Sheared Soft Matter: Paradigm for Non-equilibrium Physics

Soft matter under shear is a paradigm for fascinating non-equilibrium physics.  My talk will showcase this by taking examples from our on-going work related to (i) anomalously large fluctuations in shear rate (or viscosity) showing positive and negative values, at a constant shear stress in a jammed state [1], (ii) Spatio-temporal rheochaos  [2,3].   Our experiments on the stress induced jammed state in surfactant-based hexagonal phase and colloidal glass show that the shear rate exhibits large fluctuations, including several negative values. The Probability Distribution Function (PDF) of the global power flux to the system is consistent with non-equilibrium steady state fluctuation theorem of Gallavotti and Cohen.  Interestingly, the non- Gaussian PDF shows a ‘Universality’ class.

In the last few years, rheochaos – irregular time dependence in the stress at constant shear rate or vice versa, has been established to be a flow instability that occurs practically at zero Reynolds number in systems who flow curve is intrinsically non-monotonic or multivalued.  Our experiments using time-dependence of stress and polarized light scattering establish the spatially inhomogeneous orientational dynamics for shear thinning worm-like micellar gels en route to spatio-temporal chaos.  

In the last part of my talk, I will present our recent flow experiments on two-dimensional films at liquid-liquid interface, revealing shear banding and jamming transitions [4].

[1] S. Mazumdar and A.K. Sood, Phys. Rev. Lett. 101, 078 301 (2008) and unpublished results.
[2]  R. Ganapathy and A.K. Sood, Phys. Rev. Lett.  96, 108 301 (2006).
[3]  R. Ganapathy, S. Mazumdar and A.K. Sood, Phys. Rev. E 78, 021 504 (2008).
[4]  R. Krishnaswamy, S. Mazumdar, Vikram Rathee and A.K. Sood (2009).