Human red blood cells (RBCs) are subjected to high viscous shear stress, especially during microcirculation, resulting in stable deformed shapes such as parachute or slipper shape. Those unique deformed RBC shapes, accompanied with axial or nonaxial migration, cannot be fully described according to traditional knowledge about lateral movement of deformable spherical particles. Although several experimental and numerical studies have investigated RBC behavior in microchannels with similar diameters as RBCs, the detailed mechanical characteristics of RBC lateral movement—in particular, regarding the relationship between stable deformed shapes, equilibrium radial RBC position, and membrane load—has not yet been fully described. Thus, we numerically investigated the behavior of single RBCs with radii of 4 $\mathsf{\mu }$m in a circular microchannel with diameters of 15 $\mathsf{\mu }$m. Flow was assumed to be almost inertialess. The problem was characterized by the capillary number, which is the ratio between fluid viscous force and membrane elastic force. The power (or energy dissipation) associated with membrane deformations was introduced to quantify the state of membrane loads. Simulations were performed with different capillary numbers, viscosity ratios of the internal to external fluids of RBCs, and initial RBC centroid positions. Our numerical results demonstrated that axial or nonaxial migration of RBC depended on the stable deformed RBC shapes, and the equilibrium radial position of the RBC centroid correlated well with energy expenditure associated with membrane deformations.

Microorganisms are known to alter their motility in response to external stimuli. A typical example is the response known as taxis, which includes behaviors such as moving toward a light source (positive phototaxis) or away from it (negative phototaxis). In this study, we focused on bioconvection induced by the negative phototaxis of a Euglena suspension exposed to strong light from the bottom. Recent studies have revealed that Euglena bioconvection induced by negative phototaxis exhibits a localized structure. In a previous study, we found that a single Euglena bioconvection spot (EBC spot) maintains its structure in a cylindrical container. Such a localized structure is not exclusive to Euglena but has also been reported in red tide algae, suggesting that this is a universal feature of bioconvection phenomena. In this paper, we investigated the formation conditions of EBC spot through experiments and numerical simulations. In the experiments, we examined the variation in EBC spot formation as a function of suspension height and average cell density, revealing that the critical average cell density increases with suspension height. In the numerical simulations, we conducted bioconvection simulations based on a model of Euglena motility that incorporated three types of behavior: the negative gravitaxis, negative phototaxis, and behavior of moving toward darker areas. Our model successfully reproduced the localized state of a two-dimensional EBC spot. Furthermore, calculations corresponding to the experimental conditions of suspension height and average cell density revealed that the critical average cell density for bioconvection decreases as the suspension height increases.

The elasto-inertial effects on particle focusing in a square-tube flow were investigated experimentally and numerically. Microscale experiments using spherical particles in dilute polymer solutions demonstrated that the particles are focused on the midline and/or the diagonal in a downstream cross section, depending on the polymer concentration. Numerical computations based on the finitely extensible nonlinear elastic–Peterlin (FENE-P) model for the viscoelastic flow reproduced these focusing patterns. It was revealed that the transitions among the patterns are accounted for by the elastic forces due to the first normal stress difference and the polymer elongation, which are the essentials of the viscoelastic fluid.

An Imaging Plate (IP) has been developed as an image storing medium for use in medical X-ray diagnosis. We confirmed the IP to be a good medium not only for X-ray radiation but also for vacuum ultraviolet (VUV) and ultraviolet (UV) radiation. We found that the photo-stimulated luminescence (PSL) intensity showed a linear response to UV photon numbers in the range of 10^4.5, and that showed saturation as the irradiated photon number increased. The sensitivity curve of the IP showed a peak at the wavelength of 200 nm and decreased at longer wavelengths. When the wavelength of the irradiated UV radiation was longer than 320 nm, the PSL intensity was too low to detect with our system. This means that the cut-off wavelength of the sensitivity was measured as 310 nm.

The inertial migration of neutrally buoyant spherical particles in viscoelastic fluids flowing through square channels is experimentally and numerically studied. In the experiments, using dilute aqueous solutions of polymers with various concentrations that have nearly constant viscosities, we measured the distribution of suspended particles in downstream cross-sections for the Reynolds number ( $\textit{Re}$ ) up to 100 and the elasticity number ( $El$ ) up to 0.07. There are several focusing patterns of the particles, such as four-point focusing near the centre of the channel faces on the midlines for low $\textit{El}$ and/or high $\textit{Re}$ , four-point focusing on the diagonals for medium $\textit{El}$ , single-point focusing at the channel centre for relatively high $\textit{El}$ and low $\textit{Re}$ , and five-point focusing near the four corners and the channel centre for high $\textit{El}$ and very low $\textit{Re}$ . Among these focusing patterns, various types of particle distributions suggesting the presence of a new equilibrium position located between the midline and the diagonal, and multistable states of different equilibrium positions were observed. In general, as $\textit{El}$ increases from 0 at a constant $\textit{Re}$ , the particle focusing positions shift from the midline to the diagonal in the azimuthal direction first, and then inward in the radial direction to the channel centre. These focusing patterns and their transitions were numerically well reproduced based on a FENE-P model with measured values of viscosity and relaxation time. Using the numerical results, the experimentally observed focusing patterns of particles are elucidated in terms of the fluid elasticity-induced lift and the wall-induced elastic lift.

Microorganisms are known to alter their motility in response to external stimuli. A typical example is the responseknown as taxis, which includes behaviors such as moving toward a light source (positive phototaxis) or away from it(negative phototaxis). In this study, we focused on bioconvection induced by the negative phototaxis of a Euglenasuspension exposed to strong light from the bottom. Recent studies have revealed that Euglena bioconvectioninduced by negative phototaxis exhibits localized structure. In a previous study, we found that a single Euglenabioconvection spot (EBC spot) maintains its structure in a cylindrical container. Such a localized structure is notexclusive to Euglena, but has also reported in red tide algae, suggesting that this is a universal feature ofbioconvection phenomena. In this paper, we investigated the formation conditions of EBC spot phenomena throughexperiments and numerical simulations. In the experiments, we examined the variation in EBC spot formation as afunction of suspension height and average cell density, revealing that the critical average cell density increases withsuspension height. In the numerical simulations, we conducted bioconvection simulations based on a model ofEuglena motility that incorporated three types of behavior: the negative gravitaxis, negative phototaxis, andbehavior of moving toward darker areas. Our model successfully reproduced the localized state of a two-dimensional EBC spot. Furthermore, calculations corresponding to the experimental conditions of suspension heightand average cell density revealed that the critical average cell density for bioconvection decreases as thesuspension height increases.