University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Calendar

Thursday, September 25th 2014
1:25 pm:
Condensed Matter Seminar in 210 Physics
Speaker: Paul Chaikin, NYU
Subject: Classical Wigner Crystals on flat and curved surfaces, topological defects, `pleats‘ particle fractionalization and grainboundary dynamics.

Charged colloidal particles present a controllable system for study a host of condensed matter/many body problems such as crystallization. 2D crystals are invariably hexagonal. Hexagons perfectly tile a flat plane but a soccer ball requires exactly 12 pentagons dispersed among the hexagons on its curved surface. Pentagons and hexagons are positive and negative topological charges, disclinations, sources for positive and negative curvature. But we have discovered that “Pleats”, grain boundaries which vanish on the surface (and play a similar role to fabric pleats) can provide a finer control of curvature.
We experimentally investigate the generation of topological charge as flat surfaces are curved. For positive curvature, domes and barrels, there is one pentagon added for every 1/12 of a sphere. Negative curvature is different! For capillary bridges forming catenoids, pleats relieve the stress before heptagons appear on the surface. Pleats are important for controlling curvature from crystals on surfaces, to the shape of the spiked crown of the Chrysler building.
Adding a particle to a flat surface produces an interstitial - usually an innocuous point defect. On a curved surface interstitials are remarkable, forming pairs or triplets of dislocations which can fission dividing the added particles into fractions which migrate to disclinations.
We have also used “Topological Tweezers” to create grains and study the dynmaics of dislocations in gran boundaries.

Faculty Host: E. Dan Dahlberg

The weekly calendar is also available via subscription to the physics-announce mailing list, and by RSS feed.