Investigation On Interfacial Instability And Coupling In Liquid-driven Flow Focusing

Flow focusing(FF)is a typical kind of capillary flow,in which one phase of fluid flows through a capillary tube and gets focused by another fluid at an orifice.Under certain conditions,a steady cone can be formed facing the orifice,and a thin liquid jet,will be emitted from the tip of the cone and ultimately break up into droplets due to iinterfacial instabilities.The flow focusing method is able to produce droplets,bubbles,particles and capsules at micro-and nano-scale,and is of great significance in various applications such as pharmacology,biology,chemistry,industry and agriculture.This work employs the experimental,numerical and theoretical methods to study the interfacial instability and coupling in liquid driven flow focusing.Moreover,the jet breakup in flow focusing system upon actuation is investigated numerically.The results and conclusions are briefly given as follows:(1)In experiments and numerical simulations,the flow modes in FF are obtained and their corresponding doimains arc distinguished.For the liquid cone established in front of the focusing orifice,the effect of liquid flow rate on the stability of the cone is theoretically analyzed.Meanwhile,the flow field in the vicinity of the cone and the effect of process parameters on the cone flow profile are studied numerically.For the droplet generation at the exit of the focusing orifice,the nonlinear breakup dynamics in the dripping mode is observed,and the effect of liquid viscosity on nonlinear dynamics is discussed.In the jetting mode,the spatial evolution of jet velocity profiles is numerically analyzed.Moreover,the transition from jetting to dripping is studied.The scaling analysis on the liquid jet indicates that inertia plays a significant role on maintaining a stable jet.In the spatio-temporal instability theory,the numerical velocity profile is utilized as the basic flow.Both methods can predict the jetting/dripping transition boundary accurately.(2)Breakup of the liquid jet int,o droplets upon actuation in FF is studied numerically.Through varying the actuation frequeney f and amplitude A,four typical states of jet breakup are observed.It is foumd that the and monodispersity can be manipulated by adjusting f andl A.In partieular,with a certain range of f and A,the jet breakup has the same frequeney with the external perturbation and uniform droplets can be formed.We observe that there exists a cut-off f,beyond whieh the perturbation hardly afffect the jet,breakup,even with large A.This is found to be assoc.iated with the eritical condition for onset of Rayleigh-Plateau instability.In addition,the reservoir effect of the cone in FF effectively reduces the influence of the perturbation to the liquid jet,accounting for the presence of jetting at large A.Finally,we compare the jet breakup under different actuation phases or waveforms,which can serve as a guidance for practical applications of on-demand droplet generation.(3)The coupling of interface instability between coaxial jets in coaxial flow focusing(CFF)is studied.Experiments show two typical interface coupling situations as the flow rate ratio between the inner jet and the coaxial jet(denoted by ?Q)varies.Moreover,the effect of ?Q on the jet breakup length and the compound droplets size and morphology is investigated,which agrees well with the theoretical prediction.Numerical simulations reproduce the experimental results of coaxial jet breakup,and the flow fields under different interface coupling situations are analyzed.The results exhibit the dynamic process of coaxial jet breakup and provide the evolutions of velocity profiles along the liquid jet.The effect of ?Q on the diameters of coaxial jet and the size of inner droplets presented in the numerical simulations is in consistent with the theoretical prediction.Finally,we carry out the temporal instability analysis on the coaxial jet,which are further compared with two simplified single-phase jet model for the coaxial jet as ?Q ?0 or 1.(4)The flow modes in CFF and the instability of coaxial jets are studied.Three kinds of dynamic behaviours of the coaxial cone are observed in experiments,and four typical flow modes based on a stable cone are further discovered,in which the inner-outer dripping(DD)and inner-outer jetting(JJ)modes are s-tudied systematically.Numerical simulations reproduce the JJ and DD modes,and indicate a phase diagram of the two modes combining with the expceriments.Through data analysis,we find that the transition from J J to DD mode is trig-gered by the instability of the inner interface.Based on this perception,the J J to DD transition is studied through dimensional analysis on the inner jet,which indicates that the jet inertia and the shear stress would act corporately to main-tain a stable jet.This conclusion differs from that in FF,where the inertia plays a significant role in maintaining the stable jet.Moreover,based on the numerical velocity profiles,the linear temporal and spatio-temporal instability analysis on the coaxial jet is carried out,which predicts the effect of process parameters.(5)Breakup of the coaxial liquid jet into compound droplets upon actuation in CFF is studied numerically.Through varying the frequency f and amplitude A,the inner and outer liquid jcts exhibit four typical breakup styles,respcctively.We emphatically study the effect of f on breakup of the coaxial jet.Particularly,within a certain range of frequency,both the inner and out.er jets break up at the same frequency with the external perturbation and generate uniform "single-encapsulated" droplets.Numerical simulations show the effect of f on the size and productivity of the uniform droplets,and give the cut-off f beyond which the perturbation hardly affect the jet breakup.The cut-off f is directly related to the critical condition of Rayleigh-Plateau instability.Moreover,we study the effect of?Q on the morphology of "single-encapsulated" droplets,and show the dynamic process of the unstable coaxial cone at large A.Finally,we propose a method to fabricate controllable "multi-encapsulated" droplets by adding intermittent pulse to the sinusoidal actuation,and analyze the feasibility of the method.