Toru Maekawa

We investigate two-dimensional cluster structures composed of ferromagnetic colloidal particles, based on a flexible chain model, by the configurational-bias Monte Carlo method. We clarify the dependence of the probabilities of the creation of different types of clusters on the dipole-dipole interactive energy and the cluster size.

We study the rheological and magnetic characteristics of a magnetic fluid. The system, which we investigate, is as follows. Ferromagnetic particles are dispersed in a solvent, which is subjected to both ac magnetic and shear flow fields. The translational and rotational motions of particles are calculated by the Brownian dynamics method based on Langevin equations and the rheological and magnetic characteristics of the magnetic fluid system are estimated. First, we investigate the rheological and magnetic characteristics of the system in a dc magnetic field and then we analyze the effect of an ac magnetic field on those characteristics. We find that the negative viscosity effect is induced at a certain frequency range of the ac magnetic field. We also find that there are two main mechanisms responsible for the occurrence of negative viscosity. (1) Resonance between the rotational motions of the dipoles of particles and the fluctuation of ac magnetic fields occurs when applied magnetic fields are weak compared to the shear rate, in which case particles can still rotate in magnetic fields. Beyond this resonance frequency, negative viscosity appears. (2) The magnetic dipole moments of particles are forced to stay in the direction of the magnetic field when strong magnetic fields are applied in relatively low shear flow fields. However, negative viscosity occurs when the frequency of external magnetic fields exceeds a critical value, in which case the dipoles rotate continuously in a shear flow without stopping. In both cases, the mean angular velocity of the particles becomes higher than that of the solvent.

We investigate the possibility of growing a uniform binary compound crystal in space, proposing a new crystal growth method. We develop a numerical calculation method for the growth of binary crystals, in which convection induced by temperature and concentration differences in the solution is taken into account. How to determine the shape and movement of the solution-crystal interface during the crystal growth is clearly explained for binary crystals, which is more complicated than for single-component crystals. The boundary fit method is employed to solve this moving boundary phase transition problem. The calculation method is applied to the crystal growth analysis of an InAs-GaAs binary semiconductor and the effect of buoyancy convection induced under microgravity conditions on the crystal growth process is investigated. It is found that the concentration field is disturbed and, as a result, the solution-crystal interface is deformed by buoyancy convection, even when the gravitational acceleration is as low as 10^-6 g, which is supposed to be the gravity level in the International Space Station which will start operation in 2004. It is also found that the direction of the residual gravity has a strong effect on the concentration field in the solution and the crystal growth process. Next, we analyse the influence of g-jitters and the Sorét effect on the crystal growth process. In fact, it is found that g-jitters and the Sorét effect have little influence on the macroscopic crystal growth process. The dependence of the generation of supercooling in the solution on convection is also investigated. It is found that supercooling is not induced by convection if residual gravity is 10^-6 g. Finally, we discuss the possibility of growing high-quality InGaAs crystals of uniform compositions in space.