Measurement of the contact angle

Contamination of the droplet by adsorption of impurities from the gas phase tends to reduce 0 if yM and/or ySL is reduced and jSA remains more or less constant. 

A solid surface, even when apparently smooth, may have impurities and defects that vary from place to place on the surface and from sample to sample. Roughness reduces 0 when the value on a smooth surface is 90° and increases it when the value there is 90°. 

Contact angles on finely divided solids are more difficult to measure, but are often more desired and more important than those on large solid surfaces. One method of obtaining such contact angles is to pack the powder into a glass tube and measure the rate of penetration of the liquid into it (Brail, 1974). The distance of penetration l in time t of a liquid of surface tension yM and viscosity T is given by the modified Washburn equation (Washburn, 1921)  

This method is not reliable for dilute solutions of surfactants in many cases since it depends upon knowing the (constant) value of y. If adsorption of the surfactant 

Adsorption of the surfactant onto the solid also makes this an unreliable method for determining the wetting effectiveness of dilute surfactant solution for powdered solids. Because of the small ratio of solution volume to solid-liquid interface, solutions that contain highly surface-active material that adsorbs well at the solid-liquid interface are rapidly depleted of surfactant and may penetrate more slowly than solutions of weakly surface-active material. 

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Measurement of the contact angle at a solid-liquid interface is a widely used method for the determination of the surface energy of solid polymers. Fowkes [1] first proposed that the surface energy of a pure phase, y y could be represented by the sum of the contribution from different types of force components, especially the dispersion and the polar components, such that ... 

The simplest technique introduced by Young as early as 1805 [18] is the measurement of the contact angle as a measure of surface tension and surface energy [1,19, 20,21], In many cases this gives an indication of surface composition and can be used to observe changes in composition, structure and/or roughness at the surface during a particular surface treatment. A quantitative description or distinction between different parameters is hardly possible in most cases. 

Self-assembled monolayers are formed spontaneously by the immersion of an appropriate substrate into a solution of active surfactant in an organic solvent. After the substrate is immersed for a time from minutes to hours, it is rinsed with ligroin, methanol, distilled water, and dried in a steam of nitrogen. An apparent effect of the monolayer coating is the drastic change in wettability of the surface so that the measurement of the contact angle can be considered as an effective way to detect the formation of the SAMs. 

One of the most common ways to characterize the hydrophobicity (or hydrophilicity) of a material is through measurement of the contact angle, which is the angle between the liquid-gas interface and the solid surface measured at the triple point at which all three phases interconnect. The two most popular techniques to measure contact angles for diffusion layers are the sessile drop method and the capillary rise method (or Wihelmy method) [9,192]. 

Arce etal. [140, 141] have studied the dynamics of 1-dodecanethiol and bu-tanethiol SAMs on Au(lll), applying ex situ and in situ STM. The potential of zero charge for the thiol-modified Au(lll) electrode was determined for self-assembled monolayers of octade-canethiolate (—0.52 V), undecanethiolate (—0.49 V), propanethiolate (—0.3 V), and H, //,2//,2 //-perfluorodecanethiolate (1.04 V) ]142]. The potentials (expressed versus Ag]AgCl]saturated KCl electrode) were determined from the measurements of the contact angle for a droplet of 0.1 M NaCl04 aqueous solution. 

In printing, a film of ink is formed by wetting the surface with the compression force of the rollers. This force spreads the ink over the surface and into any capillaries that may be present. Spreading and penetration are controlled thermodynamically and kinetically. Measurement of the contact angle can be used to determine the thermodynamics of wetting. This angle can be used also to determine the contribution that polarity and dispersive forces of the liquid make to the wetting of the surface. 

By means of Eq. (8.9) the surface tension of the solid can be calculated from measurements of the contact angle. If Js is known, Eq. (8.7) provides the possibility of calculating the interfacial tension vcl- The contact angle can be predicted for solid-liquid systems by means of Eq. (8.10). 

This equation permits the derivation of yd and yps via measurements of the contact angles 6 of two liquids if yLV, yfv and ypv of both liquids are known. 

One of the most important thermodynamic characteristics of foam films is the contact angle appearing at the contact of the film with the bulk phase (solution) from which it is formed. The measurement of the contact angle and the film tension related to it is an... 

The presence of pore network effects and interconnectivity can of course strongly influence the reliability of the data obtained via both techniques. Therefore development of new models describing these effects combined with the measurement of the contact angle will strongly contribute to assess the PSD from MIP measurements. 

One or two additional functional groups introduced via other a-hydrosulfide bolaamphiphiles can be characterized by measurements of the contact angle 0a of water and hexadecane (= HD) droplets, depending on the hydrophobicity and the position of the end groups (Table 1). They also attest to the short-range... 

Even when the principle of the method seems to be very simple, the measurement of the contact angle at high temperatures using this method is rather difficult for several reasons. The accuracy and reliability of this method is very low, in general an error up to 20% may occur. The low reproducibility has many reasons. 

It should be noted that a reduction of y v uot always accompanied by a simultaneous reduction of yg. and hence it is necessary to obtain information on both interfacial tensions this, in turn, means that measurement of the contact angle is essential in the selection of wetting agents. Measurements of ygL and y y should be carried out under dynamic conditions (i.e., within very short times). In the absence of such measurements, the sinking time (as described above) could be applied as a guide for wetting agent selection. 

Measurement of the contact angle between the solid and different liquids, and application of equations which express the surface tension of the solid as a function of the contact angle and the surface tension of the liquid. 

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