Although membranes are widely used for seawater desalination, wastewater treatment, drinking water production, and many other industrial and medical applications, a major obstacle for the efficient application of membrane technology is the phenomenon of membrane fouling. Fouling results in deterioration of membrane performance and ultimately shortensmembrane life.Thus, understanding the causes of membrane fouling and developing strategies for fouling control and cleaning are major challenges. Adhesion of particles on the membrane surface is the main cause for fouling.The property of adhesion is also important in membrane science for fabricating composite membranes. Adhesion is defined as the physical attraction or joining of two substances, especially the macroscopically observable attraction of dissimilar substances. There are many techniques to study adhesion, namely pull-off tests, interfacial fracture tests, blister tests, mapping of interfacial properties, probe modification, and scratch tests. Van derWaals forces are always present between molecules or between particles and may be attractive or repulsive [1, 2], depending on whether they are working between like materials or dislike materials. For like materials, the van derWaals forces are always attractive; however, repulsive forces are predicted for certain dissimilar material combinations . Van derWaals forces have been used to explain why neutral chemically saturated atoms congregate to formliquids and solids.These forces are a main reason for fouling of membranes. Using AFM, repulsive van derWaals forces can be measured with higher precision than attractive van derWaals forces [3, 4]. Membrane separation processes are used to separate ions, colloids, and biological molecules from the fluid stream. For optimum operation, the membrane has to possess physical properties, giving appropriate interactions with solutes in the process stream. The most important properties are pore size distribution, surface morphology, appropriate electrical double layer interactions, and minimum adhesion (fouling) . Within the framework of the DLVO theory  by the interplay between van der Waals forces and the electrical double layer force, surface interactions can be explained. (DLVO theory is an acronym for a theory of the stability of colloidal dispersions developed by Derjaguin, Landau, Verwey, and Overbeek [6, 7]. The theory was developed to predict the stability against aggregation of electrostatically charged particles in dispersion .) AFM has been widely used to measure DLVO-type interactions [5,8–12] between a single colloid particle and a normally flat surface, as a function of separation distance. The electrical properties of membrane surfaces have been most commonly evaluated by electrokinetic techniques such as streaming potential measurements , but they have some limitations .The adhesion of many polymers is still not clear at a nanometric scale. Surface and interface properties can be modified by a change in chemical composition or structure. Several devices for measuring surface forces have been developed, including the surface force apparatus [13, 14], the force balance , the osmotic stress method, and the total internal reflectance microscope . But in all these methods there are limitations. Atomic force microscopy is now used for the measurement of adhesion forces. In fact, atomic force microscopic studies can be divided into topographical applications (imaging mode) and force spectroscopy, or so-called atomic force spectroscopy (AFS), i.e., measuring forces as a function of distance . In the former, one generates an image of the sample surface to observe its structural or dynamic features, which has been employed very successfully on a wide variety of surfaces, including polymers [18–20] with resolutions in the micrometer to subnanometer ranges, thus facilitating imaging at the submolecular level. There are two methods to measure adhesion forces by AFM: Contact mode AFM In contact mode AFM, the tip is mechanically contacted with the sample surface under a defined applied force.This applied force can be estimated from a force–distance curve, which is obtained by extending the tip to the surface to make contact between the tip and the surface, followed by retracting the tip. Figure 7.1 shows the force–distance curve.There is no interaction between the tip and the surface when the tip is far away from the surface (A in Fig. 7.1). When the tip is close to the surface, there is an attractive force between them. At some point, the force gradient becomes larger than the spring constant of the cantilever, so the tip snaps to the surface (B–C). Once the tip is in contact with the surface, cantilever deflection will increase as the end of the cantilever is brought closer to the sample. If the cantilever is sufficiently stiff, the probe tip may indent the surface at this point. In this case, the slope or shape of the contact part of the force curve can provide information about the elasticity of the sample surface. Extending the tip (along line C–D) results in loading (repulsive) forces to the surface.These repulsive forces are usually used as a feedback parameter for the AFM system to obtain surface morphology.
KeywordsAdhesion Force Composite Membrane DLVO Theory Ether Sulfone Surface Force Apparatus
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