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We show, using a model coupling mass transport and liquid theory calculations for a charge-stabilized colloidal dispersion, that diffusion significantly limits measurement times of its Equation Of State (EOS), osmotic pressure vs composition, using the osmotic compression technique. Following this result, we present a microfluidic chip allowing one to measure the entire EOS of a charged dispersion at the nanoliter scale in a few hours. We also show that time-resolved analyses of relaxation to equilibrium in this microfluidic experiment lead to direct estimates of the collective diffusion coefficient of the dispersion in Donnan equilibrium with a salt reservoir., Comment: 7 pages, 4 figures

International audience; We report the fabrication of permeable membranes for microfluidic dialysis applications in poly(dimethylsiloxane) (PDMS) channels. A maskless UV projection device was used to photo-pattern long hydrogel membranes (mm-cm) with a spatial resolution of a few microns in PDMS chips integrating also micro-valves. We show in particular that multi-layer soft lithography allows one to deplete oxygen from the PDMS walls using nitrogen gas flow and therefore makes possible in-situ UV-induced polymerization of hydro-gels. We also report a simple surface modification of the PDMS channels leading to strongly anchored hydrogel membranes that can withstand trans-membrane pressure drops up to 1 bar without leakages. We then measured the Darcy permeability of these membranes and estimated their cutoff by measuring the kinetics of diffusion of macromolecules of different sizes through the membrane. Finally, we illustrate the opportunities offered by such microfluidic chips for dialysis applications by observing in real time the crystallization of a model protein in a chamber of a few nanoliters.