The principle of osmotic compression is shown in Fig 1(a). A dilute formulation contained within a milliliter dialysis bag is immerged within a reservoir containing a stressing solution (generally polymers). The porosity of the dialysis membrane retains large colloidal species (10-1000 nm), but still allows fluxes of solvent and molecular solutes. The osmotic pressure difference across the membrane then drives a flow to equilibrate chemical potentials of the molecular species. Besides providing an easy way to concentrate dilute formulations without tedious hand-made mixing, this technique also leads to estimates of the equation of state of the complex fluid, i.e. the relation between the osmotic pressure pi and composition.

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