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    COLLOQUIUM OF THE LABORATORY FOR COMPUTER DESIGN OF MATERIALS
        Institute for Computational Sciences and Informatics
	                      CSI 898-Sec 001  
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 ORDER OUT OF CHAOS: A SELF-ORGANIZED TWO-DIMENSIONAL COLLOID CRYSTAL

			   Hollis Wickman
   National Science Foundation and Naval Research Laboratory

Novel precipitation and interfacial phenomena have recently been observed
in a simple chemical system:  Ca(OH)2(aq), water, and air. The behavior
depends on the reaction of calcium ions with dissolved carbon dioxide to
precipitate calcium carbonate, a chameleon-like material that appears in
minerals (marble, Iceland spar, travertines, etc.) and biominerals (bone,
nacre, teeth) in different crystal polymorphic forms (calcite, aragaonite,
vaterite), and is of major industrial importance as a component in
commercial products, boiler scale, etc. It is known that calcium carbonate
precipitate crystallites self-organize at the air/water interface to form
fractal structures that are modeled as cluster-cluster aggregates. One part
of the presentation describes a simple method for "morphing" the interface
fractals with respect to crystallite polymorph and fractal dimension. More
importantly, we have discovered conditions under which a new sub-population
of carbonate crystallites appears at the interface, initially, in a
disordered distribution over the surface. Remarkably, the crystals then
slowly self-organize into a hexatic, two-dimensional fluid crystal, which
we call a w-layer. The lattice spacing between crystallites is typically
seven times the 25 micron diameter of the crystallites making up the
w-layer. We are not aware of any previous reports of such phenomenon. The
crystals composing the w-layer have morphology remarkably different from
the common calcium carbonate polymorphs. The chemical composition and x-ray
structure of the w-layer crystals are under investigation, progress in this
area will be reported. The w-layer forms concomitantly with, but distinct
from, the previously studied fractal calcite structures. It exhibits many
properties not observed in other two- or three-dimensional colloid
crystals. Finally, it interacts with the fractal calcite over time in a
complex mixing process. Candidate models to describe the w-layer include a
Wigner lattice and a system of particles interacting via a pronounced
secondary minimum in the interparticle interaction potential. 

	               Monday , September 8 1997
                                 4:30 pm
                        Room 206, Science & Tech. I

Refreshments will be served.
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(http://www.csi.gmu.edu/lcdm/seminar/schedule.html)