The Surface Slit Viscosimeter and the Viscosity of Monolayers

Abstract

PERHAPS the greatest advantages of the surface viscosimeter which makes use of the flow of a film through a rectangular slit are (1) that the theory is relatively simple, and (2) since there are no moving parts, the substrate, usually water, is not set in motion by the apparatus, but only by the film. In a recent letter in NATURE¹, Harkins and Myers show that the flux of the film through a narrow slit varies inversely as the cube (a³) of the diameter of the slit. Since this neglects the effect due to the viscosity of the substrate, η₀, it seems to be essential to develop the complete theory, and this has been done by one of us (Kirkwood). For a slit of length l, depth h, and width a, the area flux A, is given by where α = (f₂–f₁)/l is the pressure gradient in the slit. If h/a is large relative to unity, this may be represented by a practically adequate approximation as.