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Condensation on slippery asymmetric bumps

Abstract

Controlling dropwise condensation is fundamental to water-harvesting systems1,2,3, desalination4, thermal power generation4,5,6,7,8, air conditioning9, distillation towers10, and numerous other applications4,5,11. For any of these, it is essential to design surfaces that enable droplets to grow rapidly and to be shed as quickly as possible4,5,6,7. However, approaches4,5,6,7,8,10,11,12,13,14,15,16,17,18,19,20,21 based on microscale, nanoscale or molecular-scale textures suffer from intrinsic trade-offs that make it difficult to optimize both growth and transport at once. Here we present a conceptually different design approach—based on principles derived from Namib desert beetles3,22,23,24, cacti25, and pitcher plants17,26—that synergistically combines these aspects of condensation and substantially outperforms other synthetic surfaces. Inspired by an unconventional interpretation of the role of the beetle’s bumpy surface geometry in promoting condensation, and using theoretical modelling, we show how to maximize vapour diffusion flux20,27,28at the apex of convex millimetric bumps by optimizing the radius of curvature and cross-sectional shape. Integrating this apex geometry with a widening slope, analogous to cactus spines, directly couples facilitated droplet growth with fast directional transport, by creating a free-energy profile that drives the droplet down the slope before its growth rate can decrease. This coupling is further enhanced by a slippery, pitcher-plant-inspired nanocoating that facilitates feedback between coalescence-driven growth and capillary-driven motion on the way down. Bumps that are rationally designed to integrate these mechanisms are able to grow and transport large droplets even against gravity and overcome the effect of an unfavourable temperature gradient. We further observe an unprecedented sixfold-higher exponent of growth rate, faster onset, higher steady-state turnover rate, and a greater volume of water collected compared to other surfaces. We envision that this fundamental understanding and rational design strategy can be applied to a wide range of water-harvesting and phase-change heat-transfer applications.

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Figure 1: An overview of our approach.
Figure 2: Design of, and droplet growth on, convex bumps.
Figure 3: Capillary-driven transport.
Figure 4: Coupled fast growth and transport.

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Acknowledgements

We thank M. Khan, J. Alvarenga, D. Daniel, S. H. Kang, M. Hoang, and J. Timonen for discussions and technical assistance. This research was supported by the Department of Energy/ARPA-E award number DE-AR0000326. N.H. thanks the Research Experiences for Undergraduates programme supported by the National Science Foundation award number DMR-1420570.

Author information

Authors and Affiliations

Authors

Contributions

K.-C.P., P.K. and J.A. conceived the research. J.A. supervised the research. K.-C.P., P.K. and J.A. designed the slippery asymmetric bumps and the experiments. K.-C.P., P.K., N.H., D.F. and J.C.W. carried out the experiments. All authors analysed data. K.-C.P. built the analytical and numerical models. K.-C.P., P.K., A.G., D.F. and J.A. interpreted data and wrote the paper.

Corresponding authors

Correspondence to Kyoo-Chul Park or Joanna Aizenberg.

Ethics declarations

Competing interests

J.A. and P.K. are founders of the start-up company SLIPS Technologies, Inc.

Supplementary information

Supplementary Information

This file contains Supplementary Text and Data, Supplementary Tables 1-3 Supplementary Figures 1-10 and Supplementary references – see contents for details. (PDF 2349 kb)

Fast droplet growth on a PDMS-coated spherical-cap-shaped bump compared to a flat region with the same height

This video demonstrates the fast droplet growth on a PDMS-coated spherical-cap-shaped bump (left) compared to a flat region with the same height (right). The diameter of the largest droplet on the apex of the bump is greater than that of a droplet on the flat region. This video corresponds to Fig. 2b of the main text. (MOV 2687 kb)

Bump without additional roughening by sandpaper still exhibits greater droplets on its apex compared to roughened flat surfaces with the same height

This video demonstrates that the bump without additional roughening by sandpaper still exhibits greater droplets on its apex compared to the roughened flat surfaces with the same height, thus ruling out the importance of the micro/nano surface roughness to the observed preferential droplet growth at the apex of the structures. This video corresponds to Supplementary Fig. S3. (MOV 3923 kb)

Condensed water droplets moving against gravity on an asymmetric bump

This video demonstrates that condensed water droplets move even against gravity on an asymmetric bump. The droplet moves toward the wider flat area of the slope such that it no longer overlaps with the curved regions and further grows by coalescing with the smaller droplets on the way. This video corresponds to Fig. 3d of the main text. (MOV 4376 kb)

Condensed water droplets move down the widening slope of the slippery asymmetric bumps when the slope is tilted 45 or 90 degrees relative to gravity

This video demonstrates that condensed water droplets move down the widening slope of the slippery asymmetric bumps when the slope is tilted 45 (left) or 90 degrees (right) relative to gravity. In both cases, the droplet moves toward the wider flat area of the slope such that it no longer overlaps with the curved regions and further grows by coalescing with the smaller droplets on the way, similar to Supplementary Video 3. This video corresponds to Fig. 3e of the main text. (MOV 1461 kb)

Condensed water droplets do not show directed transport on slippery bumps that have a rectangular, rather than an asymmetric widening, slope.

This video demonstrates that condensed water droplets do not show directed transport on slippery bumps that have a rectangular, rather than an asymmetric widening, slope. Without the asymmetry, the droplets fall off the side of the bump and slide next to it in the direction of gravity, whether the bump slope is aligned with (left) or against (right) gravity. (MOV 5214 kb)

Exemplary array of the slippery asymmetric bumps shows a significantly greater volume of water collected at the bottom of the surface, compared to the flat slippery surfaces.

This video demonstrates that an exemplary array of the slippery asymmetric bumps (left) shows a significantly greater volume of water collected at the bottom of the surface, compared to the flat slippery surfaces (right). This video corresponds to Fig. 4c of the main text and Supplementary Fig. S9. (MOV 5705 kb)

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Park, KC., Kim, P., Grinthal, A. et al. Condensation on slippery asymmetric bumps. Nature 531, 78–82 (2016). https://doi.org/10.1038/nature16956

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