Effect of temperature and surfactants on evaporation and contact line dynamics of sessile drops

Researches on droplet evaporation has a vast range of applications in different fields. The mechanisms of droplets evaporation have been a topic of investigation for decades. The droplet after spreading on the substrate exhibits constant evaporation rate for a constant base diameter and decreasing contact angle; followed by another evaporation mode with constant contact angle and decreasing base diameter; and finally, evaporation with decreasing of both contact angle and base diameter. In the present study, the evaporation rate and mechanisms influenced by surfactant and temperature effects are analysed. The evaporation was demonstrated to be related to the dynamics of the sessile droplet spreading ability and the depinning event. It remains complex to predict the evaporation of the sessile droplet due to the continuous change in surfactant local concentration. Moreover, the temperature change modifies the surface tension and the induced inner flows. These two questions find answers in the triple line dynamic which controls the evaporation rate. The forces controlling the depinning with surfactant was quantified. A clear constant force was found following depinning. Such dynamic equilibrium demonstrates also the effect of the friction, surface roughness and moreover the limit of surfactant concentration in the triple line direct vicinity. The evaporation rate shows two particularities: one is the classical decrease of the evaporation rate after depinning; the second in case of surfactant, there is an enhancement appearing at later droplet lifetime, which could be a consequence of film evaporation contributing to the evaporation near the triple line. The local evaporation does not change significantly despite the change in the contact angle with respect to surfactant concentration. The substrate temperature enhances evaporation rate. The coupled temperature and surfactant effect show a local evaporation rate decreasing with surfactant concentration increase at ambient temperature (around 20 degrees C) and enhancement with surfactant concentration at higher temperature.

Automated summary

Research on droplet evaporation has various applications and the mechanisms of droplet evaporation have been researched for decades. This study analyzes the effects of surfactant and temperature on the evaporation rate and mechanisms. The evaporation is influenced by the dynamics of droplet spreading ability and the depinning event. It is complex to predict the evaporation due to continuous changes in surfactant concentration. Temperature change modifies surface tension and induced inner flows. The depinning force with surfactant was quantified, and a constant force was found. The evaporation rate shows a classical decrease after depinning and, in the case of surfactant, an enhancement appears at a later droplet lifetime. Local evaporation does not change significantly with respect to surfactant concentration.