Preparation of a superhydrophobic surface by mixed inorganic-organic coatingMichele Ferrari, Francesca Ravera, and Libero Liggieri

Citation: Applied Physics Letters 88, 203125 (2006); doi: 10.1063/1.2205725 View online: http://dx.doi.org/10.1063/1.2205725 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/88/20?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Structural tunability and switchable exciton emission in inorganic-organic hybrids with mixed halides J. Appl. Phys. 114, 233511 (2013); 10.1063/1.4851715 Facile creation of bio-inspired superhydrophobic Ce-based metallic glass surfaces Appl. Phys. Lett. 99, 261905 (2011); 10.1063/1.3672036 In situ, noninvasive characterization of superhydrophobic coatings Rev. Sci. Instrum. 82, 045109 (2011); 10.1063/1.3579498 Direct nanoimprint of inorganic-organic hybrid glass J. Vac. Sci. Technol. B 24, 1402 (2006); 10.1116/1.2201457 Transparent hybrid inorganic/organic barrier coatings for plastic organic light-emitting diode substrates J. Vac. Sci. Technol. A 23, 971 (2005); 10.1116/1.1913680

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APPLIED PHYSICS LETTERS 88, 203125 �2006�

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Preparation of a superhydrophobic surface by mixedinorganic-organic coating

Michele Ferrari,a� Francesca Ravera, and Libero LiggieriCNR-Istituto per l’Energetica e le Interfasi, via De Marini 6, 16149 Genova, Italy

�Received 14 December 2005; accepted 22 April 2006; published online 19 May 2006�

In recent years, superhydrophobic surfaces, with a water-contact angle greater than 150°, haveattracted great interest for both fundamental research and practical applications. Due to the smallarea these surfaces show when in contact with water, interactions with aqueous environment areusually strongly reduced. An original methodology based on a mixed inorganic-organic coating ina multistep procedure is described here allowing a superhydrophobic surface to be prepared startingwith a mechanical treatment of a glass surface. © 2006 American Institute of Physics.�DOI: 10.1063/1.2205725�

The water repellence properties of surfaces have arisengreat interest both from fundamental and application purposeresearch.1 Such surfaces are important in practical applica-tions such as glass covers for solar cells, satellite dishes,microfluidics, windshields of automobiles, roofing, eye-glasses, and generally anywhere that reduced wettability oradhesion is desirable �small contact area between liquid andsurface is an important factor contributing to reduction inadhesion�.2 For these surfaces the solid-liquid-vapor tripleline is characterized by a large contact angle, which is themain parameter utilized do quantify the wetting properties. Itis known that surface energy, surface roughness, and homo-geneity are the three main factors that control the wetting ofa solid.3 Indeed so far the highest contact angle for water ona smooth surface obtained only by lowering the surface en-ergy is about 120°. Thus a high degree of water repellencecannot be achieved only by chemical treatments.

Surfaces with contact angles of larger than 150°, show-ing then a superhydrophobic behavior are always character-ized by low surface energy coupled with high roughness atnanometric scale. The fine surface roughness produced by afractal structure, for example, can be a dominant factor inincreasing the contact angle.4 Many researches have focusedon producing superhydrophobicity by different methods uti-lizing both ordered and not ordered structures. Nature offerswell-known examples of that and the dual-size structure hasbeen found to be a key feature in the high hydrophobicity ofthe lotus leaves, especially for low rolling off angles of waterdrops. By the mimicking of such characteristics the way wasopened to different approaches and preparation paths of su-perhydrophobic surfaces.5–7 Contact angles of more than170° have also been obtained for water by increasing theroughness of low surface energy materials through chemicaltreatment or photolithographic methods.8,9

The effect of surface roughness on hydrophobicity hasbeen explained by two different theories. According to themodel developed by Wenzel, the thermodynamic contactangle is corrected by a multiplicative roughness factor de-fined as the ratio by the actual area of the rough surface andthe geometric projected area. In this approach it is assumedthat the space between the protrusions on the surface is filled

a�Author to whom correspondence should be addressed; electronic mail:

m.ferrari@ge.ieni.cnr.it

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by the liquid; this model predicts that both hydrophobicityand hydrophilicity are reinforced by the roughness.10,11

In a different perspective Cassie and Baxter12 considerthat air is trapped in the space between the protrusions sothat the correction to the thermodynamic contact angle de-pends on the fractions of fluid area in contact, respectively,with the solid and with trapped air. Here an originalmethodology13 for the preparation of superhydrophobic coat-ing of a glass substrate is reported, based on a mixedorganic-inorganic coating. In spite the obtained coating mayalso show interesting oleophobic properties, here we focuson water repellence.

