Wetting Studies

Qualitatively similar results were obtained for the 76Cu-24Ti alloy, Pd/Si alloys, and Cu-Ni-Ti alloys those that contain Ti or Zr wet and spread on the AIN substrates, whereas those that do not contain Ti or Zr (e.g., 94Pd-4Si-2Al) ball up and fall off the AIN after cooling.  

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Other noncontact AFM methods have also been used to study the structure of water films and droplets [27,28]. Each has its own merits and will not be discussed in detail here. Often, however, many noncontact methods involve an oscillation of the lever in or out of mechanical resonance, which brings the tip too close to the liquid surface to ensure a truly nonperturbative imaging, at least for low-viscosity liquids. A simple technique developed in 1994 in the authors laboratory not only solves most of these problems but in addition provides new information on surface properties. It has been named scanning polarization force microscopy (SPFM) [29-31]. SPFM not only provides the topographic stracture, but allows also the study of local dielectric properties and even molecular orientation of the liquid. The remainder of this paper is devoted to reviewing the use of SPFM for wetting studies. 

The MAMs were found to be closely packed. NEXAFS and FTIR spectroscopy studies revealed that the molecular tilting angles relative to the surface normal varied from 4° to 21° as a function of Ax. In wetting studies it was found that the highest water contact angles ( 131°) with the lowest contact angle hysteresis... 

Cohen et al. [361] studied, amongst other things, the influence of heat on monolayers of OTS and of arachidic acid adsorbed on surfaces of aluminium oxide formed on aluminium surfaces. They used wetting studies and infrared spectroscopy and concluded that, whereas arachidic acid layers deteriorated in an irreversible manner at about 100°C, OTS layers survived intact to about 140°C. This result is attributed by these authors to polymerisation of the latter material but could also be due to a rather different bonding mechanism between the organic material and the aluminium oxide. 

The wetting balance technique is a variant of the maximum pull (or detachment) method used to measure liquid-vapour surface tensions (Keene 1993). It is nowadays widely employed in the electronics industry to quantify wetting of solders, but has also been used for wetting studies in metal/ceramic systems (Naidich and Chuvashov 1983b, Nakae et al. 1989, Rivollet et al. 1990). As compared to the sessile drop method which needs planar substrates, solids of various geometry can be studied by this technique. 

Although some of the concepts established for metal/ oxide systems are also valid for non-oxide ceramics, there are other concepts which are specific to these kinds of ceramics, owing to their predominantly covalent (SiC, BN, AIN) or metallic (TiC, TiN, WC) character. These materials seldom can be obtained as the high-purity monocrystalline specimens desirable for fundamental wetting studies. Usually, they are sintered materials with impurity contents higher than 0.1% and they often contain open porosity. Further difficulties arise from the high oxidization tendency of many of them, the presence of an oxide layer dramatically changing their wettability by liquid metals. 

Garcfa-Payo, M.C., Izquierdo-Gil, M.A., and Femandez-Pineda, C. Wetting study of hydrophobic membranes via Uquid entry pressure measurements with aqueous alcohol solutions, J. Colloid Interface Sci., 230, 420, 2000. 

Having mentioned several times the use of surfactant solutions in wetting studies, we now consider specifically some of the effects their presence can have on contact angles and wetting. The action of surfactants derives from their adsorption at the various interfaces and the resultant modification of interfacial tensions. In terms of the Gibbs equation, the relationship between the specific adsorption of a solute, G, and surface tension is given by... 

De Ruijter, M. J. D., Blake, T. D., and Coninck, J. D. 1999. Dynamic Wetting Studied by Molecular Modeling Simulations of Droplet Spreading. Langmuir 15 7836. 

B. Bhushan and Y. C. Jung, Wetting study of patterned surfaces for superhy-drophobicity, Ultramicroscopy, 107,1033-1041 (2007). 

Most equilibrium wetting studies centre around measurements of the contact angle - the smaller the angle the better the liquid is said to wet the solid. Typical examples are given in Table 11.1 for water, with a surface tension of 72 mN m, on various substrates. 

Today, the field of wetting studies benefits from the insights of sophisticated analytical techniques yet, meaningful information is stiU obtained on adsorption and wetting phenomenons relying on simple apparatus and techniques to measure surface tensions and contact angles. It is our purpose here to explain how this approach enables characterization of the wetting behavior of surfactant solutions. 

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