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134 GLOSSARY

{Yidc p. 105.) Szegvari defines this phenom* terms of a yield-value rather than a modulus. ^Cn°ⁿ in Fluidity H The reciprocal of viscosity i_Qv\ unit being the rhe. (The symbol | is unfortun V^its in generał use for several other ąuantities.)

Laminar FIow. Some authors use this term describe any | telescopic | type of flow I others restri t its use to truły Newtonian behaviour.    ^ct

Magmoid. A pasty or doughy materiał, showing both fluid and solid properties.    ⁸

Mesomorphic. A condition in which long, rod-like molecules are orientated in ranks and rows. When the molecules fit end to end, the condition is called “ smectic ” ; when this does not occur, | nematic.” Mobility (/i). The slope of the Bingham flow curve. It corresponds to the fluidity of a true fluid, The (ratę of shear) / (shearing stress)—(yield-value). Has the same dimensions as fluidity.

Plasticity. The capacity to stand moulding and retain shape under the force of gravity. This should not be confused with other colloidal properties with which it is partially correlated, such as the plastic moisture rangę, stiffness, dye adsorption, etc, It has no definite dimensions and no exact definition. (Vide pp. 62, 66.)

Pług Flow. The case where a materiał, flowing through a tubę, slides as a solid pług, e.g., tooth-paste, as it comes out of a Container. Anomalous flow near the wali of a capillary tubę, under streamline flow conditions, produces a phenomenon which is mathematically similar; but this is clearly distinguishable physically, and is known as the " o-phenomenon.” (Vide p. 34.)

Pseudo-plastic. A materiał which shows no yield-value, but whose viscosity falls progressively with rising stress.

Relaxation Time. This may be defined as the reciprocal of the ratę of dissipation of the logarithm of stress with time. It should not be confused with the ratę of recoyery on the release of stress in cases of elastic after-effect. It may be considered as the time taken

total internal stress to dissipate to i/e of its for ynder constant strain conditions. [Vide p. 48.) ^vachortness. 1 tendency to tear easily. No exact fi ition is possible. The property is a function of Mh. tensile strength and ductility, and the term appears pi| _used in different senses in different industries. ^tC>SloW Yield. Graduał permanent deformations, which tahe place in a materiał under the influence of a stress S-s than its normal yield-value.

¹ Stiffness (Pseudo-viscosity). The reciprocal of mobility, sometimes mistakenly confused with plasticity. It has the dimensions of a viscosity, and is defined as the ratio of (shearing stress — yield-value) to ratę of shear.

Strudural Viscosity. A fali in viscosity induced by a rise in shearing stress.

Thixotropy. A reversible sol-gel transformation, induced by shaking and subseąuent standing. “ False body ” describes any partially reversible decrease in consistency produced by shaking; but the two terms are often used synonymously. (From the Greek, 0t£is = touch, rpeirw = change.)

True Fluid. A materiał which obeys Newton’s assumption that ratę of shear is proportional to shearing stress.

Turbulence. The condition in which vortices occur in a flowing materiał. This generally occurs when the Reynolds’ number exceeds a certain value. This number is a function of the velocity of flow, the dimensions of the channel and the kinematic viscosity. “ Structural turbulence ” refers to a local turbulent condition, believed to be set up by anisotropic particles rotating under a viscous couple.

Viscosity (rj). The term was apparently first introduced by Ben Jonson. It is now normally used to refer to the ratio of shearing stress to ratę of shear in fluids, where this ratę is a constant independent of stress. Definition in the case of non-Newtonian materials is still a subject of controversy. Dimensions Mass X Length^-1 X Time"¹. Unit, the poise