Contact angle selected values

The adsorption of block copolymers from a selective solvent was considered by Ligoure (1991). He predicted the existence of surface micelles (see Fig. 3.22) in the case when the block interacting unfavourably with the solvent only partially wets the surface. The model predicts a critical surface micellar concentration (csmc) that differs from the bulk cmc. When the contact angle, which characterizes the interfacial interactions between the copolymer, adsorbing surface, and solvent is lower than some universal value, surface micelles were predicted to appear at a lower copolymer concentration than bulk ones. Experimental results on surfaces are discussed in Section 3.8.4. 

Table 5. The values of contact angles of selective moistening of oil drop on glass...

Two different liquids may compete with each other, if each of them individually wets a solid surface. In this case the resulting contact angle corresponds to that of a better wetting liquid with nature similar to that of the solid surface. If the surface is better wetted by water than by a hydrocarbon ( 0<9O°), it is referred to as hydrophilic (oleophobic) in the case when the surface is better wetted by non-polar hydrocarbon (0>9O°), it is referred to as hydrophobic (oleophilic). For selective wetting, as opposed to wetting in air, the contact angle, 0, can assume any value between 0° and 180°. When 0 = 0°, the more polar liquid spreads over the solid surface, forcing the less polar liquid away when 0 = 180° the situation is opposite the non-polar liquid phase completely forces the polar liquid away from the solid surface. 

The phase that is similar to the stabilizing reagent tends to become a dispersion medium, as clearly seen in the example of emulsions stabilized by finely dispersed powders. Such stabilization is possible under the condition of a finite selective wetting of powder, i.e. at finite values of contact angle, 0° < 0<180°. The powders are able to stabilize the phase that poorer wets the particles, while the liquid that is more similar to the powder becomes the dispersion medium. The reasons of such behavior are apparent from Fig. VIII-12. If water droplets covered by hydrophobic powder, such as e.g. carbon black, are placed into hydrocarbon phase (oil), the layer of carbon black due... 

The main uncertainties in mercury porosimetry are related to the selection of values for 7 and 0. A surface tension of 480 mJ m is usually assumed, but mercury is easily contaminated with impurities that significantly modify its surface tension. Likewise, a contact angle of 140° is typically assumed but it can significantly change from solid to solid, and can vary depending on the physical and chemical state of the solid surface under concern. Lowell and Shields, in their well-known monograph [60], have described in detail the fundamentals, practical aspects and interpretation of mercury porosimetry results. 

Multicomponent systems may also involve the selective adsorption of one component at the SL interface. Since the component that lowers the interfacial tension will be preferentially adsorbed, the rate of the adsorption process can affect the local tension and the contact angle. In many systems, the rate of adsorption at the solid surface is found to be quite slow compared to the rate of movement of the SLV contact line. As a result, the system does not have time for the various interfacial tensions to achieve their equilibrium values. Most surfactants, for example, require several seconds to attain adsorption equihbrium at a LV interface, and longer times at the SL interface. Therefore, if the hquid is flowing across fresh solid surface, or over any surface at a rate faster than the SL adsorption rate, the effective values of olv and osl (and therefore 6) will not be the equilibrium values one might obtain from more static measurements. More will be said about dynamic contact angles in later chapters. 

To explore the entire energy landscape, Ecav could be plotted as a function of and f. The extrema on this landscape will be the equilibrium solutions, discussed above, of which some will be the stable solutions. In this work, we will select a probable path in this landscape. The values of Ecav will be plotted for this path. To this end, assume that the liquid-air interface is first in the Cassie-Baxter state. As the liquid-air interface moves toward the other equilibrium states, the value of 9u will increase until it reaches the next available equilibrium state given in Table 1. At each intermediate state, between the equilibrium states, it will be assumed that the interface is spherical, the gas pressure is such that it is in accordance with the ideal gas law (equation (16)) and that the interface is in mechanical equilibrium (equation (17)). Thus, equations (16) and (17) are satisfied, however, equation (18) is not. This implies that in the intermediate states the local contact angle condition... 

Interfadal tension between two fluid phases is a definite and accurately measurable property depending on the properties of both phases. Also, the contact angle, depending now on the properties of the three phases, is an accurately measurable property. Experimental approaches are described, e.g., in Refs. 8,60, and 63 and in Ref. 62, where especially detailed discussion of the Wilhehny technique is presented. Theories such as harmonic mean theory, geometric mean theory, and acid base theory (reviewed, e.g., in Refs. 8, 20, and 64) allow calculation of the soHd surface energy (because it is difficult to directly measure) from the contact angle measurements with selected test liquids with known surface tension values. These theories require introduction of polar and dispersive components of the surface free energy. 

Stepwise Pressurisation. A Micromeritics Pore Sizer 9310 was used in conjunction with Micromeritics software and a PC XT computer over the pressure range 1.6 psia to circa 30,000 psia. The pressure values were defined from a selectable Pressure Table within the program. The default values for mercury contact angle and mercury surface tension were 130 and 485 mNm respectively. The stepwise pressurisation equilibrium times can be selected from zero to 30+ seconds, the default time being 10 seconds. 

The coating system which was best suited in terms of anti-adhesive properties was to be tested for its colourability with dyes and pigments to provide for decorative effects, where required. Since the lowest value for the clear coating was found in compositions containing CgFi7C2H4Si(OR)3, these systems were selected to be imparted with different colourants. The contact angles of the coloured nanocomposite layers towards water and di-iodomethane as well as the surface energy values calculated from them are also included in Table 6.3. Soluble dyes did not affect... 

---