Rheological and interfacial characterization of menstrual fluid

Menstrual fluid is a complex biological fluid composed of blood, endometrial tissue, vaginal epithelial cells, cervical mucus, and microbiota. Its physical properties remain largely unexplored, despite their relevance for applications such as the in vitro testing of absorbent products. Current industrial simulants are typically simplified water- or blood-like fluids that may not adequately reproduce the complex behavior of real menses during absorbency testing. Here, we present the first systematic characterization of the bulk and interfacial properties of menstrual fluid, including shear rheology, small-amplitude viscoelasticity, extensional dynamics, and surface tension. Measurements were performed on 228 samples collected during 20 menstrual periods from 12 healthy volunteers. Our results show that menstrual fluid viscosity is approximately fourfold higher than blood and forty-fold higher than water. Menstrual fluid behaves as a structured, non-Newtonian fluid exhibiting yield stress, shear-thinning, and extensional viscoelasticity, with a predominantly solid-like response in the linear viscoelastic regime. Surface tension is significantly lower than that of water and comparable to that of venous blood, although with a broader range. Rheological and interfacial properties systematically change with the menstrual flow rate, defined here as the mass of fluid discharge per unit time. Together, these findings provide a quantitative description of the rheological and interfacial properties of menstrual fluid, supporting future research in female hygiene and health care, as well as applications in menstrual product testing.