ICTS

Abstract: School textbooks tell us that the main difference between a solid and a liquid is the ability of the former to retain its shape. Any attempt at changing the shape of a solid is resisted by an internal elastic stress unless the deformation crosses a limiting value; at which point the solid fails. This naive viewpoint, although of great practical value, is, however, fundamentally incorrect. For example, one can argue that given enough time, atoms in the solid can always rearrange to eliminate stress no matter how much, or how little, the solid is deformed. Resolution of this paradoxical result lies at the core of our understanding of the behavior of solids under deformation. Adapting ideas which were introduced recently to study glasses, we find that rigidity arises a result of a hidden first-order phase transition between phases which differ in the way they respond to changes of shape [1]. When deformed by any amount, howsoever small, the rigid solid goes into a meta-stable state analogous to superheated water. Eventually, this meta-stable state always decays by nucleating bubbles of the stable, stress-free, solid by a process very similar to how bubbles of steam appear in a kettle of boiling water. This fresh conceptual viewpoint curiously allows us to study failure of perfect crystalline solids in quantitative detail without invoking specifics of many-body, defect–defect interactions, raising hope of a more unified description of materials in the future.

[1] Nath et al. PNAS 115 E4322-E4329 (2018)

Abstract: Existence and growth of amorphous and other structural order in supercooled liquids approaching glass transition is a subject of intense research. Even after decades of work, there is still no clear consensus on the molecular mechanisms that lead to a rapid slowing down of liquid dynamics approaching this putative transition. The existence of a correlation length associated with amorphous order has recently been postulated and also been estimated using multi-point correlation functions which cannot be calculated easily in experiments. Thus the study of growing amorphous order remains mostly restricted to systems like colloidal glasses and simulations of model glass-forming liquids. We proposed an experimentally realizable yet simple correlation function to study the growth of amorphous order. We then demonstrate the validity of this approach for a few well-studied model supercooled liquids and obtain results, which are consistent with other conventional methods. Finally I will discuss role of the static length-scale associated with this amorphous order in dynamics of glassy systems with medium range crystalline order. We tried to understand the molecular mechanisms for glass transition in liquids with and without medium range crystalline order. The issue is important to answer because if the liquid eventually starts to form crystalline domains while approaching the glass transition then the complexity of the problem may simplify and may just be governed by the physics of the growing crystalline order. If not then there exists a new class of glass forming materials whose molecular mechanism for slowing down of dynamics will probably be easier to understand in terms of the dynamics of the growing medium range crystalline order. We found evidence that dynamics of glasses with medium range crystalline order are generically different from other glass forming liquids with no predominant local order.

Reference:

1. R Das, S Chakrabarty and S Karmakar, Soft Matter 13, 6929 (2017).

2. S Chakrabarty, I Tah, S Karmakar and C Dasgupta, PRL 119, 205502 (2017).

3. I Tah, S Sengupta, S Sastry, C Dasgupta and S Karmakar, PRL 121, 085703 (2018).

4. BP Bhowmik, I Tah and S Karmakar, PRE, 98, 022122 (2018).