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 (θ_a) and receding (θ_r) range. Although the term of most-stable contact angle (θ_ms) 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 θ_ms measurement methods are inadequate to describe bubbles' wetting behaviors on solid surfaces. Herein, we proposed to take θ_ms as the mean value of θ_a and θ_r, as a more accurate descriptor of gas bubbles' dynamic behaviors on nonideal solid surfaces, similar to the definition of droplets' θ_ms on solid surfaces. The feasibility and accuracy of the proposed θ_ms 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 θ_a and θ_r, reached its maximum value when θ_ms 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 θ_a, θ_r, and the liquid-gas interfacial tension (γ_lg). Experimental and theoretical data jointly confirmed that f reached its maximum value at θ_ms ∼ 90°, namely, a "super-sticky" state, which described the dynamically most sluggish movement of the bubble along the solid surface.