Transport Toward Wettable Regions

While fabricating special surfaces by silanizing only half of a glass slide, C. Casagrande noticed that a drop of water placed at the boundary between the two regions on the hydrophobic side rushes over to the hydrophilic side. Raphael and Ondarguhu have studied this phenomenon quantitatively. It is confined to the transition region and the shape of drops is complicated. 

The drop moves because the contact angle 9ea trailing edge (A) is 

We shall now examine more quantitatively the motion of drops and puddles on horizontal surfaces bearing the imprint of a wettability gradient in the. r-direction. Both 7so( ) and depend on the position. x, and 

FIGURE 10.4. PDMS drop climbing up an inclined glass slide with a wettability gradient. 

FIGURE 10.5. Drop on a surface with a wettability gradient. When the drop is at rest (a), its trailing edge, where the curvature is the highest, experiences an overpressure. The asymmetry sets the drop in motion (b). 

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