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Spatial correlations and deformation modes in sheared colloidal glasses
Chikkadi, V.K.
Publication date 2011
Link to publication
Citation for published version (APA):
Chikkadi, V. K. (2011). Spatial correlations and deformation modes in sheared colloidal glasses.
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Summary
Glasses are amorphous solids that exists in both natural and man-made forms. It is one of the oldest material to be used by mankind. Glasses are widely used for their exceptional mechanical properties. When heated to high temperatures, they become viscous and flow easily. Upon cooling, the glass solidifies mechanically. This ability of glases to change shape when heated is crucial to its utility and beauty. When we look at the beautiful stained glass windows of cathedrals or the glass art at Murano (Venice), one immediately starts wondering how glass bends and flows. This is in addition an important question from an industrial perspective that uses glass for various applications. Despite decades of research on glasses, the question remains far from being resolved.
The direct observation of the motion of molecules in a deformed glass is difficult, as the existing experimental techniques cannot resolve the dynamics of individual molecules. In this thesis, we prepared a soft glass using dense suspensions of micron sized PMMA particles that are model hard sphere systems. Similar to atoms, the hard sphere colloidal sus-pensions show fluid and crystalline equilibrium phases. However, when they are quenched to high volume fractions of particles, they exhibit slow relaxation that is hallmark of glasses. As these particles are much larger than atoms, they can be resolved individually and their trajecto-ries can be followed in three dimensions using the technique of confocal microscopy. We exploit the direct imaging of colloids to investigate the deformation of glasses at the microscopic scale.
Similarly to molecular glasses, the colloidal glasses show two different modes of deformation: a homogeneous deformation at low shear rates and a transition to inhomogeneous flow at large shear rates. The
Summary 121 sition occurs when the shear rate exceeds the inverse of the relaxation time of the quiescent glass. We visualize of microscopic fluctuations of the local strain and non-affine displacement. These reveal that the plastic deformation occurs in localized regions that are termed shear transfor-mation zones (STZ). The overall defortransfor-mation of the glass is a collective result of such STZs. At low shear rates, the STZs occur throughout the deformed volume, however, at larger shear rates, the STZs organize into a band leading to shear banding. The difference between the bands was investigated by studying different structural and dynamic order parame-ters. It transpires that the fundamental difference lies in their dynamics. By defining an order parameter that is extensive in time and space, we show that shear banding is reminiscent of a dynamic first order transition. To elucidate the bulk flow of the glasses, we have investigated the spatial correlations of strain and non-affine displacement. Our experi-ments reveal that the formation of a shear transformation zone induces a long-ranged quadrupolar strain field around it. This long-range interac-tion between the STZs gives rise to a scale-free deformainterac-tion of the glass. Further, the associated scaling exponent remains robust over a range of shear rates. At low shear rates, the spatial correlations are isotropic, and decay in the same manner in all the directions. However, at higher shear rates, the decay of the spatial correlations is anisotropic, and they are strongest in the direction of shear. This suggests that the symmetry of spatial correlations is broken at higher shear rates, leading to the forma-tion of shear bands.
Finally, we have investigated the relaxation of a quiescent colloidal glass. The regions where particles rearrange and relax were identified us-ing the relative displacements of particles. To understand the nature and origin of such rearranging regions, we have computed the low-frequency vibrational modes of the system using the covariance matrix of displace-ments. Our experiments reveal that the zones of structural rearrange-ments are strongly correlated with the quasi-localized, low-frequency modes of the system. This points to the structural origin of the re-laxation, and shows that the system indeed relaxes along the softest available modes.
