Modulation of the Coffee-Ring Effect in Particle/Surfactant Mixtures: the Importance of Particle–Interface Interactions

Abstract

We study the effect of surfactants on the deposits formed after the evaporation of colloidal suspension drops, at initial concentrations lower than the critical micellar concentrations, for various particle/surfactant mixtures. We show that the surfactant-mediated interactions between particles and the liquid-gas (LG) and liquid-solid (LS) interfaces, rather than the flow patterns, primarily define the morphology of the dry deposit in a robust and reproducible manner. For like-charged particle/surfactant mixtures, most of the particles form a ring-shaped deposit (according to the so-called "Coffee-Ring Effect"), but some particles can also be deposited inside the ring in a way that is modulated by electrostatic interactions between the particles and the LS interface. For oppositely charged systems, surfactant adsorption to the particle surface strongly affects particle-LG interface interactions, which in turn control the deposition pattern. For low surfactant concentrations, coffee-rings are systematically observed. For intermediate concentrations, the charge of surfactant-decorated particles becomes nearly neutral, and their hydrophobicity is enhanced, which promotes particle trapping at the LG interface. A particle skin is formed and its deposition upon drying leads to homogeneous disk-like patterns. For high surfactant concentrations, particle charge is reversed, and coffee-rings are observed again. Notably, this ring-disk-ring evolution of the deposition behavior as a function of surfactant concentration is observed in a variety of mixtures, regardless of particle absolute charge and surface chemistry as well as of surfactant charge and hydrophobicity. Its apparent universal character makes it a promising strategy for a robust control of particle deposition from evaporating drops.

Droplet Drying Patterns on Solid Substrates: From Hydrophilic to Superhydrophobic Contact to Levitating Drops

“…If the TPCL does not remain pinned to the substrate but slips and sticks during drying, a series of concentric rings with varying radius are formed [91,107,108].…”

“…The processes occurring during the desiccation of the sessile colloidal droplets and morphology of the resulting precipitate depend on many different factors, for example, the nature and shape of the colloidal particles [109] and their initial volume fraction [147], the presence of admixtures (e.g., surfactants) in the solution [50,107,151,152], ionic strength and pH of the solution [28], the properties of substrate (thermal conductivity, whether hydrophilic/hydrophobic) [24,85,153,154], and evaporation mode [32,36,[155][156][157].…”

“…A particle-surfactant mixture thus can modulate the capillary flow [18,107,152]. Colloidal sulphur with salt self-assembles into a wide variety of patterns, depending on conditions [158].…”

Alternative mechanism for coffee-ring deposition based on active role of free surface

“…By contrast, fewer studies have considered that the bulk flow carries particles to an inevitable intersection with the liquid-gas interface (see, e.g., Refs. [3,4,12,20,28,32,33,36,42]). Here, we demonstrate that this inevitable intersection results from the coupling of the receding of the interface with the bulk internal flow.…”

Deposition and drying dynamics of liquid crystal droplets

“…The drying phenomenon thus provides a multi-faceted testing ground for fundamental science and engineering ideas, and insights gained can influence practical applications in printing9, genotyping10 and other complex assembly and coating schemes1112. To date, drying experiments have probed water droplets containing various (mostly small) concentrations of particles1314, polymers1516, surfactants1718, added solvents1920 and salts2122. These investigations have uncovered fascinating phenomena including coffee rings1314232425, Marangoni flows5192026, electro-wetting effects27, complex deposition patterns2128 and, in a few cases, formation of concentrated phases very near the drop edge729303132.…”