Wetting of Composite Surfaces: When and Why Is the Area Far from The Triple Line Important?

Apparent contact angles are totally governed by the area of the solid surface adjacent to the triple (three-phase) line. However, apparent contact angles do not describe the wetting situation exhaustively. The wetting regime is characterized by both apparent contact angle and the energy of adhesion. The energy of adhesion in turn depends on the physical and chemical properties of the entire area underneath the droplet. We demonstrate this experimentally by preparing rough surfaces exhibiting high apparent contact angles accompanied with the high energy of adhesion leading to the high contact angle hysteresis. A droplet deposited axisymmetrically on the superhydrophobic surface comprising a nonsuperhydrophobic spot holds sticky wetting attended with high apparent contact angles.