Stiction

Stiction is the static friction that needs to be overcome to enable relative motion of stationary objects in contact.[1] The term is a portmanteau of the words static and friction,[2] perhaps also influenced by the verb stick.

Any solid objects pressing against each other (but not sliding) will require some threshold of force parallel to the surface of contact in order to overcome static cohesion. Stiction is a threshold, not a continuous force.

In situations where two surfaces with areas below the micrometer scale come into close proximity (as in an accelerometer), they may adhere together. At this scale, electrostatic and/or Van der Waals and hydrogen bonding forces become significant. The phenomenon of two such surfaces being adhered together in this manner is also called stiction. Stiction may be related to hydrogen bonding or residual contamination.

Automobiles

Stiction is also the same threshold at which a rolling object would begin to slide over a surface rather than rolling at the expected rate (and in the case of a wheel, in the expected direction). In this case, it's called "rolling friction" or μr.

This is why driver training courses teach that if a car begins to slide sideways, the driver should try to steer in the same direction as the slide with no brakes. It gives the wheels a chance to regain static contact by rolling, which gives the driver some control again. An overenthusiastic driver may "squeal" the driving wheels trying to get a rapid start but this impressive display of noise and smoke is less effective than maintaining static contact with the road. Many stunt-driving techniques are also done by deliberately breaking and/or regaining this rolling friction.

A car on a slippery surface can slide a long way if the driver "locks" the wheels in stationary positions by pressing hard on the brakes. Anti-lock braking systems use wheel speed sensors and vehicle speed sensors to determine if any of the wheels have stopped turning. The ABS Module then briefly releases pressure to that wheel to allow the wheel to begin turning again. Anti-lock brakes can be much more effective than cadence braking which is essentially a non-automatic technique for doing the same thing.

Examples

Engineering

Stiction refers to the characteristic of stop and start type motion as a force overcomes static friction and causes a part to accelerate under dynamic friction, but the force cannot keep up with the speed of the moving part so the part tends to stop again until the force catches up, and it happens repeatedly. Stiction is a problem for the design and materials science of many moving linkages. This is particularly the case for linear sliding joints, rather than rotating pivots. Owing to simple geometry, the moving distance of a sliding joint in two comparable linkages is longer than the circumferential travel of a pivoting bearing, thus the forces involved (for equivalent work) are lower and stiction forces become proportionally more significant. This issue has often led to linkages being redesigned from sliding to purely pivoted structures, just to avoid problems with stiction. An example is the Chapman strut, a suspension linkage.[3]

Surface micromachining

Stiction or adhesion between the substrate (usually silicon based) and the microstructure occurs during the isotropic wet etching of the sacrificial layer. The capillary forces due to the surface tension of the liquid between the microstructure and substrate during drying of the wet etchant cause the two surfaces to adhere together. Separating the two surfaces is often complicated due to the fragile nature of the microstructure. Stiction is often circumvented by the use of a sublimating fluid (often supercritical CO2, which has extremely low surface tension) drying process where the liquid phase is bypassed. CO2 displaces the rinsing fluid and is heated past the supercritical point. As the chamber pressure is slowly released the CO2 sublimates thereby preventing stiction.

See also

References
  1. "Stiction, n." The Free Dictionary. Retrieved 23 May 2012.
  2. "Stiction". Merriam-Webster. Retrieved 23 May 2012.
  3. Ludvigsen, Karl (2010). Colin Chapman: Inside the Innovator. Haynes Publishing. p. 121. ISBN 978-1-84425-413-2.
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