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*

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

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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.

Original languageEnglish
Pages (from-to)892-903
Number of pages12
JournalJournal of colloid and interface science
Volume622
Early online date29 Apr 2022
DOIs
Publication statusE-pub ahead of print/First online - 29 Apr 2022

Keywords

  • Droplets
  • Evaporation
  • Lubrication approximation
  • Quasi-stationary approach
  • Solutal Marangoni flow
  • Surfactants
  • Thermal Marangoni flow
  • UT-Hybrid-D

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