Particles in air are called 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 are larger than any other forces acting on it; this will make it accurately move with the flow. In general, any particle 1-2 microns in size will be small enough to move with a moderate 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 follow air can be made 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 inhaling them by wearing an N95 mask. Micron-sized solid particles can penetrate the depths of your lungs and 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  makes sense to protect yourself from breathing them, and it’s easy to do: wear a respirator mask when working with these substances.

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

Smoke Wire

Figure 1 is an example of the smoke wire technique. This is a simple technique: oil is spread on a 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, although heat from the wire may distort the flow itself. Vegetable oil works, as does any petrochemical oil. The method’s drawbacks are that the wire often sags from heating, so tension must be applied, and the smoke only lasts as long as the oil does. To overcome these problems, people have used weights to stretch the wire and automatic oiling wipers . Oil smoke can also be made en masse and injected 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 .

Titanium Tetrachloride

A beautiful, dense white smoke can be produced chemically from titanium tetrachloride (TiCl4). The liquid TiCl4 can be painted on a surface. Once the liquid evaporates, the vapor 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

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 clumps in the presence of moisture, so it may need to be baked before use

Pollen and Spores

Pine pollen can float long distances on a breeze, as allergy sufferers can tell you. Although the particles are on the order of 50 microns, they have air-filled sacs, reducing their density . Although it is available as an herbal remedy , processing for human consumption involves grinding, which destroys the air sacs. Another very low density particle is lycopodium powder, which is the spore of clubmoss. Coincidentally clubmoss is sometimes called “ground pine.” However, lycopodium powder is highly flammable due to its high fat content; it gets 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 (Figure 2).

Figure 2: Lycopodium powder on a vibrating plate bounces to regions of less motion, generally where the plate has nodes; this phenomenon is called “cymatics” . This video was part of an advertising campaign for a loudspeaker. By Susi Sie, 2014.

Fog

Liquid aerosols (fogs) are easily generated. Clouds of micron-sized droplets can be made with air blast atomizers, medical nebulizers, ultrasonic humidifiers, dry ice, vape kits,  and stage fog generators. Micron-sized droplets convect easily, and they scatter light reasonably well, although scattering efficiency drops rapidly for smaller particles. Water fog has a great advantage in that, when made from clean water, it’s the only truly safe aerosol to breathe. Unfortunately (or fortunately, if your smoke detectors are very sensitive) water fog evaporates quickly, particularly in dry climates.

Fog generators fall into two categories: spray types, in which water is torn into droplets, and condensation types, where water is vaporized and then condenses into fog.

Spray Type Generators

Air blast atomizers, medical nebulizers, and ultrasonic humidifiers all create fog using a mechanical process that essentially shreds liquid water into tiny droplets. For example, a garden spray nozzle or household spray bottle forces water through a small orifice, breaking it into a wide range of droplet sizes depending on the pressure and details of the orifice used. Sometimes additional air is injected before the orifice to enhance the breakup process. Industry uses this type of sprayer extensively, from fire hoses to fuel injectors to paint sprayers .

 

A Bernoulli atomizer is a slightly different type of air blast atomizer that uses the Venturi effect to draw liquid up into the moving air stream, which shreds it. This type of atomizer, sometimes called a jet nebulizer, is easily made from a drinking straw, as shown in Figure 3. Simply cut a few centimeters of straw off one end, and stick the short straw upright, partially submerged in the liquid. Position the rest of the straw perpendicular to the top of the vertical straw, and blow into it forcefully so that the air stream passes over the top of the vertical straw. It works best if the vertical straw blocks about 1/3 of the horizontal straw’s outlet; this creates the flow acceleration and curvature needed to drop the pressure and pull the liquid up and into the air stream. The resulting spray  easily reaches across a restaurant table. You may wish to teach this technique to the children of parents that you find annoying. Inexpensive single-use medical nebulizers for inhaled drug delivery use this same technique, with an air stream provided by a small compressor.

For flow visualization, the droplet size needs to be around one micron, so that the aerosols trace the flow faithfully. This can be achieved from a sprayer, atomizer, or nebulizer by filtering the spray after it’s generated; that’s typically done by forcing the flow around a sharp curve. Larger particles impact the wall and drain back into the reservoir, while droplets able to follow the air flow move on downstream.

