What is Stiction?

Stiction is a portmanteau of “static” and “friction” used to describe situations in which objects which are not sliding remain fixed in place. “Friction” is usually thought of in the context of moving objects, so “static” is added to make it clear that the objects are stationary, but still experiencing a form of friction. In order to release the stuck objects, it is necessary for a certain amount of pressure to be exerted to get the objects to separate. This phenomenon can be observed with a wide variety of objects, and it can become a serious issue in certain applications.

One simple example of stiction is fairly easy to demonstrate. If a piece of wood is placed on a larger piece of wood and the large piece is tilted, the small piece will usually hold its position for a moment before sliding down. While the smaller block is stuck, it is exhibiting stiction, which is eventually overcome when the larger block is tilted at a sufficient angle. The observer can usually overcome the stiction by lightly pushing at the smaller block with a finger or two.

Several different forces can be involved in stiction. In a simple example of stiction, sometimes a hard drive fails as a result of components sticking together. This is more common when a computer has been left sitting for an extended time without being powered up. Stiction can also be observed with some other types of moving parts which are left motionless, sometimes requiring great force to get the parts to separate.

One area in which this phenomenon can become an issue is in the engineering of parts, including electronic systems, on a microscopic level. The level of machining and the detail is so fine on such systems that stiction can become a recurrent issue. In fact, some are treated with anti-stiction coating to address problem in advance. Such coatings are developed by companies which work with microelectromechanical systems (MEMS) and other projects which are conducted on a microscopic level.

Understanding how stiction works can be important to controlling the movement, or lack thereof, of various objects. Researchers who are interested in this phenomenon may develop controlled experiments which are designed to explore static friction and look into ways in which it can be overcome, sometimes with the goal of meeting a specific design or engineering need, and sometimes out of a more general interest.