Direct measurement of the nanobubble-induced weak depletion attraction between a spherical particle and a flat surface in an aqueous solution

Previously, it has been shown that stable nanobubbles (similar to 150 nm) free in an alkaline alpha-cyclodextrin (alpha-CD) solution can be removed by repeated filtration and regenerated by air injection. Recently, we found that the existence of nanobubbles can induce attraction between a spherical latex particle (radius similar to 3.2 mm) and a flat hydrophilic surface. Using total internal reflection microscopy (TIRM), we are able to directly measure such a weak interaction (down to similar to 0.1 mu N m(-1) within a range up to 200 nm). The weak attraction disappears as soon as the nanobubbles are removed. Our results suggest that the origin of such weak attraction comes from the exclusion or depletion of the charged nanobubbles from the gap between the particle and surface; namely, an imbalance between the osmotic pressures inside and outside the gap.