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, for fields ranging from biomaterials to geology, energy technologies medicine. Non-invasive viscosity measurements typically require integration times seconds. Here, we demonstrate measurement speeds reaching 20 ?s, with uncertainty dominated by thermal molecular collisions the first time. We achieve this using instantaneous velocity a trapped particle in optical tweezer. To resolve develop structured-light detection system that allows tracking over femtometre length scales 16-ns timescales. Our results translate static averaged one may be dynamically tracked on timescales dynamics. This opens pathway new discoveries systems, fast dynamics phase transitions dissipation motor molecule stepping violations fluctuation laws equilibrium thermodynamics. A structured-light-based tweezers enables ultrafast determination.