Advanced understanding of stickiness on superhydrophobic surfaces (original) (raw)

This study explores how contact angle hysteresis and titling angle relate with stickiness on superhydrophobic surfaces. The result indicates that contact angle hysteresis could not be mentioned as a proper factor to evaluate the surface stickiness. By analyzing the system pinning force of droplet placed on a titled surface, we concluded that both solid fraction and surface geometric factor are the critical factors determining the surface stickiness. O ver last decades, some special surfaces in nature feature high water contact angle (.150u) attracted enormous interests for both fundamental research and practical application 1-8. Some of these surfaces, lotus leaves for instance, not only exhibit high water contact angle, but also have extremely low sliding angle, which cause water droplets to bead and roll off from the surface by slightly titling it 1. This behavior provides extreme water repellency and self-cleaning characteristics 2-5. Meanwhile, some other surfaces, i.e. rose-petal surface, with large water droplet sliding angles have the ability to make water droplet pinned on the surfaces at any tilting angle (TA) 6. Such high-adhesion surfaces also possess many potential applications, no-loss fluid transportation for example 7,8. According to the different values of TA, the above mentioned surfaces fall into two categories, which are ''slippery'' and ''sticky'' ones, featuring low TA and high TA respectively, all these surfaces are uniformly named as ''superhydrophobic'' surfaces in litheture though. Meanwhile, people found that there exists a wide range of ''metastable'' contact angles when a liquid meniscus scans the solid surface 9. Because there are free energy barriers which exist between these metastable states, a true ''equilibrium'' contact angle is almost impossible to measure in real time. The famous Wenzel's and Cassie's theories are therefore only valuable in predicting the thermodynamically stable contact angles in theory 10,11. Therefore, contact angle hysteresis (CAH) defined as the difference of advancing angle and receding angle is usually measured to fully characterize any surface 12. Recently, however, it seems CAH has a new function and already somehow been treated as a criterion to evaluate the stickiness of superhydrophobic surfaces by researchers and students 13-16. People believe that increase in stickiness will absolutely result in a corresponding increase in CAH. As mentioned in the previous section, TA value has widely been used to describe the surface stickiness. Since both these two parameters have been mentioned in publications to describe the same physical phenomena, it is also naturally concluded by researchers that TA and CAH share the same changing tendency with the stickiness 15. However, we achieved an opposite conclusion according to our recent experimental investigation. We fabricated several posts arrays with different solid fractions. The TA increases with the solid fraction, which proves that the stickiness/pinning force increases with solid fraction 17-19. Nevertheless, the CAH shows a decreasing tendency while the solid fraction increases. This result is of particular interest from theory point of view. It reveals that CAH may not be the proper factor to describe the surface stickiness, and it is not necessary to take extra effects to make low CAH surface to achieve good slip performance.