Connecting bulk rheology, structural transitions and heterogeneous flow in Pluronic F127 micellar cubic liquid...

Understanding "yielding" of gels and other soft materials is critical for successful performance in many industries and research fields. However, it is known that not all yielding is the same, and that local structures and microscopic dynamics can be highly heterogeneous. Combining bulk measurements and direct visualization should enable us to map local flows and structural heterogeneities, and to connect their evolution with bulk rheological properties.

We study the underlying microscopic dynamic behaviours of an F127 micellar cubic phase gel using rheo-optics. Bulk rheology experiments under large and small deformations are performed with fluorescence and polarized microscopy at the edge of the gap. Large amplitude oscillatory shear results are analyzed using Fourier-Transform rheology and the Sequence of Physical Processes.

The structure-rheology analyses enable the correlation of the onset of nonlinearities with banding instabilities and the flow transition with the "slip-stick" theory previously proposed for cubic phases. Under large deformations, fracture is not consistent with the expected idealized yield behaviour. Birefringence analyses enable mapping the structural heterogeneity and the kinetics of structural transitions during the crystallization of the cubic structure. Combination of the flow and birefringence results reveals a connection between structural heterogeneity, transition kinetics, and fracture under large deformations.