The theory of absolute surface shear viscosity V. The effect of finite ring thickness

In the theory of Couette surface viscometers it is convenient to idealize a rotating ring thrust into a fluid interface supporting an adsorbed surface film as making a knife-edge contact of zero thickness. The validity of this assumption is examined by calculating from theory the flow patterns to be expected when the ring makes a contact of finite thickness with the interface. The zero thickness idealization is found to be valid for films of moderate to large surface viscosity, but it fails badly for films of low surface viscosity and places a limit upon the sensitivity of the ring method.