrolyte solutions by means of the molecular dynamics simulations
要旨：
Current interest of our group is centered on the electrokinetic phenomena caused by interaction between the electrical double layer formed near solid-liquid interface and external forces. Aiming at comprehensive understanding of the phenomena and their automotive applications, we are attempting to develop a framework of computer simulations. Among the approaches at different length scales we employ, the main topic in my talk will be a methodology for analyzing dynamics of a liquid electrolyte solution in confined geometries at a scale of nano meter, along with some numerical results. Specifically, an aqueous NaCl solution in nanochannels with negatively charged surfaces is considered, and electrokinetic flows of the solution are studied using molecular dynamics (MD) simulations. The transport coefficients that characterize the response to weak external forces, such as the electric and pressure fields, are obtained in the linear regime using a Green-Kubo approach. The non-equilibrium MD simulations with explicit external fields are also carried out, and the flow rates are directly compared. Using the numerical method, the influence of the surface charge density and the salt concentration on the flow properties is investigated, and an anomalous reversal of the electrokinetic flows observed in a certain parameter range is discussed.
日時：2014年8月27日（水）13:30
場所：理学部5号館413号室
講師：Anael Lemaitre (Laboratoire Navier (UMR 8205), Universite Paris-Est, France)
題目：Structural relaxation is a scale-free process
要旨：
We show that in deeply supercooled liquids, structural relaxation proceeds via the accumulation of Eshelby events, i.e. local rearrangements that create long-ranged and anisotropic stresses in the surrounding medium. Such events must be characterized using tensorial observables and we construct an analytical framework to probe their correlations using local stress data. By analyzing numerical simulations, we thus demonstrate that events are power-law correlated in space, with a time-dependent amplitude which peaks at the alpha relaxation time. This effect, which becomes stronger near the glass transition, results from the increasingly important role of local stress fluctuations in facilitating relaxation events. Our work thus precludes the existence of any lengthscale beyond which the relaxation process decorrelates and shows that instead relaxation arises from the scale-free accumulation of events with a tensorial signature.
日時：2014年5月14日（水）15:30
場所：理学部5号館413号室
講師：Mohammad Reza Mozaffari (University of Qom, Qom, Iran )
題目：Pair interaction between colloidal particles in nematic liquid crystal
要旨：
Structure of colloidal particles in anisotropic solvent with long-range orientational ordering, such as nematic liquid crystals, has attracted great attention in science and technology. The nematic director is distorted from its uniform orientation in the bulk due to anchoring on the surface of the colloidal particles. These elastic distortions create topological defects around the particles and induce long and short range anisotropic interactions between the particles. Depending on the anchoring type and the colloidal geometry, the particle-defect can induce a long-range dipolar-dipolar, quadrupolar-quadrupolar and/or dipolar-quadrupolar potential.
Experimentally, the colloidal interactions in nematic liquid crystals are studied using optical or magneto-optical traps. Smalyukh et al.[Phys. Rev. Lett. 95, 157801 (2005)] have measured the angular and the radial components of the force between two particles with planar anchoring in a nematic liquid crystal as a function of the interparticle separation and the angle with the bulk nematic. They showed that the particles aggregate in chains directed at nearly 30 degree with respect to the nematic direction. Lapointe et al.[Science 326, 1083 (2009)] indicated that the elastic colloidal interactions in nematic liquid crystals are sensitive to the colloids shapes. Equilibrium director field around platelet equilateral polygonal colloids exhibit dipolar symmetry if they have odd number of the edges and quadrupolar symmetry if it is is even. This is giving rise to dipolar and quadrupolar elastic colloidal interactions, respectively.
Studing the degenerate planar anchoring on the spherical particles is numerically interest and hard in comparison to the normal anchoring. Here, the interaction between two spherical colloidal particles with degenerate planar anchoring in a nematic media has been studied by numerically minimizing the bulk Landau-de Gennes free energy and the surface energy by the mean of a finite element method. The results indicate that the particles form an angle 28+-2 degree with respect to the far-field nematic direction that is consistent with Smalyukh research. We have also investigated interactions between two long parallel prism particles when their long axes are perpendicular with far-field nematic direction. Using adaptive finite element method the 2D Landau-de Gennes free energy with strong planar anchoring has been minimized. In agreement with Lapointe’s results, interaction between two triangle prisms has dipolar symmetry while the square prisms show quadrupolar symmetry.