Toru Hyakutake

Characteristics of reflected gas molecules at a solid surface

The Couette flow and a thermal problem of a rarefied nitrogen gas between two platinum walls are considered to investigate the characteristics of the reflected gas molecule at a solid surface. The analysis is based on the molecular dynamics (MD) method for the gas-wall interaction together with the direct simulation Monte-Carlo (DSMC) method for the motion of gas molecules. The accommodation coefficients of momentum, translational, and rotational energies of the molecule are obtained. The velocity and rotational energy distributions of the molecule at the wall surface are also obtained. It is found that the Maxwell-type distribution function consisting of specular and diffuse reflections well describes the distribution function of the reflected molecules if the accommodation coefficient involved is chosen properly. It is also found that the flow and temperature fields subject to the Maxwell-type reflection conditions decomposed into each direction of the space coordinates result in good agreements with th...

Lattice Boltzmann simulation of blood cell behavior at microvascular bifurcations

Lattice Boltzmann simulations of a particulate suspension in plane Poiseuille flow and two-dimensional bifurcation flow are conducted to investigate the blood cell behavior at microvascular bifurcations. The simulation results of Poiseuille flow at a low hematocrit reveal that particles are concentrated between the centerline and the wall. This phenomenon indicates the occurrence of the Segre-Silberberg effect. On the other hand, as the hematocrit increases, this effect disappears and the number of particles flowing near the wall and the centerline increases. In the case of bifurcation flow, at a high bematocrit, the fractional particle flux to a daughter branch is almost similar to the fractional bulk flow to the same branch. However, at a low hematocrit, the fractional particle flux against the fractional bulk flow increases.

Screening of sperm velocity by fluid mechanical characteristics of a cyclo-olefin polymer microfluidic sperm-sorting device

The microfluidic sperm-sorting (MFSS) device is a promising advancement for assisted reproductive technology. Previously, poly(dimethylsiloxiane) and quartz MFSS devices were developed and used for intracytoplasmic sperm injection. However, these disposable devices were not clinically suitable for assisted reproduction, so a cyclo-olefin polymer MFSS (COP-MFSS) device was developed. By micromachining, two microfluidic channels with different heights and widths (chip A: 0.3 × 0.5 mm; chip B: 0.1 × 0.6 mm) were prepared. Sorted sperm concentrations were similar in both microfluidic channels. Linear-velocity distribution using the microfluidic channel of chip B was higher than that of chip A. Using confocal fluorescence microscopy, it was found that the highest number of motile spermatozoa swam across the laminar flow at the bottom of the microfluidic channel. The time required to swim across the laminar flow was longer at the bottom and top of the microfluidic channels than in the middle because of the low fluid velocity. These results experimentally demonstrated that the width of microfluidic channels should be increased in the region of laminar flow from the semen inlet to the outlet for unsorted spermatozoa to selectively recover spermatozoa with high linear velocity.

Numerical simulation of red blood cell distributions in three-dimensional microvascular bifurcations

We constructed three-dimensional microvascular bifurcation models using a parent vessel of diameter 10μm and investigated the flow behavior of the red blood cells (RBCs) through bifurcations. We considered symmetric and asymmetric model types. Two cases of equal daughter vessel diameter were employed for the asymmetric models, where the first was 10μm, which is the same as the parent vessel and the second was 7.94μm, which satisfies Murrays law. Simulated blood flow was computed using the lattice Boltzmann method in conjunction with the immersed boundary method for incorporating fluid-membrane interactions between the flow field and deformable RBCs. First, we investigated the flow behavior of a single RBC through microvascular bifurcations. In the case of the symmetric bifurcation, the turning point of the fractional plasma flow wherein the RBC flow changed from one daughter vessel to the other was 0.50. This turning point was however different for asymmetric bifurcations. Additionally, we varied the initial offset of RBCs from the centerline of the parent vessel. The simulation results indicated that the RBCs preferentially flow through the branch of a larger flow ratio. Next, we investigated the distribution characteristics of multiple RBCs. Simulations indicated that the results of the symmetric model were similar to those predicted by a previously published empirical model. On the other hand, results of asymmetric models deviated from those of the symmetric and empirical models. These results suggest that the distribution of RBCs varies according to the bifurcation angle and daughter vessel diameter in a microvascular bifurcation of the size considered.

