Xie-Zhen Yin

Spraying modes in coaxial jet electrospray with outer driving liquid

Coaxial jet electrospray is a technique to generate microencapsules, which uses electric forces to create a coaxial microjet from two immiscible liquids. Compound droplets with narrow size distribution are produced after the jet breaks up. In this paper, the spraying modes are investigated experimentally with proper flow rates of the inner and outer liquids. Ethanol/glycerol/tween mixture (outer liquid) and cooking oil (inner liquid) are fed into the gap between outer and inner capillaries and the inner capillary, respectively. The spraying modes presented in our experiments are “dripping mode,” “dripping mode in spindle,” “cone-jet mode,” “pulse mode in cone,” and “multijets mode” sequentially, as the applied voltage increases. The region of stable cone-jet mode extends with decrease of the outer liquid flow rate and increase of the inner one. It is found that the spray phenomena are mainly determined by properties of the outer liquid, which is viscous and electric conductive enough. A rudimentary physical model is developed, in which both the viscosity and liquid interface tension are taken into account.Coaxial jet electrospray is a technique to generate microencapsules, which uses electric forces to create a coaxial microjet from two immiscible liquids. Compound droplets with narrow size distribution are produced after the jet breaks up. In this paper, the spraying modes are investigated experimentally with proper flow rates of the inner and outer liquids. Ethanol/glycerol/tween mixture (outer liquid) and cooking oil (inner liquid) are fed into the gap between outer and inner capillaries and the inner capillary, respectively. The spraying modes presented in our experiments are “dripping mode,” “dripping mode in spindle,” “cone-jet mode,” “pulse mode in cone,” and “multijets mode” sequentially, as the applied voltage increases. The region of stable cone-jet mode extends with decrease of the outer liquid flow rate and increase of the inner one. It is found that the spray phenomena are mainly determined by properties of the outer liquid, which is viscous and electric conductive enough. A rudimentary physic...

Coupling modes between two flapping filaments

The flapping coupling between two filaments is studied theoretically and experimentally in this paper. A temporal linear instability analysis is carried out based on a simplified hydrodynamic model. The dispersion relationship between the eigen-frequency ω and wavenumber k is expressed by a quartic equation. Two special cases of flapping coupling, i.e. two identical filaments having the same length and two filaments having different lengths, are studied in detail. In the case of two identical filaments, the theoretical analysis predicts four coupling modes, i.e. the stretched-straight mode, the antisymmetrical in-phase mode, the symmetrical out-of-phase mode and the indefinite mode. The theory also predicts the existence of an eigenfrequency jump during transition between the in-phase and out-of-phase modes, which has been observed in previous experiments and numerical simulations. In the case of two filaments having different lengths, four modes similar to those in the former case are identified theoretically. The distribution of coupling modes for both the cases is shown in two planes. One is a dimensionless plane of S vs. U , where S is the density ratio of solid filament to fluid and U 2 is the ratio of fluid kinetic energy to solid elastic potential energy. The other is a dimensional plane of the half-distance ( h ) between two filaments vs. the filament length ( L ). Relevant experiments are carried out in a soap-film tunnel and the stable and unstable modes are observed. Theory and experiment are compared in detail. It should be noted that the model used in our analysis is a very simplified one that can provide intuitional analytical results of the coupling modes as well as their qualitative distributions. The factors neglected in our model, such as vortex shedding, viscous and nonlinear effects, do not allow the model to predict results precisely consistent with the experiments. Moreover, the Strouhal numbers of the flapping filaments are found to be generally around a fixed value in the experiments for both cases, implying that the filaments try to maintain a lower potential energy state.

Modes in flow focusing and instability of coaxial liquid―gas jets

Six flow modes are distinguished in the flow-focusing experiments of a liquid jet forced by a high-speed air stream. The domains of the modes are identified on the parameter space of the liquid flow rate Q 1 and the gas pressure drop Δp g . The disturbance wavelength and breakup length L of the jet are also measured. A theoretical model considering axisymmetric disturbances is proposed, and a basic velocity profile of hyperbolic-tangent function is utilized. The linear temporal and spatio-temporal instability analyses are carried out using the Chebyshev collocation method. The effects of the flow parameters and the velocity profile on the flow instability are discussed. The temporal instability analysis demonstrates that the interfacial shear causes the instability of short waves and retards the instability of long waves. Moreover, the spatio-temporal instability analysis gives the transition boundary between the absolute and convective instability (AI/CI). The most unstable wavelength predicted by the temporal instability analysis and the AI/CI boundary predicted by the spatio-temporal instability analysis are in good agreement with the experimental results.

Axisymmetric and non-axisymmetric instability of an electrified viscous coaxial jet

A linear study is carried out for the axisymmetric and non-axisymmetric instability of a viscous coaxial jet in a radial electric field. The outer liquid is considered to be a leaky dielectric and the inner a perfect dielectric. The generalized eigenvalue problem is solved and the growth rate of disturbance is obtained by using Chebyshev spectral collocation method. The effects of the radial electric field, liquid viscosity, surface tension as well as other parameters on the instability of the jet are investigated. The radial electric field is found to have a strong destabilizing effect on non-axisymmetric modes, especially those having smaller azimuthal wavenumbers. The helical mode becomes prevalent over other modes when the electric field is sufficiently large. Non-axisymmetric modes with high azimuthal wavenumbers may be the most unstable at zero wavenumber. Liquid viscosity has a strong stabilizing effect on both the axisymmetric and non-axisymmetric instability. Relatively, the helical instability is less suppressed and therefore becomes predominant at high liquid viscosity. Surface tension promotes the instability of the para-sinuous mode and meanwhile suppresses the helical and the other non-axisymmetric modes in long wavelength region.

