Yoshihide Mawatari

Drying Characteristics of Porous Materials in a Fluidized Bed under Reduced Pressure

Abstract The drying of porous materials immersed in the fluidized bed under reduced pressure was performed, and the results were compared with those of hot air drying. The pressure in drying chamber was changed (5.0–101.3 kPa) and the effect of it was examined. The temperature of the sample center becomes lower as the pressure in drying chamber decreases, and the temperature in fluidized-bed drying is higher than that in hot air drying at the same pressure. The effect of pressure in drying chamber on the sample temperature is significant for different temperatures of drying gas.

Measuring the Drying Rate of Liquid Film Coatings Using Heat Flux Method

We propose a simple method of determining solvent drying rates from heat flux measurements across thin liquid films. The theory is based on quasi-steady conductive heat transport through coatings, combined with simultaneous heat and mass transfer in the gas phase. The measured evaporation rates well reproduce conventional gravimetric measurements with an uncertainty of less than 5%. Drying experiments also revealed that the proposed method is robust in systems with high levels of fluctuation and thus provides an alternative tool for monitoring drying kinetics in forced air flows.

Bubbling and Bed Expansion Behavior Under Vibration in a Gas-Solid Fluidized Bed

The effect of vibration on the flow patterns and fluidization characteristics including the minimum fluidization velocity (u mf ), the void fraction (e mf ) at u mf and the bed expansion ratio were examined. The powders used were spherical glass beads and their diameters were 6, 20, 30, 60 and 100 μm. For group A powders, the manner in which the vibration affects the bubble formation was examined from the bed expansion ratio and the index of n/4.65. The area of the homogeneous fluidization region was also observed. The homogeneous fluidization region was broadened at a certain vibration strength, where the value of n/4.65 was a minimum. The bubble formation was observed even for 20 μm powder (group C), at large vibration strengths and at high gas velocities. Under such conditions, the bed expansion ratio increased suddenly due to bubble formation. The bubbles broke the irregular bed structure, including various properties of agglomerates. Although the channel breakage was dominant flow pattern for group C powders, the bubbles also played an important role in the improvement of the fluidization.

A model for estimating agglomerate sizes of non-magnetic nanoparticles in magnetic fluidized beds

The behavior of SiO2, TiO2 and ZnO non-magnetic nanoparticles and the effects of processing parameters on agglomerate sizes were investigated systematically in a magnetic fluidized bed (MFB) by adding coarse magnets. A mathematical model is developed based on energy balance among the agglomerate collision energy, magnetic energy, energy generated by turbulent shear and cohesive energy to predict the agglomerate sizes. The results showed that slugging of the bed disappeared and the measured agglomerate sizes decreased, so that the fluidization quality of non-magnetic nanoparticles was significantly improved by adding coarse magnets due to introduction of magnetic field. The average agglomerate sizes predicted by this model are in agreement with the experimental data.

Transition between Condensing and Air Flow Drying of Thin-Film Coatings

The transport characteristics were investigated in the bench-top condenser dryer, which is a new and air-free drying system for advanced coating applications. The measured solvent drying rate agrees with the one-dimensional diffusion theory in the early drying stages, showing the validity of the previously proposed diffusion models in a quantitative sense. The combination of the condenser dryer with the air flow system revealed a characteristic transition in the diffusion kinetics between the convection-drying and the condensing-drying regimes. The drying of immiscible ternary solution coatings showed a transition in the phase-separated morphologies from the isolated droplets to the elongated stripe patterns.

Drying-Induced Hierarchical Dimple Patterns on Partially Miscible Polymeric Films Under Ordered Convections

We describe the formation of hierarchical dual dimple patterns on polymeric sublayers by evaporation-induced interfacial stress. As water vapor condenses on partially miscible volatile sublayers, the resulting unstable wet-on-wet configurations trigger droplet assembly under ordered convection, giving rise to hexagonal traces of surface dimples, which coexist with highly ordered dimple arrays. Monitoring the time-dependent Marangoni numbers proved that the interfacial stress plays a crucial role in the transition between hierarchically and uniformly ordered dimple patterns.

Light-Tunable Solvent Drying in Photo-Responsive Solution Coatings

Solvent drying rate of azobenzene/acetone solution coating was tunable by linearly polarized UV light. The light exposure promotes the photo-induced conformational change of azobenzene from its trans to its cis form, as well as the molecular alignment perpendicular to a polarization direction. Drying experiments of the photo-responsive solutions revealed that angled irradiation of s-polarized light showed a peculiar angle-dependent drying behavior, while no dependence was observed for the p-polarized light irradiation. The photo-induced solvent drying was observed in a particular range of azobenzene weight fractions and UV light intensities, suggesting a promising way to tune solvent drying rates by light.

Agglomerating fluidization of nanoparticles in the vibration or magnetic field

The agglomerate characteristics of SiO2, TiO2, ZnO nanoparticles in the vibration or magnetic field are studied with respect to bed pressure drop and the average agglomerate sizes. The results showed that slugging of the bed disappeared and the measured agglomerate sizes decreased, so that the fluidization quality of nanoparticles was significantly improved due to introduction of vibration or magnetic field. A model of energy balance is proposed to estimate the agglomerate sizes in the vibration or magnetic field, respectively. The equilibrium agglomerate sizes calculated by this model are in reasonable agreement with the experimental values.

Cracking in Drying Silica‐Polymer Films: Morphology Transitions

We performed direct imaging of crack nucleation and propagation in drying aqueous silica‐poly(vinyl alcohol) suspension coatings. The suspension films are dried on a transparent hot stage to acquire time‐lapse video images, which enable us to measure the growth rate of nucleating and connecting cracks as a function of time. The direct imaging reveals a morphology transition from network‐ to star‐like patterns as decreasing particle/polymer weight ratio: the propagating cracks contact with each other at higher weight ratios, whereas they stop propagating in a finite drying time and remain as distinct patterns at lower particle contents. The measured nucleation rate of the distinct crack exponentially decays with an increase in particle contents.