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We have developed a poly(dimethylsiloxane) (PDMS) microfluidic chip to studythe directional drying of a colloidal dispersion confined in a channel. Ourmeasurements on a dispersion of silica nanoparticles once again revealed thephenomenology commonly observed for such systems: the formation of a poroussolid with linear growth in the channel at short times, slowing down at longertimes as the evaporation rate decreases. The growth of the solid is alsoaccompanied by mechanical stresses that are released by the delamination of thesolid from the channel walls and the formation of cracks. In addition to theseobservations, we report original measurements using hydrophilic filler in thePDMS formulation used (Sylgard-184). When the PDMS matrix is in contact withwater, water molecules pool around these hydrophilic sites, resulting in theformation of microscopic water clusters whose size depends on the waterpotential $\psi$. In our work, we have used these water clusters to estimatethe water potential profile in the channel as the porous solid grows. Using atransport model that also takes into account solid delamination in the channel,we then linked these water potential measurements to the hydraulic permeabilityof the porous solid. These measurements finally enabled us to show that theslowdown in the evaporation rate is due to the invasion of the porous solid byair/water nanomenisci at a critical capillary pressure $\psi_\text{cap}$.

Directional drying of a colloidal dispersion: quantitative description  with water potential measurements using water clusters in a  poly(dimethylsiloxane) microfluidic chip