Curvature‐Driven Reversible In Situ Switching Between Pinned and Roll‐Down Superhydrophobic States for Water Droplet Transportation

Abstract

Artifi cial superhydrophobic surfaces [1][2][3][4][5][6][7][8][9][10] with water contact angles (CAs) greater than 150 ° have been intensively investigated due to their unique "anti-water" property that could be utilized in a wide range of applications. [11][12][13] Recent development of intelligent devices, such as microfl uidic switches and biomedicine transporters, makes strong demands on surface wettability control, therefore, responsive surfaces have become a signifi cant issue for superhydrophobic studies. Up to now, various smart surfaces have been successfully developed as reversible switches for wettability control through a micronanostructured surface on a responsive material. [14][15][16][17][18][19][20][21][22][23][24][25] These unique tunings of surface wettability greatly contributed to refi ned control of surface wettability. With the thorough understanding of superhydrophobic phenomenon, superhydrophobic surfaces have been classifi ed into fi ve states [ 26 ] according to the details of CA hysteresis, which have been well verifi ed on different samples based on experimental results. [ 1 , 8 , 27-29 ] Superhydrophobic surfaces in different states show distinctive advantages in varied applications. Hence, efforts have also been devoted to precise tuning between different superhydrophobic states. For example, Lai et al. [ 23 a] investigated superhydrophobic surfaces with controlled adhesion to water droplets by using different kinds of rough surfaces. Li et al. [ 23 b] observed reversible switching between a transitional state (sliding angle of 75 ° ) and the Wenzel superhydrophobic state (high adhesion force) by changing the temperature. This inspired no-loss microdroplet transfer and trace-liquid reactor applications, [ 15 ] which usually need precise control of water droplet movement on the same surface from "roll-down" to "pinned" superhydrophobic states. Nevertheless, this no-loss transfer of a given water droplet requires a sensitive in situ tuning of surface wettability. Jiang et al. have reported an in situ control of magnetic droplet movement using extra magnetic fi eld, where the tuning was based not on pure water droplets, but on magnetic liquids. [ 27 ] From the practical point of view, it is still worth pointing out that the above-mentioned tuning approaches usually depend on harsh tuning conditions, such as UV irradiation, [ 18 ] electrical current, [ 19 , 21 ] a wide range of temperature, [ 23 ] or treatments by chemical solvents. [ 22 , 25 ] They may be not suitable for mild condition applications. For example, enzymes or biological cells in microfl uidic devices would be seriously affected under UV irradiation, temperature change, or addition of chemical substances. In addition, most of these tunable surfaces are based on artifi cially introduced material compositions or particular material species, [18][19][20][21][22][23][24][25] such as azobenzene and metal oxides, which suffer from poor biocompatibility. Therefore, it is urgently desirable to fi nd a simple, environmentally...

Pneumatic smart surfaces with rapidly switchable dominant and latent superhydrophobicity

“…On the other hand, surface structure reconfiguration is an appealing alternative to realize dynamic wettability control. Typically, switchable wetting behaviors have been achieved by bending a soft substrate to a large degree 27 and by fabricating recoverable structures with shape memory polymers. 19 Although these pioneering works have provided helpful insights into the controllable switching of surface dewetting properties, there is still a lack of efficient and secure strategies that can realize surface switching on demand.…”

Facile fabrication of functional PDMS surfaces with tunable wettablity and high adhesive force via femtosecond laser textured templating

“…On the other hand, hydrophobic surfaces with a high adhesive force (rose-petal-like surfaces) have also raised great academic interests recently due to their promising applications in biochemical separation, functional coating, and transport of microdroplets. [19][20][21] Efficient fabrication method to generate such surfaces with both high water repellency and strong adhesion needs to be explored.…”

Mechanical stretch for tunable wetting from topological PDMS film

“…17 In the previous work by some of the current authors, a curvature-driven in situ switching between pinned and rolldown superhydrophobic states was proposed. 18 In the work, a poly(dimethysiloxane) (PDMS) surface with a regular array of pillars was prepared, which exhibited curvature-dependent contact angle, adhesion force and sliding angle. Based on this unique switching, an in situ "mechanical hand" for no-loss water droplet transportation is proposed.…”