There are many choices available for particles for flow visualization in water. The primary requirement is that the particles be 100 microns or less in diameter, and that they not contribute any undesirable physics, such as by dissolving or being toxic. Neutrally buoyant particles are desirable, but not often required. There are two main categories: solid particles, and hydrogen bubbles. We’ll start with solids.
Rheoscopic Fluids
If the particles are oblong, shiny and densely seeded, you get a rheoscopic fluid, literally ‘current showing’. The particle orientation, and thus the shine, depends on the fluid motion to orient the particles; it won’t work in a stagnant flow. Exactly how the orientation depends on the flow state is complex, limiting this technique to qualitative applications to date. According to mica or aluminum flakes were used for this purpose throughout most of the 20th century. However, mica and aluminum are much denser than water, and even 100 micron particles tend to settle out rapidly, limiting their use to relatively high speed flows. Still, metal oxide coated mica flakes are chemically inert, nontoxic and refractory (don’t burn or melt). They are used extensively in paint pigments, and are sold inexpensively in art supply shops and online as beautifully glittery powders. Jaquard Pearl Ex Interference Blue is one of my favorites.
In the mid 1960s Kalliroscope, a fluid developed by artist Paul Matisse, was popular. It contained crystalline guanine, derived from fish scales, which was shiny, had a small particle size ( 60 microns) and a density not much more than water’s. However, it became quite expensive, and production ceased in 2014. An inexpensive alternative, discovered by Borrero-Echeverry, Crowley and Riddick in 2018 is microscopic stearic acid crystals, which is easily and cheaply extracted from shaving cream. It is not quite as shiny as Kalliroscope, but its density is largely identical to water, and solutions of it retain their optical properties for years without settling out. One slight disadvantage to be aware of is that the crystals change phase/melt at temperatures above ~50 C. Figure 1 shows flow around a circular cylinder using stearic acid crystals derived from shaving cream.
Solid particles for water
Some of the particles appropriate for water have already been discussed in the context of particles for air: alumina which sinks over time, and pine pollen and lycopodium powder, which float on the water surface. Tap water may contain sufficient particulates that no additional particles are needed, especially if laser light sheets are used for illumination.
If neutral buoyancy is required, corn starch powder generally has a range of densities close to that of water. It is inexpensive and nontoxic, so it can be dumped down the drain without worry for the environment. However, take care to keep concentrations dilute so that particle-particle interactions don’t occur, unless you are really trying to create oobleck, with all of its fascinating non-Newtonian behavior .
If a specific density particle is required, glass or polymer microspheres are available with various diameters and optical properties, but these tend to costly, at $10 to >$200 per gram . You won’t want to dump these down the drain.
Hydrogen Bubbles