Ultra-Slippery Hydrophilic Surfaces by Hybrid Monolayers

Slippery solid surfaces with low droplet contact angle hysteresis (CAH) are crucial for applications in thermal management, energy harvesting, and environmental remediation. Traditionally, reducing CAH has been achieved by enhancing surface homogeneity. This work challenges this conventional approach by developing slippery yet hydrophilic surfaces through hybrid monolayers composed of hydrophilic polyethylene glycol (PEG)-silane and hydrophobic alkyl-silane molecules. These hybrid surfaces exhibited exceptionally low CAH (<2°), outperforming well-established homogeneous slippery surfaces. Molecular structural analyses suggested that the remarkable slipperiness is due to a unique spatially staggered molecular configuration, where longer PEG chains shield shorter alkyl chains, thus creating additional free volume while ensuring surface coverage. This was supported by the observation of decreased CAH with increasing temperature, highlighting the role of grafted chain mobility in enhancing slipperiness by self-smoothing and fluid-like behaviors. Furthermore, condensation experiments demonstrated the exceptional performance of the hydrophilic slippery surfaces in dew harvesting due to superior condensation nucleation, droplet coalescence, and self-sweeping efficiency. These findings offer a novel paradigm for designing advanced slippery surfaces and provide valuable insights into the molecular mechanisms governing dynamic wetting.