**3.3 Nanostructured surfaces with bimodal roughness**

Surfaces with bimodal I roughness (Figure 6c) were created by adding 65 nm particles subsequently to the 470 nm particles (cf Figure 1c,d). The degree of substrate coverage with 470 nm particles was of 46.9 ± 1 % on this sample. This approach is similar to that of Takeshita et al. (2004) using poly(ethylene terephthalate) conditioned with PAH and adhesion of 350 nm carboxylated polystyrene latex particles followed by 100 nm particles. However, the layer obtained in that case was not regular, despite sonication of the solution to prevent latex aggregation. We attribute this to the fact that the samples were rinsed with water before drying.

Surfaces with bimodal II roughness (Figure 6d) were prepared by adsorbing PAH after the formation of the first adhering layer (cf Figure 1b,c,e,f). In this way, 65 nm particles were adhering not only to the glass substrate but also to the 470 nm particles, providing raspberry-like structures. This procedure was repeated five times independently and 3 to 15 samples were prepared in each experiment, demonstrating the repeatability as well as the reproducibility of the method. Figure 6 (d, e) presents the extreme results obtained, corresponding to degrees of coverage by 470 nm particles of 50 ± 3 % and 22 ± 1 %, respectively.

Fabrication of Surfaces with Bimodal Roughness Through Polyelectrolyte/Colloid Assembly 67

substrate orientation (upward, downward, vertical), convection, particle concentration and contact time. The selection of the best approach depends primarily on the particle size, which is critical in the range of 100 nm, and secondarily on particle density and on the

The support of the Foundation for Training in Industrial and Agricultural Research (FRIA), of the Belgian National Foundation for Scientific Research (FNRS), of the Région Wallonne and of the Federal Office for Scientific, Technical and Cultural Affairs (Interuniversity Poles

Adamczyk, Z., Zembala, M., & Michna, A. (2006). Polyelectrolyte adsorption layers studied by streaming potential and particle deposition. *J. Colloid Interface Sci.,* 303, 353-364 Adamczyk, Z., Zembala, M., Kolasinska, M., & Warszynski, P. (2007) Characterization of

Agheli, H., Malmstrom, J., Hanarp, P., & Sutherland, D.S. (2006). Nanostructured biointerfaces. *Mater. Sci. Eng. C: Biomimetic Supramol. Syst.*, 26, 911-917 Barthlott, W., & Neinhuis C. (1997). Purity of the sacred lotus, or escape from contamination

Bertrand, P., Jonas, A., Laschewsky, A., & Legras R. (2000). Ultrathin polymer coatings by

Boonaert, C.J.P., Dupont-Gillain, C.C., Dengis, P.B., Dufrêne Y.F., & Rouxhet, P.G. (1999).

Boonaert, C.J.P., Dufrêne, Y.F., & Rouxhet, P.G. (2002). Adhesion (primary) of

Bravo, J., Zhai, L., Wu, Z., Cohen, R.E., & Rubner M.F. (2007). Transparent superhydrophobic films based on silica nanoparticles. *Langmuir,* 23, 7293-7298 Caillou, S. , Gerin, P.A., Nonckreman, C.J., Fleith, S., Dupont-Gillain, C.C., Landoulsi, J.,

Changui, C., Doren, A., Stone, W.E.E., Mozes, N, & Rouxhet, P.G. (1987). Surface properties

Chen, K.M., Jiang, X., Kimerling, L.C., & Hammond, P.T. (2000). Selective self-organization of colloids on patterned polyelectrolyte templates. *Langmuir,* 16, 7825-7834

polyelectrolyte multilayers on mica and oxidized titanium by steaming potential and wetting angle measurements. *Colloid Surf. A: Physicochem. Eng. Asp.,* 302, 455-

complexation of polyelectrolytes at interfaces: Suitable materials, structure and

Cell separation, flocculation, In: *Encyclopedia of bioprocess technology: Fermentation, biocatalysis, and bioseparation,* M.C. Flickinger, S.W. Drew (Eds.), pp. 531-548, John

microorganisms onto surfaces, In: *Encyclopedia of environmental microbiology,* G.

Pancera, S.M., Genet , M.J., & Rouxhet, P.G. (2008). Enzymes at solid surfaces: nature of the interfaces and physico-chemical processes. *Electrochim. Acta,* 54, 116-

of polycarbonate and promotion of yeast cell adhesion. *Journal de Chimie Physique et* 

desired degree of coverage.

**5. Acknowledgements** 

**6. References** 

460

122

of Attraction Program) is gratefully acknowledged.

in biological surfaces. *Planta,* 202, 1-8

Wiley & Sons, Inc., New York

*de Physico-Chimie Biologique*, 84, 275-281

properties. *Macromol. Rapid Commun.*, 21, 319-348.

Bitton (Ed.), pp. 113-132, John Wiley & Sons Inc., New York

Owing to the possible interest of nanostructured surfaces in the field of biointerfaces, samples with adhering particles were incubated in phosphate buffer saline (the main constituent of culture media) for 24 h at 37°C, followed by rinsing with water, rinsing with isopropanol and drying. The result obtained with a bimodal II roughness is presented in Figure 6f. The observed morphology is similar to that of the sample not exposed to buffer (Figure 6d), demonstrating the robustness of the protocol and of the nanostructured surface obtained.

Recently published works aimed at creating this kind of roughness using different approaches. Suspensions of raspberry-like particles were prepared by styrene polymerization on silica particles (Perro et al., 2006; Reculusa et al., 2002). In another study, silica particles having different sizes and bearing functional groups were firstly synthesized independently and then mixed to react together. The obtained aggregates were then grafted on a specific substrate to obtain a dual-size roughness surface (Xiu et al., 2006). In another approach, a layer of silica particles in hexagonal close packing was created on a substrate, and gold nanoparticles were formed on the top of the silica spheres by sputtering (Ming et al., 2005). The method used in the present work has several advantages, such as the use of components which are commercially available (polycation, latex particles), and a simple procedure (sequential steps of polycation adsorption and colloid adhesion) which does not require sophisticated devices or complex reactions. The protocol could be extended to other particle sizes for obtaining a broader panel of roughness, the density of the two types of particles could be tuned, and architectures could be elaborated with more than two particle sizes. The bimodal surfaces of type II roughness mimic the particular topography observed on Lotus leaf. A superhydrophobic surface is thus expected to be obtained after treatment with compounds conferring a low surface energy (Bravo et al., 2007).
