Fabrication of COC micromodels with wettability heterogeneities: method and influence on fluid transport†

Wettability plays a key role in multiphase fluid flow through porous media, significantly influencing geological processes such as CO₂ sequestration, groundwater remediation, or oil recovery. Micromodels, i.e. microfluidic porous media, have advanced the study of fluid flows in porous media by enabling direct visualisation of these processes. However, the influence of wettability heterogeneities on fluid flows in porous media remains underexplored in the literature, with studies focusing primarily on homogeneous wettabilities. In this study, we propose a complete method to manufacture micromodels with controllable, heterogeneous wettabilities. This work is at the crossroads of three different fields: microfabrication, surface treatment and fluid transport in porous media. The micromodels are made from a transparent polymer, cyclic olefin copolymer (COC), using hot-embossing. A plasma enhanced chemical vapor deposition (PECVD) process with a tetraethyl orthosilicate (TEOS) precursor is then used locally to reduce the COC's wettability. The durability, degree, and localisation of the deposition are quantitatively assessed with scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS), IR spectroscopy, and contact angle measurements. Our fabrication method successfully produced mixed-wet micromodels with easily controllable wettability patterns. Additionally, our study also presents a qualitative analysis of the impact of wettability heterogeneities on multiphase flows for oil, water, and water-in-oil emulsion injections. The location of the treated surface areas is shown to strongly impact emulsion stability and transport.