Adhesion Force Measurements

Due to the high practical relevance of adhesion forces in industrial and everyday applications, a broad spectrum of experimental methods to measure adhesion forces has been established and there are, for example, standardized procedures such as peel tests for adhesive tapes or tack tests for pressure-sensitive adhesives. We will focus here on some representative examples of experimental work targeted toward a fundamental understanding of contact mechanics and adhesion phenomena. 

Another method to study the contact mechanics and adhesion behavior of soft solids is the so-called JKR test using elastomeric poly(dimethylsiloxane) (PDMS) lenses that are brought in contact with flat surfaces or with each other [884]. The soft PDMS ensures almost ideal JKR behavior of the contacting surfaces. Apphed load, indentation and contact radius, and neck shape can be determined simultaneously, which allows comparison with the JKR predictions. The surfaces of the lenses can easily be modified by treatment with an oxygen plasma to induce a silica-like surface that can then be modified using silane chemistry. As long as these layers are kept thin, the mechanical properties will still be dominated by the bulk PDMS. This type of experiments have been used extensively to study the influence of separation rate on adhesion (for a review, see Ref. [885]). 

Rimai et al. used a scanning electron microscope to directly image the contact radii of microparticles deposited on surfaces. For glass beads on polyurethane surfaces, the relation between sphere size and contact radius was well described by JKR model [886], whereas for polystyrene beads on silicon wafers [887], only inelastic contact models could explain the observed relation. 

This spread of results is not a deficit of the method but an inherent property of the system studied. Even for the very smooth silica spheres, surface inhomogeneities lead to broad distribution of adhesion forces and is a general feature of particle adhesion [889]. 

Dynamic AFM methods also allowed to measure single-atom adhesion [300,895] and separate the different contributions such as van der Waals forces and chemical 

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The flotation of mica has been correlated to the adhesion force measured from surface force (SFA—see Section VI-4) experiments although, to these authors, it is clear that dynamic effects prevent an absolute comparison [69, 70],... 

Thundat T, Zheng X-Y, Chen G Y, Sharp S L, Warmack R J and Schowalter L J 1993 Characterization of atomic force microscope tips by adhesion force measurements App/. Phys. Lett. 63 2150... 

Fig. 27—Friction and adhesion forces measured on AFM show different dependences on reiative humidity (from Ref. [23]).

Figure 6.9 A schematic of the micromechanical force measurement (left) and video images of hydrate particles during each stage of the adhesive force measurement. (From Taylor, C.J., Adhesion Force between Hydrate Particles and Macroscopic Investigation of Hydrate Film Growth at the Hydrocarbon/Water Interface, MS Thesis, Colorado School of Mines, Golden, CO (2006). With permission.)...

Fig. 20 a Change in adhesion forces measured by SFM using chemically modified tip during the time-dependent etching process of polyethylene films, b Adhesion titration curves of PE films unmodified ( ) and etched with chromic acid for 45 min ( )... 

The Pulsed Force Mode allows investigations of the tip-sample interactions on a more quantitative level. For PFM measurements, the Dimension 3100 SFM was equipped with a Pulsed Force Mode box (WITec Instruments, Ubn, Germany). For adhesive force measurements, a standard micro-fabricated silicon cantilever with aluminum reflex coating (Olympus, AC240TS) with spring constants of about 2 N/m was used. To choose cantilevers with similar spring constants, their resonance frequencies were measured [13]. All measurements are performed under ambient conditions using the same set point of 0.5 V. 

The -propanol adsorption from the gas phase significantly decreases the adhesion force between the silicon oxide surfaces measured with AFM. Fig. 7 illustrates the adhesion force measured with a single tip (2.2 N/m) at various partial pressures of n-propanol. The adhesion force decreases 40% compared with the dry Ar case upon initial introduction of n-propanol partial pressure. This reduction is not as drastic upon further increase of the n-propanol partial pressure. A sudden change in adhesion with only 10% partial pressure indicates that a few monolayer thick n-propanol film, as shown in Fig. 4, is sufficient to reduce the adhesion between the silicon oxide surfaces. This behavior is in sharp contrast to the relationship between the adhesion force and the water adsorption isotherm. In the case of water, the adhesion force increases several fold when the relative humidity increases from zero to... 

Fig. 7 Adhesion force measured with AFM as a function of n-propanol partial pressure. The data are normalized to the dry Ar case, which is measured before the n-propanol exposure. Data are acquired with a single tip (2.2 N/m).

The ratios (FadhpoMsx/fhdhsi) (where fadhpoMsx and Fadhsi represent the adhesion force measured on PDMS and siheon substrates, respectively, and X... 

A typical fiber was 60 pm in diameter, about the same as a human hair, and the adhesion force measured was 30 pN. This force was readily determined by the 5 mm deflection of the fibre just before detachment. The interesting feature of the experiment reported by Tomlinson was the sudden attachment of the fibers when contact was approached. The surfaces leaped into contact He took trouble to apply no force pushing the surfaces together, and concluded that the molecular... 

In a recent study Raj et al. presented the first direct study of adhesion forces, by colloidal force microscopy, between smooth PLA films representing the polymer matrix, and a microbead of cellulose that mimics the cellulose material in flax fibers [65]. Normalized adhesion force measurements demonstrated the importance of capillary forces when experiments were carried out under ambient conditions. Experiments, conducted under dry air allowed for the deduction of the contribution of pure van der Waals forces, and the results, through the calculation of the Hamaker constant, show that these forces, for the PLA/cellulose/air system, were lower than those obtained for the cellulose/cellulose/air system and hence underlined the importance of optimizing the interface among these materials. The study demonstrated the capacity of AFM to probe direct interactions in complex systems by adjusting the nature of the surface and... 

The chemical modification by silani2a-tion (or other chemical reactions) of carbon, oxide, or metal electrode surfaces [21, 22] or SAM formation on gold surfaces with thiol or disulfide compounds [23] has been utilized for the tip functionalization. The systematic chemical derivatization of the tips was carried out with silane [10, 24-27] or thiol [17, 18, 20, 28-37] derivatives. Today, chemical differentiation of the terminal groups by FFM [5-20,28, 36, 37] or adhesive force measurements [17, 18, 20, 24-28, 30-37] is called chemical force microscopy (CFM) [17]. Adhesive and frictional forces can be mapped in x-y planes as CFM images. The adhesive... 

Fig. 3 A 5 X 5 rm (256 x 128) image of (a) an adhesive force map obtained by PFM-AFM and (b) a histogram of the adhesive forces measured in Fig. 3 (a), using the same CH3 tip on the same patterned surface that was observed in Fig. 2.

These adhesion forces measurements between functionalised tips and surfaces were then used as routine operation in order to validate the grafting of functional groups on tips. It is worth noting that the standard deviation is more important in N2 (20 to 25%) than in water (10 to 15%) due to capillary forces, which are not completely suppressed in N2. 

Adhesion Force Measurements on Polypropylene Surfaces The results obtained on polypropylene surfaces are presented in Fig. 4. 

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