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Directional motion of evaporating droplets on gradient surfaces
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Image of FIG. 1.
FIG. 1.

SEM images of the gradient surfaces with nonuniform micropillar arrays. (a) Surface_a: the micropillar diameter a is 10 μm and height h is 20 μm. The center-to-center spacing s varies from 10 to 70 μm, resulting in solid fraction φ from 0.786 to 0.012 and water static CA from 121° to 170°. (b) Surface_b: the micropillars of the same height have diameters of 4-20 μm and center-to-center spacings of 5-76 μm. The resulted solid fraction φ and static CA range from 0.503 to 0.054 and 124° to 163°.

Image of FIG. 2.
FIG. 2.

(a) and (b) Time-lapse images of an evaporating droplet on gradient surfaces showing directional CL receding from the higher static CA side to the lower static CA side on Surface_a and from the lower static CA side to the higher static CA side on Surface_b. (c) and (d) Time evolutions of the CA and contact diameter on Surface_a and Surface_b.

Image of FIG. 3.
FIG. 3.

Schematics of an evaporating droplet on a micropillar gradient surface, where the pillar diameter gradually increases from the left to the right side. In the inset, the blue dash lines and numbers indicate the receding of the CL from one pillar to another during the evaporation. The CL first separates from the previous pillar to position “1,” and then jumps to the next pillar (at position “2”).

Image of FIG. 4.
FIG. 4.

A diagram showing the model prediction of directional droplet evaporation. In regions I and III, the CL moves in the direction of the wettability gradient, while in region II, the CL moves against the direction of the wettability gradient. The symbols show the dynamic processes of droplet evaporation on seven fabricated surfaces including all the regions. The arrow of each data trace indicates the CL moving direction. For instance, surface 1, the pillar diameter a is 10 μm constantly, and the center-to-center spacing s varies from 10 to 32 μm from L to R (Table I). While a L/a R ( = 1) remains unchanged, s L/s R changes as the CL recedes. Starting from s L/s R = 0.313, the CL of the droplet recedes from the right side (s R = 32 μm) and jumps to the next (s R = 30 μm), thus s L/s R becomes 0.333. The experimental results of seven fabricated surfaces are all consistent with the theoretical predictions.


Generic image for table
Table I.

Parameters of the surfaces.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Directional motion of evaporating droplets on gradient surfaces