Shiri Samira, Murrizi Armela, Bird James C
Department of Mechanical Engineering, Boston University, Boston, MA 02215, USA.
Micromachines (Basel). 2018 Nov 2;9(11):566. doi: 10.3390/mi9110566.
A water drop can bounce upon impacting a superhydrophobic surface. However, on certain superhydrophobic surfaces, a water drop will stick rather than bounce if it is sufficiently hot. Here, we aim to better understand the mechanisms that can lead to this bouncing-sticking transition. Specifically, we model two potential mechanisms in which a superhydrophobic surface could trap a sufficiently hot drop within milliseconds: melting of microtextured wax and condensation of the vapor within the superhydrophobic texture. We then test these mechanisms through systematic drop impact experiments in which we independently vary the substrate and drop temperatures on a waxy superhydrophobic Nasturtium leaf. We find that, whenever the surface or the drop is above a microtexture-melting temperature, the drop sticks. Below this temperature, a critical temperature threshold for bouncing can be predicted and controlled by considering the relative timescales between condensation growth and drop residence time. We envision that these results can provide insight into the design of a new class of superhydrophobic surfaces to act as a rapid thermal fuse to prevent drops that exceed a critical temperature from bouncing onto a thermally sensitive target.
水滴撞击超疏水表面时会反弹。然而，在某些超疏水表面上，如果水滴足够热，它会粘附而不是反弹。在这里，我们旨在更好地理解导致这种反弹-粘附转变的机制。具体来说，我们模拟了两种潜在机制，在这两种机制中，超疏水表面可以在几毫秒内捕获足够热的水滴：微纹理蜡的熔化和超疏水纹理内蒸汽的凝结。然后，我们通过系统的水滴撞击实验来测试这些机制，在实验中，我们在蜡质超疏水旱金莲叶上独立改变基底和水滴的温度。我们发现，只要表面或水滴高于微纹理熔化温度，水滴就会粘附。在这个温度以下，可以通过考虑凝结增长和水滴停留时间之间的相对时间尺度来预测和控制反弹的临界温度阈值。我们设想，这些结果可以为设计一类新型超疏水表面提供见解，这类表面可作为快速热熔断器，防止超过临界温度的水滴反弹到热敏目标上。
