Abstract
Condensation of water from the atmosphere on a solid surface is an ubiquitous phenomenon in nature and has diverse technological applications, e.g. in heat and mass transfer. We investigated the condensation kinetics of water drops on a lubricant-impregnated surface, i.e., a micropillar array impregnated with a non-volatile ionic liquid. Growing and coalescing drops were imaged in 3D using a laser scanning confocal microscope equipped with a temperature and humidity control. Different stages of condensation can be discriminated. On a lubricant-impregnated hydrophobic micropillar array these are: (1) Nucleation on the lubricant surface. (2) Regular alignment of water drops between micropillars and formation of a three-phase contact line on a bottom of the substrate. (3) Deformation and bridging by coalescence which eventually leads to a detachment of the drops from the bottom substrate. The drop-substrate contact does not result in breakdown of the slippery behaviour. Contrary, on a lubricant-impregnated hydrophilic micropillar array, the condensed water drops replace the lubricant. Consequently, the surface loses its slippery property. Our results demonstrate that a Wenzel-like to Cassie transition, required to maintain the facile removal of condensed water drops, can be induced by well-chosen surface hydrophobicity.
Highlights
To achieve efficient water-drop condensation, researchers have implemented diverse types of superhydrophobic surfaces
Superhydrophobic surfaces are patterned with hydrophobic micro- or nano-structures such that air is entrapped below a deposited water drop placed on top[17,18,19,20]
liquid-impregnated surfaces (LIS) are promising for efficient water condensation, their application remains limited by insufficient understanding
Summary
To check the wetting property of the flat hydrophobic and hydrophilic SU-8 surfaces, the static contact angles of water (θW) and ionic liquid (θIL) drops were measured (OCA35; DataPhysics, Germany). To simultaneously measure the shape of the condensing drops and the lubricant, a laboratory-built laser scanning confocal microscope with a blue laser (wave length: 473 nm, power: 25 mW, Cobolt, Sweden) and a 40 × /0.85 dry objective lens (Olympus, Japan) was used (Fig. 1a). To initiate water condensation on the LIS, a temperature control stage operated by gas cooling and a humidity control cell (cylindrical cell with diameter 50 mm and height 60 mm) were mounted on the confocal microscope. The ionic liquid filled the space between the pillars (the filling height was 10 μm) but did not cover the top faces of the micropillars. The nucleation density and the shape of the drops were identical within our experimental accuracy between successive measurements on the identical surface
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