Bubble-particle attachment science: study on hydrophobic force between bubble and particle based on AFM and DWFA

To explore the origin mechanism of long-ranged and short-ranged hydrophobic force in bub-ble-particle system, atomic force microscopy (AFM) and flotation dynamic wetting film apparatus (DWFA) were used to measure the hydrophobic force between air bubble and the same hydrophobic glass substrate. AFM results showed that the force between hydrophilic glass substrate and bubble was always repulsive since the both van der Waals force and double layer force were repulsive. There was no attractive force triggering film rupture in the system. The film between hydrophobic glass and bubble was unstable. When 19.3 nN loading force was reached, a significant jump-into contact was observed in the force curves. The hydrophobic force was attenuated by a single exponential model with 3.50 nm decay length. Film ruptured at 32.96 nm critical thickness. This kind of hydrophobic force tented to be a short-ranged force, originated from the entropy effect due to the re-arrangement of interface water molecules. DWFA results also showed that the film between hydrophilic glass and bubble was thermostatically stable. 133 nm equilibrium film thickness was reached when the total disjoining pressure was equal to the Laplace pressure inside air bubble. Also, the film between hydrophobic glass and bubble was unstable. The liquid film thinned rapidly and ruptured at the critical thickness of 185 nm. The decay length of hydrophobic force was 47.30 nm and was a kind of long-ranged force, originated from the nanobubble cavitation effect on solid-liquid interface. The bubble used in AFM and DWFA experiments was micro-meter and millimeter, respectively. The hydrophobic force was influenced by the bubble size and related with the capillary wave on bubble surface. Under the attraction force, more intensive interface wave was formed on the big bubble surface. Due to the thermodynamic instability of water molecules at hydrophobic interfaces, this kind of interface wave induced the precipitation of cavitation bubble on the hydrophobic interface and increased the range of hydrophobic force. © 2019, Editorial Office of Journal of China Coal Society. All right reserved.