The preparation consists of different operational stages.�1� A glass slide is first ground by 1000 grit sand paper for2 min to increase the surface roughness, rinsed in ethanol,and then in pure water in a ultrasonic bath. �2� The glass isthen cleaned with H2SO4 98%, rinsed in water, and dried inan air flush. �3� A hexane dispersion of Degussa fumed silicananoparticles of primary size of 16 nm treated with dimeth-yldichlorosilane is applied by dipping and a slow withdraw-ing and drying. �4� Finally, a polymeric film of FC735�Acota Ltd., UK� is applied by dipping and slow withdraw-ing and drying.

FC735 is a low surface energy commercial blend of fluo-roacrylate polymer and fluoroalkylethers showing both hy-drophobic and oleophobic features. It has been selected be-cause of its properties in terms of time stability, chemicalinertia, and viscosity. This latter is very important for an easyapplication and the formation of suitable thickness films.

The described preparation allows well defined and re-peatable two scale nano/microstructure to be obtained. In-deed, scanning electron microscopy �SEM� observationsshow a fractal-like structure with similar morphology at dif-ferent scales �Figs. 1�a� and 1�e�� which appears to be homo-geneous throughout the glass surface �Figs. 1�a� and 1�b��.The assembled nanoparticles are homogeneously covered bythe polymer and distributed presenting only very few localdefects �larger pores�.

The mechanical treatment allows the particles to beentrapped into the “network” carved by the sand paper ontothe glass surface, avoiding their aggregation during thesolvent evaporation step. Indeed, in absence of grindingtreatment the particles aggregate in disordered “islands”

�Figs. 1�c� and 1�d��.

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The structure observed by atomic force microscopy�AFM� �Nanoscope III, Digital Instruments� in contact mode�Fig. 2� reveals the presence of a kind of “secondary, rough-ness” a particular topographic feature based on dual-sizeroughness that can give an explanation to the enhanced hy-drophobicity observed: a primary or coarse-scale roughstructure of about 200 nm–1 �m coupled with a finer onearound or on the top of the coarse one in the range of40–80 nm.

To check the wetting properties, measurements of theadvancing-receding contact angle were carried out by meansof a computer assisted drop shape apparatus, equipped withthe ASTRA software developed at the authors’ laboratory andallowing a frame rate acquisition of more than 10 Hz. In this

FIG. 1. SEM observations ��a� and �b�� coating with surface treatment �bar,1 and 10 �m�; ��c� and �d�� coating without treatment �bar, 1 and 10 �m�;�e� magnification of �a� �bar, 200 nm�.

FIG. 2. AFM image �contact mode� with vertical bar of 82 nm/div, hori-2

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zontal bar of 2 �m/div, and scanning area of 10 �m .

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case the sampling rate was 1 Hz. The sample cell was lodgedin a thermostatic chamber, carefully protected by environ-mental dust and previously saturated by water in order toavoid evaporation effects throughout the experimentalcampaign.

High purity grade water, produced by a MilliQ �Milli-pore� ion-exchange purifier with a microfiltration stage, wasutilized for the measurements.

The advancing-receding contact angle were obtained bymeans of a Hamilton programmable dispenser at a slowspeed of volume variation in order to provide a quasistaticmeasurement in the range of increment of 20% of the dropvolume.

The choice of the capillary was carefully made to reduceas much as possible the impact with the drop and then a steelneedle of 0.21 mm of diameter was used for the measure-ments. The initial volume was 5 �l. The capillary remainedinside the drop during the measurement. The measurementshave been performed and repeated in different points of thesurface in order to assess the homogeneous character of thefilm deposition. The surface appears to be not influenced bythe previous presence of the drop. The drop shape indicatesthe superhydrophobic conditions of the surface �Fig. 3�.

Table I summarizes the measured contact angle forglasses coated according to �i� the complete procedure,�ii� coated without grinding �preparation procedure withoutstep �1�, and �iii� simply coated with hydrophobic polymer�only steps �2� and �4� of the preparation procedure�. Theproposed coating procedure provides a very high degree ofhydrophobicity, with a contact angle comparable with thehighest values reported in the literature.

The comparison with the performance of the sample pre-pared without grinding shows that this unique feature ismainly due to homogeneity of the coating at microscopicscale. The hystheresis from these data appears very low andin the order of less than 2° in a time window around a thou-sand of seconds. In this time frame no evaporation effectshave been observed.

FIG. 3. Water drop on the superhydrophobic surface bar of 1 mm.

TABLE I. Contact angles for coatings obtained under different conditions.

Advancing CA �°� Receding CA �°�

Grinding 170±1 168±1No grinding 138±3 129±3FC735 only 102±1 98±1ct to the terms at: http://scitation.aip.org/termsconditions. Downloade

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203125-3 Ferrari, Ravera, and Liggieri Appl. Phys. Lett. 88, 203125 �2006�

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In summary, an original but simple path to prepare asurface with high hydrophobic properties has been proposedby using a composite film deposition starting with a me-chanical treatment of the solid substrate. The repeatability ofthe procedure and its performance allow to forecast a devel-opment of the method with the aim of improving the filmformation and its stability under different conditions such astemperature or type of liquid. Further investigations are un-der way to assess the oleophobic properties.

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