Precise characterization of micro rotors in optical tweezers

TL;DR

This study uses optical tweezers and laser back scattering to precisely analyze the rotation and torque of microscopic asymmetric objects, including biological cells, revealing detailed rotational dynamics and potential for measuring physical properties.

Contribution

It introduces a novel optical tweezer-based method to characterize micro rotor rotation, torque, and inertia, including low reflectivity biological samples, with detailed spectral analysis.

Findings

Detected fundamental and harmonic rotational frequency peaks.

Demonstrated characterization of biological cell rotors.

Enabled measurement of average torque and insights into hydrodynamics.

Abstract

We present an optical tweezer based study of rotation of microscopic objects with shape asymmetry. Thermal fluctuations and rotations are simultaneously monitored through laser back scattering. The rotation results in a modulation in intensity of the back scattered light incident on a quadrant photo detector. This results in the manifestation of peaks at a fundamental rotational frequency and at integer harmonics, superimposed on a modified Lorentzian in the power spectrum. The multiple peaks indicate that the rotations are periodic but with varying angular velocity. We demonstrate the use of video microscopy for characterization of low reflectivity rotors, such as biological cells. The methods also enable a measurement of the average torque on the rotor, and in principle, can reveal information about its principal moments of inertia, and the role of hydrodynamics at micron levels