Robust omniphobic surfaces

Abstract

Superhydrophobic surfaces display water contact angles greater than 150 degrees in conjunction with low contact angle hysteresis. Microscopic pockets of air trapped beneath the water droplets placed on these surfaces lead to a composite solid-liquid-air interface in thermodynamic equilibrium. Previous experimental and theoretical studies suggest that it may not be possible to form similar fully-equilibrated, composite interfaces with drops of liquids, such as alkanes or alcohols, that possess significantly lower surface tension than water (gamma(lv) = 72.1 mN/m). In this work we develop surfaces possessing re-entrant texture that can support strongly metastable composite solid-liquid-air interfaces, even with very low surface tension liquids such as pentane (gamma(lv) = 15.7 mN/m). Furthermore, we propose four design parameters that predict the measured contact angles for a liquid droplet on a textured surface, as well as the robustness of the composite interface, based on the properties of the solid surface and the contacting liquid. These design parameters allow us to produce two different families of re-entrant surfaces- randomly-deposited electrospun fiber mats and precisely fabricated microhoodoo surfaces-that can each support a robust composite interface with essentially any liquid. These omniphobic surfaces display contact angles greater than 150 degrees and low contact angle hysteresis with both polar and nonpolar liquids possessing a wide range of surface tensions.

Fig. 1.

Critical role of re-entrant texture. (A and B) Droplets of water (colored with methylene blue) and rapeseed oil (colored with oil red O) on a duck feather. (C and D) Schematic diagrams illustrating possible liquid-vapor interfaces on two different surfaces having the same solid surface energy and the same equilibrium contact angle (θ), but different geometric angles (ψ). (E) An SEM micrograph of an electrospun surface containing 44.4 wt% fluorodecyl POSS and possessing the beads-on-strings morphology. The inset shows the molecular structure of fluorodecyl POSS molecules. The alkyl chains (R_f) have the molecular formula −CH₂CH₂(CF₂)₇CF₃. (F) An SEM micrograph of a microhoodoo surface (with W = 10 μm, D = 20 μm and H = 7 μm). The samples are viewed from an oblique angle of 30°.

Fig. 2.

Design parameters for a robust composite interface. (A) A schematic illustration of the electrospun surface, highlighting the expected liquid-vapor interface with a liquid having an equilibrium contact angle θ < 90°. The important surface texture parameters R, D, h₁, and h₂ are also shown. R_sag is the radius of curvature of the sagging composite interface. The electrospun surface typically possesses lower values of the robustness parameter H* in comparison to the parameter T*. (B) A schematic illustration of the microhoodoo surface with a small pore-depth (h₂). The important surface-texture parameters, R, D, H, and W, shown in the figure, can be varied independently. Such a microhoodoo surface typically possesses lower values of H* in comparison to T*. (C) A schematic illustration of the microhoodoo surface with a larger pore-depth (h₂) developed by increasing the microhoodoo height (H). Such a microhoodoo surface typically possesses lower values of T* in comparison to H*.

Fig. 3.

Imbuing oleophobicity to natural surfaces. (A) Droplets of rapeseed oil (γ_lv = 35.7 mN/m), colored with oil red O, on a duck feather dip-coated in a solution of fluorodecyl POSS. (B) Droplets of octane (γ_lv = 21.7 mN/m) on a lotus leaf dip-coated in a solution of fluorodecyl POSS. (C) Droplets of water (γ_lv = 72.1mN/m), methylene iodide (γ_lv = 50.1 mN/m), methanol (γ_lv = 22.7 mN/m), and octane (γ_lv = 21.7 mN/m) on a lotus leaf surface covered with electrospun fibers (beads-on-strings morphology) of PMMA + 44 wt% fluorodecyl POSS. A reflective surface is visible underneath all droplets, indicating the presence of microscopic pockets of air and the formation of a composite interface (21).

Fig. 4.

Controlling the morphology of electrospun surfaces. (A–C) SEM micrographs of the various electrospun fabric textures for the PMMA + fluorodecyl POSS − 44wt% blend, produced by varying the concentration of the electrospinning solution. The insets show droplets (droplet volume V ∼ 2 μl) of hexadecane (γ_lv = 27.5 mN/m; θ = 80°) on each electrospun surface.

Fig. 5.

Omniphobicity of electrospun fabrics. The apparent advancing (filled symbols) and receding (hollow symbols) contact angles as a function of liquid surface tension for the beads-only, beads-on-strings, and fibers-only electrospun surfaces, respectively. The surfaces contain either 16.7 wt% or 44.4 wt% fluorodecyl POSS.

Fig. 6.

Omniphobicity of microhoodoo arrays. (A) The apparent advancing and receding contact angles on a silanized microhoodoo surface. The inset shows droplets of heptane (red), methanol (green), and water (blue) on the microhoodoo surface. (B) A series of images obtained using a high-speed digital video camera that illustrates the bouncing of a droplet of hexadecane on a silanized microhoodoo surface. (C) A series of images (obtained over a period of 5 min), showing the evaporation of a droplet of methanol under ambient conditions, on a microhoodoo surface. (Scale bar, 1 mm). (D) A master curve showing the measured (filled symbols; denoted −M in the legend) breakthrough pressures for a number of microhoodoo and electrospun surfaces with various alkanes and alcohols, scaled with the breakthrough pressure of octane on the electrospun beads-only surface containing 44.4 wt% POSS, as a function of the robustness factor A*. Our predictions (hollow symbols; denoted −P in the legend) for the breakthrough pressures are also shown.