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• A steady cone-jet occurs in a greater range of liquid flow rates for a hemispherical capillary. • Varicose and whipping regimes are easily obtained with a hemispherical nozzle. • The transition from varicose and whipping usually depends on interfacial forces on the jet. • A general parameter to determine varicose, transition, and whipping regimes is presented. A cone-jet regime in electrohydrodynamic atomization (EHDA) is widely applied into printing, electro-spinning, spray-coating, and biological mass spectrometry. Regulating different instabilities of jet is crucial for optimizing its operating parameters. In present work, an atypical, hemispherical capillary with inner diameter of 0.40 mm and cap size of 6.20 mm was employed as atomization nozzle, through which a steady cone-jet regime was confirmed versus suitable feeding liquid flow rate and electric potential. A permanent and highly charged jet emits from a Taylor cone apex and further disintegrates into fine highly charged drops in the form of varicose or whipping instabilities. The results indicate that a steady cone-jet is observed in the specific ranges of the feeding liquid flow rate, electric potential and conductivity. The typical breakup instabilities including varicose and whipping are clearly observed with an increase in liquid flow rate for a fixed electric potential for most of liquids, and the transition from varicose to whipping instability easily occurs. For a steady cone-jet, the jet breakup instabilities usually depends on the competition between interfacial forces on the electrified jet. The transition process involving a low-viscosity jet with a large flow rate is observed, and the general parameter G = Γ 2 δ μ 0.52 is calculated to account for surface charge, surface tension, and viscosity effects, where Γ is the electric force to surface tension ratio and δ μ is the inertia to viscous force ratio. Finally, the varicose, transition regime, and whipping are classified with first and second critical threshold values G = 40 and G = 102, respectively. Meanwhile, as the electrified jet transforms from varicose to whipping instabilities, the jet breakup length shows a trend of increasing first and then decreasing. [ABSTRACT FROM AUTHOR]

• The necking time is independent of electric field in a reasonable range of flow rate. • The transition from dripping to jetting is enhanced by electric field for relatively small tube. • A model for the transition from periodic dripping to jetting is established in an electric field. • A critical criterion for the transition is presented and in reasonable agreement with experiments. A liquid is issued from the outlet of the capillary tube vertically, and drop formation may be in the form of periodic dripping, dripping faucet and jetting, which usually depends on flow rate or Weber number. We consider the critical Weber number (W e C = ρV 0 2 D / σ) at which the transition from periodic dripping to jetting occurs in the presence of an electric field, where high-resolution images are recorded. The necking time is mainly related to nozzle diameter and surface tension, and independent of electric potential and flow rate (in a reasonable range).The first threshold limit decreases with an increase in electric potential, where the dripping faucet is clearly affected and the point of drop detachment gradually increases. When the drop formation is in the form of dripping faucet, the transition from dripping faucet to jetting regime is enhanced by electric potential, where a sudden transition could be observed in an extremely limited range of liquid flow rate. The critical criterion (Weber number) is presented to determine the transition from dripping to jetting in the presence of an electric field. The critical Weber number is in a reasonable agreement with the experiment, although the predicted results are quantitatively divergent with experimental data. [ABSTRACT FROM AUTHOR]

