Ultrafast viscosity measurement with ballistic optical tweezers

Viscosity is an important property of out-of-equilibrium systems such as active biological materials and driven non-Newtonian fluids. Noninvasive viscosity measurements typically require integration times of seconds. Here we demonstrate measurement speeds reaching twenty microseconds, with uncertainty dominated by thermal molecular collisions for the first time. We achieve this using the instantaneous velocity of a trapped particle in an optical tweezer, combined with a structured-light detection system. This opens a pathway to new discoveries in out-of-equilibrium systems.