Applied Mechanics of Materials Laboratory, Department of Mechanical Engineering, Temple University, 1947 North 12th Street, Philadelphia, PA 19122, USA.
Soft Matter. 2018 Nov 7;14(41):8276-8283. doi: 10.1039/c8sm01688g. Epub 2018 Oct 15.
Water droplet transport on fibers is of great importance for achieving high water collection efficiency from fog. Here, we exploit a new droplet sliding mechanism to accelerate the droplet coalescence and collection for highly efficient fog harvesting by coating hydrophilic microfibers with superhydrophobic layers of assembled carbon nanoparticles. We find that during the initial water collection, unlike the pinned droplets having axisymmetric barrel shapes wrapped around uncoated microfibers, the hanging droplets on coated microfibers with non-wrapping clamshell shapes are highly mobile due to their lower contact hysteresis adhesion; these are observed to oscillate, coalesce, and sweep the growing droplets along the horizontally placed microfibers. The driving force for droplet transport is mainly ascribed to the coalescence energy release and fog flow. After introducing small gravity force by tilting coated microfibers with a small angle of 5°, we find that it can effectively drive the oscillating mobile droplets for directional transport by rapidly sweeping the droplets with a much higher frequency. Finally, the water collection rate from fog on uncoated microfibers over a prolonged duration is found to be improved over 2 times after superhydrophobic coating, and it is further enhanced over 5 times after a small tilting angle of 5°.
水滴在纤维上的输运对于实现雾中高效集水非常重要。在这里，我们利用一种新的液滴滑动机制，通过在亲水微纤维上涂覆组装的碳纳米粒子的超疏水层来加速液滴的聚合并提高高效雾采集效率。我们发现，在最初的集水过程中，与被包裹在未涂层微纤维上的具有轴对称桶形的固定液滴不同，具有非包裹蛤壳形状的涂层微纤维上的悬挂液滴由于其较低的接触滞后附着力而具有很高的流动性；这些液滴会发生振荡、聚并，并沿水平放置的微纤维扫过不断生长的液滴。液滴输运的驱动力主要归因于聚并能释放和雾流。通过将涂覆有微纤维的小角度（5°）倾斜，引入小的重力，我们发现它可以通过快速扫过液滴以更高的频率有效地驱动振荡移动的液滴进行定向输运。最后，我们发现，在经过超疏水涂层处理后，未涂层微纤维上雾的集水速率在长时间内提高了 2 倍以上，而在小倾斜角度 5°后，集水速率进一步提高了 5 倍以上。
