Thermodynamic analysis on wetting states and wetting state transitions of rough surfaces

Determining the equilibrium wetting states and exploring the conditions and mechanisms of the wetting state transition from the Cassie-Baxter (CB) state to the Wenzel (W) state (CB-W transition) have been a central topic in the study of superhydrophobic behavior on rough or textured surfaces. Although considerable progress has been made, some issues regarding this topic are still not completely understood. In this study, a systematic thermodynamic analysis has been performed to address several key issues related to this topic. Generalized theoretical expressions for determining the equilibrium wetting states (the threshold Young contact angle of the CB region) and evaluating the stability of the CB state (the energy barrier separating the CB and W states and the critical pressure for the CB-W transition) have been derived. Applying these expressions to four types of surfaces built with protrusions in paraboloid, truncated cone, inverted truncated cone and flat-top pillar shapes, the wetting equilibrium and resultant wetting states have been studied. The physical meanings of the threshold Young contact angle, the roles and mechanisms of the energy barrier and critical pressure in stabilizing the CB state have been discussed. Finally, a general guidance for achieving robust superhydrophobicity on the studied surfaces has been given.

Combination of active behaviors and passive structures contributes to the cleanliness of housefly wing surfaces: A new insight for the design of cleaning materials

Evolutionary pressure has pushed many extant plants and animals to develop micro/nanostructures on their surfaces to keep them clean. These structures have become ideal models for bio-inspired design. Although microstructures on biological surfaces have been widely studied, little attention has been paid to the combined role of microstructures and animal's active cleaning behaviors in keeping their surfaces clean. In this study, we explored the relationship between these micro/nanostructures and wettability as well as the role of the housefly cleaning behaviors in keeping their wings clean. Hierarchical structures consisting of microscale macrotrichias with nanoscale grooves on the wings were observed under scanning electron microscope. The wings were hydrophobic (CA = 133.3°) but with high adhesion to water (CAH = 87.5°), indicating that they were non-self-cleaning surfaces. Macroscale droplets standing on the wings could be best described as being in a transitional wetting state between Wenzel and Cassie-Baxter states due to the presence of the nanoscale grooves, which increased the resistance to water penetration. The hydrophobicity decreased (CA = 109.9°) when the nanostructures were removed by coating the wings with a thick layer of polydimethylsiloxane (PDMS). The houseflies could highly efficiently remove the microscale droplets atop the macrotrichias, and reduce bacterial contamination on their wings through grooming and flutter activities. These active cleaning behaviors could offset the absence of self-cleaning properties and play a key role in keeping the wings clean. The results indicate that housefly wings could be used as a template for the design of special functional surfaces. The present findings not only improve our understanding of the wettability and cleaning properties of natural surfaces, but also provide important insights into the design of bio-inspired materials.

Progress in understanding wetting transitions on rough surfaces

The abrupt change in the apparent contact angle occurring on a rough surface is called wetting transition. This change may be spontaneous or promoted by external stimuli such as pressure or vibration. Understanding the physical mechanism of wetting transitions is crucial for the design of highly stable superhydrophobic and omniphobic materials. Wetting regimes occurring on rough surfaces are introduced. Experimental methods of study of wetting transitions are reviewed. Physical mechanisms of wetting transitions on rough surfaces are discussed. Time and energy scaling of wetting transitions are addressed. The problem of the stability of Cassie wetting on inherently hydrophobic and hydrophilic surfaces is discussed. The origin and value of a barrier separating the Cassie and Wenzel wetting states are treated in detail. Hierarchical roughness increases the value of the energy barrier. The stability of Cassie wetting observed on re-entrant topographies is explained. The irreversibility of wetting transitions is explained, based on the asymmetry of the energy barrier, which is low from the side of the metastable (higher-energy) state and high from the side of the stable state. The critical pressure necessary for a wetting transition is introduced. The problem of "dimension" of wetting transition is discussed. Reducing the micro-structural scales enlarges the threshold pressure of a wetting transition. The roles of gravity and air compressibility in wetting transitions are treated. The dynamics of wetting transitions is reviewed. The results of molecular simulations of wetting transitions are presented. The trends of future investigations are envisaged.