The advantages of using transparent flow boundaries and liquids with matched index of refraction are demonstrated for internal flow research, when the internal boundaries of the flow are of complicated shape. In particular, a new technique involving aqueous solutions of zinc iodide is described, as applied to the study of secondary flows in aerodynamic devices used in ink jet printing. Zinc iodide solutions offer large freedom in the choice of index of refraction, without being excessively viscous. When used in conjunction with transparent conducting electrodes, the solutions afford internal electrochromic flow visualization as well. The electrochromic property is demonstrated for the case of classical circular Couette flow. In contrast with many other candidate liquids, zinc iodide solutions are safe and compatible with transparent plastics. These properties extend the scope of optical flow diagnostics, such as laser Doppler velocimetry.