Melting of colloidal crystals

Crystal melting is affected by many factors such as defects, surfaces, dimensionality, lattice structure and particle interaction, and thus exhibits rich phenomenology. It is usually a first-order phase transition which currently lacks a fundamental theory. Despite numerous studies over the past century, melting remains poorly understood at the microscopic level. Micrometer-sized colloidal particles can be viewed as "big atoms" which can assemble into various phases. Their thermal motions can be directly imaged and tracked using optical microscopy. Hence, colloids are powerful model systems for studying phase transitions. Recent progress made in fabricating tunable micrometer sized colloidal particles has enabled the direct visualization of crystal melting with single-particle dynamics. The observations have greatly expanded our understanding of melting kinetics, such as:

• homogeneous melting can be described accurately by classical nucleation theory at weak superheating, however, strong deviations are also observed under strong superheating, mainly due to the coalescence of nuclei;
• liquid nuclei are triggered by local particle-exchange loops rather than any defects.

Detailed experimental observations and theoretical understanding can be found in those publications:

• Imaging the homogeneous nucleation during the melting of superheated colloidal crystals, Science 338, 87 (2012) doi: 10.1126/science.1224763
• Direct observation of liquid nucleus growth in homogeneous melting of colloidal crystals, Nature Communications 6, 6942 (2015) doi: 10.1038/ncomms7942
• Modes of surface premelting in attractive colloidal crystals. Nature, 531, 485-488 (2016).doi: 10.1038/nature16987

A review on melting of colloidal crystals, including homogeneous and heterogeneous nucleation of 3D crystals, 2D melting and surface premeltings, etc.

• Melting of colloidal crystals. Advanced Functional Materials, 26, 8903-8919 (2016).doi: 10.1002/adfm.201603077

When two nuclei grew and the distance between them was reduced, a liquid channel developed and facilitated their coalescence.

Copied from Nature Communications 6, 6942 (2015)

Two-step nucleation mechanism in solid-solid phase transitions

Nature Materials 14, 101-108 (2015)

The microscopic kinetics of ubiquitous solid–solid phase transitions remain poorly understood. By using single-particle-resolution video microscopy of colloidal films of diameter-tunable microspheres, we found that transitions between square and triangular lattices occur via a two-step diffusive nucleation pathway involving liquid nuclei. The nucleation pathway is favored over the direct one-step nucleation because the energy of the solid/liquid interface is lower than that between solid phases. Based on the theoretical understanding of this pathway, our findings suggest that an intermediate liquid should exist in the nucleation processes of solid–solid transitions of most metals and alloys, and provide guidance for better control of the kinetics of the transition and for future refinements of solid–solid transition theory.

Detecting Microbial Dysbiosis Associated with Pediatric Crohn Disease Despite the High Variability of the Gut Microbiota

Cell Reports 14, 945-955 (2016)

Codes for MMICA can be found on Github.

Summary

The relationship between the host and its microbiota is challenging to understand because both microbial communities and their environments are highly variable. We have developed a set of techniques based on population dynamics and information theory to address this challenge. These methods identify additional bacterial taxa associated with pediatric Crohn disease and can detect significant changes in microbial communities with fewer samples than previous statistical approaches required. We have also substantially improved the accuracy of the diagnosis based on the microbiota from stool samples, and we found that the ecological niche of a microbe predicts its role in Crohn disease. Bacteria typically residing in the lumen of healthy individuals decrease in disease, whereas bacteria typically residing on the mucosa of healthy individuals increase in disease. Our results also show that the associations with Crohn disease are evolutionarily conserved and provide a mutual information-based method to depict dysbiosis.

Highlights

• Mutual information distinguishes Crohn disease and controls by using the microbiome
• Stool and ileal microbiomes contain the same information about Crohn disease
• Microbes more abundant in ileum than in stool positively correlate with Crohn disease
• Statistical power to detect association varies greatly among commonly used methods

Fungal microbiota profile in newly-diagnosed treatment-naive children with Crohn disease

Journal of Crohn's and Colitis, 11(5) 586-592, (2017)

Our aim was to define the characteristics of fungal microbiota in newly diagnosed treatment-naive children with Crohn's disease (CD).

We found that CD-associated fungi varied with the level of phylogenetic tree. There was no significant difference in abundance between normal and inflamed mucosa. Significantly abundant CD-associated taxa included Psathyrellaceae, Cortinariaceae, Psathyrella, and Gymnopilus. However, there was no significant difference in fungal diversity between CD and controls. Further studies with larger sample size and including functional analysis are needed to clarify the significance of the fungal community in the pathogenesis of CD.