Fog represents a large, untapped source of potable water, especially in arid climates. Numerous plants and animals use textural as well as chemical features on their surfaces to harvest this precious resource. In this work, we investigate the influence of surface wettability characteristics, length scale, and weave density on the fog harvesting capability of woven meshes. We develop a combined hydrodynamic and surface wettability model to predict the overall fog collection efficiency of the meshes and cast the findings in the form of a design chart.Two limiting surface wettability constraints govern re-entrainment of collected droplets and clogging of mesh openings. Appropriate tuning of the wetting characteristics of the surfaces, reducing the wire radii, and optimizing the wire spacing all lead to more efficient fog collection.We use a family of coated meshes with a directed stream of fog droplets to simulate a natural foggy environment and demonstrate a five-fold enhancement in the fog-collecting efficiency of a conventional polyolefin mesh. The design rules developed in this work can be applied to select a mesh surface with optimal topography and wetting characteristics to harvest enhanced water fluxes over a wide range of natural convected fog environments.

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Designing multifunctional surfaces that have user-specified interactions with impacting liquids and with incident light is a topic of both fundamental and practical significance. Taking cues from nature, we use tapered conical nanotextures to fabricate the multifunctional surfaces; the slender conical features result in large topographic roughness, while the axial gradient in the effective refractive index minimizes reflection through adiabatic index-matching between air and the substrate. Precise geometric control of the conical shape and slenderness of the features as well as periodicity at the nanoscale are all keys to optimizing the multifunctionality of the textured surface, but at the same time, these demands pose the toughest fabrication challenges. Here we report a systematic approach to concurrent design of optimal structures in the fluidic and optical domains and a fabrication procedure that achieves the desired aspect ratios and periodicities with few defects and large pattern area. Our fabricated nanostructures demonstrate structural superhydrophilicity or, in combination with a suitable chemical coating, robust superhydrophobicity. Enhanced polarization-independent optical transmission exceeding 98% has also been achieved over a broad range of bandwidth and incident angles. These nanotextured surfaces are also robustly antifogging or self-cleaning, offering potential benefits for applications such as photovoltaic solar cells.

We estimate the effective Navier-slip length for flow over a spray-fabricated liquid-repellent surface which supports a composite solid-air-liquid interface or 'Cassie-Baxter' state. The morphology of the coated substrate consists of randomly distributed corpuscular microstructures which encapsulate a film of trapped air (or 'plastron') upon contact with liquid. The reduction in viscous skin friction due to the plastron is evaluated using torque measurements in a parallel plate rheometer resulting in a measured slip length of b slip z 39 mm, comparable to the mean periodicity of the microstructure evaluated from confocal fluorescence microscopy. The introduction of a large primary length-scale using dual-textured spray-coated meshes increases the magnitude of the effective slip length to values in the range 94 mm # b slip # 213 mm depending on the geometric features of the mesh. The wetted solid fractions on each mesh are calculated from free surface simulations on model sinusoidal mesh geometries. The trend in measured values of b slip with the mesh periodicity L and the computed wetted solid-fraction rf s are found to be consistent with existing analytic predictions.

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