Multi-scale visualization of desorption in clay-coated microfluidic channels: Effect of flow dynamics and porous geometry

• Developed a novel microfluidic platform functionalised with transparent synthetic smectite clay to mimic clay-rich porous media. • Enabled direct, spatially resolved visualization of desorption processes using fluorescence imaging at the pore scale. • Demonstrated how flow rate and porous geometry interact to control microscale desorption dynamics. This study investigates desorption dynamics in clay-rich porous media with multiple scales of pore size through a microfluidic approach that enables spatially resolved pore-scale observations of transport processes. Desorption, the release of previously sorbed substances from surfaces into surrounding fluid, is critical for contaminant transport, remediation strategies, and resource recovery in environmental systems. While microfluidic devices offer substantial advantages for studying transport processes in porous media, realistically replicating natural surface characteristics in traditional micromodels remains challenging. Geomaterial microfluidics, achieved by coating conventional substrates with rock or soil minerals, offers a powerful tool for visualising pore-scale mass transport and solid-fluid interactions. A key challenge in employing geomaterial-coated micromodels to explore sorption-desorption is the opacity of most geomaterial minerals, hindering visualization of mass concentration changes within porous media. This research introduces a streamlined clay coating procedure to functionalise polydimethylsiloxane (PDMS) microfluidic channels with transparent synthetic smectite clay, mimicking the physicochemical properties of clay porous media, enabling direct visualization of desorption processes across various flow conditions and porous geometries. Tracer flow tests conducted in a series of clay-coated microfluidic channels revealed the influence of fluid flow conditions and porous geometry on the microscale desorption behavior. Desorption of fluorescein, used as a model sorbate, was observed via fluorescence imaging, enabling visualization and quantification of concentration changes over time with high spatial resolution. The findings demonstrate that desorption behavior is influenced by the intricate interplay between fluid flow condition and porous geometry. While increasing flow rates accelerate desorption, this does not necessarily improve overall recovery efficiency (the proportion of previously sorbed substance that can be recovered). Lower flow rates result in longer times to achieve complete desorption, where no recoverable sorbate remains, but may reduce residual mass concentration at exhaustive desorption, highlighting the importance of optimizing flow conditions for efficient contaminant recovery. This work provides insights into transport phenomena relevant to efficient recovery of valuable substances from water, supporting circular economy principles through resource reuse while minimizing harmful by-products. By addressing the previously underexplored desorption dynamics in recovery processes, our findings contribute to developing sustainable treatment and recovery technologies for water management and environmental remediation.