Evaporation of Droplets on Superhydrophobic Surfaces: Surface Roughness and Small Droplet Size Effects
ABSTRACT 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.
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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. · 5.08 Impact Factor
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ABSTRACT: The contact line dynamics during the evaporation of nano-droplets sessile on textured solid substrates is investigated using molecular dynamics method. Our simulations reproduce the nanoscale physical phenomena such as the constant contact angle (CCA) and constant contact radius (CCR) evaporation modes which have been reported in recent experiments. The results also show that the localized liquid–solid interaction in the vicinity of the contact line dominates the wetting behavior on the textured surfaces, which also indicates a molecular origin of the pinning force. Increase of the characteristic width of the inhomogeneity enlarges the energy barrier to jumping out a hydrophilic textured element, which further results in a larger pinning force. For the evaporation of a nano-droplet on the surface with large pinning force, the anchoring of the contact line and the CCR mode are observed at the first onset. While on the textured surfaces with smaller characteristic width of the inhomogeneity, only the CCA and mixed modes can be found during the evaporation. This work provides a foundation on which the sessile nano-droplet evaporation can be understood from atomistic scale.Soft Matter 05/2013; 9(24):5703-5709. · 4.15 Impact Factor
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ABSTRACT: Vibrations of small (microlitre) sessile liquid droplets were studied using a simple optical deflection technique. The droplets were made to elongate in one direction by taking advantage of the anisotropic wetting of the liquids on structured diffraction grating surfaces. They were vibrated by applying a puff of nitrogen gas. Motion of the droplets was monitored by scattering laser light from their surfaces. The scattered light was collected using a photodiode and the resulting time-dependent intensity signals were Fourier transformed to obtain the vibrational response of the drops. The vibrational spectra of elongated sessile drops were observed to contain two closely spaced peaks. A simple model which considers the frequency of capillary wave fluctuations on the surfaces of the drops was used to show that the vibrational frequencies of these peaks correspond to standing wave states that exist along the major and minor profile lengths of the droplets.Langmuir 03/2013; · 4.38 Impact Factor