Study On Droplet Dynamics And Manipulation In Microchannels

In recent years,as a highly efficient energy-saving,safe and controllable,green and environment-friendly chemical process,microchemical technology has attracted widespread attention in both academia and industry.Meanwhile,the micro-magnetofluidics combining microfluidics with magnetic particles was developed and applied widely in biological,medical and chemical fields.In the present study,the processes of formation,breakup of ferrofluid droplets and coalescence of glycerol aqueous droplets were investigated experimentally using a high-speed digital camera.The formation of the ferrofluid droplet in mineral oil in a Y-shape microchannel was investigated.Three types of flow regimes were observed: slug flow,dripping flow and jetting flow.The droplet size decreased with increasing the flow rate of the continuous phase,the capillary number of the continuous phase and the ratio of two phase flow rates.Based on the two-phase flow rates,capillary number of the continuous phase and the magnetic Bond number,a new correlation for predicting the droplet size was proposed.The breakup dynamics of ferrofluid droplet in a microfluidic T-junction was investigated.Three types of droplet breakup regimes at the T-junction were observed: breakup with permanent obstruction,breakup with part obstruction and breakup without obstruction.The droplet breakup processes with permanent and part obstruction could be divided into squeezing stage,transition stage and pinch-off stage.The minimum width of droplet neck could be scaled as power-law relationships with time for squeezing stage and with remaining time for the last two stages.The droplet breakup process without obstruction had only last two stages,and the dynamics of the liquid-liquid interface was similar to that in the other two breakup patterns.The results showed that the magnetic field could affect the dynamics of droplet breakup.The manipulation of a droplet at a microfluidic T-junction was achieved via adding glycerol aqueous solution into the droplet with the same solution.Four coalescence types of droplets at the T-junction were observed: tail coalescence,slipping coalescence,tail coalescence with a new droplet formation and non-coalescence with a new droplet formation.Taking the three-phase flow rates and the initial droplet length into consideration,two correlations for predicting the length of the coalesced droplet of direct coalescence and slipping coalescence were proposed.The formation of alternating droplets in a T-junction microchannel was investigated.Two types of alternating droplets formation regimes at the T-junction were observed: normal generation and upstream droplet squeezing generation.The ferrofluid droplet formation process was divided into the expansion stage and pinch-off stage.The speed of ferrofluid tip increased with the flow rates.By adjusting the flow rates of the two phases and the magnetic flux intensity,the 1:1 alternating droplets which were composed of a glycerol aqueous solution droplet and a ferrofluid droplet could be obtained,and the range of the size ratio of the two droplets was 0.85-1.25.