Recent progress in experiments for sessile droplet wetting on structured surfaces

Impacting Dynamics of Droplets on Cylindrical Superhydrophobic Surfaces Decorated with a Convex Ridge

This work investigates the dynamic behavior of droplets on superhydrophobic cylindrical surfaces with a convex ridge through experimental analysis, focusing on the effects of varying the diameter ratio (D* = D/D0) and the ridge width ratio (δr = r/D0). Impact morphology diagrams are established to reveal the morphology transition of the droplet as a function of D* and δr. The splash threshold is obtained, and the energy loss during the collision process is analyzed by examining the recovery coefficient and the splitting angle, with the splitting threshold found to be dependent on δr. Additionally, the presence of the convex ridge modifies the anisotropic behavior of the droplet, leading to a 35% reduction in contact time compared to that of cylindrical superhydrophobic surfaces without a ridge. Analysis of the contact time reveals that azimuthal splitting time has the greatest influence on the overall contact time, with shorter azimuthal splitting time resulting in reduced contact time. This research aims to better understand the impact dynamics of droplets on cylindrical surfaces with a convex ridge, providing potential applications for anti-icing technologies.

Wetting Effect on Patterned Substrates

A droplet deposited on a solid substrate leads to the wetting phenomenon. A natural observation is the lotus effect, known for its superhydrophobicity. This special feature is engendered by the structured microstructure of the lotus leaf, namely, surface heterogeneity, as explained by the quintessential Cassie-Wenzel theory (CWT). In this work, recent designs of functional substrates are overviewed based on the CWT via manipulating the contact area between the liquid and the solid substrate as well as the intrinsic Young's contact angle. Moreover, the limitation of the CWT is discussed. When the droplet size is comparable to the surface heterogeneity, anisotropic wetting morphology often appears, which is beyond the scope of the Cassie-Wenzel work. In this case, several recent studies addressing the anisotropic wetting effect on chemically and mechanically patterned substrates are elucidated. Surface designs for anisotropic wetting morphologies are summarized with respect to the shape and the arrangement of the surface heterogeneity, the droplet volume, the deposition position of the droplet, as well as the mean curvature of the surface heterogeneity. A thermodynamic interpretation for the wetting effect and the corresponding open questions are presented at the end.

Flotation surface chemistry of water-soluble salt minerals: from experimental results to new perspectives

The flotation separation of water-soluble salt minerals has to be conducted under the condition of saturation in brines which represents a challenging but exciting topic of colloid and surface chemistry. Despite several proposals on explaining the success of this industrial application for many decades, our understanding of the flotation separation is still far from complete yet, owing to the complexity of the highly selective collection of salt crystals by air bubbles in brines. Here, we thoroughly review the experimental results for halogen, oxyanion, and double salts and match them with the proposed theories on the flotation of soluble salts to identify the agreed and disagreed cases. The experimental results show that the flotation of these salts varies from collectors (surfactants applied to control the crystal hydrophobicity) to collectors and is strongly affected by the brine ion composition and pH conditions. We find some exceptional flotation results that cannot be simply explained by the crystal surface charge and wettability. Furthermore, we outline several disputes and discrepancies between the experiments and the theories when different collectors are applied. Apart from the extensive consideration of surface hydration, the presence of external ion species exhibits ubiquitous effects on the surface properties of salt crystals and the colloidal properties of collectors. We conclude that the interactions between salt ions, water molecules, collectors, and salt crystals must be considered more thoroughly, and the activity of collectors at the air-liquid interface should also be the focus. Advanced techniques such as molecular dynamics simulation, atomic force microscopy, X-ray photoelectron spectroscopy, and sum-frequency generation spectroscopy are expected to be promising research tools for future studies.

Droplet Evaporation Dynamics on Hydrophobic Network Surfaces

Surface modification, such as hydrophobic network modification, is very promising technology to control droplet dynamics, heat transfer, and evaporation. However, fundamental mechanisms of how these chemically patterned surfaces affect the droplet evaporation dynamics and predictions of evaporation rates are still lacking. In the present work, we systematically investigated the full process of droplet evaporation dynamics on hydrophobic network surfaces and distinguished four different stages: constant contact line (CCL) stage, constant contact angle (CCA) stage, pattern-pinning (PP) stage, and moving contact line (MCL) stage. We further developed a general model considering the pinning and depinning forces to accurately predict the evaporation transition from PP to MCL stages (i.e., critical receding contact angle, θ_cr). As for the influence of the chemically patterned surface on the evaporation rate, a corrected contact line length was considered and combined with the well-known Rowan and Erbil's models. Finally, a general model was thus proposed and showed successful predictions for the evaporation durations of each stage.