Controlling Octagon-to-Square Wetting Interface Transition of Evaporating Sessile Droplet through Surfactant on Microtextured Surface

Producing and maintaining specific liquid patterns during evaporation holds great potential for techniques of printing and coating. Here we report the control over the evolution of surfactant solution droplets on the micropyramid substrates during evaporation. The polygonal droplet shape is achieved during the drying rather than solely at the beginning. As the initial surfactant concentration is 0.04 mM, the droplet maintains its initial octagonal shape throughout the lifetime. Interestingly, the initial octagonal shape transforms into a square during the evaporation as the initial surfactant concentration reaches 0.8 mM. These findings can shed light on wetting pattern control for complex solutions required in various applications

Octagonal Wetting Interface Evolution of Evaporating Saline Droplets on a Micropyramid Patterned Surface

Textured surfaces have been extensively employed to investigate the dynamics, wetting phenomena, and shape of liquid droplets. Droplet shape can be controlled via the manipulation of topographic or chemical heterogeneity of a solid surface by anchoring the three-phase line at specific sites. In this study, we demonstrate that droplet shape on a topographically patterned surface can be modified by varying the concentration of salt potassium chloride (KCl) in the droplet solution. It is found that at the beginning of evaporation the octagonal shape of the solid–liquid interface is changed to a rectangle with corners cut upon increasing the salt concentration. Such a variation in the solid–liquid interface versus the salt concentration is explained by the analysis of free energy difference. It indicates that the increases in solid–liquid and liquid–vapor surface tensions by raising the salt concentration result in a favored extension of the three-phase line intersecting the micropyramid bottom sides than the counterpart intersecting the micropyramid diagonal edges. The saline droplets experience a pinning stage at first and a depinning one afterward. The onset of depinning is delayed, and at which the instantaneous contact angle is larger upon raising the salt concentration. The three-phase line which intersects the micropyramid diagonal edges recedes ahead of the one along the micropyramid bottom sides, making the octagonal wetting interface evolve toward a circle. A close view at the droplet edge indicates that the three-phase line repeats “slow slip–rapid slip” across row by row of micropyramids during the depinning stage

Octagonal Wetting Interface Evolution of Evaporating Saline Droplets on a Micropyramid Patterned Surface

Textured surfaces have been extensively employed to investigate the dynamics, wetting phenomena, and shape of liquid droplets. Droplet shape can be controlled via the manipulation of topographic or chemical heterogeneity of a solid surface by anchoring the three-phase line at specific sites. In this study, we demonstrate that droplet shape on a topographically patterned surface can be modified by varying the concentration of salt potassium chloride (KCl) in the droplet solution. It is found that at the beginning of evaporation the octagonal shape of the solid–liquid interface is changed to a rectangle with corners cut upon increasing the salt concentration. Such a variation in the solid–liquid interface versus the salt concentration is explained by the analysis of free energy difference. It indicates that the increases in solid–liquid and liquid–vapor surface tensions by raising the salt concentration result in a favored extension of the three-phase line intersecting the micropyramid bottom sides than the counterpart intersecting the micropyramid diagonal edges. The saline droplets experience a pinning stage at first and a depinning one afterward. The onset of depinning is delayed, and at which the instantaneous contact angle is larger upon raising the salt concentration. The three-phase line which intersects the micropyramid diagonal edges recedes ahead of the one along the micropyramid bottom sides, making the octagonal wetting interface evolve toward a circle. A close view at the droplet edge indicates that the three-phase line repeats “slow slip–rapid slip” across row by row of micropyramids during the depinning stage