Direct thickness measurement of doctor-bladed liquid film on gravure roll surface

We provide experimental evidence that the liquid film remaining on a doctor-bladed gravure roll surface significantly influences the liquid transfer from the engraved roll surface to a moving substrate. The local free surface profiles of doctor-bladed liquid films were directly measured on both tri-helical grooves and lands (the nongrooved areas between grooves) at different roll speeds and blade thicknesses. The liquid film covered the roll surface with a finite thickness on the lands below the critical capillary number. An increase in the capillary number led to a transition from a fully filled to a starved configuration, in which the liquid barely remained on the lands and partly filled the grooves with a concave surface profile. The liquid transfer ratio monotonically decreased with a decrease in the liquid thickness on the gravure roll surface, obeying a single curve for different blade thicknesses. Furthermore, the decrease in liquid thickness promoted a flow instability transition from unstable dripping though a stable state with a uniform surface to a cascade.