Abstract
AbstractSurfaces that entrap air underwater serve numerous practical applications, such as mitigating cavitation erosion and reducing frictional drag. These surfaces typically rely on perfluorinated coatings. However, the non‐biodegradability and fragility of the coatings limit practical applications. Thus, coating‐free, sustainable, and robust approaches are desirable. Recently, a microtexture comprising doubly reentrant cavities (DRCs) has been demonstrated to entrap air on immersion in wetting liquids. While this is a promising approach, insights into the effects of surface chemistry, hydrostatic pressure, and cavity dimensions on wetting transitions in DRCs remain unavailable. In response, Cassie‐to‐Wenzel transitions into circular DRCs submerged in water are investigated and compared with those in cylindrical “simple” cavities (SCs). It is found that at low hydrostatic pressures (≈50 Pa), DRCs with hydrophilic (θo ≈ 40°) and hydrophobic (θo ≈ 112°) make‐ups fill within 105 and 107 s, respectively, while SCs with hydrophilic make‐up fill within <10−2 s. Under elevated hydrostatic pressure (P ≤ 90 kPa), counterintuitively, DRCs with hydrophobic make‐up fill dramatically faster than the commensurate SCs. This comprehensive report should provide a rational framework for harnessing microtexturing and surface chemistry toward coating‐free liquid repellency.
Highlights
For the superhydrophilic make-up and at a nominal hydrostatic pressure of ≈50 Pa, doubly reentrant cavities (DRCs) and simple cavities (SCs) got filled within ≈10 s and
For the hydrophobic make-up, we found that wetting transitions at elevated pressures were significantly faster in the DRCs than in SCs; the breakthrough pressures for the DRCs were significantly lower than those for the SCs
Our high-resolution experiments with confocal laser scanning microscopy (CLSM) revealed that as the liquid penetrates inside the DRCs, the primary meniscus remains pinned at the DR edge; as the pressure increases, the curvature of the spheroid-shaped water–air interface keeps increasing until the water touches the cavity floor
Summary
We realized arrays of DRCs and SCs on SiO2/Si wafers using microfabrication protocols that we have reported recently[19,20,21,47] (Figure 1). The advancing water meniscus was stabilized at the DR edges and air was entrapped inside the cavities In this configuration, DRCs were nearly saturated with water vapor, which led to the formation of continuous wetting films onto the superhydrophilic cavity surface[51] (Figures 2D and the inset and Figure 3A–H). The drop-wise condensation eventually led to the formation of a film inside the cavity, which rose upward to merge with the primary meniscus stabilized at the doubly reentrant edge This capillary condensation of water displaced the air inside the cavity, because of the low solubility of the air in water under our experimental conditions[52] (Figure S5, Supporting Information). A tiny fraction of air was trapped underwater, such that the water meniscus penetrated to a depth of ≈15 μm In this case, Cassie-to-Wenzel transitions took place within 2 days (Figure S6, Supporting Information).
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