Study On The Interaction Mechanism Between Heterogeneous Surface With Discrete Hydrophilic Dots And Air Bubble

Froth flotation is a separation technique rely on surface hydrophilic and hydrophobic differences,is widely used for the separation and concentration of valuable components from a variety of minerals,precipitates,and even proteins and microorganisms.As the basic unit of flotation,the solid-bubble interaction(bubble mineralization)is the key to flotation.Therefore,study of the solid-bubble interaction is of great importance for understanding flotation process,regulating flotation process and promoting the development of flotation technology.The surface heterogeneity of mineral particles is common in nature from the macroscale to the nanoscale,which could be attributed to the association with other minerals as well as the defect of crystal ranging from interstitial impurity atoms to missing or misplaced atoms in the crystal lattice.Otherwise,The heterogeneities of mineral surfaces are also caused by selective adsorption of flotation reagents on the mineral surface,oxidation and non-uniform dissociation of functional groups located at the surface,or slimes on the mineral surface.Especially for coal,which is a sedimentary rock mineral transformed from biomass through biochemical and physical chemical actions,it is a mixture material which comprises of organic and inorganic substances.And there is no absolute“pure”monomer in coal.However,up to now,the effect of heterogeneity on flotation is often ignored by scholars.Researches about the mechanism of solid-bubble interaction in flotation often regard the solid surface as homogeneous in chemical components.Better understanding of the interaction mechanism between heterogeneous surface and air bubble may potentially offer a new thought for altering the interactions by a particular design of the surface,and further intensify the flotation of difficult-to-float minerals.In this study,two groups of smooth heterogeneous samples which comprises of discrete hydrophilic dots on an interconnected hydrophobic surfaces were fabricated with micro-electromechanical system(MEMS)techniques.Effect mechanisms of surface heterogeneities on wetting characteristics were revealed.And the pseudo-line tensions on heterogeneous surfaces were also stuied.The collision and adhesion process between free-rising bubbles and heterogeneous surfaces were studied by using a high-speed camera.Effect of surface heterogeneities on the thinning,rupture and expansion kinetics of the wetting film during bubble heterogeneous surface collision were revealed.At last,a microbalance together with a high speed video were used to quantitatively characterize the interaction force during bubble-heterogeneous surface continuous interaction.The interaction mechanism between heterogeneous surface and bubble was clarified from the perspective of quantitative analysis.Effect mechanisms of surface heterogeneities on wetting characteristics were revealed.For heterogeneous surfaces with different hydrophilic area fractions,during advancing contact angle measurements,the hydrophilic area fractions had nearly no restricting influence on the retreating behavior of the contact line,causing a nearly constant value of advancing contact angle.Conversely,during receding contact angle measurements,ever low hydrophilic area fraction would cause the three-phase contact line pinning,resulted in a decrease in the receding contact angle.The validity of the line-fraction Cassie equation in calculation of contact angles on heterogeneous surfaces was also verified in this study.For heterogeneous surfaces with the same hydrophilic area fraction but different hydrophilic dot sizes,the advancing contact angles were also nearly constant because almost no energy barrier was aroused by the hydrophilic region,which would prevent the movement of contact line.However,the greater energy barriers caused by larger hydrophilic dots led to a slight decrease in the receding contact angles.Therefore,the bubbles are more easily to adhere to the heterogeneous surface with low hydrophilic area fraction or small hydrophilic dot,while the detachment performs of bubbles from heterogeneous surfaces were independent to the characteristics of heterogeneous surfaces.Meanwhile,the thermodynamic stability of bubbles on heterogeneous surfaces was analyzed.It was found that either the pinning or the contact line moving continuously mode is thermodynamically favored.However,a mixed mode is never thermodynamically favored over the two extreme modes.The effect of the surface heterogeneties on the pseudo-line tensions were eliminated.For heterogeneous surfaces with different area fractions,the results showed that the pseudo-line tension increased with the the hydrophilic area fraction decrease and the vector pointing to the liquid phase.It can be infered that the pseudo-line tension will promote the adhesion behaviour between bubble and heterogeneous surface with smallhydrophilicareafraction.Meanwhile,amodel:(?) base on the line-fraction Cassie equation was developed to estimate the pseudo-line tension on the heterogeneous surface with discrete hydrophilic dots distribution on a continuous hydrophobic background.The validity of the theoretical equation was proved by comparison the theoretical calculations to experimental measurements.The thinning,rupture and expansion kinetics of the liquid film during bubble heterogeneous surface collision were revealed.In the same measurement system,the forces between bubbles and the heterogeneous surfaces during collisions are almost the same.Thus,the collision behaviours(before bubble rupture)between bubbles and the heterogeneous surfaces are almost independent of the surface.However,the time elapsed from the first collision moment to the bubble rupture moment were decreased as the increase of the hydrophilic area fraction on heterogeneous surface.More hydrophobicity of samples resulted in bigger area of the interfacial gas enrichment in the form of a dense gas layer or rearrangement of water moleculars,which will drive the liquid film into a metastable state.And further aroused the rupture of the bubble more easily under samples.However,the behavior of the TPC line varied on different heterogeneous surfaces after bubble rupture.For heterogeneous surface with different area fractious,it was found that the terminal diameter and average expansion velocity decreased with the area fraction increase.For heterogeneous surface with different dot sizes,the terminal diameter and average expansion velocity decreased with the increase in the dot size of heterogeneous surface.Measurement and analysis of forces during continuous interaction between heterogeneous surfaces and air bubbles.For heterogeneous surfaces with different hydrophilic area fractions,the snap-in force(F_in),the maximum adhesion force(F_max)and the pull-off force(F_off)increased as the decrease of the hydrophilic area fraction.The contact diameter at snap-in point,maximum adhesion point and pull-off point increased with the hydrophilic area fraction decrease,respectively.The values of force at snap-in point and maximum adhesion point were consistent with the changes of contact angle.At the pull-off point,there is no significant correlation due to the influence of pulled-off bubble states.Meanwhile,it was proved that the interaction force between air bubble and heterogeneous surface can be calculated by using the theoretical formula,which comprises of the interfacial tension and capillary force.This paper contains 71 figures,11 tables and 178 pieces of references.