Surface roughness contact angle affected

Test methods used to determine the uniformity of substrates are numerous and vary with the type of material. They are generally the same tests used to characterize the material or to determine its fundamental physical properties. Tests that are commonly employed are hardness, tensile strength, modulus, and surface characteristics such as roughness or contact angle with a standard liquid. Often a test similar to the nonvolatile test mentioned above is used to determine if there are any compounds in the substrate that are capable of out-gassing on exposure to elevated temperatures. Moisture content of certain hydroscopic polymers, such as nylon and polycarbonate, is also known to affect adhesion. 

At present, many authors I20-I26 follow another concept From the plot of pj p [M /t) versus the surface tension of the liquids, the geometric factor K is calculated for those liquids that should wet the solid completely. By inserting this K value and [t]/p- ] M2/t for these liquids into Eq. (13). their contact angles 0 are calculated and used for the interpretation of the solid-vapor surface tension of the porous material. This procedure is dubious, because it can be expected that the contact angles, calculated from the Washburn equation, are affected by roughness and porosity. If we apply this procedure to the PTFE powder for hexadecane, a contact tingle 0 = 88 would be obtained. However, it is well known that the contact angle of hexadecane on a flat and smooth... 

Current theories to explain hysteresis of contact angles are primarily based on the concepts of surface roughness, surface heterogeneity, friction, and adsorption phenomena. Unintentional adsorption, or contamination—the result of inadequate experimental technique—is, however, the most frequent explanation. As all systems involving solids are subject to the reasons indicated above for hysteresis, we chose the system mercury-benzene-water, which should be affected only by adsorption phenomena, controllable under proper experimentation. An additional advantage is the fact that all interfacial tensions involved can be measured. 

Characterization of the membrane surface It should be emphasized that the properties of the membrane surface strongly affect membrane performance. Contact angle is often used as a measure of surface hydrophilicity or hydrophobicity. X-ray photoelectron spectroscopy (XPS) provides the data on atomic compositions at the membrane surface. Recently, attentions have been focused on the nodular structure as well as the roughness at the membrane surface that can be measured by atomic force microscopy (AFM). 

Standpoint, in practice, the type of topography present may carry with it certain practical consequences. For example, if the surface, rather than being randomly rough, has pores, crevices, capillaries, or other structures that have their own characteristic wetting and penetration properties, the apparent contact angle will be affected by the thermodynamics and kinetics associated with such structures. 

It is well-known that the roughness of a solid surface affects the equilibrium contact angle. Modify Equations 2.2 through 2.6 using Wenzel s approach to include the effects of surface roughness. Note the following ... 

Surface morphology can also affect the detachment of a gas bubble. Equation 4 is only suitable for a clean, smooth surface. When a water droplet completely wets a rough surface, the effect of roughness on the contact angle is given by the Wenzel equation [11] ... 

In addition to its influence on surface reactivity, surface structure is also seen to affect wettability on the micrometer scale, as is best illustrated by the lotus effect (see Chapter 3b). The lotus leaf is superhydrophobic, i.e. has a water contact angle of about 160°, thanks to the combination of the waxes on the surface with a characteristic dual micrometer- and nanometer-scale surface topography. Without the structure, the wax chemistry would only impart mild hydrophobicity to the surface. Superhydrophobicity comes about only when a water droplet is in contact with a rough surface with a substantial enclosure of air beneath the drop (Figure 9). This is the so-called Cassie-Baxter state, named after the authors of the work that described the contact angle of water droplets in this state by means of the equation ... 

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