Colloid probe technique

The colloid probe technique was first applied to the investigation of surfactant adsorption by Rutland and Senden [83]. They investigated the effect of a nonionic surfactant petakis(oxyethylene) dodecyl ether at various concentrations for a silica-silica system. In the absence of surfactant they observed a repulsive interaction at small separation, which inhibited adhesive contact. For a concentration of 2 X 10 M they found a normalized adhesive force of 19 mN/m, which is small compared to similar measurements with SEA and is probably caused by sufactant adsorption s disrupting the hydration force. The adhesive force decreased with time, suggesting that the hydrophobic attraction was being screened by further surfactant adsorption. Thus the authors concluded that adsorption occurs through... 

In 1991, the same author helped to develop a new experimental procedure, called the colloid probe technique , which is now widely used to measure the interaction forces between colloidal surfaces (see Ducker et ah, 1991). 

The second device with which surface forces can be measured directly and relatively universally is the atomic force microscope (AFM) sometimes also called the scanning force microscope (Fig. 6.8) [143,144], In the atomic force microscope we measure the force between a sample surface and a microfabricated tip, placed at the end of an about 100 //,m long and 0.4-10 //,m thick cantilever. Alternatively, colloidal particles are fixed on the cantilever. This technique is called the colloidal probe technique . With the atomic force microscope the forces between surfaces and colloidal particles can be directly measured in a liquid [145,146], The practical advantage is that measurements are quick and simple. Even better, the interacting surfaces are substantially smaller than in the surface forces apparatus. Thus the problem of surface roughness, deformation, and contamination, is reduced. This again allows us to examine surfaces of different materials. 

Some of the disadvantages were overcome by the use of the colloidal probe technique to measure adhesion forces (review Ref. [216]). The colloidal probe technique offers the advantage that the same particle can be used for a series of experiments and its surface can be examined afterwards. The accessible range of particle size is typically limited to a range between 1 /zm and 50 pm. The tedious sample preparation, limits the number of different particles used within one study, for practical reasons. Therefore the colloidal probe and centrifugal methods complement each other. 

The JKR theory predicts correct contact radii for relative soft surfaces with effective radii larger than 100 /an. This was shown in direct force measurements by the surface forces apparatus [217, 218] or specifically designed systems. For smaller spheres it was verified using the colloidal probe technique [219],... 

In experiments with friction force microscopy, the tip forms a contact of a few nanometers in diameter with the substrate, a so-called nanocontact. In reality, friction of macroscopic bodies is determined by the interaction via m/crocontacts. One possibility of extending the method of friction force microscopy to larger contact areas is the use of the colloidal probe technique, where a small sphere is attached to the end of an atomic force microscope cantilever (see Section 6.4). Even for microcontacts, the proportionality between the true area of contact and the friction force was observed (see example 11.1). 

Example 11.4. McGuiggan et al. [492] measured the friction on mica surfaces coated with thin films of either perfluoropolyether (PFPE) or polydimethylsiloxane (PDMS) using three different methods The surface forces apparatus (radius of curvature of the contacting bodies R 1 cm) friction force microscopy with a sharp AFM tip (R 20 nm) and friction force microscopy with a colloidal probe (R 15 nm). In the surface force apparatus, friction coefficients of the two materials differed by a factor of 100 whereas for the AFM silicon nitride tip, the friction coefficient for both materials was the same. When the colloidal probe technique was used, the friction coefficients differed by a factor of 4. This can be explained by the fact that, in friction force experiments, the contact pressures are much higher. This leads to a complete penetration of the AFM tip through the lubrication layer, rendering the lubricants ineffective. In the case of the colloidal probe the contact pressure is reduced and the lubrication layer cannot be displaced completely. 

Up to date, besides the SFA, several non-interferometric techniques have been developed for direct measurements of surface forces between solid surfaces. The most popular and widespread is atomic force microscopy, AFM [14]. This technique has been refined for surface forces measurements by introducing the colloidal probe technique [15,16], The AFM colloidal probe method is, compared to the SFA, rapid and allows for considerable flexibility with respect to the used substrates, taken into account that there is no requirement for the surfaces to be neither transparent, nor atomically smooth over macroscopic areas. However, it suffers an inherent drawback as compared to the SFA It is not possible to determine the absolute distance between the surfaces, which is a serious limitation, especially in studies of soft interfaces, such as, e.g., polymer adsorption layers. Another interesting surface forces technique that deserves attention is measurement and analysis of surface and interaction forces (MASIF), developed by Parker [17]. This technique allows measurement of interaction between two macroscopic surfaces and uses a bimorph as a force sensor. In analogy to the AFM, this technique allows for rapid measurements and expands flexibility with respect to substrate choice however, it fails if the absolute distance resolution is required. 

Membranes are very finely porous structures and like all such porous structures used in an industrial context are susceptible to fouling caused by adhesion of components of the materials being processed. This fouling can be minimised or avoided if suitable polymers are used in membrane manufacture. However, the selection of membrane polymers suited to particular separations has until now been a matter of experience (and failure) rather than science. However, an AFM used with the colloid probe technique [23] can provide a rapid means of assessing the adhesion of solutes to membrane materials and is hence a powerful tool for the membrane technologist. 

The first reported uses of an AFM to study EDL interactions using the colloid probe technique were the studies by Ducker et al. [29,301 and by Butt [311... 

Andersson, K.M. and Bergstrom, L., DLVO interactions of tungsten oxide and cobalt oxide surfaces measured with the colloidal probe technique, J. Colloid Interf. Sci.. 246, 309, 2002. 

