A
material’s perspective: Developing a transient framework for understanding
nonlinear rheological responses
Simon Rogers
University of Illinois at Urbana-Champaign, USA
Abstract
Modern society relies on soft materials, which are
important for foods, consumer products, biological materials, and energy and
environmental applications. The interactions that hold soft materials together
are often comparable in magnitude to the thermal energy, making them especially
susceptible to weak forces. In order to develop functional soft materials, they
need to be processed far from equilibrium. Despite recent progress, we still do
not understand how molecular-scale behavior informs macroscopic properties in
these systems. Of particular interest is the transient nonlinear rheology,
where stresses and deformations can induce massive molecular reorganizations
that manifest as transformations in the macroscopic material properties.
Transient conditions are encountered in most biological situations, and well as
industrial flows including startup and cessation, which often dictate the
success of a product or process.
One particularly interesting class of materials undergoes
changes that transform their physical behavior from that of solids to that of
liquids. These so-called yield stress fluids have been studied for over a
hundred years, and still present significant scientific and engineering
challenges. Yield stress fluids have a wide variety of microstructures,
including filled polymer systems, colloidal glasses, and jammed microgels, and yet present a consistent rheology. Phenomena
such as the overshoot in the loss modulus in a strain amplitude sweep, viscosity
bifurcation and avalanches under stress-controlled tests, and the presence of
apparent yield strains are all typical features of yield stress fluid rheology.
In this talk, I will present a rheological framework for
understanding yield stress fluid rheology that is commensurate with recent rheo-SANS studies of self-assembled soft systems. The new
approach describes responses in terms of instantaneous recoverable and
unrecoverable strains that can be determined by iteratively performing
constrained recovery steps during traditional rheological characterizations. I
will show how the results of these new experiments elucidate the physics
underlying yield stress fluid phenomena, and also provide insight into the
phenomenon of mechanical memory observed in colloidal glasses, emulsions, and
foams. The lessons learned from these experimental results have led to the
development of a simple rheological model that accurately predicts yield stress
fluid behavior across a wide range of situations. The new model does not
contain features that have been considered crucial to understanding yield
stress fluids, and yet does a better job of describing real behaviors than
current state-of-the-art models. Taken
together, these studies are providing a rational route toward understanding and
designing structure-property-processing relationships for yield stress fluids.
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