A Jet Viscometer with Variable Rate of Shear

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Abstract
A viscometer consisting of a glass jet which is inserted in a closed system is described. With this instrument high rates of shear of the order of magnitude of 100,000 sec.−1 can be reached for hydraulic oils. Observations on straight mineral oils of sufficiently high viscosity to exclude turbulence, on refined rape oil, and on castor oil, showed that Poiseuille's law was obeyed, i.e., the rate of flow was strictly proportional to the pressure difference applied to the jet. On the other hand, for colloidal solutions temporary viscosity reductions became apparent when 1/t, the reciprocal value of the time of flow of a given volume, was plotted against p, the pressure difference, and when it was ascertained that the lowest value which the viscosity would reach was still high enough to exclude the occurrence of turbulent flow. Whenever the condition of turbulent flow applied, the curvature of the (1/t)—p curve was in the opposite direction so that in those cases in which the temporary viscosity reduction would bring the system within the range of turbulent flow a change in direction and curvature of the (1/t)—p curve would take place with the result of either indicating an asymptotic approach to a straight line or, in more pronounced cases, showing a point of inflection. For colloidal solutions the results suggest an analytical criterion for assessing the transition from laminar to turbulent flow at high rates of shear. In the laminar region log (pt) was found to decrease linearly with increasing values of the logarithm of the mean rate of shear. With the instrument described here laminar flow was found to persist when Reynolds' numbers up to about 44,000 were encountered. However, the critical Reynolds' number is not necessarily the same for the same oil and two different jets as the beginning of turbulence may depend upon the geometrical shape of the jet.
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