Dynamic light scattering studies on the sol-gel transition of a suspension of anisotropic colloidal particles

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Abstract
Structural relaxation in amorphous systems is an area of much current interest. Many studies have been devoted to the sol-gel transition in systems based on polymers @1-3#, natu- ral gelatine @4#, and gels based on spherical colloids @5-7#. Here we report on the sol-gel transition in a system of charged disks with an aspect ratio of 25, suspended in water. In such systems the orientational degrees of freedom play a crucial role not only in the dynamics but also in the static structure of the gel. Molecular dynamics simulations on hard disk systems reveal a rich phase diagram with nematic and cubatic liquid crystalline phases @8#. However, before these liquid crystalline phases can be formed the system enters a glassy phase or gel. Current opinion favors the ''house of cards'' structure for the gel @9#, which implies a random structure with short range orientational order. On the average the disks are oriented with their positively charged rim to- wards the negatively charged base of their neighbor. Within this view the orientational degrees of freedom will play a crucial role in the formation of the gel. Structure and dynamics in the neighborhood of the tran- sition can be studied best by using the noninvasive technique of light scattering, dynamic ~DLS! and static. DLS probes the density correlation function describing the time evolution of the density fluctuations. The striking feature, largely emerging from DLS measurements around the gel point @4-6#, is the marked similarity of the behavior of the corre- lation function with the scenarios given by the mode cou- pling theory of Gotze for the structural glass transition in molecular systems @10-13#. In the mode coupling scenarios there are two characteristic algebraic relaxation processes: a faster b relaxation, followed by the slower a relaxation. The relaxation processes are ''stretched'' towards longer and longer time scales as the transition is approached. In the glass or gel phase only the b relaxation remains, which is observed as an algebraic decay towards a plateau value. This plateau reflects the onset of structural arrest at the gel point, which is observed as a pure elastic contribution to the scat- tering, the equivalent of the Debye-Waller factor. The relevant quantities to be measured are the two alge- braic exponents governing the a and b relaxation, and the fraction of frozen-in density fluctuations which is considered the order parameter of the transition. However, an accurate determination of these quantities by light scattering is far from trivial @14#. One of the experimental difficulties en- countered is elastic scattering in the gel phase. This static scattering is observed as ''speckle'' and acts as a spatially fluctuating intrinsic local oscillator. In the intensity correla-