Superhydrophobicity, i.e. the ability of various surfaces to cause water droplets to bead up and roll off their surface, is a common observation in nature. In an attempt to develop bio-mimetic superhydrophobic surfaces, numerous researchers have investigated the superhydrophobic character of various natural surfaces, such as the lotus leaf and duck feathers. In their classic paper, Cassie and Baxter¹ noted, “The duck is generally regarded as having attained perfection in water-repellency, and it is usually taken for granted that the duck uses an oil or similar coating with larger contact angles than any known to man. In actual fact, the duck obtains its water-repellency from the structure of its feathers.”

The synergistic effects of surface texture and relatively low surface energy enable duck feathers to trap pockets of air underneath droplets of a high surface tension liquid like water (γ = 72.1 mN/m) and form a stable composite solid-liquid-air interface that resists wetting. However, the feathers natural surface chemistry and texture are insufficient to withstand the decrease in free energy arising from the spreading of lower surface tension fluids such as oils or alcohols. Thus, as is tragically evident after an oil-spill, low surface tension liquids like gasoline and rapeseed oil (γ = 35.7 mN/m) completely wet the surface of a duck feather.

Designing and producing textured surfaces that can resist wetting by such low surface tension fluids has been a significant challenge in materials science, and no examples of such materials exist in nature. In this work we develop various surfaces possessing re-entrant texture² that can support composite solid-liquid-air interfaces, even with extremely low surface tension liquids such as pentane (γ = 15.7 mN/m) - the lowest surface tension alkane in liquid state at atmospheric pressure. Further, to aid the systematic engineering of non-wetting surfaces, we develop four design parameters that allow us to provide an a priori estimation of both the apparent contact angles, as well as the robustness of the composite interface, obtained with a particular contacting liquid.

Based on these design parameters, we produce electrospun polymeric (poly methyl methacrylate - PMMA) fiber mats containing extremely low surface energy, fluoroalkyl-substituted silsequioxane molecules (FluoroPOSS).^2,3 Numerous fluoroPOSS molecules have recently been synthesized³ in which the rigid 2 nm silsesquioxane cage is surrounded by different organic groups including 1H,1H,2H,2H-tridecafluorooctyl (fluorooctyl POSS) and 1H,1H,2H,2H-heptadecafluorodecyl (referred to as fluorodecyl POSS). The fluorodecyl POSS molecules were found to have the lowest surface energy amongst the various POSS molecules synthesized and were used exclusively in this study.

The low surface energy of fluorodecyl POSS molecules, as well as their small size, enables them to migrate (bloom) to the fiber-air interface during the electrospinning process. As a consequence, electrospun fibers (with radius ~ 500 nm) containing as low as 5 vol% POSS can display extremely high contact angles (> 150°) and support a robust composite interface with a wide range of polar and non-polar liquids. We suggest the term ‘omniphobic', i.e. surfaces that resist wetting by all liquids, to describe such surfaces. In addition, we describe a simple ‘dip-coating' process that enables us to provide a conformal coating of PMMA - fluoroPOSS blends on any surface that inherently possesses re-entrant texture, such as duck feathers or commercially available fabrics. The synergistic effect of roughness, re-entrant texture of the preformed substrate, and the low surface energy of fluoroPOSS-PMMA blends, imbues omniphobicity to the dip-coated surfaces, enabling them to support a composite interface with even the lowest surface tension liquids.

References:

1. Cassie, A.B.D. & Baxter, S. Wettability of porous surfaces. Trans. Faraday Soc. 40, 546–551 (1944).

2. Tuteja, A. et al. Designing superoleophobic surfaces. Science 318 (5856), 1618-1622 (2007).

3. Mabry, Joseph M. et al. Fluorinated Polyhedral Oligomeric Silsesquioxanes (F-POSS). Ange. Chem. Int. Ed. 47 (22), 4137-4140 (2008).