Multi-Scale Mechanism Of Droplet/Bubble Interaction On Discrete Textured Surfaces

Flotation is one of the most economical and effective methods to deal with fine-grained hard-to-float minerals,and the difference in surface wettability of mineral particles is the key factor determining the flotation efficiency.In the flotation process,the surface wettability of mineral particles is not only affected by the surface chemical properties,but also the surface physical morphology plays an important role in the wettability.At present,a consensus has been reached on the method of enhancing flotation through the chemical regulation of interfacial forces.However,the physical control of the surface wettability of mineral particles has not attracted enough attention.The wetting and adhesion behavior of droplets/bubbles on physically heterogeneous surfaces is still unclear.The mechanical mechanism and microscopic mechanism of the interaction process need to be further studied.In this thesis,samples with heterogeneous discrete textures with different surface morphologies were designed and prepared.Using high-speed dynamic photography technology,the wetting characteristics,the movement of the three-phase contact line（TPCL）and the collision adhesion process of droplets/bubbles on different discrete textured surfaces were analyzed from the macro scale;We used a microbalance adhesion-desorption force measuring instrument to reveal the interaction mechanism between droplets/bubbles and discrete textured surfaces from a mesoscopic mechanical scale.Additionally,we clarified the wetting and adhesion mechanism of droplets/bubbles on discrete textured surfaces and the mechanism of micro-nano desorption mechanics using molecular dynamics simulation methods from a microscopic scale.Finally,we conducted flotation experiments on quartz sand particles under surface roughness control,and analyzed the mechanism of the effect of surface roughness on the particle-bubble adhesion process based on the E-DLVO theory.The main conclusions of the thesis are as follows:The macroscopic wetting and adhesion behaviors of droplets/bubbles on discrete heterogeneous textured surfaces were elucidated.The surface wettability of the discrete texture was found to be negatively correlated with the surface area fraction but positively correlated with the surface roughness.Surface physical morphology mainly influenced the receding contact angle of droplets.During droplet rolling wetting on discrete textured surfaces,the rear contact line first detached from the solid surface,and then the front contact line adhered to the solid surface.The wetting process of the droplet was achieved by continuously lowering the gas-liquid interface to the next texture while keeping the three-phase contact line fixed.During dewetting,the three-phase contact line showed a periodic movement pattern of"unpinning-pinning-unpinning-pinning."Furthermore,theoretical prediction models were established for the critical rolling angle of both anisotropic and isotropic discrete textures,and the relationship between the critical rolling angleαof discrete texture and droplet volume V,Young’s contact angleθ_Y,and area fraction f was derived.The adhesion behavior of bubbles in the liquid phase was found to be analogous to the wetting behavior of droplets in the gas phase.The mesoscopic mechanical interaction mechanism between droplets/bubbles and discrete textured surfaces was revealed.The interaction force between a droplet and a hydrophobic/hydrophilic discrete textured surface is positively correlated with the surface area fraction,but negatively correlated with the roughness for hydrophobic surfaces and positively correlated with the roughness for hydrophilic surfaces.When a droplet interacts with a groove textured surface,the normalized three-phase contact line lengthδ₂ is not a constant value at different characteristic interaction points,while it is the same on a square columnar textured surface.Additionally,when the wetting anisotropy of a droplet on a groove textured surface is minimized,the interaction force between the droplet and the solid surface is maximized,and the wetting state of the droplet is most stable.Therefore,increasing the wetting anisotropy of droplets on surfaces is one way to improve surface hydrophobicity,and the pinning action of the bubble on the discrete textures surface is the core factor that determines the interaction force between the two.The wetting and adhesion mechanism of droplets/bubbles on discrete textured surfaces and the mechanism of micro-nano desorption mechanics are clarified.The surface area fraction and roughness of the hydrophilic/hydrophobic discrete textured surfaces determine the interaction force between the liquid droplet and the solid surface.The wetting radius of droplets on both the striped and hydrophobic square columnar textured surfaces exhibits scale invariance with respect to the wetting time,and the wetting index is close to the prediction of the MKT theory.However,on the hydrophilic square columnar textured surface,there is a difference between the wetting index and the theoretical prediction.Furthermore,due to the effect of wetting anisotropy,the"pinning"effect of droplets on the hydrophilic square columnar textured surface is more pronounced than that on the striped textured surface,resulting in a higher detachment force acting on droplets on the hydrophilic square columnar textured surface than on the striped textured surface.The effect of particle surface roughness on the flotation effect of quartz sand was studied,and the interaction energy between particles with different rough protrusions and air bubbles was calculated based on E-DLVO theory.The increase in surface roughness effectively decreases the wetting rate of quartz particles and enhances their flotation rate.The theoretical calculations indicate that surface protrusions significantly increase the hydrophobic interaction energy between particles and bubbles,thus enhancing the hydrophobic attraction between them.Rough particles with larger surface protrusions need to overcome a lower adhesion energy barrier during adhesion to bubbles,which also increases the probability of adhesion between rough particles and bubbles.This thesis is based on the preparation of hydrophilic/hydrophobic surfaces using discrete texture.Through the multiscale study of the wetting and adhesion characteristics and mechanical behavior between droplets/bubbles and discrete textured surfaces,the research reveals the strengthening mechanism of surface physical heterogeneity on particle wetting and floatability.The research results not only contribute to further improve the flotation interface mechanics theory system,but also provide key technical support for enhancing mineral separation process.This thesis contains 146 figures,19 tables and 201 pieces of references...