Interrogating the Ballistic Regime in Liquids with Rotational Optical Tweezers

TL;DR

This paper introduces a novel rotational optical tweezer technique to measure ballistic particle-liquid interactions at microscopic scales, enabling direct observation of rotational hydrodynamics and rapid viscometry.

Contribution

The study presents a new method for probing the ballistic rotational regime in liquids using high-bandwidth polarization measurements of birefringent probes in optical traps.

Findings

Validated rotational hydrodynamic effects in a new parameter space

Achieved calibration-free viscometry with less than 50ms data

Enabled ultra-fast microrheometry in out-of-equilibrium systems

Abstract

Accessing the ballistic regime of single particles in liquids remains an experimental challenge that shrouds our understanding of the particle-liquid interactions on exceedingly short time scales. We demonstrate the ballistic measurements of rotational probes to observe these interactions in the rotational regime within microscopic systems. This study uses sensitive high-bandwidth measurements of polarisation from light scattered by orientation-locked birefringent probes trapped within rotational optical tweezers. The particle-liquid interactions in the ballistic regime are decoupled from the optical potential allowing direct studies of single-particle rotational dynamics. This enabled us to determine the dissipation of rotational inertia and observe and validate rotational hydrodynamic effects in a previously inaccessible parameter space. Furthermore, the fast angular velocity thermalisation time enables calibration-free viscometry using less than 50ms of data. This methodology will provide a unique way of studying rotational hydrodynamic effects and enable ultra-fast microrheometry in systems out-of-equilibrium.