Jean Léonard Marie Poiseuille

Jean Léonard Marie Poiseuille was born in Paris in 1797 and trained as a physician at the École Polytechnique and then the Paris medical faculty, graduating in 1828. His dissertation was on the forces driving venous and arterial circulation — an unusually quantitative question for a French medical thesis of the period, and the start of a lifelong obsession.

Rather than go into clinical practice, Poiseuille spent his career in a lab. Between 1838 and 1846 he ran meticulous experiments pushing water, mercury, and various salt solutions through fine glass capillaries, measuring flow rate as a function of applied pressure, tube length, and tube radius. In 1840 he published the result now universally known as Poiseuille's law: for laminar flow of a Newtonian fluid through a cylindrical tube, volumetric flow rate scales as R⁴·Δp/(8ηL). The fourth-power dependence on radius is the key: any branched transport system — lungs, circulatory tree, water mains — is dominated by its narrowest vessels.

Poiseuille never wrote down a partial differential equation; the theoretical derivation of his law from Newton's viscosity ansatz was later completed by Hagen in Germany and by Stokes, which is why the result is sometimes called the Hagen-Poiseuille equation. But the phenomenon and the scaling are his. The SI-adjacent unit of viscosity, the poise, is named after him.