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The fundamentals of granular mixing are not well understood, particularly in comparison with fluid mixing. We consider the mixing in two-dimensional tumbling mixers with a view to reveal the role of advection in the dynamics of mixing. Computations and experiments for noncircular mixers show chaotic advection, which results in faster convective mixing as compared to circular mixers. Chaotic advection dominates in large mixers, the norm for industrial processes, whereas particle diffusion is important for small—laboratory scale—mixers. The results are important for the design and scale-up of industrial mixers.