Dear Colleagues,

Seals are indispensable elements in almost every kind of technical system and natural organism. Their purpose is to either protect a system against the ingress of detrimental substances from the outside, or to prevent the exit of fluids into the environment, also known as "leakage" or "emissions". Although their proper functioning is of vital importance in most applications, seals tend to be regarded as a commodity and too trivial to receive special attention from the general public. However, sealing is generally an auxiliary, yet crucial, function, which cannot be taken for granted.

For a long time, sealing was treated like a craft and was afforded little to no scientific attention. It took long time to recognize that dynamic seals are primarily a tribological system. Wear and proper material selection was and will always be an issue, but it happened very late that the role of hydrodynamic lubrication in contacting dynamic seals and the analogies with hydrodynamic plain bearings were finally recognized. Lubricant films are very thin, which is necessary to minimize leakage, but this forces seals to operate in or close to the mixed lubrication regime. Given the otherwise inevitable, albeit small, leakage, "zero leakage" requires a hydrodynamic or aerodynamic reverse pumping mechanism, which transports the previously leaked fluid back into the working space of the machinery. These effects are still not fully understood.

A specific issue of sealing technology is the compatibility between seal materials, especially elastomers, and lubricants, calling for an interdisciplinary approach by chemists, physical chemists, and tribologists.

In summary, all of this comprises enough motivation for a Special Issue of Lubricants!

Prof. Dr. Gerhard Poll
Guest Editor

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