Phase Transition

Background

With appropriate particle size, time scale and adjustable potential, colloidal particles are viewed as a decent ‘simulation system’ for the true atoms. And with our powerful confocal microscopy, we can get the position and single-particle level kinetic behavior of every particle in the colloidal system. That provides great convenience for studies about some statistical phenomenon like the phase transitions. Based on this background, we have been carrying out series of experiments in colloidal system for an accurate and innovative understanding of the structures and dynamics throughout the complex transition process.

Finished Work

Kinetic Pathway of Homogeneous Crystallization

In this project, we visualize the kinetic pathway of homogeneous crystallization in experiment. This phase transition process consists of the formation of precursors caused by density fluctuation in the system and a subsequent transformation to crystalline nucleus. With the help of confocal microscopy, we obtain the structural information at single-particle-level and define local structure through a new method of bond orientational order calculation we modify in this work. We clarify precursors and nucleus into 3 different kinds according to their local order and summarize the relationship between them.
This work reveals the kinetic pathway of homogeneous crystallization, which is essential and helpful for the further research of crystallization.

More details in:
Nature Physics, 2014, 10(1): 73-79.

Crystallization of Polymorphic Systems

Polymorphic system refers to a system with more than two stable crystalline states in the phase diagram. In this work, we experimentally reveal the crucial roles of competing ordering in the initial selection of polymorphs and the final grain boundary motion toward the most stable state. To access the spatiotemporal fluctuations of local symmetries in crystallization near the bcc.-fcc. border, we develop a unique method to correctly identify local order structures by removing distortions due to positional fluctuations.
Our research reveal how computation affects crystal nucleation and ordering toward final crystalline state.

More details in:
Science Advances, 2020, 6(27), eaaw8938.

Thermally-activated Pathways of Diffusionless Solid-to-Solid Transition

Solid-to-solid transitions usually occur via athermal nucleation pathways on pre-existing defects due to immense strain energy. However, we experimentally find both athermal and thermally-activated nucleation pathways controlled by the softness of parent crystal using an in-situ single-particle-level observation method. Moreover, we reveal three new transition pathways including: ingrain homogeneous nucleation driven by spontaneous dislocation generation, heterogeneous nucleation assisted by remelting grain boundaries and wall-assisted growth.
Our results shed light on the control of solid-to-solid transitions through the parent phase’s softness and defect landscape.

More details in:
Nature Communications, 2021, 12(1): 40-42.

Kinetics of Crystallization in Spherical Confinement

Crystallization under geometrical confinement is of fundamental importance in condensed matter physics and material science. In this work, we access the crystallization kinetics at the single-particle level by encapsulating charged colloids in emulsion droplets. We find rapid formation of ‘skin’ layers with an icosahedral arrangement of defects under geometrical frustration effect, followed by interior ordering and slow ripening. The final morphologies are determined by dynamical interplay between the system-independent ‘skin’ layer formation and the structural transformation towards the most stable solid far from surface.
This work is important for the structural design of nanoscale crystals since we reveal the crucial role of kinetics in morphological selection under geometrical constraint.

More details in:
Nature Physics, 2021, 17(1): 121-127.

Fast Crystal Growth at Ultra-low Temperatures

Slow liquid diffusion and geometric frustration brought by a rapid, deep quench are believed to inhibit fast crystallization and promote vitrification. In this project, we report fast crystal growth in supercooling charged colloidal systems and show that it occurs via wall-induced barrierless ordering. This ordering process consists of two coupled steps, including a diffusionless collective step-like advancement of the rough interface that disintegrates frustration, as well as a subsequent defect repairing inside newly formed solid phase that controls crystal quality. We also show that the intrinsic mechanical instability of a disordered glassy state subject to the crystal growth front allows for domino-like fast crystal growth even at ultra-low temperatures.
Our findings contribute to a deeper understanding of fast crystal growth and is useful for applications related to vitrification prevention and crystal-quality control.

More details in:
Nature Materials, 2021, 20(10): 1431-1439.