Yielding processes in a colloidal glass of soft star-like micelles under large amplitude oscillatory shear (LAOS)

The understanding of yielding and flow of a colloidal glass under large amplitude oscillatory shear (LAOS) represents a motivating challenge. Monitoring the higher harmonics in the stress signal by Fourier-transform (FT) rheology may provide useful insight on the progressive transition from linear to nonlinear viscoelastic response. However, the physical interpretation of FT-rheology data is still not obvious. Here we study the process of yielding in a colloidal glass formed by star-like block copolymer micelles with LAOS experiments and interrogate the nonlinear intracycle stress response by FT analysis and decomposition to an orthogonal set of Chebyshev polynomials [Ewoldt, R. H., et al. J. Rheol. 52(6), 1427-1458 (2008)]. Such approach provides a robust framework enabling us to map out a rich phenomenology of intracylce nonlinearities that may relate to distinct physical mechanisms. We find that the nonlinearities during yielding are represented by intracylce shear thickening/thinning and strain hardening/softening of the viscous and elastic response of the system, respectively. We suggest that the underlying mechanisms are related to cage breaking and reformation as well as stress storing and relaxation within the period of oscillatory shear which are affected by an interplay between shear and Brownian motions and thus relate to Peclet number variation with strain and frequency. (C) 2010 The Society of Rheology. [DOI: 10.1122/1.3483610