Bubble Adhesion to Superhydrophilic Surfaces

This paper reports experimental results and analysis of adhesion forces of captive air bubbles of varying volumes (8-20 μL and Bond number on the order of 1) to superhydrophilic surfaces, and are compared to hydrophilic surfaces. A polished bare silicon substrate and a micropillared silicon substrate with apparent contact angles of sessile water droplets of 40° and 4° were used as the hydrophilic and superhydrophilic surfaces, respectively. Theoretically, the adhesion force of an air bubble to an inclined surface depends on the contact width and the contact angle hysteresis (or the cosine difference of the contact angles between uphill (advancing) and downhill (receding) sides, also called maximum and minimum angles, respectively). The results show that the sliding angle, contact angle hysteresis, and adhesion force are much lower on the micropillared superhydrophilic surface than on the hydrophilic surface, which is due to the entrapped water layer on the micropillared superhydrophilic surface. On both the hydrophilic and superhydrophilic surfaces, the adhesion force slightly decreases with bubble volume. Although the contact width increases with an increase in bubble volume, the contact angle hysteresis decreases more significantly so that the adhesion force is effectively reduced with an increase in bubble volume. This is attributed to the effect of the buoyancy force, which becomes more pronounced with an increase in bubble volume (i.e., larger Bond number) and depends on the shapes of air bubbles formed on the surfaces with different wettabilities.