Particles in air are aerosols, and may be liquid or solid. For flow vis purposes, we’ll call solid particles smoke, and liquid particles fog. Now, how do we make or get aerosols? Let’s start with smoke.

Smoke

For a solid particle to be a good flow tracer, it needs to be small enough that drag forces on an individual particle will be larger than any of the other forces acting on it; this will make it move accurately with the flow. Generally, any particle that is one or two microns in size will be small enough to move with a moderate speed air flow (10 m/s), although it may not follow the smallest scales (tiniest eddies) of a highly turbulent flow.

Solids that are small and/or light enough to be good tracers in air can be generated in various ways: combustion, a chemical process, ground up from a mineral or even grown on trees. Caution! whatever the source, you should protect yourself from breathing them with an N95 mask. Micron sized solid particles can penetrate into the depths of your lungs, where they can cause serious respiratory illnesses: asthma, bronchitis, lung inflammation and eventually lung cancer or chronic obstructive pulmonary disease leading to heart failure. Any solid particle 2.5 micron or smaller falls into the classification of PM2.5 air pollution whose adverse effects on human health are well known. It only makes sense to protect yourself from breathing it, and it’s easy to do: wear a mask when working with these substances.

Figure 1: Smoke wire visualization of airflow over a mannikin in a model operating room. By Jeff Payne, James Kostrzewa, Thomas Rachlin, James McNeill for Team Second assignment, 2009.

Figure 1 is an example of the smoke wire technique. This is a simple technique: an oil is spread on at thin wire which is heated by an electrical current until the oil smokes. The resulting particles, presumably soot, are small enough to convect with air flows, however the heat from the wire may distort the flow itself. Vegetable oil can be used, or any petrochemical oil. The main drawbacks from this method are that the wire often sags from heating, so tension must be applied, and the smoke only lasts as long as the oil does. A variety of systems have been used to overcome this problems such as weights to stretch the wire and automatic oiling wipers . Oil smoke can also be made in quantity and injected in streams into a wind tunnel; many famous images were made by F.N.M Brown at the University of Notre Dame through the 1960s using this technique .

A beautiful, dense white smoke can be produced chemically from titanium tetrachloride (TiCl4). The liquid TiCl4 can be painted on a surface. The liquid evaporates, and the vapor then reacts with water vapor in air to form micron sized titanium dioxide particles. Unfortunately, this reaction also creates hydrochloric acid vapor, which is corrosive to metals and human lungs . Nevertheless, this technique has been used to mark boundary layer flows .

Alumina (aluminum oxide) is often used for particle tracking in combustion flows, since it is refractory. It is readily available in micron and submicron sizes, since it is commonly used as an inexpensive polishing powder. It tends to clump in the presence of moisture, so it may need to be baked before use

Pine pollen can float long distances on a breeze, as allergy sufferers can tell you. Although the particle size is on the order of 50 microns, it has air-filled sacs, reducing its density. It is available as an herbal remedy . Another very low density particle is lycopodium powder, which is the spore of a clubmoss. Coincidentally clubmoss is sometimes called  a ‘ground pine’. However, lycopodium powder is highly flammable due to its high fat content, and has been used as a flash powder in magic acts and physics demonstrations . Video artist Susi Sie used lycopodium powder to great effect in an advertisement that combines a breeze with cymatics.

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