Dynamics of drop-induced lubricant separation and depletion on oleophilic polymer brushes

Controlling the wettability and friction of solid surfaces is crucial for many situations in nature and technology. Liquid-infused surfaces such as Slippery Liquid Infused Porous Surface(SLIPS), polymer gels and polymer brushes, have attracted particular attention since their introduction because of the low contact line friction and near-zero contact angle hysteresis. However, the softness of the substrate leads to more complex wetting characteristics of the droplets on the surface, e.g., substrate deformation and lubricant separation, forming wetting ridges at the contact line. Although phase separation and the formation of wetting ridges have been widely noted in SLIPS and polymer gels, this phenomenon remains unobserved and uninvestigated in polymer brushes with much thinner thickness( a few nanometers to hundreds of nanometers). Here, we demonstrate that the extraction of a wetting ridge induced by a water drop deposited onto a layer of oil-infused polymer brushes leads to a significant depletion of the oil saturation both under and next to the drop. Comparison of the experimental data to a numerical model allows to extract saturation-dependent transport parameters for the lubricant within the brush layer. This is important not only for understanding the oil diffusion process in the brush layer, but also for the insights into how to control the stability of lubricants in the future.