Colloid-polymer mixtures can undergo spinodal decomposition into colloid-richand colloid-poor regions. Gelation results when interconnected colloid-richregions solidify. We show that this occurs when these regions undergo a glasstransition, leading to dynamic arrest of the spinodal decomposition. Thecharacteristic length scale of the gel decreases with increasing quench depth,and the nonergodicity parameter exhibits a pronounced dependence on scatteringvector. Mode coupling theory gives a good description of the dynamics, providedwe use the full static structure as input.
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