  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.
