Durable, optically transparent, superhydrophobic polymer films

Transparent and superhydrophobic free-standing films would be useful for extending non-wetting properties to optical applications especially those exposed to flowing water. However two challenges, transparency and durability, have proven difficult to overcome. Although a high degree of surface roughness is essential to sustain droplets of water in the Cassie state, this same roughness can lead to significant scattering and loss of optical clarity. Reducing roughness to a length scale below 1/4 the wavelength of light reduces scattering, but can result in loss of durability. In this paper we describe a novel and simple lamination approach for treating free-standing films to become superhydrophobic, with high transparency and durability, by partially embedding hydrophobic silica nanoparticles into the surface of a cyclic olefin polymer (COP) film. Partial embedding insures that the particles are firmly adhered while their nano-scale roughness is exposed, resulting in excellent superhydrophobicity (contact angles > 160 degrees and roll off angle < 10 degrees), high transparency (over 87% transmission at 500 nm) and durability under flowing water (> 6 h at volumetric flow rate of 70 L.min(-1)). The effect of processing parameters including the particle coaling thickness and lamination conditions on the optical properties, hydrophobicity as well as mechanical durability of films are systematically studied.