Department of Mechanical Engineering, Boston University, Boston, MA 02215.
Department of Mechanical Engineering, Boston University, Boston, MA 02215
Proc Natl Acad Sci U S A. 2017 Jul 3;114(27):6930-6935. doi: 10.1073/pnas.1700197114. Epub 2017 Jun 19.
The ability to enhance or limit heat transfer between a surface and impacting drops is important in applications ranging from industrial spray cooling to the thermal regulation of animals in cold rain. When these surfaces are micro/nanotextured and hydrophobic, or superhydrophobic, an impacting drop can spread and recoil over trapped air pockets so quickly that it can completely bounce off the surface. It is expected that this short contact time limits heat transfer; however, the amount of heat exchanged and precise role of various parameters, such as the drop size, are unknown. Here, we demonstrate that the amount of heat exchanged between a millimeter-sized water drop and a superhydrophobic surface will be orders of magnitude less when the drop bounces than when it sticks. Through a combination of experiments and theory, we show that the heat transfer process on superhydrophobic surfaces is independent of the trapped gas. Instead, we find that, for a given spreading factor, the small fraction of heat transferred is controlled by two dimensionless groupings of physical parameters: one that relates the thermal properties of the drop and bulk substrate and the other that characterizes the relative thermal, inertial, and capillary dynamics of the drop.
增强或限制表面和撞击液滴之间的传热能力在从工业喷雾冷却到寒冷雨中动物的热调节等应用中非常重要。当这些表面具有微/纳米结构和疏水性或超疏水性时，撞击液滴可以迅速扩散并在被困的气穴上反弹，以至于它可以完全从表面弹开。预计这种短接触时间会限制传热；然而，热交换的量以及各种参数的精确作用，例如液滴大小，尚不清楚。在这里，我们证明当液滴弹开时，毫米大小的水滴与超疏水表面之间交换的热量将减少几个数量级，而不是当液滴粘住时。通过实验和理论的结合，我们表明超疏水表面上的传热过程与被困气体无关。相反，我们发现，对于给定的扩展因子，传递的热量的一小部分由两个无量纲物理参数分组控制：一个与液滴和基体的热性质有关，另一个与液滴的相对热、惯性和毛细动力学有关。
