Evaporating droplets on oil-wetted surfaces: Suppression of the coffee-stain effect

Abstract

<jats:p>The evaporation of suspension droplets is the underlying mechanism in many surface-coating and surface-patterning applications. However, the uniformity of the final deposit suffers from the coffee-stain effect caused by contact line pinning. Here, we show that control over particle deposition can be achieved through droplet evaporation on oil-wetted hydrophilic surfaces. We demonstrate by flow visualization, theory, and numerics that the final deposit of the particles is governed by the coupling of the flow field in the evaporating droplet, the movement of its contact line, and the wetting state of the thin film surrounding the droplet. We show that the dynamics of the contact line can be tuned through the addition of a surfactant, thereby controlling the surface energies, which then leads to control over the final particle deposit. We also obtain an analytical expression for the radial velocity profile which reflects the hindering of the evaporation at the rim of the droplet by the nonvolatile oil meniscus, preventing flow toward the contact line, thus suppressing the coffee-stain effect. Finally, we confirm our physical interpretation by numerical simulations that are in qualitative agreement with the experiment.</jats:p>

On explosive boiling of a multicomponent Leidenfrost drop

“…Consequently, the emulsified drop completes the oil encapsulation and becomes a water-in-oil Leidenfrost drop. We note in passing that a similar wrapping-up phenomenon has also been observed in other fluids systems by Li et al (30), Tan et al (31), and Kadota et al (21). where r b is the horizontal extent of the drop bottom surface, and µv , ρv , and ρ l are the dynamic viscosity of vapor, the vapor den-sity, and the liquid-phase density of the evaporating component (water in stage 3 and oil in stage 4), respectively.…”

“…Remarkably, the oil cap then gradually spreads over the drop surface (the third image in Fig. 1B), as also seen for a drop on an atmospheric surface (30,31). The process of the third stage is sketched in Fig.…”

Particle Size Determines the Shape of Supraparticles in Self-Lubricating Ternary Droplets

“…Specifically, the flow of water that is directed toward the air–water–oil contact line can carry particles away from the air–water interface, hindering the formation of the shell. 48 …”

On the compound sessile drops: configuration boundaries and transitions

“…In principle, the configuration transition can also result from the variation of interface tensions, e.g. using surfactants as in the experiments (Li et al 2020b) to control the morphological configuration for sessile drops.…”