Osmotic Stress on chip: an innovative tool to screen complex fluids at the nanoliter scale – OSMOCHIP

Solvay is facing today with an increasing demand for studying new formulations with limited environmental footprints and for a wide range of applications. The elaboration of their phase diagram is simple but tedious, since it consists in mixing chemicals by hand or using labscale mixing apparatus, and in monitoring the state of the mixture often only visually. These day-to-day tasks are associated to large costs and long times, typically 20-80 k€ and 1-4 months, and suffer from severe limitations for handling concentrated mixtures, often viscous or gelled. A common strategy to overcome these limitations relies on continuous approaches such as solvent exchange by dialysis between the formulations under study and reservoirs of known osmotic pressure. This osmotic compression technique provides an easy way to screen continuously formulations without tedious hand-made mixing, but also leads to fundamental data such as Equations of States EOS (for non-aggregated mixtures). These data are also fundamental inputs to optimize any processes driving soft matter out-of-equilibrium (drying, coating, membrane-based separation, etc.). Despite its relevance for R&D, osmotic compression is rarely used due to the very long equilibration times (from days to weeks) and the difficult use of in-situ measurements.

OsmoChip aims to develop microfluidic chips mimicking osmotic compression at the nanoliter scale. The small scales combined with large surface-to-volume ratios will lead to unprecedented improvements in the time scale of measurements (2 to 3 orders of magnitude). Such tools would also make possible in-situ measurements, such as small-angle X-ray scattering, spectroscopy, transmission microscopy… to probe the microstructure of the formulation during its compression. These tools will thus open the way to rapid high-throughput screening of formulations (up to 100 samples/day/chip), thus decreasing significantly current R&D bottlenecks. Beyond these industrial benefits, our microfluidic technique will provide EOS of complex fluids at an unprecedented level of accuracy. The rapidity and the reproducibility of these measurements will be significantly enhanced as compared to macroscopic osmotic compression owing to the outstanding control of mass transfer at small scales. Moreover, real-time measurements of relaxation towards equilibrium will yield out-of-equilibrium quantities such as the collective diffusion coefficient of the formulation. Macroscopic measurements of this coefficient are difficult and scarce, but they are again of particular interest for processes driving soft matter out-of-equilibrium.

To reach our challenging objectives, OsmoChip is organized around 3 workpackages. WP1 concerns the integration of membranes with different pore sizes, permeability, and chemical nature, within a wide range of chips depending on the application. WP1 is devoted to development of microfabrication protocols and to extensive characterization of the membranes. WP2 will exploit these devices to measure EOS of complex fluids (equilibrium states) and transport coefficients from time-resolved measurements of the relaxation towards equilibrium. The aim of WP2 is to optimize microfluidic chips to perform osmotic compression in the more appropriated way. WP2 will also tackle fundamental issues related to the confinement and the anisotropy of osmotic compression, using adequate modeling and in-situ characterizations. WP3 will deliver microfluidic platforms for handling industrial formulations (from colloidal dispersions to surfactant/polymer solutions) and screening their phase diagrams at a high throughput. WP3 will include the development of both the automated platforms and the associated in-situ measurements. The deliverable of WP3 is the successful demonstration of these tools on several key products from Solvay, particularly concerning the screening of dispersants for aqueous dispersions of pigments.