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, and for fields ranging from biomaterials to geology, energy technologies and medicine. Non-invasive viscosity measurements typically require integration times of seconds. Here, we demonstrate measurement speeds reaching 20 μs, 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. To resolve the instantaneous velocity we develop a structured-light detection system that allows particle tracking over femtometre length scales and 16-ns timescales. Our results translate viscosity from a static averaged property to one that may be dynamically tracked on the timescales of active dynamics. This opens a pathway to new discoveries in out-of-equilibrium systems, from the fast dynamics of phase transitions to energy dissipation in motor molecule stepping and to violations of fluctuation laws of equilibrium thermodynamics.