Real-space fluorescence recovery after photo-bleaching of concentrated suspensions of hard colloidal spheres

Abstract

In concentrated suspensions of fluorescent colloidal hard spheres (close to and above the glass transition density), we bleached part of the system in cube shaped regions using high intensity laser light. Recovery of these bleached cubes was followed in real space using confocal scanning laser microscopy (CSLM). This method provides mean squared particle displacements up to timescales that are three orders of magnitude beyond those available by present experimental techniques. We show that, above the (hard sphere) glass transition density, particles move over distances of the order of their own diameter on timescales of 10 6 to 10 8 Brownian times. Moreover, the mean squared displacement, hx 2 i, shows powerlaw behavior over seven time (t) decades: hx 2 i / t (0.30AE0.05) . This behavior is different from earlier observations by dynamic light scattering. It is argued that these differences are caused by gravity effects, as the only difference between the systems is the buoyant mass of the colloids.

Gravity-Induced Aging in Glasses of Colloidal Hard Spheres

“…1(b) we have plotted DLS data from van Megen [19]; these data clearly are consistent with ours under normal gravity conditions. Our results indicate a strong influence of gravity on particle displacements; preliminary indications of this have been reported in [20]. This is surprising, as the gravitational length in our system, even under normal gravity (where h 13 m), equals many particle diameters.…”

“…The method is described in detail in [20], and is only summarized here. Using a CSLM, profiles were bleached in the form of cubes with linear sizes on the order of 10 to 100 particle diameters in the focal plane, and order 10 diameters in thickness.…”

“…However, due to the limited coherence time of lasers, only time scales on the order of hours can be reached by this technique. In order to be able to reach longer time scales, which is necessary for the kind of concentrated systems we are interested in, we applied real-space fluorescent recovery after photobleaching as recently developed by us for colloidal systems and reported in [20]. This method makes use of confocal scanning laser microscopy (CSLM), and allows following particles over time windows that are up to 3 orders of magnitude longer than those reached by DLS.…”

“…By integrating the fluorescent intensities along the sides of the bleached cubes, intensity profiles were obtained. The decay length of the fluorescent intensity profile, defined by the length where the intensity is half the (normalized) intensity of the unbleached part of the sample, x 1=2 , is related to the mean squared displacement hx 2 i by [20] …”

Synthesis of fluorescent silica-coated gibbsite platelets

“…By bleaching a well-defined pattern, the recovery speed of the fluorescent intensity in that region is a measure for the collective motion of the platelets [21]. A rough estimate for the diffusion coefficient, using randomly oriented infinitely thin plates, yields [22] …”