In the present work,the interaction mechanism of specific polyelectrolyte multilayers (PEMs),fabricated by layer-by-layer deposition of polydiallyldimethylammonium chloride (PDDA) and poly(sodium 4-styrenesulfonate) (PSS),is studied using atomic force microscopy.The underwater oil-repellency of PSS-capped PEMs was further explored by measuring the interaction forces between tetradecane droplets and PEMs-coated silica substrates under various salinities.The force curves were analyzed following the Stokes-Reynolds-Young-Laplace theoretical model.Desirable consistency was achieved between the experimental and theoretical calculations at low NaCl concentrations (0.1 mM and 1 mM);however,underestimation of the attractive force was found as the NaC1 concentration increases to moderate (10 mM) and high (100 mM) levels.Discrepancy analyses and incorporated features toward a reduced surface charge density were considered based on the previous findings of the orientation of anionic benzenesulfonate moieties (Liu et al.in Angew Chem Int Ed 54(16):4851-4856,2015.https://doi.org/10.1002/anie.201411992).Short-range steric hindrance interactions were further introduced to simulate “brush” effect stemming from nanoscale surface roughness.It is demonstrated in our work that the PSS-capped PEMs remains a stable underwater lipophobicity against high salinity,which renders it potential application in surface wetting modification and anti-fouling.