Events

Controlling Glass Time Through Entropy and Elasticity

Glasses have been considered ``Humankind's most important material''[1]. The silicate glasses of optical fibers that cross oceans for communication, the vaccine vials that tolerate extremely low temperatures, and their protective glass coatings have played prominent roles in our pandemic survival. Despite these engineering advances, some aspects of glass transition physics remain mysterious. For example, when nanometer layers of glass coatings are stacked in a device geometry, their properties are highly influenced by the kinetics of their neighboring layer, leading to unpredictable properties. In contrast, glass surfaces can behave liquid-like instead of rigid, resulting in flow in layers as thick as 30 nm, well beyond the intermolecular interaction range of ~1 nm. 

Glasses produced by physical vapor deposition (PVD) onto rigid temperature-controlled substrates have been shown to use this surface equilibration mechanism to access low-energy states, producing glasses with high density and improved stability. This process can also imprint structural features that don’t otherwise exist in liquid-cooled glasses.  The most optimized stable PVD glasses have been shown to have properties analogous to glasses aged for ~1-100 million years. I will discuss how, through molecular design, we can unravel some of the details of this mechanism and understand the glass transition process more broadly. I will also discuss how the elasticity of the support substrate can play a key role in the surface mobility and surface equilibration process, at distances ~200 nm away. Counterintuitively, a soft substrate can promote the formation of denser and more stable glasses, resulting in another leap in equilibration speed. A glass deposited on a soft substrate within ~3 hrs requires significantly slower deposition on a rigid substrate, taking well over ~3,000 years [2]. I will discuss how this phenomenon can be related to the dissipation of elastic deformation through the soft substrate, and quantify the role of elasticity in the vitrification of glassy materials.  

[1] https://www.theatlantic.com/technology/archive/2018/\\04/humankinds-most-important-material/557315/

[2] Luo, P., Wolf, S. E., Govind, S., Stephens, R. B., Kim, D. H., Chen, C. Y., Nguyen, T., Wąsik, P., Zhernenkov, M., McClimon, B. & Fakhraai, Z. High-density stable glasses formed on soft substrates. Nature Materials, (2024)