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In atomic and most molecular materials the nucleation of crystallites is very fast and therefore hard to observe. However, suspensions of colloidal particles are good model systems to study the growth of crystals (e.g. hard spheres, see figure below). The nucleation times are of the order of hours, so there's enough time to observe the whole growth process in detail. We use PMMA particles with a diameter of the order of a micron as a model system to study homogeneous nucleation and growth of crystals. Our particles have been made by Andrew Schofield in Prof. P.N. Pusey's group at the University of Edinburgh. The particles are sterically stabilized by a short surfactant and their interaction closely approximates that of hard spheres.
We use light scattering techniques as well as confocal microscopy to study the nucleation and growth of colloidal crystals.
We work hard to answer questions concerning the properties of crystal nuclei: How do crystal nuclei form from the metastable liquid? What is their structure? (The smallest nuclei are too small to be studied by Bragg scattering.) What is their shape? What are the properties of the solid-liquid interface? How fast do they grow? What is their critical size? ...
Click here to download a paper with many of our results obtained with confocal microscopy.
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Static small angle light scattering is one of the methods we use to study the formation and growth of crystal nuclei from the metastable liquid. In such measurements we probe density fluctuations during the crystallization process. The nucleation sites have a somewhat higher density than the surrounding liquid and give rise to a peak at low q-values. After some time the peak appears and grows while it slowly shifts to lower q-values. Later its intensity decreases while it still shifts to lower q. From this time dependence we get information about the crystallization rate, the time dependence of the crystal size, and the factors limiting the growth of the crystallites. With this method the results are always an average over an ensemble of many nuclei. Next to small angle light scattering we also use Bragg scattering to study the growth of crystals. Many of our light scattering experiments at Harvard are done in the context of the PCS experiment aboard the international space station (ISS). During PCS (Physics of Colloids in Space), the growth of crystals and gels is being studied by different light scattering techniques. |
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In the picture on the left a typical measurement of the small angle peak is shown. The numbers in the legends are the time in seconds after mixing the sample. |
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We use a laser scanning confocal microscope from Noran Instruments to observe crystallization in suspensions of fluorescently labeled PMMA beads with a diameter of roughly 2 microns. With the confocal we can take 3d images of a small volume in the sample. Therefore, we are able to study individual very small crystal nuclei. By taking images two or three times every minute we can watch how crystallites form and evolve with time. We use local bond order parameters to determine where crystal nuclei have formed due to structural fluctuations in the metastable liquid. The ray-tracing image below is an example for this: Particles with a crystal-like surrounding are represented by the large red spheres while the smaller yellow spheres (not drawn to scale) represent particles in the liquid. | |
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| Have a closer look at some crystalline clusters that we have observed in our samples. |
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This web page is maintained by:
Urs Gasser
40 Oxford Street
ESL 201
Cambridge, MA 02138
617-496-7451
gasser@deas.harvard.edu