The dynamic structure factor of fractal colloidal gels is shown to exhibit a stretched exponential decay to a finite plateau with an exponent of about 0.7. The value of the plateau depends on both initial particle volume fraction f 0 and scattering wave vector. We show that this behavior results from the contribution of internal elastic modes of many length scales, and present a model which accounts for the data. From the observed plateau we determine that the very small elastic modulus scales as G ϳ f 3.9 0 , in agreement with predictions, and with direct mechanical measurements. [S0031-9007(97)05169-7] PACS numbers: 82.70.Dd, 61.43.Hv, 63.50. + x, 82.70.Gg Colloidal aggregates form fascinating structures; despite the apparent disorder of their shape, they possess a remarkable degree of symmetry, and can be well described as fractals [1]. This scale invariance has facilitated the description of their structure and its relationship to the kinetics of their formation [2]. One of the unique features of a fractal structure is that its density decreases as its size grows; as a result, colloidal aggregates ultimately gel to form a very weak solid, comprised of a connected, disordered network that fills all space [3][4][5]. If the aggregation is predominantly diffusion limited, the average clusters that form the gel are surprisingly uniform in size, resulting in a strong peak in the static light scattering intensity at low angles [3,5]. By contrast, if the aggregation is reaction limited, the large polydispersity in the cluster size precludes the low angle scattering peak [4]. Suspensions with exceedingly low initial particle volume fraction f 0 can gel, provided buoyancy matched particles are used to avoid sedimentation. The resultant solid is a very interesting material; although it is a highly disordered network, the scale invariant structure is nevertheless well determined. This makes colloidal gels ideal models for the study of the internal dynamics and related mechanical properties of disordered networks in general. Unfortunately, however, such gels are so weak that it is very difficult to measure their properties with mechanical techniques.In this Letter, we overcome this experimental limitation by using dynamic light scattering (DLS) to measure the internal dynamics of fractal colloidal gels, and develop a model that allows us to determine their mechanical properties. We show that colloidal gels exhibit unusual behavior. At early times the dynamics appear as anomalous or "stretched" diffusion with an exponent independent of scattering wave vector q or initial particle volume fraction f 0 ; this is in sharp contrast with polymer gels, where stretched diffusive behavior is observed only after an initial regime of simple exponential decay [6,7]. For colloidal gels, it is still possible to define an effective diffusion coefficient, which scales as q 22 ; this is in sharp contrast with DLS from internal motions of polymers, where a q 23 behavior is expected [8]. For colloidal gels formed from higher f 0 , the dyn...

A liquid-glass transition was observed experimentally in a new system, an oil-in-water emulsion. Dynamic light scattering was employed to obtain the intermediate scattering function f(q,t) for a range of volume fractions and scattering vectors q. The results are compared with predictions of the mode coupling theory. While the usual idealized version of the theory provides accurate fits to the data on the liquid side of the transition, fits for volume fractions near the transition and in the glass phase were found to require the extended version, presumably due to an additional decay mechanism related to the deformability of the oil droplets.

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