Numerical analysis of low-Reynolds-number flow past an array of pillars with surface slip

In the development of micro- and nano-fluidic devices incorporating nanopillar structures to create nanoscale confined flow paths, it is important to consider the slip velocity on the pillar surface, which reflects the local wettability. In this work, flow past an array of pillars with surface slip at a low Reynolds number is numerically investigated for different pillar configurations to quantify the effect of slip on the flow rate. Furthermore, the influence of spatially varying slip distributions on the flow rate and direction is examined, which may provide a basis for passive flow control in micro- and nano-fluidic devices. The simulations are carried out using an immersed boundary projection method developed to impose the Navier boundary condition and accurately reproduce slip velocity on complex surfaces. The results show that the flow rate increases monotonically with slip length and sufficient enhancement can be achieved when the slip length is on the order of the pillar size. In addition, surfaces with localized slip regions can effectively improve the flow rate when slip is applied to areas that correspond to high slip velocity in the fully slipping case, yielding performance comparable to uniformly slipping surfaces. For asymmetric slip-length distributions, the flow is found to incline with respect to the driving direction despite the geometric symmetry of the pillar configurations. These findings suggest that controlled slip-length distributions, due to spatially varying wettability on nanopillar surfaces, can be a powerful design parameter for advanced flow manipulation in micro- and nano-fluidic devices.