• The onset and termination electric potential to form the steady cone-jet electrospray increase as conductivity increasing. • The lower and upper limitation of liquid flowrate to form the steady cone-jet electrospray decrease as conductivity increases. • The varicose instability can transform into whipping instability as the electrical conductivity increasing. • The jet breakup instability transition mechanism is interpreted by theoretical analysis. • The whipping to similar micro-dripping instability can be observed for EG with higher conductivity. In present work, the effect of the electrical conductivity on the steady cone-jet electrospray are experimentally investigated, where the operating diagram and the jet breakup instability are extensively examined. A hemispherical tube is employed as spraying nozzle severed as high electric potential electrode in a needle-plated electrode configuration compared to the common and sheathed tubes. The cone-jet electrospray is visualized by a high-resolution camera and extensively analyzed offline. The aqueous solution in ethanol, Diethyleneglycol (DEG) and Ethylene glycol (EG) are atomized with varied electrical conductivity under different high electric potentials and flowrates. Experimental results indicate that the cone-jet electrospray can operate in the certain ranges of operating parameter for all types of capillaries. The operating diagram for the hemispherical tube is significantly broader than other tubes because of various electric field strength distribution. The onset electric potential to determine a steady cone-jet electrospray gradually increases and the termination value also increases as the electrical conductivity increases. The lower and upper flowrate to form a steady cone-jet electrospray also increase as the electrical conductivity increases. The jet breakup instability can transit from varicose to whipping due to low the reference flow rate as electrical conductivity increases, which is agreement with theoretical analysis. In particular, for EG, the transition from whipping to similar micro-dripping is observed as the electrical conductivity increases, while the transition also occurs as flowrate increases for EG with higher electrical conductivity. The increase in the electrical conductivity and flowrate elongates the cone due to EHD body force, which causes the electric field varying, further resulting in the difference in jet breakup instability. Moreover, the electrical conductivity also plays an important role in jet breakup length. The instability variation of jet with conductivity (DEG), q V =1000 nL/s, ϕ =8.00 kV (Hemispherical nozzle with D o =0.70mm); The variation with liquid flow rate of EG with higher conductivity (K =8.5 × 10−4 s/m, ϕ =10.20 kV, (Hemispherical nozzle with D o =0.70mm) [Display omitted] [ABSTRACT FROM AUTHOR]

• The stable cone-jet occurs in a certain range of electric potential and flow rate. • The electrified meniscus forms appears successively convex, linear sides and concave with an increase in electric potential. • The jet breakup transits from varicose to kink instability with an increase in flow rate. The cone-jet is one of the most interesting means in electrohydrodynamic atomization (EHDA) for wide applications because of highly charged and monodisperse drops. The pulsating and stable cone-jet modes in EHDA of diethyleneglycol (DEG) are observed by means of high-speed photography technology, as well as the breakup modes transition of electrified jet from varicose to kink instability. The conductive liquid is driven to form a liquid cone at the outlet of the capillary tube, and form a permanent jet from the apex of the cone by the tangential electric stress. With an increase in electric potential, the cone angle and jet length increase. The electrified meniscus and the cone may appear convex, linear sides and concave in cone jet regime, which depends on the static equilibrium of forces exists at each point of electrified liquid surface. The jet breakup modes with multi-jet, varicose and kink instabilities are successively recorded as liquid flow rates increasing. The jet breakup transition depends on flow rate. The drops size rises proportionally with an increase in liquid flow rate or a decrease in electric potential in stable cone-jet mode. The dimensionless diameter d / d 0 as a function of the dimensionless flow rate q V / q m is in good agreement with the universal scaling laws. [ABSTRACT FROM AUTHOR]

• The droplet formation in electric periodic dripping regime was visualized. • The oscillation of electrified meniscus was visualized and identified. • The characteristic frequency deceases with an increase in applied potential. • The amplitude increases as electric number increasing. The dynamics of droplets and oscillation of electrified meniscus at low flow rates in an external electric field were investigated. An electric field is created by nozzle-plated electrode. The evolution of droplet formation is recorded by a high-speed digital camera. Meanwhile, the time evolution of meniscus is also focused. The critical sizes of the droplet and liquid thread were used to characterize the dynamics of droplet formation and oscillation of the meniscus. The results showed that a droplet gradually originates from a hemispherical meniscus and changes from spherical to pear-shaped. A liquid thread connects the meniscus and primary droplet, and pinches off as time advances. The droplets diameter decreases as electric potential increasing and is dependent of capillary diameter, while usually independent of flow rates. The droplet limiting length decreases, while the liquid thread diameter increases as electric Bond number increasing. The period of detachment decreases with an increase in electric potential and strongly depends on flow rate. The meniscus usually oscillates when the primary droplet detaches. The oscillation frequency increases as electric field strength increasing and is substantially affected by flow rate. An approximate equation to predict the oscillation frequency was presented, where the electric potential was taken into account. The variation of the oscillation frequency is qualitatively good agreement with experimental work. The amplitude of oscillation (or displacement) is significantly affected by electric field and increases as electric Bond number increasing. [ABSTRACT FROM AUTHOR]