Contact angle measurement adhesion force

The surface forces apparatus (Section 2.3) enables the estimation of a surface energy term, Fq (Eq. 9), providing sufficiently smooth surfaces can be produced. In recent years Chaudhury, Pocius and colleagues have made a valuable contribution to the field of adhesion by developing the technique to study energies of adhesion and of surface energies of polymers [81-85]. These SFA results provide alternatives to values based on traditional destructive tests or contact angle measurements. 

Kunz W, BeUoni L, Bernard O, Ninham BW (2004) Osmotic coefficients and surface tensions of aqueous electrolyte solutions role of dispersion forces. J Phys Chem B 108 2398-2404 Lee L-H (2000) The gap between the measured and calculated liquid-liquid interfacial tensions derived from contact angles. J Adhesion Sci Technol 14 167-185 Li ZX, Lu JR, Styrkas SA, Thomas RK, Rennie AR, Penfold L (1993) The structure of the surface of ethanol/water mixtures. Mol Phys 80 925-939 LoNostro P, Fratoni L, Ninham BW, Baglioni P (2002) Water absorbency by wool fibers hofmeiter effect. Biomacromol 3 1217-1224... 

The interaction of the solutes with the membrane surface is normally characterized by the membrane hydrophilicity/hydrophobicity and a fixed charge using a contact angle measurement and a streaming potential, respectively. In this study, AFM was used to determine the force of adhesion between a HA-coated silica probe (illustrated in Figure 5.27) and the membrane surface. A typical force measurement... 

The Lifshitz theory is insufficient to account for the behavior of the second polymer series, in particular when examined with the polar probes. However, since the SiO, probe is known to contain hydroxyl groups, it was attempted to correlate the adhesion measurements with the behavior of water on the surface as measured by water-contact angle (see Table 2). The water-contact angle measurements were observed to correlate reasonably well with pull-off forces for the second polymer series and the polar probes, suggesting that the AFM-van der Waals approach could potentially be used to provide local hydrophobicity/hydrophilicity information and thus to differentiate between polymer surfaces, as an alternative to the method described by Sinniah et al. ... 

Garbassi and co-workers [15] have the reviewed the subject of polymer surface analysis including surface characterisation techniques using a wide variety of spectroscopies and measurement of contact angles and surface force. Also, studies on surfaces and applications of surface science, including wettability, adhesion, barrier properties, biocompatibility, reduction of friction, and wear resistance, were carried out. 

The contact angle of water on SAM-coated or on uncoated silicon (table 2.4) ranged from 4° to 113° after the SAM deposition and from 34° to 103° after the force measurements. For 0 , we used the contact angles measured after the force measurements. For 02, we assumed it was 27° for Si3N4 [21], Assuming these values for the contact angles of water, i.e., 0j = 34° to 103° and 02 = 27°, then 0 -t 02 < 180°, which means the condensed capillary could generate an adhesion force. Therefore, from eq. (2.7), the adhesion force is proportional to the radius of curvature of the fitted spherical surface. 

The component of the surfaces under investigation can be determined by contact angle measurements, using a nonpolar liquid as the interaction of a nonpolar liquid is restricted to dispersive forces [83], The nonpolar liquid should have a high surface tension (such as DIM) to avoid the spreading of the liquid. In the case of negligible acid-base interactions (i.e., for nonpolar solids), the total work of adhesion between a solid and a liquid... 

Adhesion hysteresis is often observed, analogously to contact angles, as the force-area curves measured... 

A monolayer of adsorbed molecules is sufficient to mask the surface forces emanating from contacting substrates. Direct measurement of van der Waals forces (1), adhesion measurements in high vacuum (2) and contact angle measurements (3) illustrate this point. A much thicker layer is required to prevent mechanical interaction between the contacting surfaces. In the case of a sphere on a flat the film thickness must be rather greater than the diameter of the circular contact region (4) in order to supress the interaction of the substrates. 

In a recent case study (see Svendsen et al, 2007 and also Problem 6.1), in collaboration with a paint company, the adhesion of six different epoxies-silicon systems has been studied. These paints are used in marine coating systems. Some epoxies showed adhesion problems in practice while others did not. The purpose of the study was to understand the origin of these problems and whether adhesion could be described/ correlated to surface characteristics, e.g. surface tensions. An extensive experimental study has been carried out including both surface analysis (contact angle measurements on the six epoxies, surface tension of silicon at various temperatures, atomic force microscopy (AFM) studies of the epoxies), as well as measurements of bulk properties (pull-off adhesion tests and modulus of elasticity). Theoretical analysis included both estimation of Zisman s critical surface tensions and surface characterization using the van Oss-Good theory. 

Figure 6.1 The contact angle, , as a force balance. Cos d is a measure of the equilibrium between the energy of cohesion between the molecules of liquid L (horizontal arrows) and the energy of adhesion (vertical arrows) between liquid L and solid S. The solid lines represent apolar interactions and dashed lines are polar interactions. 

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