Energy injection in an epithelial cell monolayer via negative viscosity

Active fluids are driven out of thermodynamic equilibrium by internally generated forces, causing complex patterns of motion. Even when both the forces and motion are measurable, it is not yet possible to relate the two, because the sources of energy injection and dissipation are often unclear. Here, we study how energy is transferred by developing a method to measure viscosity from the shear stresses and strain rates within an epithelial cell monolayer. Surprisingly, there emerged multicellular regions in which the relationship between shear stress and shear strain rate was negatively proportional, indicating a negative viscosity. We provide direct experimental evidence that the negative viscosity results from cells aligning their stresses with the orientation of the flow. Regions of negative viscosity consistently exhibited greater cell speed and vorticity, and the cells had elevated metabolic activity, indicating that negative viscosity is a mechanism for injection of surplus energy. More broadly, our study shows that negative viscosity is a useful means of quantifying the flow of energy in active materials.