Ultrafast viscosity measurement with ballistic optical tweezers

TL;DR

This paper introduces a method for ultrafast, noninvasive viscosity measurement using ballistic optical tweezers, achieving microsecond resolution by tracking particle velocity with structured-light detection.

Contribution

The authors develop a structured-light detection system enabling instantaneous velocity measurement in optical tweezers, vastly improving viscosity measurement speed and enabling dynamic studies of out-of-equilibrium systems.

Findings

Achieved viscosity measurements in twenty microseconds.

Demonstrated measurement uncertainty limited by thermal noise.

Enabled real-time tracking of viscosity in active systems.

Abstract

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. However, noninvasive viscosity measurements typically require integration times of seconds. Here we demonstrate a four orders-of-magnitude improvement in speed, down to twenty microseconds, with uncertainty dominated by fundamental thermal noise 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 with megahertz bandwidths. 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, to violations of fluctuation laws of equilibrium thermodynamics.