In fluidized granular matter (such as rapidly flowing sand) heat can flow from colder to hotter granular temperatures, violating Fourier’s law. A simple heuristic explanation for this anomalous heat current is presented, based on the non-equilibrium nature of granular fluids. The heuristic explanation leads to a straightforward calculation of the heat current which is in good agreement with existing, more detailed calculations and with recent experiments.

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A freely-cooling granular fluid with spatially uniform temperature and density is said to be in the homogenous cooling state (HSC). The situation described here is the HSC perturbed by weak temperature and/or density gradients.

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