Please cite this article in press as: D. Orsi, et al., On the relation between hierarchical morphology and mechanical properties of a colloidal 2D gel system, Colloids Surf. A: Physicochem. Eng. Aspects (2012), doi:10.1016/j.colsurfa.2012.01.001 ARTICLE IN PRESS G Model COLSUA-17440; No. of Pages 7 Colloids and Surfaces A: Physicochem. Eng. Aspects xxx (2012) xxx–xxx Contents lists available at SciVerse ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects jo ur nal homep a ge: www.elsevier.com/locate/colsurfa On the relation between hierarchical morphology and mechanical properties of a colloidal 2D gel system Davide Orsi a , Giacomo Baldi a,b , Pietro Cicuta c , Luigi Cristofolini a,∗ a Physics Department, University of Parma, Italy b IMEM-CNR, Parma, Italy c Cavendish Laboratory, University of Cambridge, UK a r t i c l e i n f o Article history: Received 1 November 2011 Received in revised form 2 January 2012 Accepted 2 January 2012 Available online xxx Keywords: Langmuir monolayer Interfacial Shear Rheometry 2D gel Gold nanoparticles Colloids a b s t r a c t We study a bidimensional gel system formed by a Langmuir film of gold nanoparticles. Its morphology is thoroughly characterized by AFM, SEM, and ellipsometric imaging techniques and shows a complex structure with features on a hierarchy of different sizes following a Levy distribution. The mechanical response arising after the gel point is investigated by Interfacial Shear Rheometry. The film is found to be mainly elastic, with the mechanical moduli scaling as a power law of the reduced concentration, in the same way as the fluctuation time which was measured in a recent X-ray Photon Correlation Spec- troscopy experiment. The frequency dependence of the moduli is well described in the framework of the Soft Glass Rheology model (SGR) [Sollich et al. Phys Rev Lett 78, 2020 (1997)]. In this theory a power law distribution of relaxation times is postulated, whose exponent is experimentally determined for the present case. Such a distribution may reflect – in the dynamics – the hierarchical nature of the morphol- ogy of the film evidenced by microscopy. A mastercurve can be built, extending time–temperature to a time–concentration superposition principle as in [Cicuta et al. Phys Rev Lett 90, 116103(2003)]. This allows to describe the mechanical response over a frequency range slightly larger than that experimen- tally accessible. Besides the SGR component, a viscous term is always present, whose origin is investigated also taking into account samples with different preparation histories. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Many everyday life applications, as well as advanced technolo- gies, rely on the diversity of colloidal interfacial processes and properties. In particular one of the frontiers in the field of nano sci- ences is represented by the self-assembling processes of molecules, colloidal particles and bio-colloidal materials at fluid–fluid inter- faces, like in Pickering emulsions [1], and the possibility to tune their self-assembly properties [2]. In this framework, inorganic nanoparticles (NP) are particularly interesting as they can be exploited in an increasing number of technological applications, including biosensing, therapeutics [3,4] and diagnostics [5]. The basic scientific knowledge in this area feeds into many appli- cations in chemical, pharmaceutical and food industries, as well as in designing nano-devices such as sensors, assays, photonics and bio-fuel cells. These devices in turn are deployed in the areas of energy, health and environmental protection. NPs can also be used as “additives” to improve the performance of existing materials (as thermal conductivity, mechanical stability or energy transfer) ∗ Corresponding author at: Physics Department, University of Parma, Parco Area delle Scienze 7/A, 43100 Parma, Italy. Tel.: +39 0521 905276; fax: +39 0521 905223. E-mail address: Luigi.Cristofolini@fis.unipr.it (L. Cristofolini). [6], or to impart new functions to them (e.g. magnetic NPs have been investigated for drug delivery [7,8]). From the fundamen- tal point of view, NP bridge the length scales between molecular surfactants and micron-sized particles. The former usually are in dynamical equilibrium with a bulk reservoir, whereas the latter are irreversibly attached to the surface as first shown by Pieranski [9]. NPs are somehow in between the two extremes and can be in dynamical equilibrium between surface and subphase, depending on their coating and on surface pressure. New experimental and theoretical tools need to be developed to address the complex and multidisciplinary issues raised from the study dynamics in confined geometry, such as on the interface. In particular, the interaction between nanoparticles at the inter- face (not directly predictable from their behaviour in solution) is of paramount importance in determining the properties of the inter- face itself. Recently Isa et al. have been able to characterize the adsorption of NPs at the oil–water interface [10] and to measure the contact angle at the microscopic interface between colloidal particles and fluid [11], which in turn determines the interaction between particles. Looking at the surface rheological properties, Cicuta et al. have been able to formulate a time/concentration superposition principle – analogue of the time/temperature super- position principle – to the case of the shear response of colloids at an interface [12]. This allows to build a mastercurve and thus to 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2012.01.001