Formation And Flow Characteristics Of Ferrofluid Droplets In Microchannel

As a new technology,microfluidic technology has attracted the attention of scholars.Microchannels fabricated by microfabrication technology could generate and control the flow of microdroplets,which were widely used in biology,chemistry and engineering.However,ordinary droplets had the limitation of being controlled.The combination of ferrofluid and microchannel could avoid this problem.It realized the external controllable force to control the fluid in the microchannel.In this paper,the formation process of ferrofluid droplets in microchannels and the flow characteristics of droplets in single flow channel and double-branch confluence channel/were studied by using visualization platform.Firstly,T-junction microchannels were fabricated by PDMS.The formation of water-based ferrofluid droplets in T-junction microchannels was studied.It was observed that the formation flow patterns of droplets included slug flow,transition flow and dripping flow.The formation process of droplets included contraction waiting stage,expansion stage,neck deformation stage and rapid fracture stage.It was found that the width of the neck and the length of the head extension decrease with the increase of the continuous phase flow rate and increase with the increase of the dispersed phase flow rate;the width of the neck increases with the increase of the magnetic induction intensity,but the length of the head extension decreases with the increase of the magnetic induction intensity.The larger the continuous phase flow rate was,the smaller the dispersed phase flow rate was,the greater the magnetic induction intensity was,the faster the droplet formation frequency was,the smaller the droplet length was,and the larger the liquid column length was.The droplet size could be predicted according to the prediction formula:l/w(28)1.37(7)Q_cQ_d(8)^-0.²⁴Ca_c^-0.14,l/w(28)1.25(Q_c/Q_d)^-0.24Ca_c^-0.14Bo_m^-0.05?Secondly,the flow characteristics of ferrofluid droplets in the tapered wide microchannel/were studied.It was found that there were two flow patterns of single row and double row arrangement in the absence of magnetic field,and single row arrangement and coalescence in the presence of magnetic field.With the increase of flow rate ratio,the deformation degree of droplets decreased and the droplets tended to be arranged in a single direction;with the increase of magnetic induction,the deformation degree of droplets decreased and the droplets tended to coalesce,but with the increase of magnetic induction,the droplets would change from coalescence to arrangement.The coalescence of droplets included three stages:approach,extrusion and coalescence.The larger the continuous phase flow rate was,the smaller the magnetic induction intensity was,and the coalescence position gradually moved backward.Finally,the flow characteristics of ferrofluid droplets in the T-junction converging channel/were studied.It was found that the flow patterns of two droplets in the T-junction converging channel/without magnetic field were synchronous contact,asynchronous contact and asynchronous non-contact;the flow patterns with magnetic field were synchronous contact separation,synchronous contact coalescence,asynchronous contact truncation,fracture after asynchronous contact and asynchronous non-contact.With the increase of continuous phase flow rate,the coalescence position of droplets in synchronous contact coalescence moved backward and forward with the increase of magnetic induction.This paper contains 42 figures,5 tables and 109 references.