Surface roughness and contact angle

In the context of the structural perturbations at fluid-solid interfaces, it is interesting to investigate the viscosity of thin liquid films. Eaily work on thin-film viscosity by Deijaguin and co-workers used a blow off technique to cause a liquid film to thin. This work showed elevated viscosities for some materials [98] and thin film viscosities lower than the bulk for others [99, 100]. Some controversial issues were raised particularly regarding surface roughness and contact angles in the experiments [101-103]. Entirely different types of data on clays caused Low [104] to conclude that the viscosity of interlayer water in clays is greater than that of bulk water. 

Table 3. Surface Roughness and Contact Angle Hysteresis Plates...

R.N. Wenzel, Surface roughness and contact angle, /. Phys. Colloid Chem., 53, 1466-1467(1949). 

The effect of surface roughness on contact angle was modeled by several authors about 50 years ago (42, 45, 63, 64]. The basic idea was to account for roughness through r, the ratio of the actual to projected area. Thus = rA. lj apparent and similarly for such that the Young equation (Eq.-X-18) becomes... 

As can be seen in Table 6.5, ONB in APG solution of concentration C = 100 ppm took place at significantly higher surface temperatures. It should be noted that the ONB in surfactant solutions may not be solely associated with static surface tension Sher and Hetsroni (2002). Other parameters such as heat flux, mass flux, kind of surfactant, surface materials, surface treatments, surface roughness, dynamic surface tension and contact angle need to be considered as well. 

As the untreated surface and the weakly ablated one possess the same surface roughness parameter, contact angles may be compared. Hence for all the test hquids used, the contact angles are by 10-20° lower on the slightly ablated samples than on the untreated surfaces. 

For surface roughness evaluation, contact angle of water was measured on a surface coated with a water-repellent agent Heptadecafluorodecyltrimethoxysi-lane, which is a fluoroalkyltrimethoxysilane (FAS), was used as the water-repellent agent the partially hydrolyzed FAS was coated on the flowerlike AI2O3 thin films and dried at 60°C for 20 h. 

Wettability is the result of attractive forces between the adhesive and the substrate. When the adhesive spreads excellently on the surface, an increase of wetting and penetration into the surface roughness occurs. Contact angles and surface tension can be measured (Fig. 41). Wettability is crucial for good adhesion, but it is not the only measure for adhesion because other influences play important roles. 

It should be emphasized that evaporation and condensation are very complex nonequilibrium topics that are the source of many specialized textbooks. The processes of condensation and evaporation are related to physicochemical parameters, including temperature, vapor pressure, surface tension, surface energy and contact angle, surface impurities, homogeneity and roughness. Phase change is a local phenomenon, and some degree of local supersaturation is required to initiate condensation and desublimation. 

Increase adhesion tension. Maximize surface tension. Minimize contact angle. Alter surfactant concentration or type to maximize adhesion tension and minimize Marangoni effects. Precoat powder with wettahle monolayers, e.g., coatings or steam. Control impurity levels in particle formation. Alter crystal hahit in particle formation. Minimize surface roughness in milhng. 

Tab. 2.1. Results from three groups using a wide variety of SAMs as surface treatments [7c, 33d, 52], Several authors have shown loose correlations between improved OTFT performance and higher contact angle of the SAM. Varying results in similar surface treatments indicate that many mechanisms are at work, including contact angle, but probably also involving deposition conditions, surface roughness, and chemistry.

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