When 11:45 AM - 1:00 PM Apr 09, 2009
Where 2150 HH Dow
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Designing Superoleophobic Surfaces


Anish Tuteja, MIT

Superhydrophobicity, the ability of various surfaces to cause water droplets to bead up and roll off, is a common observation in nature. In an attempt to develop bio-mimetic superhydrophobic surfaces, numerous research groups have investigated the superhydrophobic character of various natural surfaces, such as the lotus leaf and duck feathers. The synergistic effects of surface texture and relatively low surface energy enable superhydrophobic surfaces to trap pockets of air underneath droplets of a high surface tension liquid like water (glv = 72.1 mN/m), and form a stable composite solid-liquid-air interface that resists wetting. However, for the various natural superhydrophobic surfaces, the 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 (glv ~ 20 – 30 mN/m). Thus, low surface tension liquids like gasoline completely wet these natural surfaces.

 

Designing and producing textured surfaces that can resist wetting by low surface tension fluids has been a significant challenge in materials science, and no examples of such surfaces exist in nature. As part of this work, we explain how a third factor, re-entrant surface curvature (apart from surface chemistry and roughness), can be used to significantly enhance liquid repellency, by studying electrospun polymeric fibers containing extremely low surface energy, fluorinated molecules (fluorodecyl POSS). Increasing the fluorodecyl POSS concentration in the electrospun fibers, which inherently possess re-entrant curvature, allows us to transition the surface wetting characteristics systematically from hydrophilic to superhydrophobic and finally to the first ever superoleophobic surfaces, i.e. surfaces that display contact angles q*> 150° with various low surface tension oils.

 

Further, to aid the systematic engineering of non-wetting surfaces, we developed 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, supported with a given contacting liquid. Based on these design parameters, we also developed model micro-textured surfaces in silicon (called micro-hoodoos) that feature re-entrant curvature. The resulting structures are the most oleophobic surfaces ever produced, displaying pentane (glv = 15.7 mN/m) contact angles greater than 160° and low contact angle hysteresis.