Pattern Formation upon Evaporation of Sessile Droplets of Polyelectrolyte/Surfactant Mixtures on Silicon Wafers.

Lionel Perrin,Ramon G Rubio,Andrew Akanno,Eduardo Guzman,Francisco Ortega

doi:10.3390/ijms22157953

Abstract

The formation of coffee-ring deposits upon evaporation of sessile droplets containing mixtures of poly(diallyldimethylammonium chloride) (PDADMAC) and two different anionic surfactants were studied. This process is driven by the Marangoni stresses resulting from the formation of surface-active polyelectrolyte–surfactant complexes in solution and the salt arising from the release of counterions. The morphologies of the deposits appear to be dependent on the surfactant concentration, independent of their chemical nature, and consist of a peripheral coffee ring composed of PDADMAC and PDADMAC–surfactant complexes, and a secondary region of dendrite-like structures of pure NaCl at the interior of the residue formed at the end of the evaporation. This is compatible with a hydrodynamic flow associated with the Marangoni stress from the apex of the drop to the three-phase contact line for those cases in which the concentration of the complexes dominates the surface tension, whereas it is reversed when most of the PDADMAC and the complexes have been deposited at the rim and the bulk contains mainly salt.

Highlights

Introduction and José SUrietaContact angle, spreading, and evaporation phenomena are ubiquitous in nature and in technological processes, such as heat exchanges, ink-jet printing, pesticide applications, cosmetics and pharmacology, the food industry, etc
The patterns formed on the silicon wafers upon the complete evaporation of sessile droplets of polyelectrolyte–surfactant mixtures present two well-differentiated regions, with the appearance of a third one for mixtures with high surfactant concentration
The relatively constant size of the ring is an indication that the coffee ring is mainly due to the poly(diallyldimethylammonium chloride) (PDADMAC) in the mixture and independent of the size of the PDADMAC–surfactant complexes formed in the mixture

Summary

Introduction

Contact angle, spreading, and evaporation phenomena are ubiquitous in nature and in technological processes, such as heat exchanges, ink-jet printing, pesticide applications, cosmetics and pharmacology, the food industry, etc. A simplified approach to the problem is sketched, where an axisymmetric fluid droplet is in equilibrium with two immiscible phases, e.g., solid and vapor or liquid and vapor, defining a three-phase contact line (TPCL). Either a thermodynamic or a mechanical approach to the problem can lead to the well-known Young equation. Γsv − γsl = γlv cos θ, Licensee MDPI, Basel, Switzerland. Where γsv , γsl , and γlv are the interfacial tensions of the solid/vapor, solid/liquid, and liid/vapor interfaces, respectively, and θ is the so-called equilibrium contact angle.

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Results

Conclusion