---
name: Rheology
slug: rheology
type: field
status: running
version: 3.2.1
released: 1929-04-01
maintainer: The Society of Rheology
dependencies:
  - fluid-mechanics
  - continuum-mechanics
  - materials-science
  - patience
license: Physical Reality Public License v1.0
tags:
  - flow
  - deformation
  - viscoelasticity
  - soft-matter
  - non-newtonian
  - stress
---

Rheology

What it actually is

The science of how matter flows and deforms under stress, which is to say: the science of things that refuse to be simply solid or simply liquid.

How it works

Everything deforms if you push hard enough, or wait long enough. Rheology quantifies that reluctance. The core loop runs as follows:

Apply a stress or strain to a material
Measure what it does over time
Build a constitutive model that describes the relationship
Watch the model fail at the boundary conditions
Publish a revised model

The governing parameter is the Deborah number: the ratio of a material's relaxation time to the observation time. High Deborah means it acts like a solid. Low Deborah means it flows. The joke is that even mountains have a Deborah number. They just require geological patience to observe.

Features

Viscosity measurement: how much a fluid resists flow. Honey is cooperative. Blood is not.
Viscoelasticity: materials that simultaneously store and dissipate energy. Silly Putty. Skin. Your career plans.
Thixotropy: materials that thin under sustained shear and recover at rest. Ketchup. Yogurt. procrastination.
Shear banding: the material's polite way of saying it will not deform uniformly, no matter what your model assumed.
Normal stress effects: why a polymer solution climbs up a stirring rod instead of flung outward. Called the Weissenberg effect. Upsetting the first time you see it.

Configuration

# rheometer_defaults.yaml
geometry: cone-and-plate        # or parallel plate, or Couette
gap_size: 50e-6                 # meters, adjust for particle size
temperature: 25.0               # Celsius, hold it constant or regret it
strain_sweep: [0.001, 100]      # percent, find the linear viscoelastic region first
frequency_range: [0.01, 100]    # rad/s
loading_protocol: gentle        # DO NOT shear-melt your sample during loading

Note: Sample loading is where most experiments die. There is no configuration flag for this. It is a skill.

Known bugs

Cox-Merz rule: empirically holds for many polymer melts. Has no rigorous theoretical basis. Used everywhere anyway.
Wall slip: the material near the instrument wall stops obeying the bulk model. Common. Annoying. Often ignored.
Evaporation: open geometries let your sample dry at the edges over long experiments. The data looks fine until it very much does not.
Thixotropic hysteresis: the material remembers its shear history. Your experimental protocol may be measuring its memory more than its properties.

Error codes

Code	Meaning
`ERR_NONLINEAR_REGIME`	You applied too much strain. Linear viscoelasticity is no longer valid here.
`ERR_SAMPLE_EJECTED`	Cone-and-plate at high shear rate. The material voted to leave.
`ERR_MODEL_DIVERGES`	Constitutive equation blew up. Check your mathematics.
`ERR_INSTRUMENT_COMPLIANCE`	The rheometer is deforming, not just the sample.

FAQ

Q: Is rheology just viscosity? A: No. Viscosity is one scalar from one type of experiment. Rheology is the full map of mechanical behavior in time and space. Calling it "just viscosity" is like calling music "just volume."

Q: What everyday materials are rheologically interesting? A: Cement, mayonnaise, biological tissue, magma, cornstarch suspensions, and the vast majority of things humans actually manufacture or eat.

Q: Why does ketchup not come out of the bottle? A: Yield stress. It will not flow below a critical stress threshold. Apply a sharp tap. You are now a practicing rheologist.

drafting spec…