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. 2016 Apr 4:6:23687.
doi: 10.1038/srep23687.

3D Imaging of Water-Drop Condensation on Hydrophobic and Hydrophilic Lubricant-Impregnated Surfaces

Tadashi Kajiya  1 , Frank Schellenberger  1 , Periklis Papadopoulos  1 , Doris Vollmer  1 , Hans-Jürgen Butt  1
Affiliations

3D Imaging of Water-Drop Condensation on Hydrophobic and Hydrophilic Lubricant-Impregnated Surfaces

Tadashi Kajiya et al. Sci Rep. .

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.

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Figures

Figure 1
Figure 1
(a) Schematic representation of the temperature and humidity controlled measurement cell in the confocal microscope. (b) Schematic and confocal images of the LIS. For a confocal image, the x−y and x−z slices are shown. The yellow region corresponds to the ionic liquid. The blue color is the reflection from the interface. The pillars and substrate appear black. The pillar geometry is characterized by width (w), spacing (s) and height (h).
Figure 2
Figure 2. 3D images and x−z cross-section of water drops condensing on an ionic liquid impregnated surface consisting of hydrophobic rectangular micropillars (w = 20 μm, s = 20 μm, h = 10 μm).
Note that the 3D image is tilted for a clear visualization. In reality, the substrate is placed horizontally. Four sequential time steps are shown. (a) 70 s: Tiny droplets nucleate on the surface of the lubricant. (b) 300 s: As the size of the drops become comparable to the pillars’ spacing, they move between the pillars and formed a regular pattern. The drop contacts the bottom of the substrate with a contact angle of θ ≈ 130°. (c) 450 s: The drop deforms to fill the free space and bridge with its neighbor. (d) 600 s: The drops coalesce and form a large drop, the center of which covers a pillar. Movies of 3D images and x−y and x−z cross sections are available in the supplemental files S1 and S2.
Figure 3
Figure 3. Averaged drop diameter and area fraction of the surface occupied by condensed droplets (φa) as a function of time.
formula image and φa are measured at a height of the pillar’s top face (z = 10 μm). The pillars had a rectangular or circular cross-section of 10 μm or 20 μm width and identical spacing.
Figure 4
Figure 4. Radius of curvature of the drop at the air/water interface (Rw) versus the radius of curvature at the water/lubricant interface (Rwl).
The red square symbols correspond to the drops floating on the lubricant and blue circular symbols correspond to the drops touching the bottom of the substrate. Data of different pillar sizes and geometries are superimposed in the graph. A linear fitting curve is plotted as dashed line with a slope of 4.1 ± 0.03.
Figure 5
Figure 5
(a–c) Top and middle row: x−y cross section of a drop condensing on a LIS with hydrophobic rectangular micropillars. The images are sliced at the top (z = 10 μm) and bottom (z = 0 μm) surfaces of the pillars and recorded after (a) 200 s, (b) 550 s and (c) 1100 s. The white scale bar corresponds to 20 μm. Bottom row: images of the x−z cross section. As the drop size increases, the drop gradually detaches from the bottom surface. (d) Plot of the relative contact areas (Abtm/Atop) versus the ratio of the drop diameter and pillar width (d/w). Abtm is the area of the bottom drop-substrate interface (z = 0 μm) and Atop is the area of the drop at the top of the pillars (z = 10 μm).
Figure 6
Figure 6
(a) Two forces acting around the pillar. The Laplace forces (FL) at the water/lubricant interface which pushes the drop downward and the force applied at the pillar perimeter that sustains the drop on the top (FS). θ is the contact angle at lubricant/water/solid boundary. (b) The apparent contact angle (θA), determined as the angle of the intersection at which the contour of the air/water interface crosses the horizontal plane at a height of the pillars’ top faces.
Figure 7
Figure 7. 3D images and x−z cross sections of water drops condensing on a LIS composed of hydrophilic rectangular pillars (w, s = 20 μm, h = 10 μm).
Images are recorded after (a) 50 s, (b) 200 s and (c) 260 s, respectively. The drops nucleate and grow preferentially on the pillar top surface. Few drops nucleate on a lubricant surface. In the later stage (c) the drops contact the pillars’ bottom surface. The water drops wet the bottom surface by displacing the lubricant, and the drops do not detach again. Movies of 3D images and x−y and x−z cross sections are available in the supplemental files S3 and S4.
Figure 8
Figure 8. Plot of the diameter (d) and height (h) of the drop as a function of volume (V).
The LISs with (a) hydrophobic and (b) hydrophilic micropillars are compared. Data for the different pillar sizes and geometries are superimposed in the graphs. (c) Apparent contact angle after the drop contacts the bottom solid surface.
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References

    1. Beer J. M. High Efficiency Electric Power Generation: The Environmental Role. Prog. Energy Combust. Sci. 33, 107–134 (2012).
    1. Udell K. S. Heat Transfer in Porous Media Considering Phase Change and Capillarity – The heat pipe effect. Int. J. Heat Mass Transfer 28, 485–495 (1985).
    1. Parker A. R. & Lawrence C. R. Water Capture by a Desert Beetle. Nature 414, 33–34 (2001). - PubMed
    1. Zhai L. et al.. Patterned Superhydrophobic Surfaces: Toward a Synthetic Mimic of the Namib Desert Beetle. Nano Lett. 6, 1213–1217 (2006). - PubMed
    1. Park K. C., Chhatre S. S., Srinivasan S., Cohen R. E. & McKinley G. H. Optimal Design of Permeable Fiber Network Structures for Fog Harvesting. Langmuir 29, 13269–13277 (2013). - PubMed

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