A photochromic flow visualization technique has been modified so as to allow the study of liquid-liquid interfaces. The technique utilizes two immiscible radiachromic dye solutions which remain colorless in the absence of ultraviolet radiation. The solutions turn dark blue when irradiated with ultraviolet light.
A nitrogen pulse laser has been constructed for use as a source of ultraviolet radiation. When sharply focused, the emitted beam of ultraviolet light produces a very narrow, well defined line in the solvent-dye solutions. The subsequent motion of the “activated trace” can then be recorded with high-speed photography for further analysis. This flow visualization technique was utilized to determine the flow fields on both sides of a forming drop’s liquid-liquid interface. A column was constructed so that liquid drops could be formed in, and ascend through a more dense surrounding liquid. Both the dispersed and continuous fluids were radiachromic. The nitrogen pulse laser was used to produce activated traces through both phases. High-speed movies were made to record the formation and subsequent motion of the traces. The high-speed movies were utilized as follows.
The activated traces were first digitized from a projection of the high-speed movies onto the screen of an x-y digitizer. Next, the location of the digitized points were corrected so as to account for optical refraction. At this point velocities near the drop interface were determined. Once the velocities had been determined, the shear stresses on both sides of the drop interface were calculated. It was found that a liquid-liquid interface can behave in a totally rigid manner, as predicted by some theoreticians.