Active microrheology to determine viscoelastic parameters of Stokes-Oldroyd B fluids using optical tweezers

Abstract: We use active microrheology to determine the frequency dependent moduli of a linear viscoelastic fluid in terms of the polymer time constant ($\lambda$), and the polymer ($\mu_p$) and solvent viscosity ($\mu_s$), respectively. We measure these parameters from the response function of an optically trapped Brownian probe in the fluid under an external perturbation, and at different dilutions of the viscoelastic component in the fluid. This is an improvement over bulk microrheology measurements in viscoelastic Stokes-Oldroyd B fluids which determine the complex elastic modulus $G(\omega)$ of the fluid, but do not, however, reveal the characteristics of the polymer chains and the Newtonian solvent of the complex fluid individually. In a recent work [Paul \textit{et al}., 2018 J. Phys. Condens. Matter \textbf{30} 345101], we linearized the Stokes-Oldroyd B fluid model and thereby explicitly formulated the frequency dependent moduli in terms of ($\mu_p$) and ($\mu_s$), which we now extend to account for an external sinusoidal force applied to the probe particle. We measure $\lambda$, $\mu_p$, and $\mu_s$ experimentally, and compare with existing the $\lambda$ values in the literature for the same fluid at some of the dilution levels, and obtain good agreement. Further, we use these parameters to calculate the complex elastic modulus of the fluid again at certain dilutions and verify successfully with existing data. This establishes our method as an alternate approach in the active microrheology of complex fluids which should reveal information about the composition of such fluids in significantly greater detail and high signal to noise.