Scattering properties and scattering kernel based on the molecular dynamics analysis of gas-wall interaction

The scattering behaviors of nitrogen molecules reflected at a platinum surface are studied by the molecular dynamics method for the gas-surface interaction. The platinum surface is assumed to physically adsorb xenon molecules. Distributions of molecular velocities after reflection are obtained for the impinging molecules with specified velocities within a small range. The scattering distribution shows a bimodal behavior consisting of diffuse reflection and a distribution shifting to the velocity distribution of the impinging molecules. Comparison between the present scattering distribution and the Cercignani-Lampis-Lord scattering kernel shows large differences. A simple scattering kernel using a bimodal distribution is proposed assuming mutual independence of distributions in each velocity component and is compared favorably to the present scattering distribution. In addition, the scattering kernel of the rotational energy of the molecule is discussed.

Effects of Curvature and Convective Heat Transfer in Curved Square Duct Flows

Non-isothermal flows with convective heat transfer through a curved duct of square cross section are numerically studied by using a spectral method, and covering a wide range of curvature, δ, 0 < δ≥0.5 and the Dean number, Dn, 0≤ Dn≤ 6000. A temperature difference is applied across the vertical sidewalls for the Grashof number Gr= 100, where the outer wall is heated and the inner one cooled. Steady solutions are obtained by the Newton-Raphson iteration method and their linear stability is investigated

Numerical study on flows of red blood cells with liposome-encapsulated hemoglobin at microvascular bifurcation

Flow analysis at microvascular bifurcation after partial replacement of red blood cell (RBC) with liposome-encapsulated hemoglobin (LEH) was performed using the lattice Boltzmann method. A two-dimensional symmetric Y bifurcation model with a parent vessel diameter of 20 mum and daughter branch diameters of 20 microm was considered, and the distributions of the RBC, LEH, and oxygen fluxes were calculated. When only RBCs flow into the daughter branches with unevenly distributed flows, plasma separation occurred and the RBC flow to the lower-flow branch was disproportionately decreased. On the other hand, when half of RBC are replaced by LEH, the biasing of RBC flow was enhanced whereas LEH flowed favorably into the lower-flow branch, because many LEH within the parent vessel are suspended in the plasma layer, where no RBCs exist. Consequently, the branched oxygen fluxes became nearly proportional to flows. These results indicate that LEH facilitates oxygen supply to branches that are inaccessible to RBCs.

Application of a numerical simulation to improve the separation efficiency of a sperm sorter

This paper describes a study in which numerical simulations were applied to improve the separation efficiency of a microfluidic-based sperm sorter. Initially, the motion of 31 sperm were modeled as a sinusoidal wave. The modeled sperm were expected to move while vibrating in the fluid within the microchannel. In this analysis, the number of sperm extracted at the outlet channel and the rate of movement of the highly motile sperm were obtained for a wide range of flow velocities within the microchannel. By varying the channel height, and the width and the position of the sperm-inlet channel, we confirmed that the separation efficiency was highly dependent on the fluid velocity within the channel. These results will be valuable for improving the device configuration, and might help to realize further improvements in efficiency in the future.

Effect of Wall Characteristics on the Behaviors of Reflected Gas Molecules in a Thermal Problem

The gas‐wall interaction is investigated for the thermal problem between two walls whose temperatures are different each other. The method of analysis is based on the molecular dynamics as well as DSMC method. The wall consists of platinum molecules and the gas is taken to be argon. Three cases of wall surface characteristics are considered: smooth surface, rough surface in molecular scale and the surface with adsorbates. The energy accommodation coefficients and the jump coefficients are obtained. The discussions are also made on the velocity distributions of the reflected molecules.

Effect of non-Newtonian fluid properties on bovine sperm motility

The swimming process by which mammal spermatozoa progress towards an egg within the reproductive organs is important in achieving successful internal fertilization. The viscosity of oviductal mucus is more than two orders of magnitude greater than that of water, and oviductal mucus also has non-Newtonian properties. In this study, we experimentally observed sperm motion in fluids with various fluid rheological properties and investigated the influence of varying the viscosity and whether the fluid was Newtonian or non-Newtonian on the sperm motility. We selected polyvinylpyrrolidone and methylcellulose as solutes to create solutions with different rheological properties. We used the semen of Japanese cattle and investigated the following parameters: the sperm velocity, the straight-line velocity and the amplitude from the trajectory, and the beat frequency from the fragellar movement. In a Newtonian fluid environment, as the viscosity increased, the motility of the sperm decreased. However, in a non-Newtonian fluid, the straight-line velocity and beat frequency were significantly higher than in a Newtonian fluid with comparable viscosity. As a result, the linearity of the sperm movement increased. Additionally, increasing the viscosity brought about large changes in the sperm flagellar shape. At low viscosities, the entire flagellum moved in a curved flapping motion, whereas in the high-viscosity, only the tip of the flagellum flapped. These results suggest that the bovine sperm has evolved to swim toward the egg as quickly as possible in the actual oviduct fluid, which is a high-viscosity non-Newtonian fluid.