| Summary: | We examine the nature of relative motion in colloidal suspensions. By distinguishing carefully between the thermodynamic pressure of a mixture, defined by Gibbs, and the pressure measured by Darcy in porous media, we resolve apparent contradictions between the results and interpretations of different experiments. We show that Fick's and Darcy's laws, two empirical equations thought to describe different and complementary physical phenomena, are in fact particular limits of a single, unifying thermodynamic equation which can be used more generally to describe transport in colloidal systems. Importantly, this equation relates macroscopically measurable quantities. We use it to provide new interpretations of experiments in ultrafiltration. © 2005 American Institute of Physics.
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