Superhydrophobic polymer-particle composite films produced using various particle sizes Ioannis Karapanagiotis, a * Panagiotis N. Manoudis, a Achilleas Savva b and Costas Panayiotou b Hydrophilic alumina (Al 2 O 3 ) nanoparticles (25, 35, and 150 nm) are dispersed in different concentrations in solutions of a commercial hydrophobic poly(alkyl siloxane) (Silres BS-290), and the suspensions are sprayed on glass surfaces. Static contact angles (θ S ), measured on surfaces of siloxane-nanoparticle composite films that were prepared from dilute dispersions, increase rapidly with particle concentration. Composite films prepared from concentrated dispersions exhibit a maximum, constant θ S (at saturation θ S is 160  ), which is not affected by the size of the particles. These films exhibit also very small contact angle hysteresis (5  ), which is also independent of the particle size. Consequently, the same superhydrophobic character can be induced in siloxane films using nanoparticles, which can range from a few up to several tenths of nanometers. However, the particle size and more precisely the particle specific surface area affects dramatically the minimum critical particle concentration, which must be used in the dispersions to induce superhydrophobicity on the surface of the composite films, that is, to achieve θ S = 150  . It is shown that critical particle concentration decreases exponentially with specific surface area. This result can be important for manufacturers of superhydrophobic surfaces who are interested in having a good control on the wettability of the composite films. Copyright © 2012 John Wiley & Sons, Ltd. Keywords: superhydrophobic surface; water repellency; nanoparticle; contact angle Introduction Superhydrophobic surfaces have attracted considerable attention because of their various potential applications. According to the ISI database, the number of published articles related to “superhydrophobic” in the last 5 years has increased from 183 in 2006 to 658 in 2010. [1] Inspired by the remarkable ability of the lotus leaf to repel water droplets, [2] several elegant methods have been developed to produce water repellent surfaces, including for instance, wet chemical reaction, [3] hydrothermal reaction, [4] electro- chemical deposition, [5] self-assembly, [6] layer-by-layer, [7] soft lithography, [8] chemical vapor deposition, [9] sol–gel, [10] polymerization reaction, [11] electrospinning, [12] and nanopar- ticle deposition, which attracted considerable attention. [13–30] Nanoparticles are used in the present study to induce superhydrophobicity. The aforementioned investigations are just a few examples of the enormous progress that has been achieved in the past decade in the production of surfaces with enhanced water-repellent properties, as described in several review articles (e.g. [1,31,32] ). Because a very promising application of the superhydrophobic materials is their use as protective coatings on outdoor surfaces against the undesir- able effects of the surrounding atmosphere (e.g. degradation effects of rainwater, airborne contaminants, etc.), the develop- ment of cost-effective methods to induce water repellency on large surfaces of different geometries and chemical composi- tions is very important. Therefore, spraying techniques, which are simple, cost effective, and can be easily used to treat large surfaces, have been suggested by many research groups— including ours—to produce superhydrophobic coatings on a large variety of surfaces. [13,14,16,23,24,26,29,30,33–36] According to our suggestion, silicon or metal oxide nanoparticles are dispersed in polymer (siloxane or acrylic) solu- tion in appropriate concentrations. [14,16] Dispersions are then sprayed on a surface, forming a polymer-particle composite film that has superhydrophobic properties because its surface exhi- bits a two-length-scaled hierarchical structure. This simple method has been successfully used to impart superhydrophobi- city on various surfaces such as aluminum, concrete, glass, silicon, silk, wood, [16] and several types of stones used in monuments and outdoor objects of the cultural heritage. [37–39] Materials with water-repellent properties, such as for instance surfactant- synthesized ormosils, [40,41] can be extremely useful in the conser- vation of stone monuments. However, the effect of the particle size on the hydrophobic properties of the composite polymer-particle films has not been investigated in our previous works; this is studied herein. Alumina (Al 2 O 3 ) particles, available in three different sizes, are dispersed in poly(alkyl siloxane) solutions in various concentrations. The wettability of the produced surfaces is investigated with contact angle measurements. * Correspondence to: Ioannis Karapanagiotis, Department of Management and Conservation of Ecclesiastical Cultural Heritage Objects, University Ecclesiastical Academy of Thessaloniki, N. Plastira 65, Thessaloniki 54250, Greece. E-mail address: y.karapanagiotis@aeath.gr a Department of Management and Conservation of Ecclesiastical Cultural Heritage Objects, University Ecclesiastical Academy of Thessaloniki, Thessalo- niki 54250, Greece b Department of Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece Surf. Interface Anal. (2012) Copyright © 2012 John Wiley & Sons, Ltd. Research article Received: 17 September 2011 Revised: 29 December 2011 Accepted: 12 February 2012 Published online in Wiley Online Library (wileyonlinelibrary.com) DOI 10.1002/sia.4930