Viscosity anomalies in liquid surface zones III. The experimental method

Abstract

Abstract A measuring apparatus is described in which two round highly polished plates A 1 and A 2 made of fused silica are approached along their common central axis under the influence of a given force. A 1 is spherically formed with a curvature radius R = 100 cm and mounted to the one end of an electromagnetically operated balance; A 2 is plane and fixed. Both fused silica plates are immersed in a liquid to be investigated in respect to the existence of an enhanced surface zone viscosity η G in the neighbourhood of a solid surface. The main measuring quantity is the velocity of approach d h '/d t of A 1 as a function of the plate distance h '. The course of motion of A 1 can be registered with sufficiently high accuracy by a combination of a displacement transducer, a strain gauge measuring bridge, and a xy ‐recorder or an oscilloscope. In this way evidence can be given concerning η G . Comparing the results of experiment with former theoretical considerations one has to realize that for h ' < 3 · 10 −4 cm the inevitable surface roughness of both silica plates must be taken into account. This can be done on the basis of the roughness profilogram of the surfaces and introducing an empirical function derived from measurements with n ‐heptane at 20°C, which shows no molecular long range order in its surface zone. The dependence of η G on h ' can be expressed by the formula with B and m as parameters, which can be obtained experimentally; η is the viscosity of the undisturbed liquid. For a given plate distance h ' = 3 · 10 −6 cm the relative surface zone viscosities of benzene at 6°C and water at 20°C are η G /7η = 4.0 and η G /η = 5.4, respectively.