pinning: one construct, many systems

In the plasticity of metals, pinning is the holding of a dislocation in place by obstacles such as solute atoms, precipitates, or other dislocations, so that stress must exceed a threshold before the line can tear free and glide. Cottrell and Bilby's 1949 Dislocation theory of yielding and strain ageing of iron explained how carbon atoms gather into an atmosphere that pins dislocations and produces a sharp yield point. The referent is an elastic line held by local disorder until a critical force unpins it, the same depinning behaviour that recurs for vortices in a superconductor and for a wetting front on a surface.[1]

Unit

The critical stress to free a pinned dislocation

Recovery means

Not applicable

Measured by

Inferred from yield and flow stress, the yield point, and strain-ageing behaviour

In type-II superconductors, pinning is the trapping of quantized magnetic flux lines on defects in the material, which keeps them from drifting under a current and so lets the superconductor carry large currents without dissipation. Anderson's 1962 Theory of flux creep in hard superconductors framed how thermal activation lets pinned flux lines slowly hop free, setting the critical current. It is the same construct as a pinned dislocation, an object held by local disorder until a critical driving force unpins it, realised here for magnetic vortices and measured by the critical current rather than a yield stress.[2]

Unit

The critical current set by how strongly flux lines are pinned

Recovery means

Not applicable

Measured by

Characterised by the pinning force density, the critical current, and the rate of flux creep

In wetting, contact-line pinning is the way the edge of a droplet or a spreading film sticks on roughness or chemical patches of a surface, so the contact angle must change past a threshold before the line jumps to a new position, the origin of contact-angle hysteresis. De Gennes' 1985 Wetting: statics and dynamics reviews how such heterogeneity pins the line and resists its motion. It is the identical depinning construct seen with dislocations and flux lines, here a liquid front held by surface disorder until a critical force frees it, measured by contact-angle hysteresis instead of a yield stress or critical current.[3]

Unit

The force or angle change needed to unpin a wetting contact line

Recovery means

Not applicable

Measured by

Quantified by advancing and receding contact angles and the resulting contact-angle hysteresis