ИСТИНА

Simulating carbonate reservoirs in microfluidic experiments requires reproducing not only the pore geometry, but also the basic physicochemical properties of the surface: a stable, optically compatible layer of CaCO₃ on the channel walls. This paper presents the testing of a previously developed method for forming such a layer directly inside silicon-glass microfluidic chips by pumping chemical agents through the chip and layer-by-layer (LbL) deposition on a pre-activated and functionalised surface [1]. After plasma cleaning, the channels are modified with an amine-containing silane, which provides nucleation centres and adhesion of inorganic deposits, and then Ca²+ and CO₃²- solutions are sequentially passed through at controlled pH, ionic strength and contact time. The number of LbL cycles determines the thickness of the carbonate film from tens of nanometres to units of micrometres. Optimisation of flow rate and temperature conditions minimises concentration gradients and achieves high uniformity along the channel length. To suppress waterite crystallisation and preferentially obtain calcite, slightly alkaline conditions and moderate salt concentrations were used, which, in combination with the amine substrate, stabilises the desired polymorph. SEM was used to control the thickness and continuity of the carbonate layer. The optical transparency of the substrate is maintained due to the thinness and uniformity of the layer, which ensures correct visualisation of fluid flow. Adhesion and mechanical stability are assessed by the critical shear stress at increasing flow rates. Resistance to washing with changes in ionic strength and pH demonstrates the suitability of the coating for long-term experimental cycles. The proposed method has significant advantages over traditional biochemical and gas diffusion approaches. Unlike biochemical methods, which require sterile conditions and do not provide stable control of the uniformity of the formed coating, the developed approach allows for a reproducible deposition process with a high degree of uniformity. Compared to gas diffusion technologies, the implemented method provides increased adhesion strength of the deposit to the walls of microchannels and eliminates the occurrence of unfavourable concentration gradients characteristic of static mineralisation processes. The resulting carbonate surfaces make it possible to carefully control wettability, interfacial tension, and adhesion phenomena in tasks involving oil displacement simulation, reactive flow, and carbonate-dependent chemistry on the surface of microfluidic chip channels. Thus, this method of in-situ LbL precipitation of CaCO₃ on pre-functionalised microchip walls forms a technologically simple, scalable and optically transparent platform that provides the ability to study carbonate systems with independent control of pore space geometry and physicochemical surface properties. References [1] Wang, W., Chang, S., & Gizzatov, A. (2017). Toward reservoir-on-a-chip: fabricating reservoir micromodels by in situ growing calcium carbonate nanocrystals in microfluidic channels. ACS applied materials & interfaces, 9(34), 29380-29386.