Surfactant Control of Coffee Ring Formation in Carbon Nanotube Suspensions

Abstract

The coffee ring effect regularly occurs during the evaporation of colloidal droplets and is often undesirable. Here we show that adding a specific concentration of a surfactant can mitigate this effect. We have conducted experiments on aqueous suspensions of carbon nanotubes that were prepared with cationic surfactant dodecyltrimethylammonium bromide added at 0.2, 0.5, 1, 2, 5, and 10 times the critical micelle concentration. Colloidal droplets were deposited on candidate substrates for printed electronics with varying wetting characteristics: glass, polyethylene terephthalate, fluoroethylene propylene copolymer, and polydimethylsiloxane. Following drying, four pattern types were observed in the final deposits: dot-like, uniform, coffee ring deposits, and combined patterns (coffee ring with a dot-like central deposit). Evaporation occurred predominantly in constant contact radius mode for most pattern types, except for some cases that led to uniform deposits in which early stage receding of the contact line occurred. Image analysis and profilometry yielded deposit thicknesses, allowing us to identify a coffee ring subfeature in all uniform deposits and to infer the percentage coverage in all cases. Importantly, a critical surfactant concentration was identified for the generation of highly uniform deposits across all substrates. This concentration resulted in visually uniform deposits consisting of a coffee ring subfeature with a densely packed center, generated from two distinct evaporative phases.

Preparing a liquid crystalline dispersion of carbon nanotubes with high aspect ratio

“…The average thickness was ≈393 nm, whereas the edge of the film was much thicker (≈999 nm). In earlier studies, the CNTs in the dispersions were observed to flow and accumulate along the droplet's edge due to the coffee ring effect [ 15 , 33 – 34 ]. As a result, the volume fraction of DWCNTs at the edge increased.…”

Gravitational effects on coffee-ring formation during the evaporation of sessile droplets

“…2008; Howard et al. 2023) and may further be enhanced by solute aggregation near the edge of the droplet (Orejon, Sefiane & Shanahan 2011; Weon & Je 2013; Larson 2014). However, at late stages of the evaporation, the contact line may depin and become mobile, moving inwards towards the droplet centre.…”

“…Throughout, we have assumed that the contact line remains pinned as the droplet evaporates. This has been shown to be a reasonable assumption for many configurations (see, for example, the experiments in Deegan et al 2000;Kajiya et al 2008;Howard et al 2023) and may further be enhanced by solute aggregation near the edge of the droplet (Orejon, Sefiane & Shanahan 2011;Weon & Je 2013;Larson 2014). However, at late stages of the evaporation, the contact line may depin and become mobile, moving inwards towards the droplet centre.…”

“…1997; Hu & Larson 2002; Kajiya, Kaneko & Doi 2008; Howard et al. 2023). We let be the radius of the contact line.…”

Three-Dimensional Deposits from Drying Particle-Laden Drops

“…At the same time, the “coffee ring” formation has also several disadvantages, such as the formation of nonuniform coating/patterning, formation of trench-like and raised ridge structures during three-dimensional (3D) printing with metal nanoparticle ink, , etc. As a result, there has been extensive research in developing strategies for arresting the coffee ring formation and ensuring a uniform and homogeneous deposition, which is critical for ensuring uniformity in various coating and patterning applications. , Some of the approaches for preventing the coffee ring formation involve the use of much smaller (nanoscopic) particles and ellipsoidal and flake-like particles, , performing the evaporation on oil-wetted surfaces, increasing the hydrophobicity (by physical and chemical modifications) of the surfaces on which the drop rests, − application of an external electric field − and acoustic waves during evaporation, controlling the interactions between the depositing particles and the solid substrate, − use of surfactants, , and many more.…”