Discussion of the impact of surfactant on the drag-reduction potential of superhydrophobic surfaces

Recent studies, Peaudecerf et al. (PNAS 2017) and Song et al. (PRF 2018), have investigated the negative effect of surfactant on the drag-reduction performance of superhydrophobic surfaces (SHS). As SHS could have a large impact in reducing energy utilisation for many internal and external flow applications, it is important to understand and predict how surfactant-Marangoni stresses affect the flow over SHS. We review experimental and numerical work which show how surfactant can deteriorate the effective slip of SHS and lead to the undesired no slip boundary condition. By studying the governing equations of surfactant transport over SHS, we show the complexity of modelling this coupled nonlinear problems where up to eleven dimensionless parameters can appear. We reveal different regimes in surfactant dynamics and highlight the important dimensionless parameters. We show the impact on the effective slip and the drag of the SHS. Finally, we discuss how surfactant effect could be mitigated through changes in the geometry and flow conditions.