Chapter 6: Spreading and Evaporation of Surfactant Drops

The simultaneous spreading and evaporation of droplets of aqueous surfactant solutions onto various substrates is reviewed in this chapter. Four different stages of the process have been found: the initial one corresponds to spreading till the contact angle, θ, reaches the value of the static advancing contact angle, θ_ad. The duration of this stage is rather short and the evaporation during this stage can be neglected. The evaporation is essential during the following three stages. The next stage after the spreading, which is referred to as the first stage, takes place at a constant perimeter and ends when θ reaches the static receding contact angle, θ_r. During the second stage, the perimeter decreases at a constant contact angle θ = θ_r for a surfactant concentration above the critical wetting concentration (CWC). The static receding contact angle decreases during the second stage for concentrations below the CWC because the concentration increases due to evaporation. During the final stage, both the perimeter and the contact angle decrease until the drop disappears. The developed theory predicts universal curves for the contact angle dependency on time during the first stage, and for the droplet perimeter on time during the second stage.