Volume 58, Issue 1, January 2014
Index of content:
58(2014); http://dx.doi.org/10.1122/1.4826939View Description Hide Description
We study the motion of a colloidal particle as it is driven by an oscillating external force of arbitrary amplitude and frequency through a colloidal dispersion. Large amplitude oscillatory flows (LAOFs) are examined predominantly from a phenomenological perspective in which experimental measurements inform constitutive models. Here, we investigate a LAOF from a microstructural perspective by connecting motion of the probe particle to the material response while making no assumptions a priori about how stress relaxes in the material. The suspension exerts nonconservative, hydrodynamic forces on the probe, while distortions in the particle configuration exert conservative forces: Brownian and interparticle forces, for example. The relative importance of each of these contributions to particle motion evolves with the degree of displacement from equilibrium. When the force on the probe is weak, the linear microviscoelasticity of the suspension is probed [see, e.g., Khair and Brady, J. Rheol. 49, 1449–1481 (2005)]. When oscillation rate is slow, the steady microrheology is probed [see, e.g., Squires and Brady, Phys. Fluids 17, 073101 (2005); Khair and Brady, J. Fluid Mech. 557, 73–117 (2006)]. This article develops a micromechanical model that recovers these limiting cases and then uses the same model to reveal the microrheology of colloidal dispersions deformed by a probe driven with arbitrary force amplitude and frequency. A chief result of this work is the discovery of a regime in which the resistance to motion of the probe particle is on average weaker than the resistance the probe experiences when deformed by high frequency oscillation. This hypoviscous effect arises when the reciprocating motion of the probe particle opens a channel free of other particles which is thus less resistive to probe motion. This effect is most apparent under the conditions of strong forces, rapid oscillation, and large extent of deformation.
Mechanical spectral hole burning in polymer solutions: Comparison with large amplitude oscillatory shear fingerprinting58(2014); http://dx.doi.org/10.1122/1.4829283View Description Hide Description
Large amplitude oscillatory shear (LAOS) fingerprinting of the nonlinear response of polymers and other complex soft matter has proven an excellent tool in the characterization of such materials. The major methods of quantitative characterization of nonlinearity using LAOS are the use of Lissajous-Bowditch (LB) curves and Fourier transform rheology (FTR). Another LAOS-based tool, mechanical spectral hole burning (MSHB), has been developed in our labs to investigate the heterogeneous dynamics of polymer melts and solutions and offers the possibility of a complementary method to the LB and FTR analyses for the fingerprinting of the nonlinear response of polymers and other complex fluids. Here, we present results from an investigation of the MSHB, LB loop, and FTR behaviors of polybutadiene and polystyrene solutions. The goal of the study was to examine similarities and differences in how these methods “fingerprint” the nonlinear response of the different polymer solutions. We find that the vertical holes in the MSHB experiments vary in intensity with the square of the sine wave pump magnitude. This is also true for the third harmonics obtained in the FTR, but the origins are possibly different for the two signatures. We have analyzed the fingerprinting categories as a function of strain amplitude and frequency from the LB loops and higher harmonics of FTR and add an additional feature to the fingerprints that are related to MSHB signatures, which are found to complement the other two methods and add new information to the fingerprinting diagrams.