Assemblies disaggregation and diffusion dictated droplet impact and wetting behaviors on hydrophobic surface

Controllable impact and wetting of droplet on the paraffin surface are easily realized by tuning the chain length of surfactant. • Chain length of surfactant affects the impact and wetting processes differently. • Surfactant with rapid diffusion rate inhibits droplet retracting during impact. • Assemblies disaggregation replenishes surfactants at interface. • Surfactant with high adsorption efficiency promotes droplet wetting. Improving the liquid deposition on plant leaves is crucial during pesticide spraying. However, the hydrophobicity of most plant leaf surfaces makes the droplet splash or run off, finally reducing the efficacy of the foliar-applied pesticide. Though some molecular additives have been employed to regulate the above process, the understanding about the mechanism and the effect of molecular structure is still limited. Herein, cationic surfactants with different double-chain lengths (8–14) are adopted to regulate the droplet impact and wetting on hydrophobic paraffin surface (simulating leaves). With the surfactant chains extending, the capacity for restraining retraction and promoting wetting increases firstly, and then decreases. It can be attributed to that the surfactant with shortest chain cannot form assemblies to inhibit retraction, as the chain length increases, the small-sized micelles reduce but the vesicles become larger and firmer thus the self-assemblies scarcely disaggregate to replenish the monomer at interface during impact. Likewise, short-chain surfactant diffuses faster to interface than long-chain surfactant due to the size effect and hydrophilic/hydrophobic ratio. Our work not only elucidates the mechanism of droplet impact and wetting dynamics by tuning the assemblies aggregation and surfactant diffusion but also promotes the promising application of surfactant in pesticide spraying.