Activating the Microscale Edge Effect in a Hierarchical Surface for Frosting Suppression and Defrosting Promotion

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Surface-enhanced Raman scattering (SERS) technique with naturally born analyte identification capability can achieve ultrahigh sensitivity. However, the sensitivity and quantification capability of SERS are assumed to be mutually exclusive. Here, we prohibit the formation of the ultrasensitive SERS sites to achieve a high quantification capability through separating the gold (Au) nanorods from approaching each other with thick metal organic framework (MOF) shells. The sensitivity decrease caused by the absence of the ultrasensitive SERS sites is compensated by the analyte enrichment function of a slippery surface. The porous MOF shell around the Au nanorod only allows analytes smaller than the pore size to approach the Au nanorods and contribute to the SERS spectrum within the complex sample, greatly enhancing the analyte identification capability. Overall, we have demonstrated an integrated SERS platform with analyte enrichment and analyte filtration function, realizing sensitive, quantitative, and size selective analyte identification in complex environments.

Quantitative and Sensitive SERS Platform with Analyte Enrichment and Filtration Function

Evaporation modes and kinetics of sessile droplets of water on micropillared superhydrophobic surfaces are experimentally investigated. The results show that a constant contact radius (CCR) mode and a constant contact angle (CCA) mode are two dominating evaporation modes during droplet evaporation on the superhydrophobic surfaces. With the decrease in the solid fraction of the superhydrophobic surfaces, the duration of a CCR mode is reduced and that of a CCA mode is increased. Compared to Rowan's kinetic model, which is based on the vapor diffusion across the droplet boundary, the change in a contact angle in a CCR (pinned) mode shows a remarkable deviation, decreasing at a slower rate on the superhydrophobic surfaces with less-solid fractions. In a CCA (receding) mode, the change in a contact radius agrees well with the theoretical expectation, and the receding speed is slower on the superhydrophobic surfaces with lower solid fractions. The discrepancy between experimental results and Rowan's model is attributed to the initial large contact angle of a droplet on superhydrophobic surfaces. The droplet geometry with a large contact angle results in a narrow wedge region of air along the contact boundary, where the liquid-vapor diffusion is significantly restricted. Such an effect becomes minor as the evaporation proceeds with the decrease in a contact angle. In both the CCR and CCA modes, the evaporative mass transfer shows the linear relationship between mass(2/3) and evaporation time. However, the evaporation rate is slower on the superhydrophobic surfaces, which is more significant on the surfaces with lower solid fractions. As a result, the superhydrophobic surfaces slow down the drying process of a sessile droplet on them.

Evaporation kinetics of sessile water droplets on micropillared superhydrophobic surfaces.

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.

Pinning and depinning mechanism of the contact line during evaporation of nano-droplets sessile on textured surfaces

Electrowetting on liquid-infused film (EWOLF): Complete reversibility and controlled droplet oscillation suppression for fast optical imaging

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Electrowetting on liquid-infused film (EWOLF): complete reversibility and controlled droplet oscillation suppression for fast optical imaging.

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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Evaporation of droplets on superhydrophobic surfaces: surface roughness and small droplet size effects.

Publisher’s Note: Evaporation of Droplets on Superhydrophobic Surfaces: Surface Roughness and Small Droplet Size Effects [Phys. Rev. Lett. 109 , 116101 (2012)]

B. L. Luk

Papers

Evaporation of droplets on superhydrophobic surfaces: surface roughness and small droplet size effects.

Publisher’s Note: Evaporation of Droplets on Superhydrophobic Surfaces: Surface Roughness and Small Droplet Size Effects [Phys. Rev. Lett. 109 , 116101 (2012)]