Contact angle microscopic

The question has been taken up again recently by L. R. White who distinguishes between the macroscopic and the microscopic contact angles (Fig. 3.9). The macroscopic contact angle 6 is governed by the slope of a... 

Fig. 3.9 Macroscopic contact angle 0, and microscopic contact angle 0,. S = solid L — liquid G = gas. (White .)...

Within the core region the profile of a drop is modified by surface forces, such as long-range van der Waals and electrostatic double-layer forces [220], These forces affect the profile in a range of 1-100 nm. They can cause a difference between the microscopic contact angle and the macroscopic one (which enters into Young s equation) [268,269], If the liquid is attracted by the solid surface and this attraction is stronger than the attraction between the... 

Figure 1.5. As Figure 1.4 but showing a microscopic contact angle different from the macroscopic 0M-... 

Although previous results [2] show a contact angle of 170° at an applied voltage of 0 V, the microscopic contact angle is expected to be closer to 180° (the Young s angle Oq) since the... 

Wetting and Spreading, Fig. 8 Schematic depiction of the contact line region on a microscopic level illustrating the distinction between the apparent (macroscopic) contact angle 6 and the intrinsic (microscopic) contact angle 6 due to surface roughness effects on the substrate... 

There has also been some discussion that the contact angle generally observed is the extrapolation of the profile of the bulk drop to the solid surface ( macroscopic contact angle). The real contact angle ( microscopic ) is quite different. Differential ellipsometry can measnre molecnlar dimensions quite well and show that the microscopic contact angles are indeed qnite different (Heslot et al., 1992). 

Young s equation did not account for the third-phase interaction in the vicinity of each interface that may change the interfacial energies. To correct this equation, line tension was introduced. Line tension is defined as the interfacial energy per unit length across the three-phase contact line. It is found that line tension is more important when radius of droplet is less than a micron (Marmur 1996), which can be found in microelectronic systems and microfluidic devices (Heine et al. 2005). Thus, modified Young s equation can be used for microscopic contact angle calculation, described as... 

In this method (Fan and Cagin 1995 Hautman and Klein 1991), a microscopic contact angle is calculated by comparing the average height of the center of mass of the liquid cluster to that of an ideal sessile drop in the shape of a sphere intersecting the surface plane. The position of the sphere relative to the plane is determined by the center-of-mass position and the condition that the volume of the sphere in the half space above the surface plane contains the correct number of liquid molecules (assuming a uniform density in the idealized drop equal to that of bulk). The equation employed is... 

Yaminsky V. (1995) Molecular mechanisms of hydrophobic transitions. InJ Drelich, JS Laskowski KL Mittal (eds), Apparent and Microscopic Contact Angles, pp. 47—95. VSP, Utrecht. 

Copolymers 7Fo3iOK-M-B2 and 7Fox20K-IP-B2 were dip-coated on cover glasses for optical microscopes. Contact angles of water on such surface were measured with Cahn DCA. The advancing contact angle of water on freshly prepared copolymer films was about 110 , which is comparable to that for Teflon (ca. 109 ). The same contact angle reduced to 95 after the films were immersed in water for two months. 

The transversality condition (2.21) at h = gives h (h ) = 0, which means the drop edge approaches the equilibrium film of thickness on the solid surface at zero microscopic contact angle. This condition allows the determination of the integration constant in Equation 2.26 as... 

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