Kitaro Adachi

EFFECT OF STEPWISE CHANGE OF DRYING RATE ON MICROSTRUCTURE EVOLUTION IN POLYMER FILMS

The effect of the drying rate of the solvent on microstructure evolution in thin films of immiscible polystyrene/polycarbonate blends was investigated by applying a stepwise change in air velocity. The polymer blend was dissolved in tetrahydrofuran and cast on a glass substrate to form a thin liquid film. The drying rate of the solvent was measured by a gravimetric technique, and the phase-separated structure in the film was observed by optical microscopy. The results indicated that cellular structures of droplet arrays were formed in the phases evolved primarily during the early stage of drying, and the droplets then coalesced in the intermediate stage to form bigger droplets. The size of the first evolved phase decreased with increasing time at a high drying rate, whereas larger droplets were obtained with longer high drying rate regimes. These results suggested that the droplets were induced by secondary phase separation which was strongly affected by the drying rate of the solvent.

Experimental investigation of air entrainment in a vertical liquid jet flowing down onto a rotating roll

Abstract The experimental results of air entrainment into a liquid jet flowing down onto a rotating roller were presented by means of the impinging jet method. The impinging jet was a substitute for a liquid curtain in an industrial coating process. Roll velocity at the onset of air entrainment was measured for different flow rates, nozzle diameters, nozzle heights, impingement angles, viscosities and surface tensions. The results indicated that the inertial force of the jet can delay the onset of air entrainment and that it acts most effectively on the dynamic wetting line when the flow rate is set to make the wetting line locate at the impingement point. The effect of inertial force decreased with further increasing flow rate and the onset velocity reached a constant value which depended only on the physical properties of the fluid. The results of dimensionless analyses indicated that there are two critical values of the Reynolds number, i.e., the one at which the onset velocity has a peak and another above which the onset velocity does not depend on the Reynolds number. In the Reynolds number above the second critical value, major forces acting on the dynamic wetting line were viscous drag force and surface tension, and the flow mechanism at the onset of air entrainment was the same as that in plunging tape flow. Application of the results to curtain coating flow indicated that the critical Reynolds number exists not only in plunging jet flow but also in plunging curtain flow.

Flow-induced stripe pattern formation in phase-separating fluids

We demonstrated that the introduction of a temperature gradient along the free surface induces a particular stripe pattern in phase-separating fluids. The horizontal temperature gradient drove lateral-periodic spiral liquid motion flowing from warmer to cooler places due to thermocapillarity. Properly chosen polymer compositions and initial film thicknesses in ternary solutions allowed us to promote a phase separation in the presence of spiral flow, which assembled the demixed polymer droplets along the flow-stagnation lines. The resulting assembled phases aligned in the temperature gradient direction and eventually formed periodic polymer stripes involving the same spacing as that of the flow axis. The critical condition for the stripe pattern formation was given by the ratio of two relevant film thicknesses, i.e. the thickness for the onset of the phase separation and that for the cessation of liquid motion.

Evaporation-induced pattern formation in polymer films via secondary phase separation

The ordered necklace-like or cellular pattern formation has been investigated for ternary polymer solutions in which primary and secondary phase separations were induced by the solvent evaporation. The patterns consisted of regular droplet arrays of various diameters, which arose in the pre-existing polymer phases via the secondary phase separation. The pattern formation was enhanced with increasing air velocity and ambient humidity, while it was suppressed with decreasing initial film thickness. The regular pattern was independent of surface wettability of the solid substrate. We ascribe the morphology change to the evaporation-induced surface tension driven convection, which re-arranged the droplet distributions from disordered to flow-induced ordered patterns.

Stripe pattern breakup in evaporating liquid layer on a plane with horizontal temperature gradient

We demonstrated that a temperature gradient along the free surface of the evaporating liquid layer induces a transition of liquid motion from the stripe-like longitudinal rolls into ordered convection cells. Experimental observations showed that the breakup of the stripe-like convection occurred at a pair of Marangoni numbers based on the two temperature gradients in the horizontal and vertical directions. We also demonstrated that silica particles accumulated by the convections showed ordered monolayer or randomly-distributed multilayers, which result from the transition of the liquid motion.