thixotropy: one construct, many systems

In rheology thixotropy is the reversible, time-dependent decrease in viscosity of a structured fluid when it is sheared, followed by a gradual recovery of that structure, and so of viscosity, when it is left at rest. Goodeve's 1939 general theory framed it as the progressive breakdown and rebuilding of an internal network, and Mewis and Wagner's review sets out the modern understanding for colloidal suspensions and the ways it is measured. The referent is a material's time-dependent structural memory under flow, the same physics whether the fluid is a paint, a drilling mud, a cement paste, or a clay.[1][2]

Unit

Rate of structural breakdown under shear and of recovery at rest

Recovery means

Structure, and hence viscosity, rebuilds over time once shearing stops

Measured by

Characterised by hysteresis loops, stepped shear-rate tests, and structural-kinetics models of viscosity versus time

In concrete technology thixotropy is the tendency of a fresh cement paste or concrete to build up internal structure and stiffen while it rests, then to flow again once it is remixed or vibrated. Roussel's model treats this as a reversible, time-dependent restructuring and shows why it matters in practice: for the pressure fresh concrete exerts on formwork, for the stability of self-compacting mixes, and for the strength of the bond between successive cast layers. It is the same reversible breakdown-and-recovery construct as in general rheology, realised in a cementitious suspension and measured through its evolving yield stress at rest.[3]

Unit

Rate at which the resting yield stress of fresh concrete grows and recovers

Recovery means

The built-up structure breaks down again on remixing or vibration, restoring flow

Measured by

Quantified by the growth of static yield stress with rest time, from rheometer or slump-based structural-build-up tests

In geotechnical engineering thixotropy is the reversible process by which a remoulded or disturbed clay gradually regains stiffness and strength while resting at unchanged water content and volume, as its particle structure re-forms. Mitchell and Soga's Fundamentals of soil behavior explains the effect and why it matters, for example in the strength gain of driven piles in clay and in the behaviour of sensitive clays after disturbance. It is the identical reversible structural-recovery construct seen in paints and cement pastes, here in a soil and measured as the regain of undrained strength over time.[4]

Unit

Regain of undrained strength over rest time at constant water content

Recovery means

The strength gained at rest is lost again on remoulding and then regained, the hallmark reversibility

Measured by

Measured as the ratio of rested to freshly remoulded undrained shear strength over time (a thixotropic strength-regain ratio)