Yukio Yamaguchi’s research while affiliated with The University of Tokyo and other places

During drying of colloidal suspensions, colloidal particles can form concentrated particle layers beneath the receding free surface. The drying rate can gradually decrease with the growth of the particle layers. We construct a model to investigate how such drying characteristics is affected by interactions between particles. In this model, the formation of the particle layers is described by Langevin dynamics simulations, and the drying rate is evaluated from the permeation resistance of the particle layers. We show that the decrease in the drying rate is suppressed when the particles form aggregates by attractive interactions. The present model would enable us to predict and control the drying characteristics through the character of colloidal particles.

During drying of binary colloidal mixtures, one colloidal particle component can segregate to the top surface. We investigate conditions where the segregation occurs through the analysis of a linearized diffusion model with Fick’s law generalized for binary colloidal mixtures. The present model is the simplest representation that includes cross-diffusion between different particle components to describe the segregation. Using the analytical solutions of this model, we classify states in terms of which the particle component segregates for the following variables: the mixture ratio of particle components, diffusion coefficients, and drying rates. The obtained state diagrams suggest how to control the segregation by designing material and operation conditions.

When suspensions containing colloidal particles of two different sizes are coated on substrates to form films by evaporating the host fluids, the smaller particles can segregate to the top surface of the films. We investigate the effects of the evaporation rate on the segregation by use of Langevin dynamics simulations. The evaporation rate is scaled by the Brownian diffusion rate of the particles, yielding a dimensionless number which we define as the particle drying Péclet number. We show that there is a Péclet number at which the segregation is the most enhanced. Our result indicates the need for the regulation of the evaporation rate to control the segregation.

Silicon microparticles were formed by reduction of silicon tetrachloride by zinc. Chemical etching of these microparticles produced microparticles with a unique trench structure that showed visible photoluminescence. While we have previously reported the radially developed crystalline structure inside the as-synthesized microparticles, their crystalline structure was not clearly understood. In this study, we directly observed the cross sections of microparticles using a transmission electron microscope. We revealed the existence of a core inside the microparticles. The core showed a well-ordered electron diffraction pattern, which was readily explained by five single crystals rotated by 72° each. We also found that the observed radial lines and bands in the microparticles were caused by dislocations, mainly along the {111} planes. We consider that preferential etching along these radial dislocations is the origin of the formation of microparticles with trench structures.

In this study, we investigated vacuum drying of droplets of colloidal suspension. Because of the loss of the latent heat of vaporization, the drying droplet was cooled and then formed ice. Colloidal fibers consisting of packed particles spontaneously formed when the droplet froze from the gas-liquid interface. Conversely, we observed formation of sponge-like porous structures of particles when the whole droplet almost simultaneously froze. However, the freezing mode was not the only factor for formation of colloidal fibers. We found that the surface coverage of particles on the gas-liquid interface was also important. Owing to drying, some particles accumulated at the interface before freezing. When the surface coverage was higher than a threshold value, formation of fibers was severely restricted even in the surface freezing mode. Our results clearly show the important roles of surface freezing and the surface coverage of particles on the gas-liquid interface in formation of colloidal fibers.

The stability of photoluminescence (PL) intensity from chemically etched silicon microparticles is studied. Etched microparticles have many narrow and deep trenches on surface. They show visible orange-red PL, which decreases in intensity during continuous excitation by ultraviolet light. The intensity of PL partially recovers when the surrounding gas is changed from air to nitrogen. Thus PL quenching consists of both reversible and irreversible processes and we propose a kinetic model that consists of two quenching paths. Adsorption and desorption of oxygen followed by irreversible oxidation of emission sites are considered in the fast quenching pathway, while the slow pathway involves transport of oxygen molecules to emission sites in trenches with poor access. Our model agrees well with experimental data and rate constants of involved processes are determined, with which we discuss kinetics in PL quenching. Possible strategy to increase PL stability is also discussed.

When cultured endothelial cells (ECs) are exposed to shear flow, initially cobblestone-like ECs spontaneously elongate and align along the flow direction, acquiring a similar architecture to that of native endothelium in blood vessels. Though previous works have revealed how individual cells sense and respond to shear flow, little is known about the contribution of cell-cell interaction to this phenomenon. Here, we reveal that the response of ECs to shear flow is cell density-dependent, and therefore possibly mediated by collective cell behavior (i.e. cell-cell interaction). A cell density threshold was identified, below which ECs never formed an aligned structure by shear flow exposure. This threshold cell density corresponded to the two-dimensional percolation threshold, suggesting that the aligned structure formation requires a connecting network of ECs. We also observed an optimum cell density at which both elongation and alignment were completed within a minimum time period. A possible mechanism of the EC response to applied shear flow is discussed.

Using amine-modified silicon quantum dots (Si-QDs) with visible photoluminescence as a building block, drug-loaded Si-QD aggregates were assembled. The aggregates were designed to break down in response to the endosomal pH decrease, which enabled selective intracellular release of the loaded drugs.

We construct a mesoscale model of colloidal suspensions that contain solutes reversibly adsorbing onto the colloidal particle surfaces. The present model describes the coupled dynamics of the colloidal particles, the host fluid, and the solutes through the Newton-Euler equations of motion, the hydrodynamic equations, and the advection-diffusion equation, respectively. The solute adsorption is modeled through a square-well potential, which represents a short-range attractive interaction between a particle and a solute molecule. The present model is formulated to be solved through direct numerical simulations. Some numerical results are presented to validate the simulations. The present model enables investigations of solute adsorption effects in the presence of a fluid flow and an inhomogeneous solute concentration distribution.