Research On The Wettability Of Hierarchically Structured Surfaces

Superhydrophobicity [with a contact angle (CA) greater than 150o] has recently attracted considerable interest for both fundamental research and practical applications. Of special interest is the fabrication of superhydrophobic surfaces with low CA hysteresis (CAH), which has promising applications ranging from self-cleaning window glasses to microfluidic devices. Superhydrophobicity with low CAH was first observed in nature on the sacred lotus (Nelumbo nucifera) which has long been the sign of purity in Asian cultures. Rainwater on a lotus leaf beads up (with a CA of about 160o) and rolls readily off (with a rolling angle less than 5o) taking powderlike contaminants along, resulting in a self-cleaning behavior. This self-cleaning behavior, termed as the Lotus Effect, stems from a hierarchical structure made of unwettable wax crystals present on the leaf surface. Hierarchical structures (optimized by natural selection) are also responsible for later found natural superhydrophobicity (of such as water strider legs, duck feather, and inset wings,) and for this reason have received particular attention.A suitable convex hierarchical structure does not only enhance the hydrophobicity, it also stabilizes the Cassie contact, and thus favors the Lotus Effect. Remarkably, a unique hierarchical structure was recently found to lead mosquito (C. Pipiens) eyes to keep the Lotus Effect even when exposed to moisture, which insure mosquitos a clear vision in humid habitats. It was demonstrated that, besides its intrinsic hydrophobicity, this unique property is attributed to the elaborate collaboration between mico- and nanostructures: close-packed first-level structures (ommatidia) at the microscale, non-close-packed second-level structures (nipples) at the nanoscale. This finding provides an inspiration for sovling antifogging problems by superhydrophobic approaches. However, the basic mechanisms of the robust Lotus Effect of mosquito eyes were not yet well understood. Based on the classical Young, Cassie and Wenzel equations, we theoretically studied the robust Lotus Effect (antifogging property) of mosquito eyes. Nanohemispheres and round nanopillars were adopted to simulate the nanonipples, respectively, and the local contact angles were assumed to satisfy the Young equation. We also assume that only if the Cassie contact occupies a lower energy comparing to other contacts, a robust Lotus Effect can be obtained. The robust Lotus Effect of mosquito eyes is unpuzzled by showing how the collaboration of close-packed first-level structures and non-close-packed second-level structures insures the Cassie a lowest energy among all possible contact states.According to Herminghaus, a suitable hierarchical structure could even render any surface (independent of the intrinsic contact angle) nonwettable as long as the roughness amplitude at small scales is sufficient to suspend a free liquid surface. Remarkably, this argument is experimentally supported by the recent achievements in water repellence through constructing intrinsic hydrophilic material with hierarchical structures. Such surfaces [including superhydrophobic La0.7Sr0.3MnO3 coatings, LaMnO3, CaBi4Ti4O15 (which are intrinsic hydrophilic) with hierarchical structures obtained by our group], however, lost their superhydrophobic properties when the water formed onto them from moisture. Those observations indicate that the Lotus Effects of those surfaces are unrobust and also depends on how the water gets onto them. Further study is still in need to obtain a robust Lotus Effect. In order to explain this phenomenae theorectically, an ideal hierarchical structure is evolved from the mosquito eye, which could suspend any liquid with a definite intrinsic contact angle and could thus lead to a superhydrophobicity by optimizing the hierarchical structure. Based on the Young, Wenzel, and Cassie equations, theoretical analysis was performed on the stability of the Cassie contact between the liquid and such a hierarchical structure. We found, remarkably, that the Cassie contact on such a hierarchical structure could be robust only if the hierarchical structure is intrinsic hydrophobic itself. In othe words, an intrinsic hydrophobic material could not lead to a robust Lotus Effect. Such a conclusion accounts for the loss of the Lotus Effect of superhydrophobic La0.7Sr0.3MnO3 coatings, LaMnO3, CaBi4Ti4O15 in the case of water vapor condensing onto them. Based on the theoretical analysis on the robust Lotus Effect of mosquito eyes and the ideal hierarchical structure surfaces, optimization criteria for hierarchical structures for robust Lotus Effect could be formulated.La0.7Sr0.3MnO3 coatings with different surface structures thus different degrees of surface roughness were prepared by annealing the coatings of nanometric power at different temperature, and the effect of the surface structures as well as roughness on the wettability is studied. La0.7Sr0.3MnO3 coatings with micro-nano hierarchical structures can be obtain by annealing the coatings composed of La0.7Sr0.3MnO3 nanopowder under certain temperature. Remarably, we found that La0.7Sr0.3MnO3 coatings display distinct wettability from superhydrophilicity of almost 0o to superhydrophobicity of more than 150o by varying the surface microstructures as well as roughness of the same material. This offers a strong support to Herminghaus's model which predicts that the material hierarchical structure determines it wettabiltiy and draw the conclusion that the surface microstructure is a key factor that could determine the wettability.The studies offered by this thesis may enrich understandings of wettablity for complex surfaces and widen the range of raw materials for superhydrophobic surfaces.