Interplay between optical, viscous and elastic forces on an optically trapped Brownian particle immersed in a viscoelastic fluid

TL;DR

This study investigates how optical, viscous, and elastic forces interact in the Brownian motion of a sphere in viscoelastic fluids, revealing that optical forces can slightly alter the fluid's mechanical response and proposing a new calibration method.

Contribution

It provides a detailed analysis of force interplay in viscoelastic fluids and introduces an alternative calibration approach for optical trapping experiments with non-Newtonian fluids.

Findings

Optical forces can modify the complex modulus at low frequencies.

Different viscoelastic solutions show varied responses to optical forces.

A new calibration methodology for non-Newtonian fluids in optical trapping is proposed.

Abstract

We provide a detailed study of the interplay between the different interactions which appear in the Brownian motion of a micronsized sphere immersed in a viscoelastic fluid measured with optical trapping interferometry. To explore a wide range of viscous, elastic and optical forces, we analyze two different viscoelastic solutions at various concentrations, which provide a dynamic polymeric structure surrounding the Brownian sphere. Our experiments show that, depending of the fluid, optical forces, even if small, slightly modify the complex modulus at low frequencies. Based on our findings, we propose an alternative methodology to calibrate this kind of experimental set-up when non-Newtonian fluids are used. Understanding the influence of the optical potential is essential for a correct interpretation of the mechanical properties obtained by optically-trapped probe-based studies of biomaterials and living matter.