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 10⁶ to 10⁸ Brownian times. Moreover, the mean squared displacement, 〈x²〉, shows power-law behavior over seven time (τ) decades: 〈x²〉 ∝ τ^(0.30±0.05). 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.