Where does viscosity collide?

In fluid mechanics, viscosity is a fluid's internal resistance to flowing when one layer is made to slide past another. Newton set out the origin idea in his 1687 Philosophiae Naturalis Principia Mathematica, proposing that the resistance between adjacent layers of a fluid is proportional to the rate at which they are sheared, the relation now called Newtonian viscosity. Stokes gave the modern mathematical treatment in his 1845 On the Theories of the Internal Friction of Fluids in Motion, folding this internal friction into the equations of fluid motion. The referent here is a physical property of matter, the drag one part of a fluid exerts on another, and it is not a property of a population or of a person.[1][2]

Unit

A fluid's resistance to shear, scaled as a coefficient in pascal-seconds

Recovery means

Not applicable

Measured by

Measured with a viscometer or rheometer as the ratio of shear stress to shear rate

In evolutionary biology, population viscosity is the tendency of individuals to stay near where they were born, so that neighbours are more likely to be genetic relatives. Hamilton introduced the idea in his 1964 The Genetical Evolution of Social Behaviour, noting that in a viscous population limited movement keeps kin together and can favour behaviour that helps neighbours. Later work in the same tradition used this to study how limited dispersal shapes cooperation and competition among relatives. The referent is a spatial and genetic feature of a group of organisms, describing how far they move and how related their neighbours are, and it is not the thickness of a fluid or a feature of one person's behavior.[3][4]

Unit

A population's degree of limited dispersal, raising local genetic relatedness

Recovery means

Not applicable

Measured by

Inferred from dispersal distances and spatial genetic structure, such as relatedness among neighbours

In behavioral neurology, viscosity names a described trait of stickiness in thought and social contact, such as prolonged, hard-to-end conversations and a slowed or repetitive style of speech, reported in some people with temporal lobe epilepsy. Waxman and Geschwind described it in their 1975 account of an interictal behavior pattern in temporal lobe epilepsy, listing viscosity as one of its features. Bear and Fedio then built a rating inventory in 1977 that measured this and related traits quantitatively. The referent is a described behavioral characteristic recorded in the clinical literature, and it is not a property of a fluid or a spatial feature of a population.[5][6]

Unit

A described behavioral trait of stickiness or perseveration in speech and interaction

Recovery means

Not applicable

Measured by

Rated in the epilepsy literature by clinical interview and inventories such as the Bear-Fedio scale