We performed experiments using a stationary plate-rotating roll system to observe air entrainment by the rotating roll into the liquid hold between the roll and flat plate both of which were set up horizontally and in parallel. We observed and measured the following points with changing liquid viscosity, gap size and roll speed: 1)the influence of the distortion and fluctuation of the meniscus line; 2)the sizes of entrained bubbles and their volumetric flow-rate; 3)the critical roll speed for the onset of bubble entrainment.
In case of low viscosity and low roll speed, the meniscus line, which was away from the point of the minimum gap, was stable and rarely fluctuant. The meniscus line moved inward with increasing roll speed, and air started to be entrained violently in large bubbles like boiling when the roll speed was over a critical value. Then, the meniscus line was almost straight, and fluctuating finely. However the fluctuation did not seem to affect the air entrainment under obs … More ervation. In case of high viscosity, air entrainment occurred uniformly and dispersedly before the meniscus line moved inwared. When the roll speed increased much more, the meniscus line moved inward, air was entrained more actively all over the meniscus line, and vortex motion due to violent entrainment was seen at several fixed points.
Entrained bubbles in case of low viscosity were a few millimeters in diameter and large enough to be visible by the naked eyes. In case of high viscosity, however, the bubbles were very fine like vapor or smoke. When the bubble size was less than 10mum, it was difficult to determine the bubble size because the air in such a small bubble dissolved very rapidly to change in size. The bubble size became smaller as the capillary number became larger, and the number of entrained bubbles per time increased. The critical roll speed for onset of air entrainment decreased clearly as the viscosity increased.