Journal of Membrane Science 315 (2008) 82–92 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci Membrane characterization by microscopic methods: Multiscale structure Y. Wyart a , G. Georges b , C. Deumi ´ e b , C. Amra b , P. Moulin a,∗ a Universit´ e Paul C´ ezanne Aix Marseille, D´ epartement Proc´ ed´ es Propres et Environnement (DPPE-CNRS-UMR 6181), Europˆ ole de l’Arbois, BP 80, Bat. Laennec, Hall C, 13545 Aix en Provence cedex 04, France b Universit´ e Paul C´ ezanne Aix Marseille, Institut Fresnel, Campus de Saint J´ erˆ ome, av. Escadrille Normandie 13397 Marseille Cedex 20, France article info Article history: Received 8 October 2007 Received in revised form 30 January 2008 Accepted 9 February 2008 Available online 16 February 2008 Keywords: AFM SEM White light interferometry Roughness Porosity abstract A great number of studies have been carried out to obtain a better understanding of membrane foul- ing so as to be able to limit its effects. The parameters studied are many and can be classified into membrane structure parameters (porosity, roughness, pore size, pore shape, pore size distribution) and membrane/effluent coupling parameters (material, surface charge, hydrophobicity, etc...). In the case of the membrane structure parameters, three types of techniques can be used: displacement techniques, tracer retention techniques and microscopic techniques. In this paper, first microscopy observation meth- ods are reviewed, and then the potential of three different techniques is studied. Scanning Electron Microscopy (SEM) provides information on surface porosity and layer thickness. The pore sizes measured with this technique were in agreement with the membrane cut off values given by the manufacturers. Atomic Force Microscopy (AFM) and White Light Interferometry (WLI) provide surface RMS roughnesses that depend on the observation scale. The RMS roughnesses that were obtained ranged between 100 and 4000 nm. For 4 unused ceramic membranes of different cut-offs and for 3 different scan sizes, the passage from one scan size to another is continuous in terms of information provided. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Membrane processes are the industrial processes whose devel- opment has been the fastest (12% growth per year) but membrane filtration is impeded by a major drawback: membrane fouling. The fouling can be either reversible or irreversible, depending on whether the membrane can be regenerated or not. This phe- nomenon entails a reduction in the production, a decline in the permeate flux, and a possible reduction in the performance of the membrane in terms of selectivity. Either a backwashing or a chemical wash will thus be necessary for the membrane to recover its initial performances. A great number of studies have been carried out in order to gain a better understanding of this phenomenon of membrane fouling so as to be able to limit its effects [1–3]. The parameters studied are many and can roughly be classified into: membrane structure parameters (poros- ity, roughness, pore size, pore shape, pore size distribution) and membrane/effluent coupling parameters (membrane material, sur- face charge, hydrophobicity, etc...). Only membrane structure parameters will be considered in this paper. The previous stud- ies carried out in this domain have focused on three types of techniques: displacement techniques [4,5], techniques of tracers’ retention and microscopic techniques [4,6]. The displacement tech- ∗ Corresponding author. Tel.: +33 4 42 90 85 01; fax: +33 4 42 90 85 15. E-mail address: philippe.moulin@univ-cezanne.fr (P. Moulin). niques require high pressures as they are used on membranes with pore sizes of about 10nm (ultrafiltration). The tracer retention techniques have been widely used, especially for defining mem- brane cut-offs. Polyethylene glycols and proteins are the most often used tracers in the case of ultrafiltration, but they are sensible to operating conditions. Advances in the study of membrane struc- ture have been made possible thanks to microscopic techniques such as Scanning Electron Microscopy (SEM) [7], Transmission Elec- tron Microscopy (MET) [8], near-field microscopy (Atomic Force Microscopy, (AFM) [9]) and Scanning Tunnelling Microscopy (STM) [10]). Among these various techniques, the most widely used are SEM and AFM. The SEM applications are varied and focus on membrane structure characterization [11], hollow fiber mem- brane fabrication [12] and the study of the fouling process [13]. Hwang and Lin [14] used observations made using SEM to qual- ify the nature of the pores of 3 microfiltration membranes with a cut-off of 0.1 m. They also observed the fouling of these mem- branes after filtration of a solution containing model particles of polymethyl methacrylate (mean diameter = 0.4 m). The major drawback of this technique is the sample preparation by gold metallization, which entails a less accurate pore size determina- tion. Atomic Force Microscopy (AFM) is a quite recent technique dating back from 1986 [15]. It was first used in 1988 to study the structure of polymeric membranes [16]. This technique can be used in three different modes: contact [17], non-contact [18] and tapping mode [19] and can be applied to all membranes, from microfiltration to reverse osmosis [20–22], for organic [23,24] 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.02.010