Surface forces measured between an air bubble and a solid surface in water

Surface forces between an air bubble and a flat mica surface have been investigated using a modified surface force apparatus. This method allows forces to be detected in two regimes: at long-range, where the bubble is distorted slightly by the surface forces, and at short-range, where the force is manifest as disjoining pressure in a thin wetting film separating the bubble from the solid. A long-range double-layer repulsion was detected, which indicates that there is a negative surface charge at the air–water interface. However, when the bubble is pressed closer to the mica, charge regulation effects cause a reversal of sign and the double-layer force becomes attractive. This causes an “adhesion” of the bubble to the mica, even when a wetting film remains between them, and a finite force is required to pull the bubble from the mica.