Liquid forced through a small orifice under pressure will come out Is a spray of fine droplets, or mist, rather than as a stream, or jet. A 'squirt gun' works the same way, as does the kitchen faucet when you turn on the tap. How come the water from them comes out in a stream instead of as a spray? An interesting question that tells there is more going on in fluid dynamics than meets the eye! The physical properties of liquids play a fundamental role. For water-based solutions in particular the properties of viscosity, surface tension, and molecular adhesion play very significant roles in the formation of an aerosol spray. As liquid begins to move inside of a tube, all of the component materials move equally and smoothly, and without turbulence. Picture the entire mass just sliding through the tube. This is called 'laminar flow'.
Materials in contact with the sides of this conduit, however, do not move as freely as do the materials in the middle of the tube. Through molecular adhesion, they experience friction by 'sticking' somewhat to the hose or pipe material. This tends to make the outermost materials move more slowly than the materials in the center of the tube, and that introduces turbulence into the flow. The faster the material moves through the tube, the greater the turbulence that is introduced. As the liquid leaves the tube, as through a faucet or a nozzle, surface tension comes into play, tending to separate the liquid stream into a line of small, round blobs. This is especially true for water, which possesses a very high surface tension. The effect is very easy to see if you just watch a smooth, single stream of water change, and break up into spherical segments. Surface tension continues to act on these bits, breaking them up into even smaller spherical bits until they hit the ground.
To make a fluid move faster through a tube, one must increase the pressure applied to it. As the liquid moves faster, the turbulence increases and it more readily separates into smaller and smaller drops. Each hole has a characteristic threshold pressure determined by its size and shape: below that pressure, the liquid stream maintains its integrity and exits as a stream. At pressures above the threshold pressure, turbulence wins and the stream exits as drops whose size is inversely related to the applied pressure. An aerosol spray can is a self-contained pressure spraying system that uses a quantity of pressurized gas to push out the liquid. Once the valve is open, the pressurized gas pushes the liquid up the tube, through the valve, and out through the small round hole of the spray nozzle. At this point, the effects of liquid flow, turbulence, pressure, and nozzle size all combine to produce a mist pattern of fine droplets.
About the Author
|Richard M. J. Renneboog, MS|
Richard M. J. Renneboog is an independent private technical consultant and writer in both chemical and computer applications. Endeavors have included preparation of scripts for instructional and promotional video, corporate website design, curriculum development for training in advanced composites technology, and development.