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Wetting and Drying of Aqueous Droplets Containing Nonionic Surfactants CE
Jing Shi1, Lisong Yang1, Colin D Bain1
1Department of Chemistry, Durham University, Durham DH1 3LE, United Kingdom.
Langmuir : the ACS Journal of Surfaces and Colloids
|April 2, 2021
Summary
Nonionic surfactants (CE) form a thin film during droplet drying, unlike SDS. This film, potentially a surfactant mesophase, influences droplet behavior and surface interactions.
Area of Science:
- Surface Science
- Colloid and Interface Science
- Physical Chemistry
Background:
- Understanding droplet dynamics is crucial for applications like printing and coatings.
- Surfactants significantly alter droplet behavior, including wetting and drying processes.
- Nonionic surfactants (CE) and anionic surfactants (SDS) exhibit different behaviors in aqueous solutions.
Purpose of the Study:
- To systematically investigate the wetting and drying of pico-liter droplets containing nonionic surfactants (polyoxyethylene alkyl ethers, CE) and compare with anionic surfactant (SDS).
- To elucidate the mechanisms behind droplet phase separation, thin film formation, and spreading inhibition.
- To explore the role of surfactant properties and concentrations on droplet dynamics and surface interactions.
Main Methods:
- Tracking the three-phase contact line (TCL) during droplet drying.
- Utilizing interferometry to study droplet morphology and film formation.
- Employing tracer particle measurements to analyze fluid flow and Marangoni effects.
Main Results:
- CE droplets undergo phase separation during drying, forming a water-rich droplet and a thin film, possibly a surfactant mesophase.
- The receding contact angle in the late drying stage of CE droplets is substrate-independent, suggesting mesophase presence.
- Both CE and SDS inhibit spreading on hydrophilic surfaces due to Marangoni contraction, with CE showing more pronounced suppression.
- CE solutions may form a viscous phase at the TCL, pinning the droplet and further inhibiting spreading.
- Mild Marangoni flows were observed for surfactant concentrations above the critical micelle concentration (CMC).
Conclusions:
- CE surfactants form a distinct thin film (mesophase) during drying, influencing droplet retraction and receding contact angles.
- Marangoni effects play a significant role in spreading inhibition for both CE and SDS, with CE exhibiting stronger effects.
- The observed phenomena are linked to surfactant phase behavior, concentration, and interactions at the three-phase contact line.
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