Takeaki Araki

Memory and topological frustration in nematic liquid crystals confined in porous materials

Orientational ordering is key to functional materials with switching capability, such as nematic liquid crystals and ferromagnetic and ferroelectric materials. We explored the confinement of nematic liquid crystals in bicontinuous porous structures with smooth surfaces that locally impose normal orientational order on the liquid crystal. We find that frustration leads to a high density of topological defect lines permeating the porous structures, and that most defect lines are made stable by looping around solid portions of the confining material. Because many defect trajectories are possible, these systems are highly metastable and efficient in memorizing the alignment forced by external fields. Such memory effects have their origin in the topology of the confining surface and are maximized in a simple periodic bicontinuous cubic structure. We also show that nematic liquid crystals in random porous networks exhibit a disorder-induced slowing-down typical of glasses that originates from activated collisions and rearrangements of defect lines. Our findings offer the possibility to functionalize orientationally ordered materials through topological confinement.

Fluid particle dynamics simulation of charged colloidal suspensions

An ew method of simulation of the dynamics of charged colloidal suspensions is formulated that is based on the fluid particle dynamics method (Tanaka and Araki 2000 Phys. Rev .L ett. 85 1338) so as to incorporate the electrohydrodynamic interactions properly. The fluid particle approximation allows us to treat dynamic coupling among motions of the three relevant elements of charged colloidal suspensions, i.e., colloidal particles, ion clouds, and liquid, in a physically natura lm anner. The validity of our method is demonstrated for a problem of the electrophoretic deposition of charged colloids. Our simulation results clearly indicate that the electro-osmotic flow causes ‘effective’ long-range attractions between charged particles of the same sign, as previously suggested by experiments and theories. Liquid suspensions containing colloids are of fundamental importance in soft matter physics, surface chemistry, biology, and industry [1–3]. In colloidal suspensions, charges play key roles in stabilizing the dispersions and also in electrically manipulating suspended particles. It is also widely known that they affect various kinetic phenomena of colloidal suspensions, such as sedimentation, rheology, and electrophoresis. When one tries to study the dynamics of charged colloidal suspensions either theoretically or numerically, the most difficult problem arises from the complex dynamic coupling among motions of the three key elements; that is ,c olloidal particles, ions, and liquid molecules. These elements are strongly interacting with each other via both electrostatic and hydrodynamic interactions. Since these static and dynamic interactions are both of long-range nature, we must inevitably deal with a very complex dynamic many-body problem. Because of these difficulties, there have so far been neither theoretical nor numerical studies that take into account the full static and dynamic coupling among the motions of these three key elements of colloidal suspensions, despite the scientific

Physical principle for optimizing electrophoretic separation of charged particles

Electrophoresis is one of the most important methods for separating colloidal particles, carbohydrates, pharmaceuticals, and biological molecules such as DNA, RNA, proteins, in terms of their charge (or size). This method relies on the correlation between the particle drift velocity and the charge (or size). For a high-resolution separation, we need to minimize fluctuations of the drift velocity of particles or molecules. For a high throughput, on the other hand, we need a concentrated solution, in which many-body electrostatic and hydrodynamic interactions may increase velocity fluctuations. Thus, it is crucial to reveal what physical factors destabilize the coherent electrophoretic motion of charged particles. However, this is not an easy task due to complex dynamic couplings between particle motion, hydrodynamic flow, and motion of ion clouds. Here we study this fundamental problem using numerical simulations. We reveal that addition of salt screens both electrostatic and hydrodynamic interactions, but in a different manner. This allows us to minimize the fluctuations of the particle drift velocity for a particular salt concentration. This may have an impact not only on the basic physical understanding of dynamics of driven charged colloids, but also on the optimization of electrophoretic separation.

