Abstract
Maintaining the non-wetting property of textured hydrophobic surfaces is directly related to the preservation of an intervening fluid layer (gaseous or immiscible liquid) between the droplet and substrate; once displaced, it cannot be recovered spontaneously as the fully penetrated Wenzel wetting state is energetically favorable. Here, we identify pathways for the “lifting” of droplets from the surface texture, enabling a complete Wenzel-to-Cassie-Baxter wetting state transition. This is accomplished by the hemiwicking of a transient (limited lifetime due to evaporation) low surface tension (LST) liquid, which is capable of self-assembling as an intervening underlayer, lifting the droplet from its impaled state and facilitating a skating-like behavior. In the skating phase, a critical substrate tilting angle is identified, up to which underlayer and droplet remain coupled exhibiting a pseudo-Cassie-Baxter state. For greater titling angles, the droplet, driven by inertia, detaches itself from the liquid intervening layer and transitions to a traditional Cassie-Baxter wetting state, thereby accelerating and leaving the underlayer behind. A model is also presented that elucidates the mechanism of mobility recovery. Ultimately, this work provides a better understanding of multiphase mass transfer of immiscible LST liquid-water mixtures with respect to establishing facile methods towards retaining intervening layers.
Highlights
Maintaining the non-wetting property of textured hydrophobic surfaces is directly related to the preservation of an intervening fluid layer between the droplet and substrate; once displaced, it cannot be recovered spontaneously as the fully penetrated Wenzel wetting state is energetically favorable
This study focuses on the fundamental aspects of solid-low surface tension (LST) liquid-water interactions and will provide insight into the mechanism that regulates the water droplet remobilization on tilted substrates
We demonstrate that an impaled droplet (Wenzel state) can be rendered mobile again on a functional superhydrophobic surface by wetting the surface with a LST liquid, i.e., hydrofluoroether (HFE), without damaging the surface
Summary
Maintaining the non-wetting property of textured hydrophobic surfaces is directly related to the preservation of an intervening fluid layer (gaseous or immiscible liquid) between the droplet and substrate; once displaced, it cannot be recovered spontaneously as the fully penetrated Wenzel wetting state is energetically favorable. Water droplet mobility is important to the functionality of many applications such as self-cleaning processes (Lotus effect)[1,2], condensation[3,4] and freezing[5,6,7,47] It is closely associated with inhibiting contact between the water molecules and the solid, which is achieved by the presence of an intervening fluid (gas or liquid) layer. An alternative approach for promoting droplet mobility can be sought by the formation of an intervening liquid layer (previous examples were gaseous layers) This can be achieved by utilizing low-surface tension liquids, with negligible solubility in water, to form an ultra-smooth, low-hysteresis lubricant layer infused into a micro/nano structured hydrophobic surface. Concerning LIS and SLIPS, initial results are promising in terms of their droplet mobility, it has been shown that potential drainage of the suffusing lubricant may occur due to gravitational effects[18]
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