  Evaporation of surfactant laden sessile droplets is omnipresent in nature and
industrial applications such as inkjet printing. Soluble surfactants start to
form micelles in an aqueous solution for surfactant concentrations exceeding
the critical micelle concentration (CMC). Here, the evaporation of aqueous
sodium dodecyl sulfate (SDS) sessile droplets on hydrophobic surfaces were
experimentally investigated for SDS concentrations ranging from 0.025 to 1 CMC.
In contrast to the constant contact angle of an evaporating sessile water
droplet, we observed that, at the same surface the contact angle of an SDS
laden droplet with a concentration below 0.5 CMC first decreases, then
increases, and finally decreases resulting in a local contact angle minimum.
Surprisingly, the minimum contact angle was found to be substantially lower
than the static receding contact angle and it decreased with decreasing initial
SDS concentration. Furthermore, the bulk SDS concentration at the local contact
angle minimum was found to decrease with a decrease in the initial SDS
concentration. The location of the observed contact angle minimum relative to
the normalized evaporation time and its minimum value proved to be independent
of both the relative humidity and the droplet volume and thus, of the total
evaporation time. We discuss the observed contact angle dynamics in terms of
the formation of a disordered layer of SDS molecules on the substrate at
concentrations below 0.5 CMC. The present work underlines the complexity of the
evaporation of sessile liquid-surfactant droplets and the influence of
surfactant-substrate interactions on the evaporation process.
