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9:52in productionCh. 1 · Experimental origin/ 9:52 · ceiling 15 min
Physics · Chemistry

Fick's laws of diffusion

Adolf Fick

Fick didn’t discover diffusion — he gave it a ruler, a clock, and a name.

Fick’s laws quantify diffusion in fluids using direct measurements of salt flux and concentration. They hold for liquids, not solids. Their derivation is experimental, their form exact, and their reach vast — but they are not universal, not theoretical first principles, and not extended beyond their empirical domain.

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Chapters & takeaways5
  1. 0:57
    Experimental origin

    Fick derived his laws in 1855 from experiment — not theory or analogy.

  2. 2:13
    The apparatus

    He measured salt flux and concentration across water-filled tubes connecting two reservoirs.

  3. 3:16
    What the laws say

    First law: flux ∝ gradient. Second law: gradient changes over time — and the two are mathematically linked.

  4. 4:48
    A boundary, not a universal law

    Fick’s work applied only to fluids — solids were excluded by consensus at the time.

  5. 6:10
    One law, two forms

    The second law is not independent: it follows from the first and matches the diffusion equation.

Worth your time?

Yes. Study the whole thing.

4.5/ 5
What works
  • fluid-diffusion
  • mass-transfer
  • quantitative-biology
What does not
  • solids
Study it if
  • chemists
  • physicists
  • biomedical-engineers
Skip it if
  • solid-state-physicists
  • geologists-studying-mantle-diffusion
The written brief1 min read

What the work claims

Diffusion in fluids follows two quantitative laws: (1) flux is proportional to the concentration gradient; (2) the gradient changes over time in a way described by a partial differential equation identical to the diffusion equation.

How it was done

Adolf Fick conducted experiments in 1855 using salt solutions diffusing between two reservoirs through water-filled tubes. His method followed Graham’s earlier work and measured concentrations and fluxes directly.

What holds up

The first law holds: diffusive flux is directly proportional to the concentration gradient. The second law holds: it correctly predicts how concentration gradients evolve over time in fluids. Both laws are mathematically linked — the second follows from the first and matches the diffusion equation.

What does not

It does not apply to solids: diffusion in solids was not considered generally possible at the time, and Fick made no claim about it.

Why it matters beyond the lab

It underpins drug delivery, dialysis, semiconductor doping, atmospheric modelling and food preservation — wherever mass moves down a concentration gradient in a fluid medium.

Is it worth your time

Yes — it is the foundational quantitative framework for diffusion in fluids, still used across chemistry, medicine and materials science. Its simplicity and empirical grounding make it worth understanding deeply.

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