John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA.
Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, Massachusetts, USA.
Nature. 2016 Mar 3;531(7592):78-82. doi: 10.1038/nature16956. Epub 2016 Feb 24.
Controlling dropwise condensation is fundamental to water-harvesting systems, desalination, thermal power generation, air conditioning, distillation towers, and numerous other applications. For any of these, it is essential to design surfaces that enable droplets to grow rapidly and to be shed as quickly as possible. However, approaches 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 beetles, cacti, and pitcher plants--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 fluxat 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.
控制液滴凝结对于集水系统、海水淡化、热能发电、空调、蒸馏塔和许多其他应用都至关重要。对于所有这些应用，设计能够使液滴快速生长并尽快脱落的表面至关重要。然而，基于微尺度、纳米尺度或分子尺度结构的方法存在固有权衡，使得很难同时优化生长和传输。在这里，我们提出了一种基于纳米比亚沙漠甲虫、仙人掌和猪笼草的原理的全新设计方法，该方法协同结合了这些凝结方面，性能明显优于其他合成表面。受甲虫凹凸表面几何形状促进凝结作用的非常规解释的启发，并结合理论建模，我们展示了如何通过优化曲率半径和横截面形状，最大限度地提高凸面毫米级凸起顶点处的蒸汽扩散通量。将这种顶点几何形状与类似于仙人掌刺的变宽斜坡集成在一起，通过创建一个自由能曲线，在液滴生长速度下降之前，将液滴沿斜坡向下驱动，从而直接将促进液滴生长与快速定向传输耦合起来。通过在向下的过程中促进聚并驱动生长和毛细运动之间的反馈，一种类似猪笼草的、具有滑润性的纳米涂层进一步增强了这种耦合。通过合理设计的集成了这些机制的凸块，即使在重力作用下，甚至在不利的温度梯度下，它们也能够生长和输送大液滴。我们进一步观察到，与其他表面相比，其生长速率的指数提高了前所未有的六倍，起始更快，稳态周转率更高，收集的水量也更大。我们设想，这种基本的理解和合理的设计策略可以应用于广泛的集水和相变换热应用。