Spontaneous coarsening of a colloidal network driven by self-generated mechanical stress

Colloidal suspensions can be regarded as an ideal model system for such key daily materials as emulsions, protein solutions, foods, and inks. When colloidal particles strongly attract each other, they aggregate, phase-separate, and sometimes form gels. The basic understanding of this spatially heterogeneous jamming process is of crucial importance from both scientific and industrial viewpoints. Usually it is believed that if colloids attract very strongly with adhesion energy more than 10 times the thermal energy, networks formed by aggregation do not coarsen with time and a stable gel is immediately formed. Contrary to this common belief, we demonstrate by numerical simulation that the coarsening of a colloidal network can proceed by self-generated mechanical stress even without any thermal noise for a system of long-range interactions: fracture-induced coarsening. This remarkable kinetic pathway of purely mechanical origin may shed new light on our basic understanding of the stability and aging (or coarsening) behaviour of colloidal gels.

Phase separation and gelation of polymer-dispersed liquid crystals

Abstract Polymerization-induced phase separation in polymer-dispersed liquid crystal is studied by computer simulations in two dimensions. The domain morphology resulting from phase separation is investigated by solving the coupled set of equations for the local volume fraction and the nematic order parameter, taking into account the viscoelastic effects and gelation due to polymerization. Comparing the morphology of phase separation by temperature quench, it is shown that the viscoelastic effects and gelation enable the polymer-rich phase to form a stable interconnected domain even when the polymer component is minority. The experimental evidence consistent with this characteristic feature is also given.

Dynamic depletion attraction between colloids suspended in a phase-separating binary liquid mixture

Understanding interactions between colloids (or nanoparticles) immersed in a phase-separating binary mixture is of both fundamental and technological importance. Here we report a novel type of interparticle attractive interaction of a purely dynamic origin, which is found by a coarse-grained numerical simulation. Due to surface wetting effects, there are strong diffusion fluxes towards particles just after the initiation of phase separation of the matrix binary liquid mixture. The flux in the region between particles soon becomes weaker than that in the other regions since the depletion zones formed around particles overlap selectively between the particles. The resulting imbalance of the diffusion flux induces interparticle attractive interactions, i.e., the osmotic force pushes particles closer. We confirm that this wetting-induced ‘dynamic’ depletion force can be stronger than a van der Waals force and a capillary force that is induced by the interfacial tension, and thus plays a dominant role in the early stage of particle aggregation. We note that this novel interaction originating from the momentum conservation law may be generic to particles acting as diffusional sinks or sources. (Some figures in this article are in colour only in the electronic version)

Surface-sensitive particle selection by driving particles in a nematic solvent

Electrophoresis and sedimentation (or ultracentrifugation) are powerful means for separating particles, proteins, and DNA, exploiting the difference in particle charge, mass, and size. Surface properties of colloids and proteins are closely related to their physical, chemical, and biological functions. Thus, the selection of particles in terms of their surface properties is highly desirable. The possibility of replacing a simple liquid like water by a complex liquid may provide a novel route to particle separation. Here we report a new principle of surface-sensitive particle selection by using nematic liquid crystal as a solvent. When we immerse a particle in nematic liquid crystal, topological defects are formed around a particle if there is a strong enough coupling between the particle surface and liquid crystal orientation (so-called surface anchoring effects). Then these defects strongly influence the motion of particles. Here we study this problem by using a novel numerical simulation method which incorporates elastic and nematohydrodynamic couplings properly. We find that the surface anchoring properties change both direction and speed of motion of a particle driven in an oriented nematic liquid crystal. This principle may be used for separating particles in terms of their surface properties.

Defect structures in nematic liquid crystals around charged particles

We numerically study the orientation deformations in nematic liquid crystals around charged particles. We set up a Ginzburg-Landau theory with inhomogeneous electric field. If the dielectric anisotropy

Defect science and engineering of liquid crystals under geometrical frustration

\varepsilon_{1}^{}

Structural and dynamical heterogeneities in two-dimensional melting

is positive, Saturn-ring defects are formed around the particles. For