Abstract
We use active microrheology to determine the frequency dependent moduli of a linear viscoelastic fluid in terms of the polymer time constant (λ), and the polymer (μp) and solvent viscosity (μs), 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(ω) 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 et al 2018 J. Phys. Condens. Matter 30, 345101), we linearized the Stokes-Oldroyd B fluid model and thereby explicitly formulated the frequency dependent moduli in terms of μp, μs and λ which we now extend to account for an external sinusoidal force applied to the probe particle. We measure λ, μp, and μs experimentally, and compare with the existing λ 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.
Highlights
The fundamental difference between liquids and solids is their response under applied shear strain - while solids store energy and are elastic, liquids dissipate energy and are viscous in nature
We have shown in a recent work that a viscoelastic fluid can be understood as a viscous solvent which contains a polymer network mixed with it
PAM to water concentration (% w/w) construction of our approach - based on linearizing the Stokes-Oldroyd B equation for viscoelastic fluids - provides for a more profound understanding about the constituents of such fluids inasmuch that it reveals the polymer and solvent characteristics separately
Summary
The fundamental difference between liquids and solids is their response under applied shear strain - while solids store energy and are elastic, liquids dissipate energy and are viscous in nature. The Maxwell model describes a time constant τM which marks a transition from the high frequency elastic nature of the sample to the low frequency viscous regime, while the Jeffrey’s model contains a zero-frequency viscosity η0 and a correction term as the background viscosity η∞ [27] These models are based on the bulk properties of the viscoelastic fluid and do not provide any information about its basic constituents. For solutions of higher polymer concentrations, the measured polymer contributions to the viscosities are not satisfactory We believe this to be due to the inherent ineffectiveness of our model in dealing with the non-linear nature of viscoelasticity or the additional complexity resulting in the superposition of several time constants and other parameters that the high concentration of polymer would induce in a fluid [8]. For linear viscoelastic fluids and for low-concentrations, our work opens a new approach in microrheology and can be used very extensively due to its simple methodology and ease-of-use
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
