Abstract
Phase diagrams reflect the possible kinetic routes and guide various applications such as the designed assembly and synthesis of functional materials. Two-dimensional (2D) materials have been a focus of ongoing research due to their wide applications; therefore, researchers are highly motivated to discover the general principles that control the assembly of such materials. In this study, we map the 2D phase diagrams of short- and long-range attractive colloids at single-particle resolution by video microscopy. Phase boundaries, including (meta)critical or triple points and corresponding real-space configurations, are precisely specified. Profiles of 2D phase diagrams with attractive interactions resemble their 3D counterparts. For short-range systems, by measuring the deepest achievable supercooled states on the phase diagram, a “crater” structure surrounding the metastable fluid-fluid critical point indicates an enhanced nucleation rate within the crater and further suggests a local minimum of the free-energy barrier for crystallization in this area. During a dense fluid-mediated two-step crystallization process, we observe that multiple crystallites could form within a single dense fluid cluster, partly due to its highly amorphous shape. This highly amorphous shape is found for all observed well-developed dense fluid clusters. It is a supplement to the multistep nucleation process. For long-range systems, equilibrium vapor-liquid coexistence is observed, which paves the way for the exploration of critical behaviors. Rigidity percolation of crystallites, and bulk fluid-solid coexistence which provide clear evidence of a possible first-order transition for 2D melting, are observed for both systems. Our experiments reveal the general features of phase behaviors shared by 2D attractive systems including graphene, protein membranes, and adsorbed nanocrystals.
- Received 29 March 2019
- Revised 24 June 2019
DOI:https://doi.org/10.1103/PhysRevX.9.031032
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
The phase diagram of 2D materials reveals general features of phase transitions and behavior in functional materials such as graphene, protein membranes, and adsorbed nanocrystals. However, despite numerous theoretical and computational studies, systematic experimental investigations of the phase behavior of 2D systems have been scarce. We map the full 2D phase diagrams of a colloid with short- and long-range attractions by video microscopy, which allows visual observations of evolving structures and configurations of each phase at single-particle resolution.
Overall, 2D phase diagrams are qualitatively similar to their 3D counterparts. For systems with short-range attraction, we confirm the enhanced nucleation rate for homogeneous crystallization surrounding the metastable critical point. During a dense two-step crystallization process, the dense fluid clusters inside a dilute fluid are in a highly amorphous shape rather than spheres or polygons as commonly presumed, and multiple crystallites can form inside a single dense fluid cluster.
For systems with long-range attraction, we observe that vapor and liquid coexist and that both the critical and triple temperatures are approximately twice the values seen in simulations. Despite their different characteristics, short- and long-range systems share some common features, such as rigidity percolation of crystallites and bulk coexistence of fluid and solid, which is evidence of a first-order transition for 2D melting.
Usually, biological and atomic or molecular systems, respectively, have short- and long-range attractions, analogous to the interactions adopted in our experiments. Our studies can provide insight into the kinetics of phase transformations and can improve the understanding of related real systems.
