A visual study of liquid motion in vertical capillaries under conditions of inverse density gradients

Abstract

A visual study applying close-up photography was used to study the motions of aqueous KMnO 4 solutions inside vertical capillaries under conditions of inverse density gradients. The method permitted the recording of the disturbance modes during the evolution of the convective instability. In the present work three types of such modes were identified. Irrespective of their sequence the final quasi-steady state was observed to be always the same. A criterion of neutral stability of these motions was proposed by Taylor for the measurement of molecular diffusivity of liquids. In this investigation, initial density gradients, capillary radius and duration of experimental runs proved to be important parameters for such measurements. It was found that Taylor's assumption of negligible axial molecular diffusion compared to convection is not valid under conditions of low initial density gradients combined with narrow capillaries. In addition, a criterion for the determination of the appropriate experimental run time was established.