rheofit

Fit flow curves — viscosity as a function of shear rate — to physically-based rheological models, and turn them into quantified material properties.

A flow curve is the fingerprint of a non-Newtonian fluid: shear thinning, yield stress, low-shear plateaus and relaxation times are all features of that one curve. Fitting it with a model whose parameters have physical meaning does three things:

• Quantifies material properties — $\sigma_y$, $\eta_0$, $\lambda$, $n$ — instead of describing curve shapes.
• Gives a concise description of the material — a handful of numbers replace hundreds of points, so samples, temperatures and batches become directly comparable.
• Feeds back to formulators — when the properties are tied to the formulation, they say immediately which ingredient or level to move to hit a material-property target.

Parameter	Reads as	Usual formulation lever
$\sigma_y$	strength of the structured network	structurant level, particle/fiber network
$\eta_0$	zero-shear / at-rest viscosity	thickener or polymer concentration
$\lambda$	relaxation time, onset of thinning	molecular weight, micelle length
$n$	how sharply it shear-thins	polymer architecture, entanglement
$\eta_{bg}$	Newtonian background flow	solvent / continuous phase

Data is read directly from TA Instruments TRIOS JSON, but fit() accepts any DataFrame with shear-rate and stress columns.

Fitting contract

Every fit minimises the relative residual $(f(\dot\gamma;p)-\sigma)/|\sigma|$, so each decade of stress counts equally and RedChi2 is dimensionless — comparable across steps, samples and models. Fits use log-space parameters, physics-informed starting values, ladder seeding from the nested parent model, a Sobol multi-start and a tight polish. Weakly identified parameters, near-degenerate Jacobians and nesting violations are reported as notes on the result.

Install

With uv (recommended)

Clone and run — uv creates the virtual environment, installs the pinned dependencies from uv.lock and installs rheofit itself in editable mode:

git clone <repo-url>
cd rheofit
uv sync

Then prefix any command with uv run (no activation needed):

uv run rheofit --demo
uv run python -c "import rheofit; print(rheofit.list_models())"
uv run jupyter lab            # notebooks: pick the .venv kernel

uv sync also installs the dev group (ipykernel, python-pptx), so notebooks and PowerPoint output work out of the box. For a lean runtime-only environment use uv sync --no-default-groups.

Activate the environment instead, if you prefer: .venv\Scripts\Activate.ps1 (Windows) or source .venv/bin/activate.

With pip

pip install -e .
pip install -e ".[pptx]"   # adds PowerPoint output

Python API

import rheofit

rheofit.list_models()
# ['carreau', 'carreau_carreau', 'herschel_bulkley', 'power_law', 'tc', 'tc_carreau', 'tccc']

rheofit.print_steps("sample.json")            # which steps exist (0-based ResultsSteps index)

df = rheofit.load_step("sample.json", 0)      # one step as a DataFrame
res = rheofit.fit(df, "tc", effort="thorough")
res["params"]   # {'sigma_y': {'value': ..., 'stderr': ...}, ...}
res["redchi"], res["notes"]

a = rheofit.analyze(                          # load -> fit -> summarise -> save
    "sample.json", steps=[0, 2], model="tc", labels=["25C", "40C"], output="png_csv",
)
a.summary      # tidy DataFrame: step, parameter, value, stderr, rel_error_pct, redchi2, quality
a.outputs      # saved PNG / CSV / PPTX paths

analyze(..., output="none") fits without writing files. Inputs may be a local path or an HTTP(S) URL. rheofit.demo_source() returns the bundled demo flow-curve file.

CLI

uv run rheofit sample.json                                     # list steps
uv run rheofit sample.json --steps 0 2 --model tc --labels 25C 40C
uv run rheofit --demo --steps 0 2 --model tccc --output both

(python -m rheofit … is equivalent inside an activated environment.)

Flags: --steps, --model, --labels, --effort {fast,normal,thorough}, --seed, --sample-name, --output {png_csv,pptx,both,none}, --demo.

Models

No yield stress	With yield stress
power_law	tc, herschel_bulkley
carreau	tc_carreau
carreau_carreau	tccc

Within each family the models form a ladder. Prefer the simplest model that fits; climb only if the residuals show structure the simpler model missed. Extra parameters buy little once RedChi2 is below 0.01, and they cost identifiability.

Layout

src/rheofit/
  io.py         TRIOS JSON reading, URL download, demo data
  models/       one module per model + _fitcore.py (shared fitting engine)
  report.py     plots, PNG scorecard, parameter summary, PPTX
  analysis.py   analyze()
  cli.py        command-line front-end

Skill

.github/skills/flow-curve-analysis/SKILL.md is the agent-facing companion to this library: it documents the models, the fitting contract and a guided interview workflow (is the sample structured? which model? which steps?), and drives the same CLI. Use the library directly, or let the skill walk you through it — they are the same code. Keep the skill in sync when the library's models, flags or outputs change.