Abstract
Controlling the droplet evaporation on surfaces is desired to get uniform depositions of materials in many applications, for example, in two- and three-dimensional printing and biosensors. To explore a new route to control droplet evaporation on surfaces and produce asymmetric particles, sessile droplets of aqueous dispersions were allowed to evaporate from surfaces coated with oil films. Here, we applied 1–50 μm thick films of different silicone oils. Two contact lines were observed during droplet evaporation: an apparent liquid–liquid–air contact line and liquid–liquid–solid contact line. Because of the oil meniscus covering part of the rim of the drop, evaporation at the periphery is suppressed. Consequently, the droplet evaporates mainly in the central region of the liquid–air interface rather than at the droplet’s edge. Colloidal particles migrate with the generated upward flow inside the droplet and are captured by the receding liquid–air interface. A uniform deposition ultimately forms on the substrate. With this straightforward approach, asymmetric supraparticles have been successfully fabricated independent of particle species.
Highlights
Evaporation of water droplets on solid surfaces is a complex process because mass and heat transfer in the liquid and air are coupled.[1,2] A droplet with suspended colloids[2−4] or dissolved molecules[5−7] generally results in ring-like depositions after drying on a solid surface, which is caused by the so-called coffee-ring effect.[1,8] It is initiated by the pinning of the droplet edge and an evaporation gradient along the droplet surface
The final deposition in drying sessile droplets is correlated with the mode of liquid evaporation,[17] particle adsorption on the liquid−air interface, particle agglomeration,[18,19] and surface tension gradients along the droplet’s surface.[20]
Lubricant-infused surfaces, consisting of micro/nano-structures filled with lubricating liquids, have been discovered with ultralow contact angle hysteresis to foreign immiscible droplets.[27−29] Yang et al have released a novel technique with capability of ultrasensitive molecular detection, which is realized by the enrichment and delivery of analytes into the detective sites based on this platform.[30]
Summary
Evaporation of water droplets on solid surfaces is a complex process because mass and heat transfer in the liquid and air are coupled.[1,2] A droplet with suspended colloids[2−4] or dissolved molecules[5−7] generally results in ring-like depositions after drying on a solid surface, which is caused by the so-called coffee-ring effect.[1,8] It is initiated by the pinning of the droplet edge and an evaporation gradient along the droplet surface. The final deposition in drying sessile droplets is correlated with the mode of liquid evaporation,[17] particle adsorption on the liquid−air interface, particle agglomeration,[18,19] and surface tension gradients along the droplet’s surface.[20] To realize uniform depositions on a substrate, approaches by controlling the evaporation process of droplets have been realized, for example, regulating flow patterns inside evaporating droplets,[6,21−23] interface deformation,[24] depinning of CLAS,[19,25] or modifying the dispersed colloidal particles.[26] Recently, lubricant-infused surfaces, consisting of micro/nano-structures filled with lubricating liquids, have been discovered with ultralow contact angle hysteresis to foreign immiscible droplets.[27−29] Yang et al have released a novel technique with capability of ultrasensitive molecular detection (down to 10−15 mol·L−1), which is realized by the enrichment and delivery of analytes into the detective sites based on this platform.[30] McBride et al.[31] demonstrated that salt crystallization is suppressed at the droplet edge of salt solution during its evaporation. Das et al.[32] reported that the coffee-ring effect can be suppressed on a silicone oil-coated surface In these cases, the lubricant film prevents pinning of the droplet edge at the substrate during droplet evaporation.
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