Illustrated Physics
physics, drawn
Concept

Rheology — where it appears

The study of how materials deform and flow, covering everything between a solid that holds its shape and a liquid that does not. Its two axes are the rate a material is deformed at and the time over which it is watched, and a quoted viscosity is a measurement condition until both are stated.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Four answers to one push. Shear stress against shear rate for four fluids. The straight line through the origin is the Newtonian definition and is the only one of the four for which the word viscosity names a number. The Bingham fluid does not move at all until the stress passes 0.4, which is why toothpaste holds a shape on a brush and why wet concrete can be stood in a heap.

The fluid that answers back

For water, stress is proportional to how fast it is sheared, and the constant of proportionality is its viscosity. For paint, blood, ketchup and cornflour in water, it is not — and once the proportionality goes, so does the idea that viscosity is a number a substance has.

fluids · Rheology Solid and liquid are answers about a duration. The relaxation time of seven materials, on a logarithmic axis spanning 39 decades, against the length of one observation. A material behaves as a solid when its relaxation time is longer than the observation and as a liquid when it is shorter, so the vertical line is what decides which — and it is a property of the observer. At 1 s, 4 of these are solids. Move the line six decades to the right and pitch joins the liquids; move it far enough left and water is a glass, which is not a figure of speech but what a picosecond pulse measures.

The liquid that remembers

Pitch shatters like glass under a hammer and flows through a funnel over a decade. Neither behaviour is the true one. What decides which a substance shows is not the substance but the length of the observation, and the ratio between the two has a name and a number.

fluids · Rheology

Named alongside it

The objects these essays reach for when they reach for this one.

ViscoelasticityViscosityConstitutive lawCreepDissipationNon-newtonianPolymerRelaxationShear stressShear-thinningStrainStress

All concepts