Vikram Thapar publishes on the Kinetics of Crystallization of Novel Solids
Vikram Thapar, a 4th year PhD student from Fernando Escobedo’s group, has published a paper titled: “Localized orientational order chaperones the nucleation of Rotator phases in hard polyhedral particles” in Physical Review Letters.
There exists a growing interest in the generation and characterization of self-assembled suspensions of polyhedral particles for a wide range of potential applications, from solar cell nanocrystal arrays, to photonic materials, to liquid armor. This interest has been partially fueled by the seemingly boundless means available today to experimentally produce nano- and micro-particles with tailored shapes and compositions. However, kinetic traps are often encountered when self-assembling particle suspensions, which can preclude the timely formation of the structures of interest. Thapar and Escobedo used advanced molecular simulation methods to study in detail the nucleation kinetics of structural order by prototypical polyhedron-shaped particles (namely, cuboctahedra, truncated octahedra, and rhombic dodecahedra) from liquid suspensions near their disorder-to-order transition concentration. Given the “roundedness” of such particle shapes, they all form an ordered phase called a “rotator” or “plastic” solid wherein particles are translationally ordered (i.e., fluctuate around fixed sites on a lattice) but are orientationally disordered (particles axes do not align in any pattern). While one would have expected these polyhedra to exhibit similar nucleation kinetics as spheres (given that only translational order is being nucleated by ‘rounded’ hard particles), Thapar and Escobedo find that the kinetics is orders of magnitude faster (for comparable degrees of supersaturation). Counterintuitively, it is the coupling between short-range spatial fluctuations of orientational order (arising from the tendency of neighbor particles’ facets to pack parallel to each other) and translational order what facilitates the growth of translationally ordered nuclei, even though the resulting bulk rotator phase has no or minimal long-range orientational order. It is expected that this catalytic entropic mechanism is general and hence play a role for many polyhedral shapes that form a rotator phase. As the first study of homogeneous nucleation of a solid in polyhedral particle suspensions, it provides a fundamental stepping stone toward understanding nucleation and entropic catalysis in self-assembled structures. From a practical viewpoint, suspensions of these polyhedral particles are predicted to form solids much faster than spheres (circumventing some of the kinetic traps often seen with the latter); some polyhedra could even be used as initiators to catalyze solid-nucleation of spheres.
Reference: Vikram Thapar and Fernando Escobedo “Localized orientational order chaperones the nucleation of Rotator phases in hard polyhedral particles”, Phys. Rev. Lett. 112, 048301 (2014). DOI: 10.1103/PhysRevLett.112.048301