Microrheology With an Anisotropic Optical Trap

Andrew B Matheson,Graham M Gibson,Manlio Tassieri,Tania Mendonca,Paul A Dalgarno,Lynn Paterson,Amanda J Wright

doi:10.3389/fphy.2021.621512

Andrew B Matheson, Graham M Gibson + Show 5 more

Open Access

https://doi.org/10.3389/fphy.2021.621512

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Abstract

Microrheology with optical tweezers (MOT) measurements are usually performed using optical traps that are close to isotropic across the plane being imaged, but little is known about what happens when this is not the case. In this work, we investigate the effect of anisotropic optical traps on microrheology measurements. This is an interesting problem from a fundamental physics perspective, but it also has practical ramifications because in 3D all optical traps are anisotropic due to the difference in the intensity distribution of the trapping laser along axes parallel and perpendicular to the direction of beam propagation. We find that attempting viscosity measurements with highly anisotropic optical traps will return spurious results, unless the axis with maximum variance in bead position is identified. However, for anisotropic traps with two axes of symmetry such as traps with an elliptical cross section, the analytical approach introduced in this work allows us to explore a wider range of time scales than those accessible with symmetric traps. We have also identified a threshold level of anisotropy in optical trap strength of ~30%, below which conventional methods using a single arbitrary axis can still be used to extract valuable microrheological results. We envisage that the outcomes of this study will have important practical ramifications on how all MOT measurements should be conducted and analyzed in future applications.

Highlights

Optical tweezers (OT), first introduced by Ashkin et al [1], exploit a tightly focussed laser beam with high intensity gradient to trap and move micro and nano-sized dielectric objects in 3D, by harnessing both the gradient and scattering forces
By using data from three different labs alongside simulations, we have explored how to correctly extract the rheological properties of the suspending media from the statistical mechanics analysis of the trajectory of a probe particle confined within an anisotropic optical trap
This latter represents an extreme case of an anisotropic trap that would not commonly be used in real Microrheology with optical tweezers (MOT) experiments, but it is simulated here to highlight any possible shortcomings of the analytical model introduced in this work to characterize asymmetric traps

Summary

Introduction

Optical tweezers (OT), first introduced by Ashkin et al [1], exploit a tightly focussed laser beam with high intensity gradient to trap and move micro and nano-sized dielectric objects in 3D, by harnessing both the gradient and scattering forces. Scientists have adopted them to study a wide range of physical and biophysical phenomena including molecular binding forces [2], organelle transport [3], flagellar motion [4], the mechanical properties of DNA [5], molecular motors [6, 7] and polymerases [8, 9], and the diffusion of proteins [10] Another successful application of optical tweezers is in the field of “hybrid” microrheology, whereby an optically trapped bead acts as a probe, revealing the thermal fluctuations of the molecules in the suspending fluid. Microrheology with OT (MOT) has been used with growing popularity to study the rheology of complex biological materials such as the vitreous humor [11], Microrheology With Anisotropic Optical Trap biopolymer networks [12], and extracellular secretions from phytoplankton [13] at the microscale.

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