Abstract
HypothesisSoluble surfactants in evaporating sessile droplets can cause a circulatory Marangoni flow. However, it is not straightforward to predict for what cases this vortical flow arises. It is hypothesized that the occurrence of Marangoni circulation can be predicted from the values of a small number of dimensionless parameters. SimulationsA numerical model for the drop evolution is developed using lubrication theory. Surfactant transport is implemented by means of convection–diffusion-adsorption equations. Results are compared to literature. FindingsIt is shown that stronger evaporation, slower adsorption kinetics and lower solubility of the surfactants all tend to increasingly suppress Marangoni circulation. These results are found to be consistent with both experimental and numerical results from literature and can explain qualitative differences in flow behavior of surfactant-laden droplets. Furthermore, diffusion also tends to counteract Marangoni flow, where bulk diffusion has a more significant influence than surface diffusion. Also, the formation of micelles is found to slightly suppress Marangoni circulation. Experimental results from literature, however, show that in some cases circulatory behavior is enhanced by micelles, possibly even resulting in qualitative changes in the flow. Potential explanations for these differences are given and extensions to the model are suggested to improve its consistency with experiments.
Highlights
The evaporation of sessile droplets is a phenomenon that has a broad practical relevance
A common issue hindering the formation of a homogeneous deposition pattern is the occurrence of the coffee-stain effect: preferential evaporation at the contact line causes a strong, outward flow resulting in a ring-like deposition pattern [9,10,11]
Besides surfactant being adsorbed onto the interface, there is surfactant dissolved in the bulk of the droplet
Summary
The evaporation of sessile droplets is a phenomenon that has a broad practical relevance. A common issue hindering the formation of a homogeneous deposition pattern is the occurrence of the coffee-stain effect: preferential evaporation at the contact line causes a strong, outward flow resulting in a ring-like deposition pattern (given a pinned contact line) [9,10,11]. The surface tension can become non-uniform as a result of nonhomogeneous changes in composition This happens for example during the drying of multicomponent droplets [21,22] or droplets with surfactants [23,24]. It may be possible to identify which parameters are relevant and what tuning they require to promote Marangoni circulation and a more homogeneous deposition pattern This is done both below and above the critical micelle concentration (CMC), where surfactant monomers cluster and form micelles. Conclusions are drawn from the obtained results
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
