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

Abstract

When a colloidal sessile droplet dries on a substrate, the particles suspended in it usually deposit in a ring-like pattern. This phenomenon is commonly referred to as the "coffee-ring" effect. One paradigm for why this occurs is as a consequence of the solutes being transported towards the pinned contact line by the flow inside the drop, which is induced by surface evaporation. From this perspective, the role of the liquid-gas interface in shaping the deposition pattern is somewhat minimized. Here, we propose an alternative mechanism for the coffee-ring deposition. It is based on the bulk flow within the drop transporting particles to the interface where they are captured by the receding free surface and subsequently transported along the interface until they are deposited near the contact line. That the interface captures the solutes as the evaporation proceeds is supported by a Lagrangian tracing of particles advected by the flow field within the droplet. We model the interfacial adsorption and transport of particles as a one-dimensional advection-generation process in toroidal coordinates and show that the theory reproduces ring-shaped depositions. Using this model, deposition patterns on both hydrophilic and hydrophobic surfaces are examined in which the evaporation is modeled as being either diffusive or uniform over the surface.

Solutal Marangoni flow as the cause of ring stains from drying salty colloidal drops

“…This phenomenon has been reported using different three-dimensional visualization techniques [35] and it is also clearly observed using our APTV measurements [20]. Recently, Kang et al [36] proposed a mechanism for the formation of ring-shaped stains in water droplets solely based on the capture of particles at the receding interface and subsequently transported along the interface until they are deposited near the contact line. The receding motion of a contact line causes -in the laboratory frame -a receding surface flow of the same magnitude [37].…”

“…This means that particles must reach the stain through the bulk, and that the mechanism proposed by Kang et al cannot properly capture the physics of ring stain formation. However, the interfacial accumulation mechanism proposed by Kang et al [36] would be valid whenever there is a dominant outward-directed interfacial flow, as in the case of salty droplets.…”

Tunable Coffee Ring Formation on Polycarbonate Nanofiber Film for Sensitive SERS Detection of Phenylalanine in Urine

“…After solvent evaporation, suspended particles are deposited at the periphery to form a coffee ring because of the capillary action and weak Marangoni flow of the solvent. 33−35 In SERS applications, the formed coffee ring pattern affects both detection sensitivity and reproducibility. For example, if a small and compact coffee ring is formed, the packing density of the metal colloids is high, which results in a strong hotspot effect.…”

Spectroscopic Detection of Glyphosate in Water Assisted by Laser-Ablated Silver Nanoparticles

“…The image was taken in one of the borders of the so called “coffee-ring” effect [35]. The EDS analysis indicates the silver composition of the sample and the image reveals a broad size dispersion of spheroid particles with roughly estimated average diameter of 100 nm.…”