This paper presents a technique for directly observing the increase in ink viscosity as ink solvent evaporates from an inkjet printhead. Depending on the balance between the generation of a solvent concentration gradient due to ink solvent evaporation and the diffusion that flattens this gradient, the ink viscosity may increase across the nozzle or only near the gas-liquid interface in particular. For printers using single-pass on-demand inkjet printheads with water-based pigment inks, it is necessary to understand the characteristics of such ink thickening phenomena in order to achieve high reliability and low ink wastage. To achieve this objective, a combination of dynamic light scattering and optical coherence tomography was used to observe the distribution and temporal changes in the motion of particles contained in the ink filled inside the nozzle, and to observe the thickening process in-situ. In particular, we attempted to understand the characteristics of the thickening phenomenon that occurs in ink as a result of evaporation by capturing changes over time in the particle diffusion coefficient and its distribution in the nozzle cross-section, including the depth and radial directions. For the observation experiment, a glass capillary was used instead of an inkjet head nozzle, and a model ink was used instead of a water-based pigment ink. Three types of glass capillaries with different inner diameters were used to compare the effects of evaporation rate. For the model ink, a colloidal solution of silica particles dispersed in a mixture of water and glycerol was prepared. The observation results showed that the particle diffusion coefficient tended to decrease and become more distributed with decreasing inner diameter in the depth direction of the glass capillary. On the other hand, in the radial direction, there was almost no deviation in the particle diffusion coefficient for any inner diameter of the glass capillary, indicating that under the present conditions, thickening progressed in a diffusion-dominated manner.

When a high temperature sphere covered with a poor conductive material is cooled by liquid, the temperature as the quench temperature which causes quenching rapid cooling of the material significantly shifts to a higher temperature by an effect of the poor material. We recently proposed a new idea[1] whether a stable phase change immediately after the contact of liquid and solid can be maintained continuously on a contact surface between liquid and solid or not. According to this idea, its condition strong depends on a corresponding critical heat flux which can be transferred from the solid to the liquid and a critical time when its phase change takes place after the solid-liquid contact. In the present study, the governing equation for among the sphere and the poor materials and cooling firstly is proposed and then its exact solutions is derived for each one. The effect of the poor material how it controls the heat flow to the liquid can be clearly explained and also the reason that the temperature at the occurrence of rapid cooling shifts to higher temperature is understood. It reveals that the critical time for the sphere covered with the Teflon becomes longer than that for bare one.