This workshop will focus on the key roles played defects in controlling the behaviour of amorphous materials, from glasses to jammed packings to active matter.

Programme

Friday 2022.09.30

08:30 Coffee/tea

09:00 Zahra Fahkraai (discussant: Rob Riggleman)

10:30 Snacks and coffee

11:00 Clare Yu (discussant: Rich Stephens)

12:30 Lunch

13:30 Dave Reichman (discussant: David Richard)

15:00 Snacks and coffee

15:30 Julia Giannini (discussant: Sean Ridout)

17:00 Free discussion

Saturday 2022.10.01

08:30 Coffee/tea

09:00 Nathan Keim (discussant: Paulo Arratia)

10:30 Snacks and coffee

11:00 Michael Falk (discussant: Damien Vandembroucq)

12:30 Free discussion

13:00 End of the meeting

Practical information:

If you wish to attend in person, please write an email with subject "DefectsWorkshop2022" to chackorn@sas.upenn.edu by the 19th of September.

Remote attendance will be possible via Zoom. To receive the link, please specify in the email body of your registration email that you will be attending remotely. Note that remote attendance will be chat-only: for logistical reasons, it will not be possible to ask questions via microphone over Zoom, but questions and comments typed in the Zoom chat will be read out.

Masks are required for in-person attendance when not eating, drinking or speaking.

Organisers: Rahul Chacko, Andrea Liu and Francesco Zamponi

Titles and abstracts:

Zahra Fahkraai

The dramatic effect of interfaces on glassy dynamics

The dynamics of glasses are dramatically influenced by their interfacial properties. While in bulk glasses local, structural signatures can be used to predict local relaxation times and describe dynamical heterogeneity, at interfaces the dynamics are modified by factors that are seemingly independent of these structural signatures. In particular, free surface dynamics can be enhanced by orders of magnitude with effects propagating over a broad distance away from the surface. In thin films and close to soft substrates these dramatic effects can propagate to length scales of 50-100 nm, far beyond the range predicted based on inter-molecular interactions and length scales of structural signatures. This enhanced mobility provides access to otherwise kinetically inaccessible states in the energy landscape, during physical vapor deposition (PVD). Through vapor deposition of molecules of various shapes, intermolecular interactions, and flexibility, we investigate the properties of glasses at low temperatures and factors that control the free surface dynamics. We demonstrate that under certain circumstances, unique phase transitions can be observed in vapor-deposited thin films at low temperatures, which can explain some of these surprising interfacial properties.

Clare Yu

Low Temperature Properties of Glasses: Why do phonons go so far?

A wide variety of amorphous materials exhibit similar behavior in their low temperature thermal properties regardless of their chemical composition. Examples include a specific heat that is linear in temperature T as well as a T² thermal conductivity. These features have been attributed to tunneling two level systems (TLS). However, the standard TLS model was not able to explain the universally small value of phonon scattering reflected in thermal conductivity, ultrasonic attenuation, internal friction, and the change in sound velocity. For example, the mean free path of phonons in amorphous SiO₂ is about 60 microns at 0.5 K. Why do phonons go so far? We present an answer based on aspects of the standard model that had either been ignored or not fully appreciated. We find good agreement between experiment and theory for a variety of individual glasses.

Dave Reichman

Localized defects in glasses: from soft modes to two-level systems

I will review our work on the in silico search for two-level systems in the energy landscape of model glass-forming liquids. The role of quench rate, inter-particle potential, and landscape search protocol in determining the density of defects will be discussed. Putative connections between soft, quasi-localized modes and two-level systems will be discussed.

Julia Giannini

Quasilocalized modes as representations of defects in passive and active structural glasses

Glasses are characterized by structural disorder and unique thermal, mechanical, and dynamical behavior. Several studies have shown that these features are governed by populations of defects hosted in the system’s disordered microstructure. In contrast to crystalline defects, glassy defects are difficult to identify from local structural information. This challenge is further complicated in active disordered solids, which have internally generated stresses, strong gradients in pressure, and noisy dynamics. In this talk, we will discuss recent work connecting the structure and dynamics of dense active matter systems with that of glasses and sheared amorphous solids. One set of studies [1,2] predicts the long-range same-time displacement and velocity correlations in systems of self-propelled particles and cell monolayers by adapting techniques originally formulated for solids and dense fluids, including normal mode analysis and continuum elasticity/hydrodynamics. Another study demonstrated a direct link between sheared solids and amorphous packings under fields of random imposed displacement in analogy to the internal driving of active solids [3]. In related work, we examined the structure and dynamics of an active particle model for human crowds, where stable disordered reference configurations are formed in the limit of persistent self-propulsion. We discovered that simple normal mode analysis is not sufficient to identify glassy defects in dense regions of these packings [4]. Therefore, our ongoing work uses anharmonic potential energy approximations to locate defects in active particle packings with global pressure gradients. These methods may be generalizable to predict dynamics in nonhamiltonian active materials.

[1] S. Henkes and E. Bertin, et. al., Nat. Comm. 11, 1 (2020).

[2] G. Szamel and E. Flenner, EPL 133, 60002 (2021).

[3] P. K. Morse and M. L. Manning, et. al., PNAS 118, e2019909118 (2021).

[4] J. Giannini and M. L. Manning, et. al., Soft Matt. 18, 1540 (2022).

Nathan Keim

Memories, annealing, and orbits with soft spots in a glassy solid

TBA

Michael Falk

A tour of the 3D local yield surface in a model amorphous solid

It has been the aim of much recent work to connect concepts regarding the physics of plasticity in glassy and amorphous materials to plastic constitutive theories. In doing so, many “plastic descriptors” have emerged to predict irreversible response to shear in atomistic models; almost all of these are inherently scalar. The scalar nature of these predictors mirrors the state variables that have been incorporated into many rate-and-state based theories of amorphous plasticity: concepts like “free volume,” “effective temperature,” and “softness". However there are strong reasons to believe that scalar measures are insufficient for describing the response of the glass structure. Here we discuss our investigations of a 3D binary Lennard-Jones glass in which we extracted the local yield stress throughout the simulated system. The shape of the resulting yield surface and the statistics of the yield events reveal interesting aspects of the underlying physics. We will use this data as a jumping off point to discuss what should supersede scalar order parameters in next-generation constitutive theories of amorphous plasticity.

This workshop is supported by the Simons Collaboration on Cracking the Glass Problem.

Simons Collaborations, made possible by support from the Simons Foundation, bring together groups of outstanding scientists to address mathematical or theoretical topics of fundamental scientific importance in which a significant new development has created a novel area for exploration or provided a new direction for progress in an established field.