When thick liquids are poured or shifted suddenly, they move slowly due to inertia, which is the resistance to sudden movement. If the same liquid is placed between two plates that are quickly turned in opposite directions, the liquid will resist the turning motion due to its shear force, or the resistance to being pulled apart. Both of these effects can be used in viscous damping equipment, which is used for shock absorbers and vibration control.
As a viscous liquid is exposed to shear, it attempts to remain in place, and will only move when the shear force overcomes the resistance. The movement of the liquid creates heat that must be removed eventually to prevent equipment damage. Large dampers often contain heat sinks or radiator fins to allow air to cool them.
Viscosity effects are velocity-dependent, which means they will show greater resistance when the movement forces are greater. Very small vibrations may not be removed well with viscous damping systems alone. Many systems use a combination of springs and viscous fluids to provide a better range of vibration reduction. Damping is commonly used in parts of the world where earthquakes are common, and buildings can be constructed with large dampers that isolate them from the ground. When an earthquake occurs, the resistance to movement limits the amount of vibration transmitted to the building structure.
A linear viscous damping system is often used in building earthquake systems. The damper typically contains a piston inside a cylinder filled with viscous fluid. Small ports or holes allow fluid to move back and forth in the cylinder, but the bulk of the fluid resists any movement of the piston. The cylinder can be connected to the building steel, and the piston shaft is connected to a foundation block installed in the ground.
Shear effects can be utilized in devices called rotary dampers, which uses a fluid between two parallel plates. Rotary dampers can be used to reduce vibration in machines and will operate as a speed brake to slow moving equipment. They can be placed under equipment in a horizontal position to dampen vibration in multiple directions.
Viscous damping uses clear liquids, such as silicones, water, or hydrocarbon oils. Mixtures of liquids and solids can have damping effects, but the effects are different. Adding solids can create a Bingham plastic effect, which is a low resistance to flow after an initial shear stress is overcome. An example is a mixture of cornstarch and water, which pours easily but acts like a solid when shaken or struck. Tomato ketchup is another Bingham plastic, which is why it stays in the bottle when turned over, but will flow easily with a shake or tap on the container.