Colloidal suspensions are widely used as a model system to study a variety of phenomena in hard condensed matter physics. The particles -  several ten nanometers to micrometers in size -  self organize into structures similar to atoms in different phases of condensed matter.  Crystalline as well as amorphous states can be prepared. Being several orders of magnitude larger than atoms, colloidal particles offer the unique possibility for studies at convenient length and time scales.
        We perform deformation experiments on colloidal suspension in the crystalline as well as the amorphous state to get deeper insight into atomic processes of plastic deformation. The face centered cubic (fcc) single crystals that we grow by slow sedimentation on a patterned template exhibit dislocations - Shockley partial dislocations, which are well known for fcc metals. We have developed a laser diffraction microscope to be used as the analogue to a transmission electron microscope to study the dislocations and follow their motion on a mesoscopic scale. In addition, we use confocal microscopy to study the defect evolution at the microscopic scale. Together, these two imaging techniques offer us the unique possibility of directly observing dislocation dynamics on different length scales. In the colloidal glass, we are able to follow the motion of the individual particles as we shear the suspension. This allows for identifying local shear events - particle rearrangement that give rise to permanent deformation.

Dislocations in Crystals

We study the nucleation and propagation of dislocations in colloidal crystals on a mesoscopic length scale using a self-made Laser diffraction Microscope (LDM) and at the single paricle level using confocal microscopy. We have studied the incorporation of dislocations in crystalline films, grown on a stretched substrate [2]. We are also studying in detail the nucleation of dislocations using a conventional sewing needle to indent an initially defect-free crystalline film [3]. This indentation experiment simulates a nano indentation experiment in atomic crystals. The figure below shows two reconstructions depicting the nucleation of a defect (a stacking fault bound by a dislocation) in the strain field under the needle. The small defect shown in the picture on the left is not stable and disappears after several minutes. The larger defect in the picture on the right is stable and stays.

Shear of Colloidal Glasses

We use a shear cell (Itai Cohen) to shear a colloidal glass and study shear events by tracking the motion of the individual particles during shearing. Figure (a) shows the particle displacements in the direction of the applied shear after 1.5% shear. Each data point  marks the displacement of an individual particle. We observe a linear shear gradient. If we focus on a smaller shear interval, we notice the existence of discrete localized events that give raise to the overall shear deformation. Figures (b) and (c) show particle positions before and after a local particle rearrangement, respectively. The red particle makes a large jump in the direction of applied shear, distorting the cage of neighboring particles.

For more information, please contact me: pschall@deas.harvard.edu

References

[1] A. van Blaaderen, R. Ruel and P. Wiltzius, Nature 385, 321 (1997).
[2] P. Schall, I. Cohen, D. Weitz and F. Spaepen, Science 305, 1948 (2004).
[3] P. Schall, I. Cohen, D. Weitz and F. Spaepen, in preparation.