Competition between thermal and surfactant-induced Marangoni flow in evaporating sessile droplets

R T van Gaalen; H M A Wijshoff; J G M Kuerten; C Diddens

doi:10.1016/j.jcis.2022.04.146

Competition between thermal and surfactant-induced Marangoni flow in evaporating sessile droplets

J Colloid Interface Sci. 2022 Sep 15:622:892-903. doi: 10.1016/j.jcis.2022.04.146. Epub 2022 Apr 29.

Authors

R T van Gaalen¹, H M A Wijshoff², J G M Kuerten¹, C Diddens³

Affiliations

¹ Department of Mechanical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, the Netherlands.
² Department of Mechanical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, the Netherlands; Canon Production Printing Netherlands B.V., P.O. Box 101, 5900 MA Venlo, the Netherlands.
³ Department of Mechanical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, the Netherlands; Faculty of Science and Technology (TNW), University of Twente, P.O. Box 217, 7500 AE Enschede, the Netherlands. Electronic address: C.Diddens@utwente.nl.

PMID: 35561609
DOI: 10.1016/j.jcis.2022.04.146

Abstract

Hypothesis: Thermal Marangoni flow in evaporating sessile water droplets is much weaker in experiments than predicted theoretically. Often this is attributed to surfactant contamination, but there have not been any in-depth analyses that consider the full fluid and surfactant dynamics. It is expected that more insight into this problem can be gained by using numerical models to analyze the interplay between thermal Marangoni flow and surfactant dynamics in terms of dimensionless parameters.

Simulations: Two numerical models are implemented: one dynamic model based on lubrication theory and one quasi-stationary model, that allows for arbitrary contact angles.

Findings: It is found that insoluble surfactants can suppress the thermal Marangoni flow if their concentration is sufficiently large and evaporation and diffusion are sufficiently slow. Soluble surfactants, however, either reduce or increase the interfacial velocity, depending on their sorption kinetics. Furthermore, insoluble surfactant concentrations that cause an order 0.1% surface tension reduction are sufficient to reduce the spatially averaged tangential flow velocity at the interface by a factor 100. For larger contact angles and smaller droplets this required concentration is larger (typically <1% surface tension reduction). The numerical models are mutually validated by comparing their results in cases where both are valid.

Keywords: Droplets; Evaporation; Lubrication approximation; Quasi-stationary approach; Solutal Marangoni flow; Surfactants; Thermal Marangoni flow.