Contact angle apparent

Fig. 5. Sessile drop on a rough surface true contact angle BTA and apparent contact angle BTH. Thick curve = surface of solid (s) thin curve = surface of liquid (1) v = vapour. T is the triple point HTR a horizontal AT a tangent to the solid surface BT a tangent to the liquid surface.

Figure 7.9 Water drop on a superhydrophobic surface showing a high apparent contact angle app- The combined effect of hydrophobicity and roughness on the right length scale, causes the Lotus effect.

Experiments on forced wetting showed that, in general, the apparent contact angle depends not only on the speed v but also on the viscosity 77, and the surface tension 7l of the liquid. 

Figure 7.13 Schematic velocity dependence of the experimentally determined apparent contact angle app-...

Especially for low capillary numbers the apparent contact angle is a function of Ca only [277]. For this reason, results on dynamic wetting are usually plotted with respect to the capillary number. 

Apparent Contact Angle and the Shape of the Contact Line... 

The phenomenon has been studied by Gao and Sonin [16] and it has been shown that the cross-sectional area and apparent contact angle of a printed bead is a function of the substrate temperature and the print-scan velocity. A quantitative analysis of jet-printed phase change materials was reported by Schiaffino and Sonin [17] for isothermal conditions. The results showed that the contact angles for the printed drops could be varied as a function of the substrate temperature. By adjusting the substrate temperature and the scanning speed, the feature size and print quality can be adjusted without having to modify the substrate surface energy. 

The effect of roughness on the apparent contact angle of a solid surface has been given quantitative form by R. N. Wenzel.8 If r, the roughness factor , is the ratio between the real and the apparent area of the surface, then... 

Another modification was performed by Cassie considering the surface heterogeneity [33]. If two phases (phases 1 and 2) with different surface energies exists on the identical surface, the apparent contact angle is described as follows ... 

When thick oxide films are formed on a sessile drop, oxidation is evident because at melting, the surface of the liquid is not smooth. However, thin oxide skins of up to a few tens of nanometers are easily deformable, allowing a smooth surface to be formed. However, even when very thin, skins do not allow metal/substrate interface to be established and as a result the apparent contact angles are as high as 160°. A classical example is pure Al on A1203 (Figure 6.20) (Brennan and Pask 1968, Eustathopoulos et al. 1974, Weirauch 1988). At room temperature, Al is instantaneously covered by an oxide layer about 2 nm thick formed by the reaction ... 

Thus, the surface of liquid Al is covered by a film of A1203 on melting and this results in an apparent contact angle of about 160° (Figure 6.20). On heating to 827°C, two different effects can be observed that depend on the value of P02 in the furnace. In a metallic chamber furnace with a low P02, a decrease of the... 

Fig. 2. A simplified drawing of the Gibbs energy curve for a real wetting system. Each minimum defines a metastable state. The lowest minimum defines the most stable state and the most stable, apparent contact angle (MSAPCA). The lowest and highest APCAs that are associated with a metastable state are the receding contact angle (RCA) and the advancing contact angle (ADCA), respectively.

WolanskyG, Marmur A. (1999) Apparent contact angles on rough surfaces the wenzel equation revisited. Colloids Surf A 156 381-388. 

The apparent contact angle is then calculated fromtan = 0.5A/(if, — i ) and steps 1 - 4 are repeated for a different value of, if the values of the given and the calculated apparent contact angles do not match. 

Figure 10 presents the interface shape of the rivulet for wall superheat as 0.5 K and Re = 2.5. Here also presented the data on pressure in liquid and heat flux density in rivulet cross-section. The intensive liquid evaporation in near contact line region causes the interface deformation. As a result the transversal pressure gradient creates the capillarity induced liquid cross flow in direction to contact line. Finally the balance of evaporated liquid and been bring by capillarity is established. This balance defines the interface shape and apparent contact angle value.For the inertia flow model, the solution is obtained from a non-stationary system of equations, i.e., it is time-dependable. In this case the disturbances in flow interface can create the wave flow patterns. The solutions of unsteady state liquid spreading on heat transfer surface without and with evaporation are presented on Fig. 11. When the evaporation is not included (for zero wall superheat) the wave pattern appears on the interface. When the evaporation includes, the apparent contact angle increase immediately and deform the interface. It causes the wave suppression due to increasing of the film curvature.

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