A scanning force instrument also allows for the acquisition of force-distance curves to characterize the local mechanical properties of the sample. Well-defined indentation experiments on soft surfaces like swollen hydrogels in aqueous media are possible with the colloidal probe technique. Raw data are assessed, for example, according to the Hertz model, with the assumption... 

Gan, Y., Franks, G.V. (2006). Charging Behavior of the Gibbsite Basal (001) Surface in NaCl Solution Investigated by AFM Colloidal Probe Technique. Langmuir Vol. 22, pp. 6087-6092. 

To measure the adhesion forces between colloids (fouling) and the surface layer, or between the surface layer and the substrate, AFM is seen as reliable and simple, as discussed earlier. Bowen et ah [36] introduced a new technique for the direct measurement of the force of adhesion of a single particle by AFM. AFM, in conjunction with the colloid probe technique, allows a direct quantification of membrane fouling through the measurement of pull-off (detachment) forces, when the probe is retracted from the surface after contact has been made. 

The forces acting between preadsorbed BSM layers have been investigated using the AFM colloidal probe technique. It has been demonstrated that the electrostatic repulsion, mainly between sialic acid groups, determines the... 

Nalaskowski, J., Drehch, J., Hupka, J., and Miller, J. D. 2003. Adhesion between hydrocarbon particles and silica surfaces with different degrees of hydration as determined by the AFM colloidal probe technique. Langmuir 19 5311. 

Investigation of cellulose systems in closer detail requires the choice of representative cellulose model surfaces for the experiment. A spin-coated cellulose surface on mica was the first model surface used in studies of forces in papermaking systems (70). This work was followed by other SFA studies using Langmuir-Blodgett (LB) films of cellulose (11-14). These films are noticeably smoother and more stable than spin-coated surfaces. In studies using the atomic force microscope (AFM) colloidal probe technique (Ducker et al. (75)), interaction forces were measured either between two cellulose beads (16,17) or between cellulose beads and spin-coated cellulose surfaces (18,19). 

In this study the AFM colloidal probe technique was used to investigate the forces between cellulose beads in aqueous solutions of simple electrolyte and xylan. Particular attention was paid to the behaviour of the cellulose beads. The adsorption kinetics and characteristics of adsorbed xylan on cellulose was studied with a quartz crystal microbalance with dissipation (QCM-D). 

The force measurements were done by the colloidal probe technique in contact mode using a NanoScope III MultiMode AFM (Digital Instruments, California) equipped with a fluid cell and a scanner E, vertical engagement, using an 0-ring. When measuring forces between cellulose beads, the bead attached to the cantilever was placed directly on top of a bead on the sample support. The position of the bead was checked using the optical microscope of the AFM instrument. 

In this present chapter, we will describe the types of surface forces which can be expected in different types of systems (classified by conditions rather than as a description of different types of forces) and will concentrate primarily on three types of surface force measuring techniques. These are the SFA of Israelachvili (and as modified by others) (44-46), the AFM colloid probe technique (these two techniques are by far the most widely exploited), and the MASIF technique which provides high-resolution data from a purpose-built non-interferometric instrument. 

Still, other results clearly show that the attractive force starts with a discontinuity in the force curve (33, 126, 127). One such example, obtained with the AFM colloidal-probe technique using silica/glass surfaces reacted with fluorinated silanes, is illustrated in Figure 20.13. 

Fortunately, AFM in conjunction with the colloid probe technique offers an alternative means of membrane surface electrical properties characterization. If a colloid probe is approached towards a surface it is possible to quantify the force of interaction. Figure 6.15 shows typical data for a Desal DK membrane, which is one of the least rough membranes [7]. 

Moreover, if the surface potential or surface charge of the colloid has been determined and the solution is of defined ionic content, it is possible to calculate the potential or charge of the surface under investigation by matching the experimentally obtained curves to theoretical calculations based on electrical double layer theory. In the example shown, the best-fit membrane surface charge was —0.00114 Cm and the best-fit membrane surface potential was —64 mV. Furthermore, an important advantage of the colloid probe technique is that it allows exploration of variations in surface electrical interactions at different points on the membrane surface, as the following section shows. 

Operation in liquid and colloid probe techniques are particular advantage of AFM. 

However, the colloid probe technique is not limited to particles. For biological studies, strategies to attach single spores [264], bacteria-coated beads [265], or single cells [266, 267] have been developed. To study forces in emulsions [268] or flotation cells such as oil drops [269, 270, 696] and bubbles have been attached to cantilevers [271]. 

A limitation of the colloid probe technique, however, is the minimum particle size that can be reproducibly attached by using optical microscopy. For spheres smaller than 1 J.m, it becomes difficult to correctly position the particle at the very end of the cantilever to avoid touching the substrate with the edge of the cantilever. In this respect, the name colloid probe is somewhat misleading since colloidal particles are usually smaller than 1 pm. Recently, there have been attempts to attach nanoparticles to the end of AFM tips either by wet chemistry [272] or by epoxy-coating of tips and dipping them into a powder [273]. 

AFM is an ideal tool for characterization of adhesion on the nanometer scale that is vital to the suction problems in Micro-Electro-Mechanical Systems (MEMS) devices. One of the key parameters in these measurements is the true contact area between the probe and the substtate. If one deals with a flat substrate and smooth spherical probe, then an appropriate model from contact mechanics can be applied to analyze the pull-off force data. Based on this method, the so-called colloidal probe technique was developed by using a micrometer-sized spherical particle glued to the end of an AFM cantilever as the force sensor. However, both the probe and the substrate usually demonsttate irregular shape and ill-defined roughness, which reduces the contact area and results in lower adhesion forces than what is expected from ideal geometries. Recently, systematic studies have been... 

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