Investigations Of Solute-Induced Marangoni Effect On A Growing Drop By PLIF/PIV Methods

In the process of mass transfer,the inhomogeneous distribution of solute on the interphase boundary will generate interfacial tension gradient.This can trigger the instability of interfacial fluid mechanics,which is called Marangoni effect.This hydrodynamic instability at interface has an important impact on the interphase mass transfer.In extraction systems,one liquid is often dispersed into drops to increase the interfacial area to improve the mass transfer efficiency.Moreover,the mass transfer in the drop formation stage can reach 10-50%or even 80%of the total extraction as reported.Therefore,this process should be fully considered when to design extraction equipment or optimize the operation.However,there are few reports on the influence of Marangoni effect on mass transfer in drop formation stage.The mechanism of interphase transfer associated with Marangoni effect in a growing drop is still not clear sufficiently and quantitatively.In our work,the concentration field and flow field of the solutal Marangoni effect in a growing drop were studied experimentally.First,a new method for directly measuring the concentration distribution of the target solute by the PLIF(Planner Laser Induced Fluorescence)technique was established.By this method,the concentration field inside a growing drop was obtained and the visualization of flow structure and development of the solute-induced Marangoni effect were realized.The influence of initial solute concentration,injection speed,needle size,continuous phase properties and solute density on the Marangoni effect during the drop formation stage was studied in the extraction system of MIBK-acetic acid-water.By analyzing the range of curvature fluctuation of the interfacial concentration contour near the drop interface,it can be accurately determined whether the Marangoni effect existed or not.The critical concentration for triggering the Marangoni effect was increased by the strength of the bulk flow inside a drop at a higher injection speed.This indicates that there is competition between bulk flow and Marangoni convection.Only when the initial solute concentration was large enough,the Marangoni effect occurred by counteracting the inhibition of bulk flow.Different needle sizes correspond to different drop shapes(especially the thickness of the drop neck)and the internal flow state inside the drop,which affected the critical solute concentration for triggering the Marangoni effect.When introduced a base in the continuous phase to neutralize the solute of acetic acid,the neutralization reaction greatly promoted the transfer efficiency of acetic acid but inhibited the Marangoni effect.The density effect of solute coupled with Marangoni effect,which further affected the distribution of solute and then influenced the location and evolution of Marangoni effect.In addition,the PIV(Particle Image Velocimetry)technique was used to study the hydrodynamic mechanism of growth of pendant Newtonian and non-Newtonian drops,and the morphological changes of the drop formation process under different experimental conditions,which gave the internal flow pattern and velocity field of the drop.On this basis,a solute was added to the drop to investigate the influence of the solute-induced Marangoni effect on the velocity field and flow pattern inside the growing drop.It was found that the flow field in the drop with Marangoni effect was unstable and vortices appeared locally.The position and size of the vortices changed at any time and the profile velocity was irregular wavy shape.The increase of the continuous phase viscosity would increase the interfacial resistance of the growth of a drop,the critical solute concentration for triggering Marangoni effect became higher,but similar streamlines still with the low viscosity liquids were found.For an elastic non-Newtonian drop,the interface was not easy to deform.When Marangoni effect occurred,the top of the drop bounced up and down,which caused the local liquid to flow up and down periodically and the flow field occupied by Marangoni flow,extensional flow and compression flow alternately.For the first time the detailed information about the concentration field and flow field of the solute-induced Marangoni effect was obtained during the drop formation stage when there was no wall effect and other external flow by experimental methods.These data can provide sound references for subsequent research on the mechanism and numerical simulation of the Marangoni effect.