# Presentation: 654039

## Colloidal Crystals out of Equilibrium: Phase-Transition Kinetics with Single-‘Atom’ Resolution Anthony  D. Dinsmore, University of Mass Amherst Physics,  DMR 0305395 & DMR 0605839

**Colloidal Crystals out of Equilibrium: ****Phase-Transition Kinetics with Single-‘Atom’ Resolution** **Anthony  D. Dinsmore, University of Mass Amherst Physics,  DMR 0305395 & DMR 0605839**

-     Melting and freezing of crystallites are everyday phenomena and the subjects of decades of research.  Yet mysteries remain about these non-equilibrium processes, in part owing to the difficulties of experimental studies of atomic systems.  Using microscopic colloidal spheres with a tunable interaction potential, we track the motions of individual spheres as they form crystals (or gels). Then, following a temperature change, the crystals sublimate into the gas phase.
-      As expected, large crystallites sublimate from the perimeter, leaving the interior well ordered.  Surprisingly, when the cluster size falls below ~30, the crystallites rapidly become amorphous throughout their area, then rapidly evaporate.  This unexpected two-stage process demonstrates the role of non-equilibrium phases in the kinetics of phase transitions [*Science* **314**, 795 (2006).]  Our experiments probe similar phenomena in the formation and aging of gels.

- Images of a single-layer crystallite of 1-m colloidal spheres, obtained with optical microscopy.  The spheres attract one another over a short range because of micelles in solution (not visible). We tune the attraction by means of temperature, and crystals can form or vanish.  Over a period of 100s, the above crystallite shrank from *N *= 49 to *N* = 31 particles.  In the subsequent 90s, it maintained *N* but began to lose crystalline order (3rd image). It then vanished within 30s.  Plots of *N* *vs*. time for many clusters in many samples show a similarly rapid drop-off when *N* < 30.

**t = 0**

**t= 100 s**

**t = 190 s**

- The plot shows that small clusters become disordered (|6|2  0) when *N* < 30; this occurs during both **sublimation** **and** **freezing** for shallow quenches.

- 0

- 0.4

- 0.8

- 20

- 40

- 60

- |**6|2

*N*

**Notes:**
The freezing and melting of crystals are fascinating phenomena that are very common in nature yet difficult to study in the laboratory.  Micron-sized particles suspended in solution (colloidal particles) serve as a useful model: Colloidal particles obey the same laws of statistical mechanics that govern how ice melts into water, but they are much larger and much slower than molecules, and hence visible with optical microscopy. By tracking the motions of thousands of individual particles, one can observe phase transitions at the single-‘atom’ level – and uncover some surprises. 

Here we have found that when sublimating (or melting) crystallites contain a fairly small number of particles, the kinetics of melting and freezing can speed up dramatically.  The change in kinetics appears to arise because the crystallites adopt structures that are not normally found in equilibrium. (In this case, a liquid phase;  for particles having a very short-range attraction, as in this work, coexistence of gas and liquid is never found in equilibrium). These results are an example of the important role of non-equilibrium states in aging and in phase transitions, which is a major focus of this NSF-supported project.  These results should also be broadly relevant to melting, freezing and annealing of atomic crystals -- especially very small ones (i.e. nanoparticles).  The results might also help pave the way to forming large crystals of proteins (which can be approximately viewed as spherical objects); forming these crystals is a major bottleneck in determining protein structures.    

## Colloidal Crystals out of Equilibrium: Phase-Transition Kinetics with Single-‘Atom’ Resolution Anthony D. Dinsmore, University of Mass Amherst Physics, DMR-0305395 & DMR-0605839 

**Education:**
-      Two graduate students (John Savage and Liquan Pei) contributed to this work.  Dr. Savage has now received his Ph.D. and moved to a postdoctoral fellowship; Mr. Pei joined the group more recently.
-      This NSF project also supports graduate student Jing Zhou and undergraduate Tim Prisk.  They have focused on using optical microscopy to measure the forces on individual particles in random, non-equilibrium granular packings in three dimensions.  Mr. Prisk recently obtained his B.S. degree and will enroll in graduate school in 2007. Mr. Zhou will shortly obtain his Ph.D. and move to a staff research position in industry, where he will work on problems involving granular materials.

**Societal Impact:**
-     We have developed a powerful model system that allows us to probe the kinetics of melting and freezing crystals and gels. These materials are very common in everyday life. The central result of this study is that the process by which very small crystallites melt or freeze is quite distinct from that of larger crystallites.  Controlling the growth of crystallites is a key step in materials science and in solving the structure of proteins in biology.