Figure 2-37-A

Liquid in a channel with grooved bottom with small deposits.

The contact angle on the channel surfaces is 60 degrees and 70 degrees on the deposit in the bottom of the grooves. The wedge half-angle is about 19.5 degrees. The rounded bottom rounds off the filament tip, but the contact angle is still plenty low enough for the liquid to fill the grooves. The side walls let the liquid rise above the grooves. One may consider this type of channel to be four V-grooves with filaments advancing, with the filament tops forming a perfectly wetting floor for a flat-bottomed channel. The perfect wetting of the flat bottom exactly cancels out the surface tension of the liquid top, so as long as the side walls are at least a little wetting, the channel will fill.