Buckling of sperm flagella in an extensional flow

Swimming cells live in dynamic fluid environments. Here, we probe the buckling dynamics of sperm flagella in an extensional flow and show good agreement between microfluidic experiments and numerical simulations. The sperm number, representing the ratio of viscous force and elastic force, characterizes the buckling behavior of the sperm flagella. We quantify the phase diagram of flagella deformation and bucking behavior for different sperm numbers and normalized strain rates. At small sperm numbers and strain rates, flagella exhibit minor elastic deflection, whereas at high sperm numbers and strain rates, multiple buckling modes occur corresponding to large deformations at multiple locations along the flagellum.