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.

Compared to the significant body of work devoted to surface engineering for promoting dropwise condensation heat transfer of steam, much less attention has been dedicated to fluids with lower interfacial tension. A vast array of low-surface tension fluids such as hydrocarbons, cryogens, and fluorinated refrigerants are used in a number of industrial applications, and the development of passive means for increasing their condensation heat transfer coefficients has potential for significant efficiency enhancements. Here we investigate condensation behavior of a variety of liquids with surface tensions in the range of 12 to 28 mN/m on three types of omniphobic surfaces: smooth oleophobic, re-entrant superomniphobic, and lubricant-impregnated surfaces. We demonstrate that although smooth oleophobic and lubricant-impregnated surfaces can promote dropwise condensation of the majority of these fluids, re-entrant omniphobic surfaces became flooded and reverted to filmwise condensation. We also demonstrate that on the lubricant-impregnated surfaces, the choice of lubricant and underlying surface texture play a crucial role in stabilizing the lubricant and reducing pinning of the condensate. With properly engineered surfaces to promote dropwise condensation of low-surface tension fluids, we demonstrate a four to eight-fold improvement in the heat transfer coefficient.

We describe a simple technique to prepare superhydrophobic and superoleophobic micro-textured surfaces by spray coating a blend of poly(methyl methacrylate) (PMMA) and the low surface energy molecule 1H,1H,2H,2H-heptadecafluorodecyl polyhedral oligomeric silsesquioxane (fluorodecyl POSS, γ sv ≈ 10 mN/m) using an air brush with a pressurized nitrogen stream. Scanning electron micrographs show the formation of micro-textured surfaces possessing re-entrant curvature; a critical feature for obtaining liquid repellency with low surface tension liquids. The surface morphology can be tuned systematically from a corpuscular or spherical microstructure to a beads-on-string structure and finally to bundled fibers by controlling the solution concentration and molecular weight of the sprayed polymer. The oleophobicity of the resulting structures is characterized by advancing and receding contact angle measurements with liquids of a range of surface tensions.

We demonstrate a reduction in the measured inner wall shear stress in moderately turbulent Taylor-Couette flows by depositing sprayable superhydrophobic microstructures on the inner rotor surface. The magnitude of reduction becomes progressively larger as the Reynolds number increases up to a value of 22% at Re ¼ 8.0 × 10 4 . We show that the mean skin friction coefficient C f in the presence of the superhydrophobic coating can be fitted to a modified Prandtl-von Kármán-type relationship of the form ðC f =2Þ1=2 from which we extract an effective slip length of b ≈ 19 μm. The dimensionless effective slip length b þ ¼ b=δ ν , where δ ν is the viscous length scale, is the key parameter that governs the drag reduction and is shown to scale as b þ ∼ Re 1=2 in the limit of high Re. DOI: 10.1103/PhysRevLett.114.014501 PACS numbers: 47.85.lb, 47.27.N-, 68.08.Bc, 83.50.Rp It is well known that superhydrophobic (SH) surfaces can reduce drag in laminar flows by presenting an effective wall slip boundary condition due to stable pockets of vapor within the asperities of the textured substrate [1,2]. The trapped vapor layer adjacent to the solid wall lubricates the fluid flow by introducing an effective slip boundary condition along portions of the wall, and consequently reduces the overall drag [3]. The magnitude of the effective slip length b in viscous laminar flows is governed by the surface feature length scale and the wetted solid fraction [4][5][6]. Measurements in various microchannel flows [7][8][9][10][11] yield values of b that are typically in the range of 10-30 μm.There is less consensus on the extent to which microscopic effective slip can influence macroscopic skin friction in turbulent flows [12,13]. Numerical simulations of turbulent channel flows indicate that the shear-free liquid-vapor interface can reduce skin friction by introducing an effective slip velocity in the viscous sublayer [14,15], and by the suppression of turbulent flow structures in the near-wall region [16]. While recent experimental studies report varying amounts of drag reduction in turbulent flows using SH surfaces [17][18][19], there are inconsistencies in the magnitude of observed drag reduction across studies, and its dependence on the slip length, surface characteristics, and Reynolds number in turbulent flow remain unclear.In this Letter, we demonstrate sustained reduction in frictional drag in turbulent Taylor-Couette (TC) flows by applying a polymeric SH coating to the inner rotor. The extent of drag reduction DR ¼ 100 × ðT flat − T SH Þ=T flat based on the inner rotor torque T , steadily increases with Re up to 22% at Re ¼ 80 000. The reduction in friction arises from finite slip effects at the moving rotor. The two key results we describe in our Letter are (i) the magnitude of drag reduction is directly related to a dimensionless slip length b þ ≡ b=δ ν , which couples the effective slip length b to the viscous length scale δ ν ¼ ν=u τ ¼ ν ffiffiffiffiffiffiffiffi ffi ρ=τ i p of the turbulent flow (where ν is the kinematic v...

We develop new criteria that describe the minimum concentration limits controlling the spinnability of dilute and semi-dilute flexible polymer solutions with high molecular weight and varying polydispersity. By asserting that the finite and bounded extensional viscosity of the solution is the key material property determining the stability of a filament during spinning, we propose a new scaling relating the minimum necessary concentration of a polymer c spin to its molecular weight M and the quality of the solvent (through the excluded volume exponent n) of the form c spin ⇠ M (n+1) . This new scaling differs from the classical interpretation of the coil overlap concentration c ⇤ or entanglement concentration c e as the minimum concentration required to increase the viscosity of the spinning dope, and rationalizes the surprising spinnability of high molecular weight polymers at concentrations much lower than c e .Furthermore, we introduce the concept of an extensibility average molecular weight M L as the appropriate average for the description of polydisperse solutions undergoing an extensiondominated spinning process. In particular it is shown that this extensibility average measure, and thus the solution spinnability, is primarily determined by the extensibility of the highest molecular weight fractions. For highly polydisperse systems this leads to an effective lowering of the minimum required concentration for successful fiber spinning (in comparison to narrowly distributed polymer solutions of similar weight average molecular weights). These predictions are validated with experimental observations of the electrospinnablity of monoand polydisperse poly(methyl methacrylate) (PMMA) solutions as well as a model bimodal blend, and through comparison to published literature data on the minimum spinnable polymer concentration for a variety of flexible long chain polymers over a range of molecular weights.