MISCIBLE DISPLACEMENTS IN CAPILLARY TUBES .1. EXPERIMENTS

Experiments have been performed, in capillary tubes, on the displaceme nt of a viscous fluid (glycerine) by a less viscous one (a glycerine-w ater mixture) with which it is miscible in all proportions. A diagnost ic measure of the amount of viscous fluid left behind on the tube wall has been found, for both vertical and horizontal tubes, as a function of the Peclet (Pe) and Atwood (At) numbers, as well as a parameter th at is a measure of the relative importance of viscous and gravitationa l effects. The asymptotic value of this diagnostic quantity, for large Pe and an At of unity, has been found to agree with that found in imm iscible displacements, while the agreement with the numerical results of Part 2 (Chen & Meiburg 1966), over the whole range of At, is very g ood. At values of the average Pe greater than 1000 a sharp interface e xisted so that it was possible to make direct comparisons between the present results and a prior experiment with immiscible fluids, in part icular an effective surface tension at the diffusing interface could b e evaluated. The effect of gravity on the amount of viscous fluid left on the tube wall has been investigated also, and compared with the re sults of Part 2. A subsidiary experiment has been performed to measure both the average value of the diffusion coefficient between pure glyc erine and several glycerine-water mixtures, in order to be able to cal culate a representative Peclet number for each experiment, and the loc al value as a function of the local concentration of glycerine, in the dilute limit.