Wetting tension

An adhesive should possess a Hquid surface tension that is less than the critical wetting tension of the adherend s surface. 

The difference yg s - y, s is known as the wetting tension. If the value lies above zero wetting will take place. In actual practice wetting of surfaces by aqueous solutions is of particular interest. But the opposite case—namely, wetting by oil—is often important too. 

Fogging is formation of small water droplets (visible condensation) on the surface of a polymer film. Undesirable effects may result from fog formation, such as reduction of clarity and dripping. Incorporation of antifogging agents eliminates the reduction of transparency by migration to the surface and increases the polymer surface critical wetting tension. This results in... 

According to the above equation, the time t that is required to wet a powder is proportional to the viscosity of the surrounding medium, the depth of the pores, and the transported volume. The wetting time, on the other hand, is inversely proportional to the wetting tension — the driving force that promotes wetting — and to the cube of the radius of the pores in the agglomerates. 

ISO 8296, Plastics - Film and sheeting - Determination of wetting tension, 1987. 

General Remarks. In the use of products containing alkyl ether sulfates, oily soil removal as well as dispersion plays an important role. The driving force responsible for the separation of oily soil from a substrate (Fig. 10) is the wetting tension j defined by equation (1) ... 

The sketch in Fig. 10 shows the equilibrium of forces with an obtuse contact angle in the oil phase (6o). In this case the wetting tension, j, of the aqueous phase is positive, which means that the adhering oil droplet is pushed together by the aqueous phase. With the increase in j the tendency of an oil droplet to be cut off and removed from a solid substrate increases. Because of this, the impeding force for the removal of oil is the interfacial tension oil/water (Yq )> which should be minimized. By minimization of the interfacial tension, moreover, the requirements for emulsification and stabilization of soil in the washing and cleaning liquid will be improved. 

The so-called wetting tension j can be defined from the following equation ... 

When two droplets - one of surfactant solution and the other of oily soil - are set on a solid surface, on the basal plane two wetting tensions jA and jB will act [3]. When the two droplets approach each other, so that a common interface is formed, at the contact line the difference of the wetting tension will act. This parameter is called oil displacement tension ... 

Figure 3.11 Wetting tension of two all-purpose cleaners versus different surfaces [11].

Figure 3.12 Two liquids A (detergent) and B (oily soil) on a solid surface (a) separated and (b) in contact, yA and yB = wetting tensions, yAB = interfacial tension, R = interfacial wetting tension [3].

Fig. 10 a Surface tension of the surfactant solution (stars, right axis) and receding contact angle of the solutions (squares, circles, left axis) on a photoresist layer processed at the threshold dose corrected by the effect of swelling, b Wetting tension y v cos 0 calculated from the values given in (a). The dashed line marks the concentration ceff. Above a, the assumed distribution of the surfactant molecules at various concentrations is drawn schematically... 

To estimate the influence of the surfactant adsorption on the capillary forces, the wetting tension yiv cos was calculated from the values given in Fig. 10a. The results drawn in Fig. 10b show for both measurement series a minimum of the capillary forces exactly at the concentration ceff. The capillary forces are reduced by about 20% compared to water. This confirms the hypothesis that the reduction of the pattern collapse is caused by a hydropho-bizing of photoresist processed with the threshold dose by cationic surfactant adsorption. Unfortunately the inverse ADS A method could not be applied at relative surfactant concentrations >0.2 since the bubbles became unstable due to the lower surface tension. Thus it cannot be estimated how the wetting tension evolves at higher concentrations. 

The wettability, which we propose to denote A immy, is directly measured by the difference between two surface tensions. This difference was called the adhesion tension by Adamson (1967) and the work of immersional wetting per unit area by Everett (1972). It has also been referred to as the wetting tension . 

Keywords Si-Surfactants / Wetting Tension / Surface Tension / Contact Angle /... 

In order to determine the contact angles (0) (Eq. 2) of aqueous surfactant solutions (c > cmc) on an apolar perfluorinated surface (FEP , Du Pont, solid surface tension 18.9 mN/m) the surface tension (y, ) and the wetting tension (a) have been measured [7],... 

Finite values of this contact angle imply incomplete wetting and droplet formation. If, on the other hand, f30 = 0, the droplets spread out over the entire wall. If the so-called wetting tension is Complete wetting takes place and a film develops. 

There is no direct way by which /gy /sL be measured, but the difference between /jy and can be obtained from contact angle measurements (= /lv COS0). This difference is referred to as the wetting tension or adhesion tension [5-7]... 

FIGURE 10.4 Cohesive energy plot of contact angle and wetting tension data for an epoxy coating. (From Hansen, C.M. and Wallstrom, E., J. Adhes. (http //www.tandf.co.uk), 15, 275-286, 1983. With permission.)... 

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