Constrained Shear in Crystals of Dimers Itai Cohen, Cornell University, DMR 0606040

1. Constrained Shear in Crystals of DimersItai Cohen, Cornell University, DMR 0606040 The Cohen group studies how crystals deform to learn more about why materials have the properties they do. It’s a tough task because atomic crystals are so small. So the group spends most of their time looking at materials called colloids – tiny particles suspended in a fluid. Colloidal crystals act similarly to their atomic counterparts but are big enough to be examined under a microscope. Crystals made from peanut-shaped colloids (Fig.1) don’t deform as easily as crystals made of spherical colloids (Fig. 2) because the peanut lobes interlock throughout the crystal. Cohen’s group is measuring the distribution of allowed shear distances in these crystals (Fig. 3) in order to determine their unique properties. If the same design principles could be mimicked in atomic systems, we would be able to make tougher metals and airplane components that are more resistant to cracks. Fig.1 Dimer shaped colloidal particles (a) Crystal of dimers (b). Fig.2 Dimer crystal under shear. Particles in orientations depicted in red would break. Fig.3 Determining Distribution of allowed shear distances. Gerbode SJ, Lee SH, Liddell CM, Cohen I, PRL101, 058302(2008) Lee SH, Gerbode SJ, John B, Escobedo F, Cohen I, Liddell CM, J. Mat. Chem. (accepted)

2. Cohen’s graduate student Leif Ristroph works with a 1st grader Partnering with a Charter School to Reach At-Risk StudentsItai Cohen, Cornell University, DMR 0606040 Professor Cohen has started a partnership between the Cornell Center for Materials Research (CCMR) outreach program and the Harlem Children’s Zone (HCZ) — a pioneering, non-profit community-based organization that works to enhance the quality of life for children and families in central Harlem. This program brings hands-on activities and lessons to interest at-risk students in science and technology. In one event, Prof. Itai Cohen and his graduate students led 130 kindergarten and first grade students as they explored the states of matter and tried their hand at classifying unusual materials. Students mixed sodium tetraborate with glue to make “silly putty,” which flows like a liquid, but bounces like a solid. Cohen has also led professional development workshops aimed at laboratory development for the HCZ middle school science teachers. Prof. Itai Cohen explains spectroscopy to 6th graders. Learn more about the Harlem Children's Zone at http://www.hcz.org/