From cooperativity in supercooled liquids to plasticity of amorphous solids
- Patrick Ilg (Swiss Federal Institute of Technology Zurich (ETHZ), Switzerland)
- Emanuela Del Gado (Department of Physics and I(SM)2, Georgetown University, USA)
- Asaph Widmer-Cooper (University of Sydney, Australia)
- Peter Schall (University of Amsterdam, The Netherlands)
A coherent understanding of the mechanical properties of amorphous materials in terms of microscopic heterogeneities in their structure is still fundamentally lacking. Important questions that are crucial for advancements in technological applications (e.g. how do cohesive interactions or detailed micro-structure end up controlling creep and mechanical aging) remain open. The latest developments on cooperative behaviour and dynamical heterogeneities in supercooled liquids and elastic heterogeneities and plasticity in amorphous solids open new possibilities to connect liquid- and solid-like aspects of these materials in a common description. However, this connection remains challenging: One reason is the huge time-scale separation between microscopic and macroscopic scales in these materials, presenting a great challenge for both simulations and experiments. Another reason is that the approaches that have been developed over the past years are rather diverse and complementary. Hence, next to the development of new ideas and techniques, there is urgent need to connect these concepts, to ultimately achieve a unifying description of this class of materials.
The workshop will discuss how new ideas and progress in our understanding may arise by bringing together the latest developments of theoretical frameworks (e.g. Random First Order Theories (RFOT), Soft Glassy Rheology (SGR), Shear-Transformation Zones (STZ), Mode-Coupling Theories (MCT)), advanced simulation techniques and experiments. More specifically, having identified characteristic microscopic processes in glassy systems, we would like to discuss their relevance for macroscopic mechanical properties.
Amorphous materials have traditionally been modeled as basically elastically isotropic and homogeneous but experiments and simulations have shown that the dynamics becomes spatially heterogeneous upon cooling from the liquid state. This problem has recently received additional attention because of new glassy materials with outstanding properties (e.g. bulk metallic glasses ) and the vast domain of applications in the soft matter field, where many systems show amorphous structures and "soft-glassy" behavior [2,3,4].
In the last few years experiments on colloidal model systems and computer simulations have uncovered several interesting microscopic mechanisms in supercooled liquids. In particular, cooperatively rearranging regions and corresponding dynamical heterogeneities have been detected and found to display characteristic changes on approaching the glass transition, including a growing dynamical length scale. A state-of-the-art review is given in the recent monograph . Whether there are underlying structural features that give rise to these dynamical properties is one of the key questions. The classical ideas of Adam-Gibbs and the more recent random first order theory (RFOT) assume a structural origin of the dynamical slowing down, whereas mode-coupling theories (MCT) do not. Recent tests of the Adam-Gibbs relation and RFOT ideas by computer simulations show some promising results [6,7,8]. New simulation methods also support the idea that dynamical heterogeneities have indeed a structural origin, although it is far from obvious. In particular, it has been recently shown that the spatial distribution of local soft modes, a purely static property of a microscopic configuration, can predict to some extent where structural relaxation will subsequently occur. This identification of a causal link between microscopic structure, heterogenous dynamics and relaxation constitutes a significant success in understanding how structure determines relaxation in a material lacking long-range order. The localised low-frequency modes are unique to disordered materials and appear to be a generic feature of glassy alloys. However, little is known about the details of the relationship between microscopic structure, the spatial distribution of the soft modes, and structural relaxation, or the contribution of the modes to the mechanical properties of the amorphous solids.
In the field of mechanics of amorphous solids significant progress has been achieved in the last years as well. Both, experiments and computer simulations have detected characteristic structural rearrangements [9,10] and avalanches  in these materials. On a microscopic scale, these amorphous materials show elastic heterogeneities. Whether and how structural relaxations can be predicted from elastic heterogeneities or other local features is intensively debated these days . The origin of mechanical instabilities that can induce yielding and flow  or the microscopic processes underlying the onset of plastic behavior in amorphous solids are fundamentally unknown . Similarly, their possible connections with structural features are far from understood. The shear-transformation zone approach seems to capture some of the observed features , being able to reproduce stress-strain relationships very well; but how this translates to specific microscopic dynamical processes and whether they can be related to specific structural features is still intensely debated [12,16,17].
Since the same material exists as supercooled liquid and amorphous solid with rather similar structures in nearby temperature intervals, it is important to understand how different microscopic mechanisms can arise and lead to significant macroscopic changes in the way the material responds to external deformation. However, there is very little fundamental understanding of these issues and it is only recently that some works are starting to explore them. In particular, possible connections between a growing length scale in supercooled liquids and the onset of plasticity have been investigated in Ref. . Ref.  shows analogies between dynamical heterogeneities in supercooled liquids and elastic heterogeneities in amorphous solids. Finally, deformations of inherent structure configurations lead to non-affine displacements that are reminiscent of dynamic cooperative rearrangements in the supercooled regime .
In view of the current situation with several recent promising results but yet unclear connections between the different directions, we are convinced that this workshop is very timely.
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