Hydrodynamic Characteristics Of Pickering Emulsion With Solid Nanoparticles In A Microchannel

The rapid advances in microchemical technology have opened up new avenues for chemical process intensification and optimization,including multiphase flow involving solid particles for e.g.heterogeneous catalysis.However,the clogging and the problem of the load/replacement of the solid catalyst limit its applications in heterogeneous catalytic systems.In the present study,a novel method based on Pickering emulsion was proposed to manipulate multiphase processes containing solid micro/nano particles in microchannels.The self-assembly of solid particles in the Pickering emulsion encapsulated emulsion droplets can reduce or eliminate the contact and adhesion between solid particles and the microchannel wall,thus alleviating or avoiding the channel blockage caused by solid particles.At the same time,the loading and transportation problems of the catalyst could be solved.In order to understand the multiphase flow mechanism of Pickering emulsion in microchannels,the flow behavior and dynamic characteristics of water-Pickering emulsion system（PES）under different operating conditions were systematically studied.Firstly,the most suitable conditions for preparing stable Pickering emulsion were selected by researching the factors affecting the stability of emulsion.The results showed that the stability of the emulsion was affected with the size,dosage and hydrophobicity of solid particles in a certain range.Therefore,20 nm Si O₂ nanoparticles were firstly modified by methyltrimethoxysilane,then used for preparing W/O Pickering emulsion with ethyl acetate as oil phase.Then,flow characteristics of three typical systems included PES,a water-suspension system（SS）formed with ethyl acetate containing suspended Si O₂ nanoparticles and a water-viscous fluid system（VFS）consisting of ethyl acetate,methyl-silicone oil and water,were also examined contrastively in a 0.3 mm wide glass T-junction microchannel whose cross section was approximately square.The results showed that flow patterns of parallel flow,slug flow,monodispersed droplet flow,and jetting droplet flow were observed under these operating conditions for PES and VFS.The size of droplet decreased with the increase of two-phase flow rate ratio,continuous phase flow rate,Ca and the ratio of viscosity,instead,increased with the increase of dispersed phase flow rate.In addition,the size of Pickering droplets could be regulated by changing the properties of solid particles,which affected the movement behavior of W/S/O dispersed droplets in PES to led to the width of droplet neck（δ）,the flow pattern and formation mechanism of droplets were also affected further.For SS,the flow patterns were unstable and droplet size changed irregularly,due to the fact that the solid particles in the suspension were easy to adhere to the inner wall of the channel and block the channel.A scaling law of the droplet size（L_D/w）was established as a function of Q_d/Q_c,μ_d/μ_c and Ca.The results showed the predicted values were in accordance with the experimental data well at±15%.Finally,the droplet formation process of the three systems and the motion behavior of W/S/O dispersed droplets in Pickering emulsion were analyzed in depth.The results showed that droplet formation was attributed to the dynamic competition among the inertia force（F_c,F_d）,the interfacial force（F_γ）,the viscous shearing force（F_μ）and the continuous phase pressure(P_neck,P_tip).In addition,the motion behavior of interface Ph_2-1was determined by drag force（F_D）,interfacial tension at the S/O interface（σ）,confinement of the microchannel wall,Basset force,virtual mass force,Magnus force,Saffman force,and collision and the repulsion forces between the adjacent emulsion droplets.It was mainly divided into two parts,one part was the T-junction intersection,the motion track was clockwise.The other part was in the main channel,the trajectory tended to be straight.