Removal of Bacteria from Solids by Bubbles: Effect of Solid Wettability, Interaction Geometry, and Liquid-Vapor Interface Velocity.

Air bubbles are a promising means of controlling fouling for a range of applications, particularly delaying fouling in marine environments. This work investigates the mechanism by which the collision of an air bubble with a solid removes adsorbed bacteria. A key feature of the work is that the numbers of bacteria were monitored via video microscopy throughout the collision; so, we were able to explore the mechanism of bacteria removal. When a bubble collides with a solid, an air-liquid interface crosses the solid twice, and we were able to distinguish the effects of the first and second air-liquid interfaces. The bacterium was allowed to adhere to smooth poly(dimethylsiloxane) and then a collision with a bubble was investigated for one of three different approach geometries: perpendicular, parallel, and oscillating parallel to the solid surface. Other factors examined were the speed of the bubble, the duration of bacterial adhesion on the solid surface, and the wettability of the solid. Surface wettability was identified as the most significant factor. When the solid dewet, almost all bacteria were removed from hydrophobic surfaces upon the passage of the first air-liquid interface. In contrast, when a thin liquid film remained between the solid and the bubble (a hydrophilic solid), variable amounts of bacteria remained. Although almost all bacteria were initially removed from hydrophobic solids, many bacteria were redeposited on hydrophobic surfaces upon the passage of the second air-liquid interface, especially when the first and second air-liquid interfaces moved in opposite directions. As described previously, a lower velocity of the bubble allows more time for the thin liquid film to drain and improved removal efficiency on hydrophilic solids. A rougher solid (8 μm diameter hemispherical protrusions) decreased the detachment efficiency because bacteria and liquid were able to shelter in concavities.