Microfluidic osmotic compression with <i>operando</i> meso-structure characterization using SAXS

We have developed a microfluidic chip for the osmotic compression of samples at the nanoliter scale, enabling the <i>in situ</i> and <i>operando</i> acquisition of structural features through small-angle X-ray scattering throughout the compression process. The design builds upon a previous setup allowing high-throughput measurements with minimal sample quantities. The updated design is specifically tailored for compatibility with a laboratory beamline, taking into account factors such as reduced photon flux and increased beam size compared to synchrotron beamlines. As a proof of concept, we performed on-chip compression of well-documented silica colloidal particles (Ludox TM-50). We demonstrated that the volume fraction could be tracked over time during compression, either by monitoring X-ray absorbance or by modeling the scattered signal. With precise control of the osmotic pressure and salt chemical potential, equations of state can be determined unambiguously from the volume fraction measurements and be interpreted with the help of the scattered intensity. These microfluidic chips will be valuable for understanding the behavior of colloidal suspensions, with applications in areas such as crystallization, nucleation, soil mechanics, control of living matter growth and interaction conditions, as well as the measurement of coarse-grained colloidal interaction potentials.