Laboratory of Thermodynamics in Emerging Technologies, Department of Mechanical and Process Engineering, Eidgenössische Technische Hochschule Zürich, CH-8092 Zurich, Switzerland.
Laboratory of Thermodynamics in Emerging Technologies, Department of Mechanical and Process Engineering, Eidgenössische Technische Hochschule Zürich, CH-8092 Zurich, Switzerland
Proc Natl Acad Sci U S A. 2017 Oct 17;114(42):11040-11045. doi: 10.1073/pnas.1705952114. Epub 2017 Sep 25.
Spontaneous removal of liquid, solidifying liquid and solid forms of matter from surfaces, is of significant importance in nature and technology, where it finds applications ranging from self-cleaning to icephobicity and to condensation systems. However, it is a great challenge to understand fundamentally the complex interaction of rapidly solidifying, typically supercooled, droplets with surfaces, and to harvest benefit from it for the design of intrinsically icephobic materials. Here we report and explain an ice removal mechanism that manifests itself simultaneously with freezing, driving gradual self-dislodging of droplets cooled via evaporation and sublimation (low environmental pressure) or convection (atmospheric pressure) from substrates. The key to successful self-dislodging is that the freezing at the droplet free surface and the droplet contact area with the substrate do not occur simultaneously: The frozen phase boundary moves inward from the droplet free surface toward the droplet-substrate interface, which remains liquid throughout most of the process and freezes last. We observe experimentally, and validate theoretically, that the inward motion of the phase boundary near the substrate drives a gradual reduction in droplet-substrate contact. Concurrently, the droplet lifts from the substrate due to its incompressibility, density differences, and the asymmetric freezing dynamics with inward solidification causing not fully frozen mass to be displaced toward the unsolidified droplet-substrate interface. Depending on surface topography and wetting conditions, we find that this can lead to full dislodging of the ice droplet from a variety of engineered substrates, rendering the latter ice-free.
从表面自发去除液体、固化液体和固体物质的形式，在自然界和技术中具有重要意义，其应用范围从自清洁到防冰和冷凝系统。然而，从根本上理解快速固化、通常过冷的液滴与表面的复杂相互作用，并从中受益，从而设计出固有的防冰材料，这是一个巨大的挑战。在这里，我们报告并解释了一种冰去除机制，该机制与冻结同时发生，驱动通过蒸发和升华（低环境压力）或对流（大气压力）从基底冷却的液滴逐渐自行脱落。成功自行脱落的关键在于液滴自由表面处的冻结和液滴与基底的接触区域处的冻结不会同时发生：冻结相界面从液滴自由表面向内移动到液滴-基底界面，在整个过程中大部分保持液态，并在最后冻结。我们通过实验观察到，并通过理论验证，相界面在靠近基底处的向内运动导致液滴-基底接触逐渐减少。同时，由于液滴不可压缩、密度差异以及向内凝固导致不完全冻结的质量向未凝固的液滴-基底界面移动的不对称凝固动力学，液滴从基底上抬起。根据表面形貌和润湿条件，我们发现这可能导致各种工程基底上的冰滴完全脱落，从而使后者无冰。
