Superhydrophobic surfaces have been receiving increasing research interest in recent years since they may be useful for many applications such as the fabrication of self-cleaning and anti-icing surfaces or the design or micro-fluidic channels with reduced liquid-flow slippage. Since superhydrophobicity arises from both surface topography and surface chemistry, a wide library of structured surfaces having well-defined surface chemistries has been developed.

Controlled nanostructures have been fabricated in silicon by first making a structured polymer thin film (by polymer demixing or block-copolymer self assembly) and then transferring the structures in the underlying substrate using deep reactive ion etching (DRIE). The structures can be tuned first via the polymer self-assembly step which allows the control of the lateral size of the structures (from tens of nanometers to tens of micrometers) and their type (pits or pillars). Second, via the DRIE step which allows the control of the depth of the structures. To investigate the effect of nanostructuring on wettability, we focused on pillar arrays obtained combining polymer demixing and DRIE. The lateral size of the structures was 400nm and their depth was ranging from 200nm to 4μm. Other structures of 80nm in lateral size were also realized using self organized block copolymer micelles films as a mask and the depth of the structures reached 200nm.

Hydrophobic surfaces with controlled contact-angle hysteresis have been fabricated by either silanizing or plasma-coating the surfaces. The wettability of each surface has been characterized by measuring the water contact angles (advancing and receding) and the rolling angle.

These tunable nanostructured surfaces with well controlled surface chemistries allow the differentiation between different wetting concepts: the superhydrophobic modes (eg. Cassie-Baxter vs Wenzel); the dynamic contact-angle hysteresis and finally the self-cleaning properties (low rolling-off angles).

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