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.