Triple line pinning

The roughness of solid surfaces affects wetting owing to two different effects the first is the fact that the actual surface area is increased and the second is pinning of the triple line by sharp edges. 

Figure 1.16. Effects of a sharp edge on the contact angle formed by a meniscus advancing on a solid surface. In (a) 1,2,3 and 4 denote the successive configurations of the liquid surface when the liquid volume is increased. With a constant volume of liquid, an advancing triple line is either pinned (b) or...

Our steering results are demonstrated using an experimentally validated numerical model [20] of droplet motion inside the UCLA electrowetting system [21, 22], This model of EWOD fluid dynamics includes surface tensimi and electrowetting interface forces, viscous low Reynolds two-phase fluid flow, and the essential loss mechanisms due to contact angle saturatimi, triple point line pinning, and the related mechanism of contact angle hysteresis. 

Watanabe M, Eiwa Y, Tokunaga S (2011) Suitable properties of ink to oeate inkjet-printed surface-directed microchaimels that utilize the pinning effect of a triple line on a rough surface. Sens Actuators B 157(2) 727-734... 

FIGURE 3.7. Relaxation of a line of triple contact pinned at defects (after Garoff and Decker)- (A) before displacement (B) after a displacement of 500 pm. 

This solution does start with zero thickness at the top of the film (x = 0). It holds if the triple line remains pinned at that spot (see chapter 3), In this case, the profile of the film near the top is parabolic. In that region, the thickness (at a given x) decreases slowly (as The solution is valid... 

The same type of analysis can be applied to sharp changes in the contact angle, such as at the boundaries between two homogeneous patches on the surface. Such boundaries also pin the triple line and provide a range of stable configurations with 0 values between 0i and 02, the contact angles in the two homogeneous patches. This is true if 0 decreases with x. Otherwise the position at the discontinuity in 0 is unstable. 

In general, the pinning effect and the corresponding stick-and-slip movement of the triple line can be attributed to singular lines on the surface where there is a rapid change in 0-a, such that 6-a decreases as x increases. 

Liu J, Mei Y, Xia R (2011) A New wetting mechanism based upon triple contact line pinning. Langmuir 27 196-200... 

In a pi molecular orbital (bonding or antibonding), the electron density is concentrated above and below an imaginery line joining the two nuclei of the bonding atoms. Two electrons in a pi molecular orbital form a pi bond (see Section 10.5). A donble bond is almost always composed of a sigma bond and a pi bond a triple bond is always a sigma bond pins two pi bonds. 

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