Viscosity anomalies in liquid surface zones III. The experimental method

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Abstract
A measuring apparatus is described in which two round highly polished plates A1and A2made of fused silica are approached along their common central axis under the influence of a given force. A1is spherically formed with a curvature radiusR= 100 cm and mounted to the one end of an electromagnetically operated balance; A2is 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 ηGin the neighbourhood of a solid surface.The main measuring quantity is the velocity of approach dh'/dtof A1as a function of the plate distanceh'. The course of motion of A1can be registered with sufficiently high accuracy by a combination of a displacement transducer, a strain gauge measuring bridge, and axy‐recorder or an oscilloscope. In this way evidence can be given concerning ηG.Comparing the results of experiment with former theoretical considerations one has to realize that forh' < 3 · 10−4cm 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 withn‐heptane at 20°C, which shows no molecular long range order in its surface zone.The dependence of ηGonh' can be expressed by the formulawithBandmas parameters, which can be obtained experimentally; η is the viscosity of the undisturbed liquid. For a given plate distanceh' = 3 · 10−6cm the relative surface zone viscosities of benzene at 6°C and water at 20°C are ηG/7η = 4.0 and ηG/η = 5.4, respectively.