Adhesion measurements microbalance

Table 3 lists some ionization properties of functionalized gold-thiol monolayers and relevant alkylsiloxane monolayers together with the appropriate bulk values. Monolayers with carboxylate terminal groups show abnormal wetting behaviour, which makes it difficult to determine accurately their surface pKa values308. Apart from contact angle titration, other methods were also used to study proton transfer equilibria at the mono-layer surfaces, such as quartz crystal microbalance (Table 3, line 1), measurements of the adhesion force between the monolayer deposited at the surface of an AFM tip and the same monolayer deposited on the substrate (chemical force microscopy, Table 3, lines 3, 4, 15), FT-IR spectroscopy (Table 3, line 7), adsorption of polyelectrolytes (Table 3, line 5) and differential capacitance measurements (Table 3, lines 12, 13). 

Two commercially available aerosol mass concentration analyzers have been described for the determination of aerosols and particulates [18], The instruments are both portable and relatively rugged and operate on ordinary commercial power. A portable system utilizing an electrostatic precipitator was described for mass concentration measurements in the range of airborne dust particles smaller than 10 /Am [19]. Chuan [20] also described a typical commercial instrument based on the piezoelectric quartz crystal microbalance for the rapid assessment of particular mass concentrations in the atmosphere. The adhesive coating used in the device is non-hygroscopic and non-reactive to the usual concentrations of pollutant gases in the atmosphere, such as CO, SO2, NOjt, and hydrocarbons. 

Corn [89] used a microbalance with a balance arm. With this particular apparatus, the quantity Ah corresponds to the deflection of the arm this arm carries a fiber of the material under investigation with a sphere at the end (Fig. III.l2). With this apparatus, adhesive forces can be measured over a wide range, from 10 to 10 dyn. The relative error of measurement is no greater than 4-10% for particles with diameters of 21-31 /xm, the error increasing to 24% when the particle size is increased to 90 jLtm. 

There are several techniques available for the measurement of the adhesion force of particles on substrates. These techniques involve both the study of individual particles (as in the microbalance technique) and of multiple particles by statistical counting (as in the centrifuge method). In each type of study the conditions of both the particles and the substrates must be known with high precision. The exact chemistry, physical size and shape, surface condition, temperature, and properties of the surrounding gas are vital parameters that must be quantified in order to obtain reproducibility. 

To gain a better understanding of what contact angles are measuring, Samuel, Zhao, and Law [26] measured the wetting (attractive) and adhesion force between water and surface 1-20 directly using a microbalance inside a tensiometer. A schematic of the measurement procedure is shown in Fig. 5.4, and details of the instrumentation... 

---