Instability of a viscous coflowing jet in a radial electric field

A temporal linear instability analysis of a charged coflowing jet with two immiscible viscous liquids in a radial electric field is carried out for axisymmetric disturbances. According to the magnitude of the liquid viscosity relative to the ambient air viscosity, two generic cases are considered. The analytical dimensionless dispersion relations are derived and solved numerically. Two unstable modes, namely the para-sinuous mode and the para-varicose mode, are identified in the Rayleigh regime. The para-sinuous mode is found to always be dominant in the jet instability. Liquid viscosity clearly stabilizes the growth rates of the unstable modes, but its effect on the cut-off wavenumber is negligible. The radial electric field has a dual effect on the modes, stabilizing them when the electrical Euler number is smaller than a critical value and destabilizing them when it exceeds that value. Moreover, the electrical Euler number and Weber number increase the dominant and cut-off wavenumbers significantly. Based on the Taylor-Melcher leaky dielectric theory, two limit cases, i.e. the small electrical relaxation time limit (SERT) and the large electrical relaxation time limit (LERT), are discussed. For coflowing jets having a highly conducting outer liquid, SERT may serve as a good approximation. In addition, the dispersion relations under the thin layer approximation are derived, and it is concluded that the accuracy of the thin layer approximation is closely related to the values of the dimensionless parameters.

Linear instability analysis of an electrified coaxial jet

A temporal linear instability analysis of an electrified coaxial jet inside a coaxial electrode is carried out in this paper. The analytic dispersion equation is derived. The characteristic of the temporal instability is investigated and effects of the parameters, including the dimensionless electrostatic force, the dimensionless velocity difference, the density ratio, the inner and the outer diameter ratios, and the Weber numbers, are discussed systematically. The axisymmetric and the helical modes are both considered. Two independent unstable modes, mode 1 and mode 2, are identified. Among all the parameters the dimensionless electrostatic force has remarkable influence on mode 1. It can change mode 1 from the Rayleigh regime to the wind induced regime, and to the atomization regime by increasing the electric-field intensity. Nevertheless, the dimensionless electrostatic force has no apparent effect on mode 2. The behaviors of mode 2 resemble those of a coaxial jet not having electric field. The surface...

Advances and applications of electrohydrodynamics

Electrohydrodynamics (EHD) has been applied in many areas, such as EHD enhanced heat transfer, EHD pump, electrospray mass spectrometry, electrospray nano-technology, etc. The basic theories underlying these applications and a brief review of the EHD advances are presented in this paper. As electrospray is an important branch of EHD, the applications of electrospray are particularly discussed. Finally, this paper gives some suggestions for the future research and development.

Instability analysis of a coaxial jet under a radial electric field in the nonequipotential case

The linear temporal stability analysis of a coaxial jet model under the radial electric field is carried out on the assumption that all free charges are “frozen” on the outer interface of the conducting liquid (i.e., the nonequipotential case) in the present paper. The analytical dimensionless dispersion relation is derived and solved numerically. Three unstable modes, namely the paravaricose, parasinuous, and transitional modes, are found in the Rayleigh regime and they change into the other two unstable modes, namely mode 1 and mode 2, in the wind induced and atomization regimes if the dimensionless electrostatic force or Weber number is sufficiently amplified. The numerical results show that the dimensionless electrostatic force as well as the Weber number inhibits the jet instability in the long wavelength region while enhances it in the short wavelength region. The relative dielectric constant of the outer liquid alters the behaviors of the nonequipotential case significantly, whereas the electrical ...

Experimental design and instability analysis of coaxial electrospray process for microencapsulation of drugs and imaging agents

Abstract. Recent developments in multimodal imaging and image-guided therapy requires multilayered microparticles that encapsulate several imaging and therapeutic agents in the same carrier. However, commonly used microencapsulation processes have multiple limitations such as low encapsulation efficiency and loss of bioactivity for the encapsulated biological cargos. To overcome these limitations, we have carried out both experimental and theoretical studies on coaxial electrospray of multilayered microparticles. On the experimental side, an improved coaxial electrospray setup has been developed. A customized coaxial needle assembly combined with two ring electrodes has been used to enhance the stability of the cone and widen the process parameter range of the stable cone-jet mode. With this assembly, we have obtained poly(lactide-co-glycolide) microparticles with fine morphology and uniform size distribution. On the theoretical side, an instability analysis of the coaxial electrified jet has been performed based on the experimental parameters. The effects of process parameters on the formation of different unstable modes have been studied. The reported experimental and theoretical research represents a significant step toward quantitative control and optimization of the coaxial electrospray process for microencapsulation of multiple drugs and imaging agents in multimodal imaging and image-guided therapy.

Response modes of a flexible filament in the wake of a cylinder in a flowing soap film

Flow induced vibrations are observed in many engineering applications. A flexible body located in the wake of an obstacle is usually forced to vibrate by the periodic vortices shedding from the obstacle. Here we focus on the response of the flexible body in the wake. Soap film tunnels are used to provide two-dimensional flow. Cylinders and flexible filaments are employed as obstacles and flexible bodies, respectively. The filaments exhibit lock-in behavior to the wake. Three response modes are found by changing the distance between the filaments and cylinders. The observations are illuminated in terms of waving plate theory.