in Wiley InterScience (www.interscience.wiley.com).We carried out numerical simulations of drying-induced surface roughening of a moving liquid layer under periodic air blowing conditions. Using spatially non-uniform Biot number profiles along the coating surface, the solutions of coupled diffusion/heat transfer equations reveal a periodic growth and relaxation in concentration gradients in the vicinity of the liquid-gas interface. The resultant interfacial stress variations promote particular asymmetric surface topographies that grow and level in a sequential manner. The unique surface roughness is identified by considering time scales required for the web motion, the stress-induced surface evolution, and the pressure-driven leveling. Predicted drying maps represent the surface roughening behavior as regions in parameter space.

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We provide experimental evidence that spontaneous cracking in silica-poly(vinyl alcohol) aqueous suspensions is significantly suppressed at high Peclet numbers (Pe). The cracking event in drying suspension coating was quantified by measuring numbers of nucleation points per unit area from acquired time-lapse video images. Direct imaging revealed that increases in Pe, i.e. the film shrinkage rate relative to the particle diffusion rate, postponed the secondary crack nucleation and the inter-crack connection to thicker films. The primary nucleation was first suppressed and then enhanced as increasing Pe, resulting in a remarkable morphology transition from ladder-like to distinct star-like cracks.

We present a lubrication model of thermo-capillary flows in moving volatile liquid film coatings. The forced air impingement from a slit nozzle onto the moving coating imposes an axi-symmetric surface temperature distribution, and thus the local surface-tension gradients on the liquid surface. Despite the symmetric temperature profile, local thickness variations became asymmetric and exhibited a particular ridge in downstream and a depression in upstream. The competing feature between the surface-tension-driven and the pressure-driven flows gives a characteristic growth and decay in the surface roughness as the temperature profile travels in the opposite direction to the moving coating. The model prediction showed that the surface roughness was first enhanced and then suppressed with increasing the impinging air velocity, suggesting some directions for achieving more uniform coatings at higher speeds.

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