Why a Lotus-like Superhydrophobic Surface Is Self-Cleaning? An Explanation from Surface Force Measurements and Analysis

The unique self-cleaning feature of the lotus-like superhydrophobic (SH) surface attracted worldwide interest in recent years. However, the mechanism of the self-cleaning phenomena remains unclear. Here, we attempt to provide a comprehensive understanding of why self-cleaning of the particles with a broad range of size can be realized on the lotus-like SH surfaces. After measurements and analysis of the force involved at the interface, we conclude that there are four main preconditions for self-cleaning: (1) contact angle (CA) > 90°, (2) low enough sliding angle, (3) low enough adhesion force, and (4) proper particle size. However, as far as the lotus-like SH surface and typical dust are concerned, all the preconditions will be satisfied automatically. We also observe that the particles with a broad range of size (from submicron level to the millimeter level) and density (virtually no limit) can be driven by a water droplet on the lotus-like SH surface. This interesting finding may be helpful for the design of novel engineering system at the micron-millimeter scale in the future.