Ultrasonic Humidifiers

Ultrasonic or “cool mist” humidifiers generate fogs by tearing droplets off a water surface, but the physics are quite different from the spray technologies described above. An ultrasonic transducer is a thin piezoelectric disk, often a couple of centimeters in diameter, that vibrates at frequencies of 1 to 2 MHz – well above the range of human hearing. When submerged in a few centimeters of water, the transducer creates a small fountain of water through a phenomenon called “acoustic streaming” . This phenomenon is a worthy subject for flow visualization and is easily observed by removing the reservoir of any humidifier. Ultrasonic pressure waves travel up the fountain and break micron-sized droplets from the water surface , forming clouds of water fog, as shown in Figure 4. The mist contains any contaminants in the water, so it’s best to use distilled water in humidifiers and keep them clean.

Figure 4: A slow motion video of acoustic streaming and atomization above an ultrasonic transducer. By Ben Howard, 2010.

Larger transducers are available (~$10 ) for generating mist above bird baths or fountains, and they offer greater flexibility for flow vis applications.

 

Condensation Fog

In contrast to fogs created by breaking up liquids, these fogs are condensed from vapor, either through a cold source, like dry ice fog, or by heating a liquid to vaporize it, before cooling it to condensation.

Dry Ice Fog

What is the white cloud you get when you submerge dry ice in water?

1. Water aerosol
2. Dry ice aerosol
3. Dry ice particles
4. Carbon dioxide droplets

 

Here’s some information that might help you answer that question. Dry ice is solid carbon dioxide, and at atmospheric pressure, it sublimates – turns from solid to gas –  at  -78 C (-109 F). So at room conditions (or even under a few feet of water), it forms a very (!) cold gas. Any water vapor contacting that cold gas will condense into liquid droplets, if not into ice crystals. In fact, dry ice submerged in cold water quickly builds up a surrounding water ice shell, which is one reason hot water works better than cold water; it takes longer for the water to cool and form the ice shell. Meanwhile, the CO₂ gas bubbles up through the water, collecting water vapor into the bubbles as it goes. Figure 5 shows the process up close in slow motion video from Professor Tadd Truscott’s Splash Lab group.

Figure 5: Dry ice submerged in water quickly converts to CO₂ gas bubbles that fill with water or ice fog. Filmed at 2000 fps with backlighting by the BYU Splash Lab, 2012.

There are serious safety concerns when working with dry ice. First, it can freeze your skin easily. Second, it must never be put in a sealed container; it expands over 1000 times when converting from solid to gas, so it will explode any sealed container. Third, CO₂ is unhealthy to breathe in concentrated amounts, so be sure to use it only in ventilated areas.

Still, dry ice fog is fun and easy to play with. Dry ice fog is colder and, thus, denser than room air, so it sinks and can even be poured like water, as shown in Figure 6. Small amounts can be purchased at most large grocery stores for around $7 per pound. If you want to achieve the kind of thick fog effect seen in stage performances, you’ll need a professional stage machine ($2000) with a 35 gallon drum of continually heated water and more than 10 pounds of dry ice .

 

Heated Fog

When a liquid is heated to vapor and then condensed into aerosol, we call it a heated fog. Stage fog machines and vape devices use this process. Stage fog machines pump “fog liquid” (water with a little glycerine (25%?) or glycol to slow the fog’s evaporation) into a heated tube. When the vapor comes out into the room, it condenses into a warm fog. These machines are pretty inexpensive ($40) around Halloween; cheaper ones generally work only in bursts and need time between to re-heat their tubes. Professional machines can run continuously but cost around $1000. The fog comes out warm and buoyant but can be cooled by passing over a bed of ice or dry ice. Some machines have a compartment for this. Although stage fog in dilute concentrations has been found to be generally safe, many theater workers have reported irritated breathing  after extensive exposure

One of my favorite flow vis demonstrations is making fog rings. Take a cardboard box about 2 feet cubed, and cut a 4-inch hole in one side. Fill the box with stage fog and then tap the box on the side. A vortex ring made of fog will pop out. You can also buy toys that make fog rings, such as the Zeroblaster, which has its own fog generator built into it .

Vape devices (a.k.a. e-cigarettes) work the same way. Wicks draw the vape liquid to an electrically-heated element, where it is vaporized before being inhaled, as shown in Figure 7. The liquid is typically glycerin or propylene glycol with a variety of additives, including nicotine and flavorings. Although vape is considered less harmful than smoking tobacco, there is growing evidence that demonstrates vaping causes a range of illnesses, including emphysema and chronic obstructive pulmonary disease (COPD) .

Oil Mists

Oil can be used in place of water in Bernoulli nebulizers and maybe in ultrasonic humidifiers. Danger! Even benign oils like baby oil or mineral oil can coat your lungs and cause disease or death. In particular, it can cause exogenous lipoid pneumonia

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