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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 be costly, at $10 to >$200 per gram . You won’t want to dump these down the drain.
Hydrogen Bubbles
In the classic film Flow Visualization by Stephen J. Kline in 1963, part of the National Committee on Fluid Mechanics Films series , flow in a water tunnel is visualized using hydrogen bubbles to illustrate the basics of streamlines, streaklines, timelines and pathlines (Figure 2). A major benefit that hydrogen bubbles have over other possible markers like a dye is that the bubbles dissolve fairly quickly, so they don’t contaminate a recirculating water channel system or water tank as a dye does. They also scatter light well, reducing the need for intense lighting. However, like other boundary marking techniques, they are best in laminar flows.
Figure 2: Flow Visualization by Stephen J. Kline, part of the National Committee on Fluid Mechanics Films series . Copyright 1963, Educational Services Incorporated.
Hydrogen bubbles are formed by electrolysis. A voltage is applied in water between a cathode and an anode; water molecules (H<sub>2</sub>O) are split into hydrogen molecules (H<sub>2</sub>) and oxygen molecules (O<sub>2</sub>) as shown in Figure 3. Note that you get twice as much hydrogen as oxygen.
Figure 4 shows a simple demonstration setup, using the graphite in pencils for both the cathode and anode, but to make hydrogen bubbles for flow visualization, a thin wire is used for the negative electrode ( the cathode, at least in galvanic cells), where hydrogen is generated, while a larger, sturdier anode where oxygen is generated is placed downstream of the region of interest. Steel wire of 2 mm diameter can be used for the negative electrode but finer (< 1µm) platinum wire is preferred . Smaller diameter wire is better because the hydrogen bubble diameters are on the order of the wire diameter, and smaller bubbles will have less buoyancy. Platinum is better than steel; it is less vulnerable to corrosion, and ductile enough to be formed into smaller diameters. However, the smaller diameter wires are much more fragile, so you’ll have to experiment to find the best combination of parameters.
A 9 volt battery can be used with a short wire, but for a larger setup such as the multiple wire rake shown in the video above, power supplies of up to 300 VDC and several amps of current will be needed. The water needs to be conductive; table salt can be added to achieve this, although sodium sulfate is reported to be better . Sodium sulfate is a laxative and is not considered hazardous by the EPA . However, electricity mixed with water can be fatal to humans. Discharging a 9 volt battery through your body won’t damage you (ever licked one?) but even a low voltage (<50 V) with sufficient current (10 mA) can hurt . Your dry skin can protect you but wet skin conducts well. Make sure you are not part of the electrical circuit!
A few more points to keep in mind: 1) Any metallic ions in the water are attracted to the electrodes, and essentially electroplate onto them them over time, which fouls them. To clean the electrodes, the current must be reversed every once in a while. 2) The cathode wire must be kept smooth and kink free because kinks will make large bubbles. 3) A certain amount of added salt will enhance bubble formation, but too much will result in overly large bubbles. 4) The chloride in table salt will make a small amount of free chlorine in the water during the electrolysis process – this is handy – it keeps your water disinfected. On the other hand, you won’t want to use the hydrogen bubble technique if studying marine animals.
Electrolytic Precipitation Technique
This method is used to precipitate metals out of a water solution . Similar to the hydrogen bubble technique it involves electrolysis. Opposite to the hydrogen bubble method, a metal wire or plate is the anode (positively charged), placed where the tracer is desired, and the cathode is downstream. The anode metal dissolves/erodes due to the electrical current, and forms a micron sized solid precipitate . This technique was used to create the most famous Karman vortex street image (by Taneda 1977) that graces the cover of Van Dyke’s Album of Fluid Motion . (Too bad it’s copyrighted and I don’t know who to contact for permission to include it here.) One advantage of this technique is that is requires only 10 VDC and 10 mA, and makes a beautiful white ‘smoke’ that follows the flow faithfully. A disadvantage is the generation of metal precipitates that must be properly disposed of.
