# Presentation: 828226

## Premelting at defects within bulk colloidal crystals Arjun G. Yodh, University of Pennsylvania, DMR-0079909 and DMR-0203378

-     In this work we gained insight into one of the most basic phenomena in physics – melting of a crystalline solid.  We showed that melting begins at defects within the crystal before spreading through the solid. Premelting is the localized loss of crystalline order at surfaces and defects for temperatures below the bulk melting transition. It can be thought of as the nucleation of the melting process.  Premelting has been observed at the surfaces of atomic crystals, but not within.  We discovered that:
-  Premelting arises at grain boundaries and dislocations within bulk colloidal crystals.
-  The degree of premelting and particle fluctuation depends on the type of defect, distance from the defect and particle volume fraction (i.e. proximity to the bulk melting point).
-     Our observations suggest interfacial free energy is the crucial parameter for premelting, in colloidal and atomic scale crystals.
*Science*, 309 1207-1210 (2005)

- Crystalline colloids premelt near defects in crystal, before the bulk crystal melts.  This figure represents a two-dimensional slice of a three-dimensional colloidal crystal.  A grain boundary, separating two crystallites with different orientations, rises from left to right.  Near the boundary it is apparent that colloidal particles (i.e. ‘big atoms’) move rapidly and exhibit liquidlike diffusion.  Red represents the most movement and violet the least movement.

**Notes:**
The crystals were equilibrium close packed three-dimensional colloidal structures made from thermally responsive microgel spheres. The thermal response of the microgel enabled precision control of particle volume fraction. Particle tracking using real-time video microscopy revealed increased disorder in crystalline regions bordering defects, the amount of which depends of the type of defect (e.g. grain boundaries, dislocations, vacancies), distance from the defect, and particle volume fraction.

## Premelting at defects within bulk colloidal crystals Arjun G. Yodh, University of Pennsylvania, DMR-0079909 and DMR-0203378

- Detail of the colloidal crystal premelting at a grain boundary. The figure shows bright field images at different temperatures (i.e particle volume fractions) of two crystallites separated by a grain boundary. (A) Sample at 27.2 oC. The solid and dashed lines show the grain boundary and a partial dislocation respectively. The grain boundary cuts the two crystals along two different planes (the yellow line has two slopes). It is composed of an array of dislocations; the two extra planes are indicated by lines in the inset. (B) Same sample at 28.0 oC. The grain boundary starts to premelt; nearby particles undergo liquid like diffusion, inset. The partial dislocation, denoted by the dashed line, is not affected. (C) and (D) The same sample at 28.1 oC and 28.2 oC, respectively. The width of the premelt region near the grain boundary increases. Scale bars are 5 m.

**Notes:**
Education:  This work was spearheaded by my PhD student Ahmed Alsayed, and my post-doctoral associate, Mohammad Islam. Ahmed is finishing up his thesis, and Mohammad will shortly be joining the faculty at Carnegie Mellon University in the Departments of Chemical Engineering and Materials Science.  Peter Collings, a Professor of Physics at Swarthmore College, was also closely involved in the project (completed during his sabbatical stay at PENN). 

Impact to Society:  Everybody is familiar with melting, but many of the details about ‘how crystals melt’ remain unknown.  This paper discovered an important melting mechanism that had been suggested theoretically, but never observed.  In particular, the crystal premelts near defects, before the bulk begins to fully melt.  These premelt regions are effectively nucleation sites where melting begins.  This new insight may be very important for processing and control of materials (both soft materials such as colloids and traditional atomic materials).