Experimental And Theoretical Investigation On Flow Focusing

Flow focusing(FF) is one of capillary flows characterized by the formation of a steady meniscus in the core of an extensional high-speed fluid focused by a small hole when a fluid is injected through a capillary needle.A very thin jet is issued from the vertex of the meniscus,passes through the hole and breaks up into monodisperse droplets at a certain distance outside the hole.The FF technique is steady,controllable and does not require rigorous conditions in producing droplets and particles down to the micrometer dimension and below with important applications in fields of chemistry,medicine,biology,industry and agriculture and so on.This dissertation is devoted to the investigation on the phenomena and rules in the liquid-gas FF.Six flow modes are distinguished in the FF experiments,the linear temporal,spatio-temporal and spatial stability analyses are carried out,the physical mechanisms in FF are explored and the comparisons between experimental and theoretical results are performed.The main contents are described as follows:1.A FF device which is convenient to observating and controlling has been developed.The effects of experimental parameters on the meniscus,jet and droplet are investigated using the developed experimental setup.Values of the jet diameter d₁ at the hole exit,the breakup length of the jet L and the disturbance wavelengthλat the jet surface are measured as the liquid flow rate Q₁ and gas pressure dropΔp_g vary.2.The flow modes and their corresponding domains are distinguished and the breakup mechanisms of them are investigated for the first time.Six flow modes obtained are respectively the cone-shaking mode,cone-adhering mode,helical jetting mode,coexisting jetting mode,axisymmetric jetting mode and dripping mode.The cone-shaking mode and the cone-adhering mode are related to the instability of the meniscus,whereas the other modes to the instability of the jet.The dripping mode is associated with absolute instability of the jet.The axisymmetric jetting mode and helical jetting mode are caused by axisymmetric and non-axisymmetric disturbances, respectively.Moreover,the coexisting jetting mode results from both axisymmetric and non-axisymmetric disturbances.3.The theoretical model in terms of linear stability analysis of FF is established. The physical model consists of a cylinder liquid jet of radius R₁ surrounded by an annular gas stream of radius R₂.Both the liquid and the gas are assumed to be viscous, incompressible and Newtonian.The effects of temperature and gravity are neglected. The main dimensionless parameters governing the phenomenon of FF include the Reynolds number Re,the Weber number We,the density ratio Q,the viscosity ratio N, and the radius ratio a.The velocity profile of hyperbolic-tangent function in two fluids satisfying all the boundary conditions is utilized,with two control parameters of the velocity at the interface Us and the slope of the liquid velocity profile at the interface K.4.Using the normal mode method and the Chebyshev spectral collocation method, the linear temporal,spatio-temporal and spatial stability analyses considering both the axisymmetric(n=0) and first non-axisymmetric(n=1) disturbances are performed,and the effects of parameters are studied.The result indicates that small(or large) We and moderate Re,as well as velocity profiles with small(or large) K and slightly moderate Us,favor the instability of long(or short) waves;the density ratio Q and the viscosity ratio N reflect the effects of outer gas on the jet instability;the radius ratio a has no influence on the jet instability when its value is slightly high.5.The most unstable disturbance wavelength obtained in axisymmetric temporal stability analysis is in good agreement with experimental measurement.The transition boundary between absolute and convective instability in axisymmetric spatiotemporal stability analysis is obtained for the first time and is in accordance with the experimental boundary between dripping and jetting.6.The spatial stability analysis considering both n=0 and n=1 shows that axisymmetric disturbances grow faster than non-axisymmetrie ones for low Weber number,while the non-axisymmetric disturbances become predominant as the Weber number increases.The Reynolds number hardly induces the change of the instability mode.The transition from the axisymmetric to non-axisymmetrie jetting mode in FF experiments is illuminated for the first time.