8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P5.154Wed 711:10-14:00Bond orientational order in randomly-packed colloidalspheresEli Sloutskin 1 and Alexander Butenko 11 Physics Department, Bar-Ilan University, Bar-Ilan University, 52900, Ramat-Gan,IsraelSystems of jammed particles are abundant, yet jamming is still poorly understood. The jammedsystems are often naively assumed to be disordered, such that only short-range correlations arepresent and all spatial directions are equivalent. Yet, the mechanical stability of these materialsimplies that a network of mechanical forces percolates through the sample. Such network maygive rise to long-range correlations, and the rotational symmetry may be broken. We directly measure,by laser scanning confocal microscopy, the positions of hard colloids, which are sedimentedby centrifugation, to form a truly macroscopic jammed matter. Initially, our colloids are suspendedin a solvent, such that their concentration determines the density of the final jammed state, playinga role of an effective (inverse) temperature. We follow the centrifugation process in motion, measuringthe density profile of our particles along the sample, as a function of the centrifugation timet; a critical time t c exists, such that the system is fully jammed for t > t c . Strikingly, while onlyshort-range positional order exists in our system, both in the fluid state and in the jammed state, theorientations of the bonds between the nearest neighbors (NN) are correlated in the jammed state,throughout the system. These extended orientational correlations give rise to a 6-fold symmetryin the jammed state, such that the rotational symmetry is broken. Moreover, this 6-fold symmetryis correlated with the direction of gravity, suggesting that the mechanical network of forces playsan important role in our system. This breaking of rotational symmetry, observed in our very simplemodel of random packing, must have an impact on a wide range of properties in other, morecomplex, randomly packed systems.154