Synergy of surface roughness and wettability gradient enables highly efficient adhesion and desorption of petroleum hydrocarbon contaminants.

In separation materials with homogeneous wettability, there exists an inherent conflict between the interfacial adhesion and desorption behaviors of oil droplets. In this study, we propose a mixed microchannel (MM) consisting of hydrophilic rough media and oleophilic media, which enables the efficient capture and rapid transport of petroleum hydrocarbon pollutants. A total of 204 oil droplets were observed for each surface condition, and the roughened medium surface increased the probability of oil droplet adhesion from 13.73% to 82.84%, while reducing the adhesion energy barrier from 1222.36 kT to 5.36 kT. Meanwhile, the wettability gradient within the MM induced a meniscus-flow region, extending the oil droplet migration distance to 2.59 times that in the oleophilic microchannel (OM) and increasing the oil-phase migration rate by 3.10%-14.49%. The synergistic coupling of surface roughness and wettability gradient overcomes the dual limitations of excessive adhesion energy barriers and high migration resistance, thereby enabling a dynamic balance between oil droplet adhesion and desorption. When the proportion of oleophilic media was 50%, the MM exhibited a higher average oil-phase separation efficiency than the OM. In addition, the pressure drop decreased by 16.38%, the effluent flux increased by 9.47%, and the oil-phase flux was simultaneously enhanced, demonstrating the synergistic advantages of surface roughness and wettability gradients in maintaining efficient separation while substantially improving flow characteristics. A produced-water sidestream experiment conducted on a Bohai offshore oil production platform further demonstrated that the average effluent oil concentration after MM treatment met the monthly average discharge limit of ≤20 mg/L specified in the Grade I standard of GB 4914-2008, with deep removal of petroleum hydrocarbon pollutants. This study provides a theoretical basis and engineering guidance for the efficient, anti-fouling separation of high-viscosity and strongly emulsified oily wastewater.