The transformation of a liquid substance into a solid glass state or vice versa is a fascinating phenomenon that has perplexed scientists for decades. However, a recent study published in Science Advances sheds light on this complex process. Conducted by Qi Zhang and a team of physicists at the Hong Kong University of Science and Technology in China, the study involves the creation of colloidal glasses via vapor deposition and the observation of their transition from a solid to a liquid state.
The researchers used a technique known as vapor deposition to create the colloidal glasses. This involves the deposition of solid particles onto a substrate in a vacuum chamber, creating a disordered structure. The team then subjected the samples to varying temperatures and observed the changes in their structure using confocal microscopy. By doing so, they were able to observe the dynamics of the glass transition process, shedding new light on this enigmatic phenomenon.
The study has important implications for a range of fields, including materials science and physics, as it provides new insights into the behavior of glasses at the molecular level. It is hoped that these findings will pave the way for the development of new materials with unique properties and applications. Overall, this study represents a significant step forward in our understanding of the complex process of glass transition and is sure to inspire further research in this area.
In their study, the researchers analyzed the structural and dynamic properties of different layers within a colloidal glass sample. By examining these properties at varying depths, they were able to identify a surface liquid layer and an intermediate glassy layer. The team then observed the behavior of individual particles within the surface liquid layer, noting various features that confirmed theoretical predictions of melting.
Specifically, the researchers focused on single-particle kinetics, examining how individual particles moved and interacted within the surface liquid layer. By doing so, they were able to gain valuable insights into the melting process and confirm the predictions made by theoretical models.
The dynamics of melting glass
The process of glass melting has long been thought to be the reverse of the glass-forming transition from liquid to glass, but recent research has shown that this is not the case. While the mechanism of glass formation has been extensively studied, the mechanism of glass melting is still in its early stages of development. One promising area of research has been the study of ultrastable glasses, which have shown a heterogenous surface melting mechanism that involves pre-melting at the surface to preempt melting from within.
Polymer scientists have focused on studying atomic and molecular ultrastable glasses, but colloids are emerging as an outstanding model system for studying glass-melting behavior. This is due to the fact that colloids consist of micron-scale particles that can be easily viewed via optical microscopy, allowing for detailed observations of thermal motions. Colloids also provide valuable microscopic information on bulk glasses, including insights into shear-induced bulk glass melting.
The exploration of thermally induced bulk or surface melting at the single particle level has been limited by the lack of colloids with tunable attraction. However, in this study, Qi Zhang and colleagues overcame this limitation by using attractive colloids to measure microscopic kinetics at different temperature ranges. The team examined the behavior of monolayer and multilayer samples subjected to slow and fast temperature changes, allowing them to study the pre-melting and melting trajectories of the colloids in detail. By using colloids with tunable attraction, the researchers were able to gain new insights into the process of melting at the single particle level. This is an important development in the field of materials science, as it opens up new avenues for understanding the complex behavior of glasses and other materials.
The glass-melting experiments: Surface pre-melting under slow temperature changes
In order to conduct their experiments, Zhang and colleagues used a 50:50 mixture of polymer spheres to prevent crystallization. They also added a dye to induce attraction between the polymethyl methacrylate spheres. By pumping the dye to the unheated region via thermophoresis, they were able to decrease the strength of the attraction while linearly increasing the effective temperature.
Using this method, the researchers were able to create both monolayer and multilayer colloids. They assembled these colloidal glasses using vapor deposition, resulting in ultrastable molecular glasses. The team then observed the behavior of the particles using optical microscopy and tracked their Brownian motions using image analysis.
The impact of slow temperature change on the structural and dynamic parameters
At a temperature of 25.3 degrees Celsius, the researchers observed complete bulk melting transitions. Prior to melting, the team had predicted that the thickness of the surface liquid would grow according to a power law, and this was confirmed through both experimental observations and simulations. The researchers also found that the local structure and dynamics were related, with the low-density region near the surface exhibiting fragile glass behavior and the high-density region near the bulk exhibiting strong glass behavior.
Interestingly, this fragile-to-strong crossover with decreasing temperature has been observed in a variety of materials, including water, metallic glasses, and organic/inorganic glasses. By focusing on the relationship between the structural and dynamic properties of bulk glass and supercooled liquid, this research has shed new light on the behavior of materials near the surface. These findings have important implications for the development of new materials with improved properties, as well as for our understanding of the fundamental principles underlying glass melting and other phase transitions.
Multilayer dynamics and temperature changes
The behavior of monolayer and bilayer colloidal crystals during pre-melting and melting was found to be distinct from one another, whereas monolayer and multilayer colloidal glasses showed similar behavior during these processes. Traditionally, crystal melting is studied by rapidly increasing the temperature beyond the melting point. To observe both melting and pre-melting processes in glass, the researchers changed the temperature rapidly.
It was observed that the glass transition temperature was lower under fast temperature changes than it was under slow temperature changes. Bilayer and trilayer glasses exhibited similar pre-melting behavior under fast temperature changes. The researchers had previously observed the consistent speed of melting in ultrastable glass using simulations, and the findings of this study confirmed those observations experimentally. These results have implications for our understanding of the fundamental principles underlying phase transitions in glass and the behavior of materials under different temperature conditions.
Cooperative rearrangement regions
Zhang and colleagues identified regions of cooperative rearrangement that play a crucial role in glass relaxation near the surface. These regions were found to consist of clusters of at least two mobile particles, which were hypothesized to have a compact core surrounded by a string-like shell.
As the effective temperature increased over time, the morphology of the material changed from compact to string-like compositions, in a manner that was consistent with predictions and observations made in bulk glasses. Heating caused the polarized cooperative rearrangement region on the surface of the monolayer glass to shift from parallel to nearly perpendicular, in order to facilitate melting. This process was reversed during the growth of the glass via vapor deposition. These findings provide insights into the microscopic processes that underlie glass transition and melting, and could have important implications for the design and development of new materials.
Outlook
In this study, Qi Zhang and colleagues used single-particle kinetics to identify two distinct surface layers in glass. The top layer remained stable at a fixed temperature, indicating pre-melting behavior instead of bulk melting. The similarity between glass and crystals was noted during the pre-melting and melting process, such as the nucleation-like bulk melting observed in ordinary glasses. However, the study of glass surface melting is still in the preliminary stage and requires further theoretical and experimental investigations at the single-particle level.
While simulations have focused on the speed of the melting front and crossover depth from surface to bulk melting, the concept of glass pre-melting has not been discussed in detail. The glass surface exhibited an additional glassy layer, which goes beyond the pre-melting theory observed in crystal pre-melting processes. Although the pre-melting/melting behaviors observed in this study are similar to those in bulk glasses, they differ from the behavioral dynamics of monolayer/bilayer crystals.
You might also be interested in reading, Revolutionizing Indoor Air Quality: York University’s Groundbreaking Solution to Measuring Pollution
