Controlling droplet deposition with surfactants

Droplets impacting on a surface are key to a wide range of applications such as spray deposition and inkjet printing. Yet, a full understanding of how they spread and retract is still lacking. Surfactants are often added to improve spreading and coverage of aqueous solutions, resulting in variations of the surface tension at timescales beyond the reach of conventional dynamic surface tension measurement methods. Here we study the impact dynamics of aqueous surfactant solutions on hydrophobic surfaces at millisecond timescales. We find that the spreading and retraction of droplets cannot be adequately described by the equilibrium surface tension. We infer the dynamic surface tension in the first milliseconds after impact from the spreading dynamics of droplets. "Slow"surfactants that take a lot of time to reach newly created droplet surface, only slightly decrease or even increase the surface tension, while "fast"surfactants, on the other hand, allow efficient wetting of aqueous solutions on hydrophobic surfaces. Our findings allow us to tailor surfactants for efficient drop deposition or spray application.