Interaction Force Between Bubble And Particle And The Thinning Dynamics Of The Thin Liquid Film

Thinning rupture of the thin liquid film between bubble and particle is the precondition for successful bubble-particle attachment.The film between bubble and hydrophobic particle is unstable and can rupture spontaneously.In contrast,the film between bubble and hydrophilic particle is thermodynamically stable.At present,the hydrophobic force as the driving force for bubble-particle attachment has reached a consensus.However,the origin of the hydrophobic force and the magnitude of hydrophobic force between bubble and particle are still unclear.On the other hand,bubble-particle attachment is drived by the combination of hydrodynamic force and surface force,always accompanied by film drainage and bubble deformation.The mechanism responsible for film thinning and rupture is not well understood.Systematic study on bubble-particle interaction and film thinning dynamics at a micro-nano scale can shed new light on flotation mineralization reaction process.It can also provide a basic theory support to flotation process intensification of difficult-to-float coal and complex refractory ore.Atomic force microscopy(AFM)was used to investigate the bubble-particle interaction and the attachment.Further,solid-solid system was selected to study the origin and behavior of the hydrophobic force.Combining force,bubble deformation and film drainage,the thinning dynamics of the thin liquid film between bubble and particle was studied by using AFM bubble probe technique and dynamic wetting film apparatus(DWFA).The hydrophobic force between bubble and particle was quantitatively characterized.Finally,film thinning dynamics between a millimeter bubble and a solid plane with different approach velocity was simulated.The spatiotenporal evolution mechanism of flotation wetting film at a wide range of Reynolds numbers was revealed.Interaction between hydrophilic particle and bubble could be described by classical DLVO theory.The forces always show repulsive and thus attachment is denied.The electric double layer on particle and bubble surface was compressed with the addition of inorganic salt ions,while the repulsive van der Waals force at short range still existed.Interaction between hydrophobic particle and bubble could be described by extended DLVO theory.The hydrophobic force was found and it was the fundamental driving force for bubble-particle attachment.The strength of hydrophobic force was positively correlated with surface contact angle.The addition of DTAB and SDS had a marked impact bubble-particle attachment behavior,depending on the introduction or disappearance of the hydrophobic force due to the change of surface contact angle.The origin and behavior mechanism of hydrophobic force in solid-solid system was identified.Hydrophobic force derived from the structural effect of the hydrophobic liquid film.Water molecules cannot form hydrogen bonds with the hydrophobic glass surface from molecular dynamics simulation.An exclusion zone above 3.5 (?) at the water-solid interface was found.Hydrogen bond cage structure in bulk water was disturbed due to the hydrophobic solid surface.As a result,the water molecules at hydrophobic interface were thermodynamically unstable.This water exclusion zone was the root cause of hydrophobic force.The short-ranged hydrophobic force was always there,while the long-ranged hydrophobic force(>50 nm)was highly system-dependent.When the solid surface was relatively rough,the gas molecules could easily aggregate and nucleate at the rough gully region,forming cavitation bubble and a long-ranged hydrophobic force.When the surface was relatively smooth,hydrophobic force showed a short-ranged force independent of approach times and gas solubility.However,the range of this kind force was still longer than that of van der Waals force.The decay length of the short-ranged hydrophobic force was within 1-2 nm range.The force curves between AFM bubble probe and glass surface with different surface hydrophobicities could be well predicted by using Stokes-ReynoldsYoung-Laplace equation.The film drainage information could also be obtained at the same time.The film between hydrophilic glass and bubble with repulsive disjoining pressure was thermodynamically stable.The film first began to thin under the Laplace pressure.When the film thickness entered into the range of repulsive van der Waals and electric double layer disjoining pressure,the film thinning rate decreased.When the disjoining pressure was equal to the Laplace pressure inside the bubble,the film reached at an equilibrium film thickness.Further driving the bubble probe,the film began to expand along the radial direction.The film between hydrophobic glass and bubble with attractive disjoining pressure was thermodynamically unstable.In deionized water,the hydrophobic force decay length between microbubble and hydrophobic glass was below 4 nm obtained from AFM force measurement,tended to be a short-ranged force.When the film thickness entered into the range of hydrophobic force,the driving pressure increased dramatically,leading to a faster drainage rate and rupture.The meahanism of hydrophobic force responsible for film rupture could be described by Frumkin-Derjaguin isotherm.The short-ranged van der Waals disjoining pressure could be overcomed the hydrophobic disjoining pressure.At the same time,the rate of system free energy became negative,inducing film rupture and forming?film.Compared with AFM force measurement method,DWFA film drainage approach found the hydrophobic force between millimeter bubble and hydrophobic glass was a long-ranged force.The hydrophobic force decay length was 47.3 nm in deionized water.Under the attraction force,stronger capillary wave formed on a big bubble.This kind of interface fluctuation may induce the formation of cavitation bubble,leading to an increase of force range.For the stable wetting film between hydrophilic particle and bubble at a wide range of Reynolds numbers,with the increase of approach velocity,the hydrodynamic force increased.At the same time,the film thickness,when the dimple was observed,was positively correlated with approach velocity.At the stage of retraction,the hydrodynamic suction effect increased with the increase of velocity.The stability of the wetting film between hydrophilic particle and bubble at a wide range of Reynolds numbers was maintained by the synergistic effect between hydrodynamic pressure and repulsive disjoining pressure.For the metastable wetting film between hydrophobic particle and bubble at a wide range of Reynolds numbers,with the increase of approach velocity,the minimum displacement when film ruptured increased.It was found that film rupture occurred at the stage of retraction,the film continued to move toward the glass surface under the hydrodynamic suction effect,entered into the range of hydrophobic force,and then ruptured.This kind of film rupture mechanism based on the synergistic effect between hydrodynamic pressure and hydrophobic disjoining pressure played an important role in bubble-particle attachment.The thesis contains 119 figures,4 forms and 209 pieces of references.