Surface wettability effects on the behavior of air bubbles injected into water flows

Any object passing through a fluid, a ship through the ocean for instance, experiences a drag force. This force acts in a direction opposite to that the object is travelling in, acting to impede its motion. This is an undesirable outcome, as additional power must be supplied to the object to overcome this force, often resulting in more fuel consumption and corresponding pollution. Any reduction in the drag force is amenable to lowering operating costs and is highly sought after in industry. The research community has been investigating a drag reduction technique for objects passing through water over the past forty years that involves injecting air bubbles onto the surface of the object. Experimentally, this technique has yielded tremendous drag reduction, but industrial application has been difficult to achieve. One reason for this is insufficient control over the size and shape of the air bubbles as they pass along the surface of the object. Drag reduction is insignificant unless an enormous amount of energy is spent to inject more air bubbles. The point at which this takes place often lends the net energy reduction to be zero or even negative, meaning more energy is spent injecting the air bubbles than the corresponding drag reduction that results. This research proposes the manipulation of solid surface wettability, meaning manipulation of the surface such that it either attracts water or repels it, as a useful method of combatting these issues.

Flat plates measuring 152 mm width by 305 mm length were treated with a laser-based surface wettability modification technique, by which water-attractive regions and water-repellant regions were imparted on the plate surface. Various patterns of water-repellant and water-attractive regions were attempted. An untreated flat plate was also included in the analysis to serve as a baseline comparison. All flat plates were placed in a water tunnel, which circulates a continuous flow of water, one at a time. Air bubbles were injected onto the flat plate surface close to its leading edge. A camera placed underneath the water tunnel captured videos of the air bubbles as they passed along the plates. Distinct behaviors of the air bubbles were observed depending on the wettability of the surface they passed over. When passing over the untreated and water-attractive surfaces, the bubbles remained small and circular. In contrast, when passing over water-repellant surfaces, the bubbles immediately formed together into one large bubble

The water tunnel experiments provided useful data of the air bubble behavior but left information gaps in certain areas that were desired. To fill these gaps, a computer model was formed that sought to simulate conditions like those of the water tunnel experiment. The information gathered from this computer model agrees with observations of the water tunnel experiments and yielded further insights and conclusions into the physics of the problem. From the information gathered, there is clear influence of surface wettability effects on the behavior of injected air bubbles. This information may be used to improve the air bubble drag reduction technique and entice future industrial application.