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The influence of surface tension and equilibrium contact angle on the spreading and receding of water droplets impacting a solid surface

An experimental investigation was performed to study the effect of surfactant on the spreading and receding of water droplets on a flat, isothermal surface. Sodium dodecyl sulfate (SDS) was added to water to lower both the surface tension and the equilibrium contact angle. Two parameters, initial velocity and initial droplet diameter, were varied. A high-speed camera recorded the droplet deformation process, including both the spreading and receding phases. Adding the surfactant had two primary effects on the spreading phase; it increased duration of the spreading phase and increased the spreading rate of the droplet. Both of these effects ultimately resulted in an increased maximum spreading diameter. In the receding phase, the surfactant decreased the rate of receding. In the initial deformation process, it was found that the droplet collapsed at nearly a constant rate, regardless of surfactant concentration. Maximum spreading diameter was found to agree with several analytical models from different sources within 10%. Instantaneous spreading diameter was compared to an energy balance model, which also showed good agreement.