Impact Dynamics of Surfactant-Laden Droplets: Hydrophobic Chain Lengths Effects

Understanding the behavior of surfactant-laden droplets impacting superhydrophobic surfaces is critical for various industrial applications. However, the influence of surfactant hydrophobic chain lengths (HCLs), which affect the diffusion coefficient of surfactants, on impact dynamics and predictive model for the maximum spreading diameter of surfactant droplets impacting superhydrophobic surfaces remain poorly understood, particularly under varying Weber numbers (We) and concentrations. This study experimentally investigates the impact dynamic of surfactant-loaded droplets with differing HCLs, focusing on their spreading, maximum spreading diameter, retraction, and oscillation behaviors. A key finding is that surfactants primarily enhance droplet deposition by inhibiting retraction stage. The deposition-enhancing effect exhibits an initial increase and subsequent decrease with increasing HCLs, governed by the interplay between dynamic surface tension, diffusion coefficient, and viscosity. Additionally, moderate increases in We favor droplet deposition and enhanced damping effect for surfactant-laden droplets impacting superhydrophobic surfaces. Finally, introducing the diffusion coefficient of surfactant through the Peclet number creates a generalized model to accurately predict the maximum spreading diameter of different surfactant droplets impacting superhydrophobic surfaces in varying impact velocities. This can be expressed as follows: βmax = 0.55Pe–0.017Re0.0679We0.246. These insights offer a deeper understanding of surfactant-laden droplet behavior on superhydrophobic surfaces and provide strategies for optimizing deposition in practical applications.