Nanodispersions of bubbles and oil drops in water

Explanations are suggested for the experimental observations that nanobubbles and oil nanodrops dispersions in water have a long life. The increased stability of nanobubbles occurs because their ascensional velocity, provided by the Hadamard-Rybczynski equation, is proportional to the square of their radius. As a consequence, for radii in the nanorange, the ascensional velocity is extremely low. In addition, the repulsive electrostatic interactions among them during their ascensional motion oppose coalescence. The presence of air in nanoemulsions increases the ascensional velocity of the nanodrops because it generates bubbles which being hydrophobic become attached to the hydrophobic drops, thus decreasing both the average density of the drop-bubble complex and the friction between the oil drop and water. As a result, the ascensional velocity is increased above the value provided by the Hadamard-Rybczynski equation. Consequently, by degassing the nanoemulsions one can increase their stability. In addition, evidence is brought for the existence of a hydrodynamic attractive force which generates coalescence. (C) 2013 Elsevier B.V. All rights reserved.