Masakazu Muto

Droplet spreading on hydrophilic solid surfaces is important in engineering processes such as semiconductor cleaning and coating, where the initial wetting behavior influences process performance. The contact angle extracted from experimental images or numerical interfaces is a finite-scale apparent quantity, and its value can depend on the observation scale used for interface fitting. This dependence is especially important on strongly hydrophilic surfaces, where the interfacial slope near the contact line is small. In this study, the observation-scale dependence of the apparent dynamic contact angle was investigated for a water droplet impacting a UV-treated hydrophilic silicon wafer. The results of high-speed imaging experiments were compared with volume-of-fluid simulations using an experimentally derived dynamic contact-angle boundary condition. The same local curve-fitting procedure and the same finite observation-scale definition were applied to both experimental images and numerical interfaces. The measured apparent contact angle depended on the observation scale; increasing the observation scale tended to reduce measurement noise, whereas an excessively large observation scale reduced the locality of the fitted interface region. The apparent dynamic contact angle extracted from the computed interface also depended on the grid resolution. Under the present conditions, grid refinement and selection of an observation scale containing sufficient interface-sampling points reduced the discrepancy between the apparent dynamic contact angles obtained from simulations and experiments. Applying the same local curve-fitting procedure and observation-scale definition to both data sets provides a common finite-scale framework for determining and comparing apparent dynamic contact angles from experimental images and numerical interfaces, complementing evaluation based on macroscopic spreading behavior.

Inkjet (IJ) printing is increasingly used not only for graphic applications but also for fabricating functional films and devices. However, the rheological behavior governing printing quality, particularly the extensional properties that enable stable jetting and precise droplet placement, remains insufficiently understood. In this study, the extensional rheological properties of dilute polyethylene oxide solutions containing glycerol were investigated using piezo-driven extensional rheometry. The glycerol content was varied to adjust the solvent viscosity, and the effect of the nozzle diameter on the material properties was examined under experimental conditions representative of IJ printing. The results show that the extensional relaxation time λE depends not only on the polymer concentration and molecular weight but also on the solvent viscosity and nozzle diameter, indicating that the characteristic length scale strongly influences the material response. Three characteristic times, namely the onset and termination of the elasto-capillary regime and the time of satellite droplet formation, were systematically organized using λE as the governing timescale of jet-breakup dynamics. The developed unified framework links the extensional relaxation time to jet-breakup dynamics while explicitly accounting for the solvent viscosity as an intrinsic material property and nozzle diameter as a characteristic length scale. Overall, these findings demonstrate that the extensional relaxation time λE serves as a key design parameter for predicting the filament lifetime and satellite droplet formation in IJ printing.

The present study proposes a novel full-field extensional rheo-optical technique to investigate the relationship between the rheological properties and internal structural deformation of complex fluids under uniaxial extensional flow. Macroscopic viscoelasticity from rheological measurements and microscopic birefringence from optical measurements are integrated to evaluate the microstructural deformation and orientation of the fluids under extensional stress. The proposed technique integrates a liquid-dripping method with a high-speed polarization camera to measure the extensional stress and flow birefringence simultaneously. In the liquid-dripping method, temporal evolution images of the liquid filament diameter for fluids dripping from a nozzle are measured to obtain the extensional stress loading on the filament. These images are acquired using the high-speed polarization camera that captures full-field two-dimensional (2D) birefringence with high spatiotemporal resolution. Wormlike and networked micellar solutions of cetrimonium bromide (CTAB) and sodium salicylate (NaSal) with varying concentrations of CTAB and NaSal are employed as the measurement targets. Consequently, we successfully visualized temporally developing images of the flow birefringence field of uniaxially extending micellar solutions induced by the orientation of micelles. Furthermore, the proposed technique supports investigating the conditions for establishing the stress–optical rule, which is the linear relationship between stress and birefringence. The stress–optical coefficient, which is a proportionality constant indicating the sensitivity of birefringence to stress, is analyzed from these measurements. The stress–optical coefficient under uniaxial extensional flow, obtained using the proposed technique, is confirmed to be comparable to that under shear flow and to depend on the number of oriented micelles.

The primary objective of this study is to enhance the understanding of hemodynamics by developing innovative experimental techniques that elucidate the interactions between solid and liquid stresses within circular vascular phantoms and their blood analogues. The non-Newtonian behavior of blood, coupled with the elastic properties of vascular structures, creates a complex fluid–structure interaction that poses challenges for the comprehensive analysis of stress fields using conventional numerical and experimental methods. To experimentally investigate the dynamic stress interactions between solids and liquids, we propose a novel noncontact, unsteady optical system for measuring dynamic stress fields. This system leverages a dynamic birefringence field measurement method that incorporates a high-speed polarization camera, as refined in our previous studies. Based on the stress-optic law, the measured birefringence field exhibited a spatial intensity distribution that mirrored that of the stress field. The validity of this relationship in fluids was confirmed by fluid birefringence field measurements taken using a rigid circular channel. This innovative approach enables the quantitative assessment of solid–liquid birefringence interactions within a birefringent polymer gel channel and a birefringent nanocellulose dispersion, simulating the elasticity of vascular walls and the shear-thinning behavior of blood, respectively. Moreover, the proposed method successfully visualizes dynamic birefringence fields within vascular phantoms and blood analogues under cardiac cycle-mimicking pulsatile flow, demonstrating its potential for unsteady stress measurements in solid–liquid systems. By overcoming the previously daunting challenge of visualizing dynamic stress interactions between solids and liquids, the proposed technique holds immense promise for advancing our understanding of hemodynamic phenomena.

To assess the extensional properties of viscoelastic liquids with low viscosity, we explored a method employing a piezoelectric drop-on-demand (DOD) head. This method ejected polymer solutions of dilute concentrations, which offered a higher suitability than the liquid dripping (LD) method. An exponentially decaying regime of filament diameter was observed, like the elasto-capillary regime of the LD method. The established power law relation between extensional relaxation time and polymer solution concentration holds in the dilute regime. The findings indicate that the filament decay behaviors observed for the DOD method with jetting flow and the LD method with dripping flow are comparable.