Understanding of Dynamic Contacting Behaviors of Underwater Gas Bubbles on Solid Surfaces.

Understanding of dynamic behaviors of gas bubbles on solid surfaces has significant impacts on gas-involving electrochemical reactions, mineral flotation, and so on in industry. Contact angle (θ) is widely employed to characterize the wetting behaviors of bubbles on solid surfaces; however, it usually fluctuates within the bubble's advancing (θa) and receding (θr) range. Although the term of most-stable contact angle (θms) was defined previously as the closest valuable approximation for thermodynamically meaningful contact angle for a droplet on a solid surface, it has not been widely studied; and the precise θms measurement methods are inadequate to describe bubbles' wetting behaviors on solid surfaces. Herein, we proposed to take θms as the mean value of θa and θr, as a more accurate descriptor of gas bubbles' dynamic behaviors on nonideal solid surfaces, similar to the definition of droplets' θms on solid surfaces. The feasibility and accuracy of the proposed θms have been evidenced by recording the bubbles' contacting behaviors on solid surfaces with varied wettabilities. In addition, it was found that the contact angle hysteresis (δ), as the difference between θa and θr, reached its maximum value when θms approached 90°, regardless of the roughness (r) of the substrates. Finally, built on the above concept, the lateral adhesion force (f) of the gas bubble on the solid interface, which worked on the three-phase contact line (TPCL) of an individual bubble on a solid surface against its lateral motion during the bubble advancing or receding process, was described quantitatively by combining θa, θr, and the liquid-gas interfacial tension (γlg). Experimental and theoretical data jointly confirmed that f reached its maximum value at θms ∼ 90°, namely, a "super-sticky" state, which described the dynamically most sluggish movement of the bubble along the solid surface.