The shear of thin liquid films

Optically smooth rubber pressed against glass provides a microconforming contact between which a thin uniform film of liquid can be sandwiched. The contact largely overcomes the problem of asperity interaction between surfaces. The rubber surface could be slid relative to the glass plate under controlled conditions and in this way it proved possible to study the shear properties of liquid films as thin as 40 Å. Results for water show that down to film thicknesses of 70 Å there is no increase in shear strength of the liquid. Below this thickness asperity interaction increasingly confuses the interpretation of measurements. If the water film contains a surface active agent, monolayer protection does much to reduce the friction between surfaces when the film is less than 70 Å. In the absence of monolayers the friction rapidly rises with decreasing film thickness towards the dry value.