DOI: 10.1002/adma.200700030 Superhydrophobic Surfaces Generated from Water-Borne Dispersions of Hierarchically Assembled Nanoparticles Coated with a Reversibly Switchable Shell** By Mikhail Motornov , Roman Sheparovych, Robert Lupitskyy , Emily MacWilliams , and Sergiy Minko* The objective of this research was the fabrication of super- hydrophobic surfaces by deposition (casting) of the aggre- gates of nanoparticles from water-born solutions with no sur- factant application. This approach to superhydrophobic surfaces is important for technologies which avoid organic sol- vents, surfactants, and applications of complex methods (e.g., lithography, microprinting, micromolding, etc.) for the fabri- cation of textured functional surfaces. The casting of textured coatings from water-born dispersions is a non-toxic and envi- ronmentally friendly method that can be conducted without expensive equipment and tools. The goal of this research was achieved by the fabrication of aqueous dispersions of hybrid responsive nanoparticles. The hybrid particles consisted of a silica core with a grafted mixed block copolymer brush of poly(styrene-block-4-vinylpyridine) P(S-b-4VP) constituting the responsive particle shell. The re- sponsive shell was used to tune and stabilize the secondary ag- gregates of the particles of the appropriate size and morphol- ogy in an aqueous environment. The suspension of the particles formed a textured hydrophilic coating on various substrates upon casting and evaporation of water. Heating the coating above the glass transition temperature of polystyrene (PS) resulted in production of the superhydrophobic material. Superhydrophobic surfaces have received much attention due to their important applications ranging from self-cleaning materials to microfluidic devices. A superhydrophobic surface has a very high advancing water contact angle, typically of 150° or higher, and a very low contact angle hysteresis, that is a very low sliding angle, typically below 15°. [1,2] The wettabili- ty of a solid surface is governed by the combination of two factors: the surface chemical composition and the surface tex- ture. [1,3–7] The surface roughness amplifies the hydrophobic behavior of hydrophobic surfaces due to two possible mecha- nisms: the Wenzel regime [6] (homogeneous wetting of a rough surface when the wetting liquid (water) penetrates surface cavities) and the Cassie regime [7] (heterogeneous wetting of a rough surface when air is trapped beneath a droplet of water and water escapes from cavities, crevices, and grooves occu- pied by trapped air). In the Wenzel regime, the water contact angle and the contact angle hysteresis increase with surface roughness resulting in a high roll-off angle. This surface is not a superhydrophobic surface because of the high wetting hys- teresis. A low wetting hysteresis is indeed obtained in the Cas- sie regime for a “composite” surface with trapped air in the surface grooves. The transition between the Wenzel regime and the Cassie regime depends on the intrinsic contact angle, as measured on the reference smooth surface of the same chemical composition as for the rough sample, and the surface texture. McCarthy and Oner [1,2] demonstrated experimentally that the Cassie regime can be approached for characteristic dimensions of surface features of the rough substrate in the range of 2–32 lm for the dominant in plane length. Investigations of superhydrophobic surfaces of plants and insects suggested that complex (fractal) surfaces with hier- archical texture are even more effective in generating super- hydrophobic wetting behavior since nanobubles of air can decorate the rough surface (surfaces with nanogrooves). [5,8] This structure results in a substantial decrease of the fraction of the solid-liquid interface. Superhydrophobic surfaces were fabricated using two major approaches. The first approach was to create a rough structure on a hydrophobic surface, [1,2,9–24] and the second approach was to decrease the surface energy of a rough surface by chemically bonding low surface-energy species to the sur- face. [9,20–23,25–27] Several variations of these approaches exist which involve the formation of superhydrophobic surfaces by deposition of micelles of block copolymers, [28] containing polydimethylsiloxane (PDMS) blocks, [29,30] as well as by the use of the LBL deposition of a mixture of polyelectrolyte-par- ticles with further treatment with organosilanes. [30–33] The sur- face morphology was generated either by deposition of nano- particles [29,30,34,35] to enhance the roughness of the coatings or by the treatment with selective solvents which provide, at the same time, surface migration of low surface energy blocks of the block copolymers. [28,30] Many of abovementioned methods for fabrication of super- hydrophobic surfaces, however, have limited practical applica- tion because most of the preparations involve etching, plasma treatment, chemical vapor deposition, electrodeposition, cal- COMMUNICATION 200 © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008, 20, 200–205 – [*] Prof. S. Minko, Dr. M. Motornov, R. Sheparovych, R. Lupitskyy, E. MacWilliams Department of Chemistry and Biomolecular Science NY Center for Advanced Materials Processing, Clarkson University 8 Clarkson Ave., Potsdam, NY 13699-5810 (USA) E-mail: sminko@clarkson.edu [**] We acknowledge financial support from the NSF award CTS 0456548, NTC award CO4-CL06, and ARO award W911NF-05-1- 0339. Supporting Information is available online from Wiley Inter- Science or from the authors.