Evaporation of Droplets on Superhydrophobic Surfaces: Surface Roughness and Small Droplet Size Effects

Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Hong Kong 999077, China.
Physical Review Letters (Impact Factor: 7.51). 09/2012; 109(11). DOI: 10.1103/PhysRevLett.109.116101


Evaporation of a sessile droplet is a complex, nonequilibrium phenomenon. Although evaporating droplets upon superhydrophobic surfaces have been known to exhibit distinctive evaporation modes such as a constant contact line (CCL), a constant contact angle (CCA), or both, our fundamental understanding of the effects of surface roughness on the wetting transition remains elusive. We show that the onset time for the CCL-CCA transition and the critical base size at the Cassie-Wenzel transition exhibit remarkable dependence on the surface roughness. Through global interfacial energy analysis we reveal that, when the size of the evaporating droplet becomes comparable to the surface roughness, the line tension at the triple line becomes important in the prediction of the critical base size. Last, we show that both the CCL evaporation mode and the Cassie-Wenzel transition can be effectively inhibited by engineering a surface with hierarchical roughness.

Download full-text


Available from: Xuemei Chen, Jul 07, 2015
61 Reads
  • Source
    • "We first designed and fabricated a hierarchical surface with nanograssed micro-truncated cone architecture (see Methods Section for details). The hierarchical surface was fabricated with a two-step process we developed previously3940. Briefly, the micro-truncated cone structure with an inclination angle of 54.7° was first created using an anisotropic wet-etching, and then nanograss arrays were etched on the whole surface using a modified deep reactive ion etching (DRIE) process4142434445. The top and base diameters of the truncated cones are ~55 and ~70 μm, respectively. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Despite extensive progress, current icephobic materials are limited by the breakdown of their icephobicity in the condensation frosting environment. In particular, the frost formation over the entire surface is inevitable as a result of undesired inter-droplet freezing wave propagation initiated by the sample edges. Moreover, the frost formation directly results in an increased frost adhesion, posing severe challenges for the subsequent defrosting process. Here, we report a hierarchical surface which allows for interdroplet freezing wave propagation suppression and efficient frost removal. The enhanced performances are mainly owing to the activation of the microscale edge effect in the hierarchical surface, which increases the energy barrier for ice bridging as well as engendering the liquid lubrication during the defrosting process. We believe the concept of harnessing the surface morphology to achieve superior performances in two opposite phase transition processes might shed new light on the development of novel materials for various applications.
    Scientific Reports 08/2013; 3:2515. DOI:10.1038/srep02515 · 5.58 Impact Factor
  • Physical Review Letters 09/2012; 109(12). DOI:10.1103/PhysRevLett.109.129904 · 7.51 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Surfaces with special wetting properties not only can efficiently repel or attract liquids such as water and oils but also can prevent formation of biofilms, ice, and clathrate hydrates. Predicting the wetting properties of these special surfaces requires detailed knowledge of the composition and geometry of the interfacial region between the droplet and the underlying substrate. In this work we introduce a 3D quantitative method for direct nanoscale visualization of such interfaces. Specifically, we demonstrate direct nano- to microscale imaging of complex fluidic interfaces using cryostabilization in combination with cryogenic focused ion beam milling and SEM imaging. We show that application of this method yields quantitative information about the interfacial geometry of water condensate on superhydrophilic, superhydrophobic, and lubricant-impregnated surfaces with previously unattainable nanoscale resolution. This type of information is crucial to a fundamental understanding as well as the design of surfaces with special wetting properties.
    ACS Nano 09/2012; 6(10):9326-34. DOI:10.1021/nn304250e · 12.88 Impact Factor
Show more