Finding the right flow for menstrual fluid models

Menstrual fluid is complex, comprised not just of blood, but also mucus, resident microbiota, and vaginal cells. But because it is biologically sourced, menstrual product testing often relies on representative model materials in addition to real menstrual fluid.

However, many of the studies that inform these models focus primarily on the fluid’s cellular, molecular, and protein composition. Fewer have examined its rheological properties, like viscosity and surface tension, which affect how it moves and transfers through absorbent materials. Pero et al. conducted the first thorough characterization of menstrual fluid’s rheological and interfacial properties.

“A scientifically grounded understanding of its physical properties helps us use that limited supply where it is most valuable to help bridge that practical gap,” said author Jan Claussen. “Real menstrual fluid can be used to anchor and validate test models, while representative fluids and conditions can enable broader, repeatable testing at the scale needed to design the best possible products.”

Using 228 samples from 12 healthy volunteers spanning 20 menstrual periods, the researchers measured menstrual fluid’s surface tension, viscosity under shear, flow initiation, filament formation, and more. Despite significant variation — not only from person to person, but also from individuals’ samples within the same cycle — a few behaviors were universal. The fluid is generally more viscous than blood and forms elastic filaments, behaving more like a solid or gel than a liquid.

“It is not equivalent to veinous blood, and neither fluid is equivalent to a simple aqueous solution,” said author Eleonora Pero. “The additional components of menstrual fluid influence how it flows, stretches, and interacts with materials.”

Capturing the full range of menstrual fluid behaviors will eventually require a larger, more diverse cohort of samples representing variations in age, hormonal status, and gynecological conditions. The group hopes their work will be a first step in defining the properties and physical attributes that representative materials should aim to mimic.

Source: “Rheological and interfacial characterization of menstrual fluid,” by Eleonora Pero, Christian Neu, Alexandra Wolf, Jan Claussen, Giovanna Tomaiuolo, and Stefano Guido, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0346620 .