What is a fluid?

   Matter in a state in which only equal pressure from all sides can be supported without immediate distortion is a fluid. Liquids and gases are fluids. When fluids move from place to place or change shape under inequalities of pressure, they are said to flow.

   Fluids have viscosity, or resistance to flow, because of frictional forces between molecules. Gases have smaller viscosities than liquids because the molecules of a gas are farther apart. The unit of measurement of viscosity is the poise. The viscosity of a fluid changes with temperature and pressure.

   Fluids flow in several ways. A particular type of flow is the result of the fluid properties, the obstacles in the path of fluid flow, and the flow velocity. Laminar flow is the movement of parallel filaments of the molecules of the fluid past each other. The entire region of flow appears to divide into layers. If a pack of cards is bent, each card slips past its neighbor somewhat as the layers of molecules in a fluid slip past each other. Turbulent flow occurs when fluids move rapidly. In turbulent flow the molecules have no regular path or constant velocity. Water in almost all rivers moves by turbulent flow. Streamline flow is similar to laminar flow except that the sheets of fluid bend around objects with smoothed outlines.

   Bernoulli's principle shows that when the velocity of a flowing stream of fluid is increased at a certain point, the pressure of the fluid is decreased at that point. Many devices, using air for the moving fluid, operate successfully because Bernoulli's principle is true. Atomizers and spray guns have a narrow-tipped barrel through which air is blown. The narrow tip forces the air to increase in velocity. Just where the air leaves the barrel tip, an open tube is placed that extends down to the liquid in the atomizer or spray gun. As low pressure is created above the open tube, liquid is sucked up the tube and out into the airstream.