Simultaneously the as prepared sample was subjected

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a. Simultaneously, the as-prepared sample was subjected to a 5 g weight and was dragged forward and backward with a speed and abrasion length of about 2 cm s - 1 and 5 cm, respectively. Figure 6 b displays the change in the water CA as a function of scratch cycles. The results show that the water CA was still close to 150 ° after 80 scratch cycles. These results display that the as-prepared superhydrophobic surface has good mechanical abrasion resistance. Furthermore, when the hydrophobic SiO 2 is in contact with a water droplet, it is picked up by the droplet and Fig. 4 EIS results of bare and the as-prepared superhydrophobic silica coatings formed on the Al surfaces in 3.5 wt% NaCl solution. a Nyquist plots, b Bode | Z | versus frequency plots and c Bode phase angle versus frequency plots Fig. 5 Time sequence of the self-cleaning process on the superhydrophobic surface at a sliding angle about 3 ° J Sol-Gel Sci Technol 123
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forms a porous shell around it. The Young–Dupre ´ equation ( D F ¼ ² p R 2 S c LV 1 þ sin h e ð Þ 2 where R S is the radius of the SiO 2 , h e is the intrinsic contact angle, D F is the net change in surface free energy and c LV is the liquid–vapor interfa- cial tensions) suggests that it is always favorable for par- ticle to spontaneously attach to the liquid–vapor interface, even if they are hydrophobic [ 43 ]. Figure 7 a shows the 2D digital image of the liquid marble constructed by hydrophobic silica. We can observe that the silica particles are random and close-packed over water surface and there are a mass of voids between microspheres. The presence of voids areas makes the inner water can evaporate and result in the instability of liquid marbles. Meanwhile, liquid marbles with porous appearance do not wet contact sub- strate because of gas film existed in porous construction. Figure 7 b is the illustration image of liquid marbles enwrapped by the hydrophobic silica, which are aggregated over the whole water drop surface. The robustness of the liquid marble was measured by evaporation rate of inner water at ambient temperature. As shown in Fig. 8 , the volume of the liquid marble was measured by the contact angle measurement system video camera until the liquid marble collapse completely at room temperature (35 % relative humidity, 28 ° C). The state of the liquid marble was recorded every 20 min. Time evo- lution of a 10 l L water marble at room temperature is shown in Fig. 8 a–f. Initially, the liquid marble was nearly spherical, and height of the marble started decreasing because of evaporation before 60 min, and buckling of the marble occurred in Fig. 8 d. Finally, in 100 min water Fig. 6 a Schematic illustration of the scratch test. b The variation in water CAs on the as-prepared superhydrophobic surface scratched by the sandpaper Fig. 7 a Surface of the liquid marble coated with silica grains as seen with a microscope, b Schematic diagram of the liquid marble Fig. 8 Horizontal profiles of the same silica liquid marble during evaporation with time obtained by contact angle measurement system video camera
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  • Fall '19
  • Surface tension, Contact angle, superhydrophobic surface, superhydrophobic coatings

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