| dc.description.abstract | Wetting phenomena of a liquid droplet on various solid surfaces are everywhere in our daily life as well as in engineering and science. In this thesis, there are four major parts:
(1)When a sessile drop encounters a pendant drop through a hole, it is generally anticipated that they will coalesce and flow downward due to gravity. However, like “wall-free” capillarity, the pendant drop may be sucked up by a sliding drop instantaneously if the radius of the curvature of the former is smaller than that of the latter. This phenomenon can be explained by Young-Laplace equation and convective Ostwald ripening. Our results indicate that superhydrophilic perforated surface can be used as an effective way for the removal of small droplets adhering to the inner walls of microchannel systems.
(2)A desert beetle tilts its body forwards into the fog-laden wind to collect water by the hydrophilic patches on its superhydrophobic back. The pinning and dewetting mechanism of a tilted drop pinned by a designed patch on a superhydrophobic surface with negligible contact angle hysteresis (CAH) is explored both experimentally and theoretically. The patch is designed in different shapes including square, rectangle and triangle. For a square or rectangular patch, the uphill contact angle (CA) of the tilted drop varies with the inclined angle (a) of the plate. The drop remains pinned until the critical inclined angle (ac) is achieved. As a=ac, the uphill CA of the drop reduces to the receding angle of the patch. The magnitude of ac grows approximately linearly with the pinning length (wp), which is related to the patch size. It is found that wp equals the side-length (w) of square or rectangular patch perpendicular to the sliding direction. While wp on square patches remains essentially unchanged before sliding, wp on the triangular patch grows with increasing a. However, the relation between sin(a) and wp for the triangular patch is consistent with that between sin(ac)and w for square and rectangular patches. Surface Evolver simulations based on free energy minimization are performed to reproduce the wetting and dewetting behavior. The simulation outcomes agree quite well with the experimental results.
(3) The CAH of acrylic glass is experimentally and theoretically studied through the compression-relaxation process of droplets by using a superhydrophobic surface with negligible CAH effect. In contrast to the existing technique in which the volume of the droplet changes during the measurement of CAH, this procedure is carried out at a constant volume of the droplet. By observing the base diameter (BD) and the CA of the droplet during the compression-relaxation process, the wetting behavior of the droplet can be divided into two regimes, the contact line withdrawal and the contact line pinning regimes, depending on the gap thickness (H) at the end of the compression process. During the compression process, both regimes possess similar droplet behavior; the contact line will move outward and the BD will expand while the CA remains at the advancing angle. During the relaxation process, the two regimes are significantly different. In the contact line withdrawal regime, the contact line will withdraw with the CA remaining at the receding angle. In the contact line pinning regime, however, the contact line will be pinned at the final position and the CA will decline to a certain value higher than the receding angle. Furthermore, the advancing pinning behavior can also be realized through a successive compression-relaxation process. On the basis of the liquid-induced defects model, Surface Evolver simulations are performed to reproduce the behavior of the droplet during the compression-relaxation process; both contact line withdrawal and pinning regimes can also be identified. The results of the experiment and simulation agree with each other very well.
(4) A superhydrophobic graphite surface has been fabricated through two facile physical steps, peeling and ultrasonicating. Peeling yields micron-scale roughening, and thus a highly hydrophobic surface is obtained. Further ultrasonicating results in a superhydrophobic surface with nanostructure embedded in microstructure. The nanostructure leads to network-like pores on the superhydrophobic film and convective Ostwald ripening is observed. Owing to their distinct resistance to liquid imbibition, contact angle hysteresis on hydrophobic and superhydrophobic surfaces is fundamentally different. Moreover, the adhesive force on a superhydrophobic surface grows with the contact time, and such aging effect is absent on hydrophobic graphite surface. | en_US |