Force measurements by AFM

The tip and sample interaction is depicted in Figure 15.14. The data of such studies have been reported on silica surfaces. These measurements showed the deflection of the cantilever between the silica plate and the glass sphere coated with octadecyltrichlorosilane. It is of interest to note that AFM is a very sensitive molecular force microscope. The forces measured by AFM are of the order of 10,000 to 1(X),000 times lower than an insect ( 1 mg) sitting on a surface ... 

AFM Atomic force microscopy [9, 47, 99] Force measured by cantilever deflection as probe scans the surface Surface structure... 

The rupture force measured in AFM experiments is given, therefore, by the average slope of the energy profile minus a correction related to the effects of thermal fluctuations. Equation (11) demonstrates that the rupture force measured in AFM experiments grows linearly with the activation energy of the system (Chilcotti et ah, 1995). A comparison of (10) and (11) shows that the unbinding induced by stiff springs in SMD simulations, and that induced by AFM differ drastically, and that the forces measured by both techniques cannot be readily related. 

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. 

The use of AFM has been expanded to measure the forces acting on an AFM tip as a function of separation between the tip and surface, but the tip geometry is often difficult to characterize and inconvenient to compare to theoretical predictions of force measurements. By attaching a colloidal size sphere (on the order of 5-10 (im in radius a) to the end of a cantilever (first accomplished by Ducker [26]), it is possible to measure the colloidal interaction between surfaces in system with a well-characterized geometry (shown schematically in Figure 4.2). The measurement of a force-distance cycle is the record of the deflection of the cantilever as the two surfaces approach through motion of the piezo stage. The separation distance between the surfaces is not measured directly... 

The adhesion forces of silica and alumina particles in the DI water and slurry solution were measured by AFM and are shown in Fig. 16.27 [66]. The smallest adhesion force, 0.38 nN, was observed between the copper surface and alumina particles in a citric acid solution at pH 6. The largest adhesion force of alumina particles, 5.83 nN, was measured in DI water. 

The materials analyzed were blends of polystyrene (PS) and poly(vinyl methyl ether) (PVME) in various ratios. The two components are miscible in all proportions at ambient temperature. The photooxidation mechanisms of the homo-polymers PS and PVME have been studied previously [4,7,8]. PVME has been shown to be much more sensitive to oxidation than PS and the rate of photooxidation of PVME was found to be approximately 10 times higher than that of PS. The photoproducts formed were identified by spectroscopy combined with chemical and physical treatments. The rate of oxidation of each component in the blend has been compared with the oxidation rate of the homopolymers studied separately. Because photooxidative aging induces modifications of the surface aspect of the material, the spectroscopic analysis of the photochemical behavior of the blend has been completed by an analysis of the surface of the samples by atomic force microscopy (AFM). A tentative correlation between the evolution of the roughness measured by AFM and the chemical changes occurring in the PVME-PS samples throughout irradiation is presented. 

In general, we determine the magnitude of pzc and iep by electrophoretic or potentiometric titration methods. In some cases, these data have been obtained by using the streaming potential measurements. In a recent stndy, the wetting properties of silanated surfaces were investigated by AFM. Also, the force measnrement on hydrogel was measured by AFM. ... 

Table 5.3 Friction force F (In terms of TMR value [V], which Is equal to the difference between lateral forces scanning left-to-right and right-to-left) of PDMS 5 and 5 for various speeds and applied loads measured by AFM (AF=0.01 V).

Table 5.4 Friction force F (in terms ofTMR [V]) of PDMS 17 after extraction of free chains, measured by AFM (AF=0.1 V)...

The force spectroscopy measurements are performed by putting an unmodified AFM tip in a very thin liquid crystal layer on a flat glass plate. Both tip and glass plate are covered with a monolayer of N,N-dimethyl-N-octadecyl-3-aminopropyltrimethoxysilyl chloride (DMOAP) to induce good homeotropic alignment of the liquid crystal. Forces acting between tip and glass plate are measured by AFM used in the force spectroscopy mode. 

This discussion about the adhesion of single molecular bonds is rather important. Looking back over the past few hundred years, it seems that our progress in adhesion study has been to understand more and more about the molecular meaning of adhesion. This has happened in several ways first, molecular contact has been achieved reliably with elastomers, with mica, with fine particles, and with atomic force probes second, very small forces have been measured by AFM and by other techniques (Section 10.3) finally, the ultimate... 

We can siunmarize at this point that the forces measured in AFM adherence force (and frequently friction) measurements depend crucially on the chemistry of the AFM tip and the surface of interest, as well as the medium in which the contact takes place. Hence by systematic variation of the tip chemistry via chemical modification/functionahzation and of the medium chemical contrast spatially resolved chemical imaging becomes possible. 

The use of a simple elastic model taking into account adhesion forces enables the quantitative measurement by AFM of the elastic modulus of polymer surfaces. Local force-modulation measurement also permits to qualitatively compare the local stiffness of polymers. These techniques associated to the imaging abilities of AFM... 

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