Capillary Jet Research Articles

The research of interaction of the capillary discharge with metal foils

The properties of capillary discharge under its interaction with various metal foils are analysed. Spectral composition of capillary discharge jet is investigated. Upon jet interaction with metal foils, plasma domains occur. The properties of glowing plasma domains, which occur in a constant magnetic field, are analysed. The possible internal structure of plasma domains is analysed.

Electrical and Spectroscopic Diagnostics of Atmospheric Pressure DBD Plasma Jet

Atmospheric pressure capillary dielectric barrier oxygen discharge plasma jet is developed to generate nonthermal plasma using unipolar positive pulse power supply. Both electrical and optical diagnostic techniques were used to characterize the produced plasma as functions of applied voltage and gas flow rate. Electrical diagnostics indicated that the discharge frequency decreased with gas flow rate but increased with the applied voltage. Analytical results obtained from the optical emission spectroscopic data revealed the gas temperature, excitation temperature and electron density. Gas temperature was found to decrease with increasing oxygen flow rate but increase linearly with applied voltage. The produced plasma was applied preliminarily to study the inactivation yield of Fusarium oxysporum fungus infected potato samples.Journal of Bangladesh Academy of Sciences, Vol. 40, No. 1, 23-36, 2016

High-brightness EUV source based on laser plasma using a liquid-metal droplet target

We present the study of a source of extreme ultraviolet (EUV) radiation based on laser plasma generated due to the interaction of radiation from a nanosecond Nd : YAG laser with a liquid-metal droplet target consisting of a low-temperature eutectic indium – tin alloy. The generator of droplets is constructed using a commercial nozzle and operates on the principle of forced capillary jet decomposition. Long-term spatial stability of the centre-of-mass position of the droplet with the root-mean-square deviation of is demonstrated. The use of a low-temperature working substance instead of pure tin increases the reliability and lifetime of the droplet generator. For the time- and space-averaged power density of laser radiation on the droplet target and the diameter of radiating plasma , the mean efficiency of conversion of laser energy into the energy of EUV radiation at equal to 2.3 % is achieved. Using the double-pulse method, we have modelled the repetitively pulsed regime of the source operation and demonstrated the possibility of its stable functioning with the repetition rate up to for the droplet generation repetition rate of more than , which will allow the source brightness to be as large as .

A hybrid flow focusing nozzle design to produce micron and sub-micron capillary jets

Flow focusing is providing new tools in Analytical Chemistry for its unique combination of simplicity, droplet size control just above the micrometer scale, and aerosol beam collimation. However, the most interesting droplet size range lies at the stability limit of the standard flow focusing operating mode. Nevertheless, that stability limit can be forced down to produce jets on the submicrometer scale by underpinning the cone-like meniscus at the tip of a hypodermic needle. Using a nebulizer design based on that concept, we show that the flow focusing steady jetting regime extends down to the limit imposed by the instability of jet itself, even for Reynolds numbers as low as 10−2. This stability enhancement leads to droplet sizes well below those previously obtained with flow focusing. In this work, we experimentally determine the influence on the jet's radius of the liquid flow rate and viscosity, as well as the focusing orifice. We analyze the instability mechanisms that impose the minimum values of the issued flow rate and jet's size. Finally, the jet's breakup and the size of the resulting micron-sized droplets are examined from high-speed optical imaging.

Spatial modes in one-dimensional models for capillary jets.

One-dimensional (1D) models are widely employed to simplify the analysis of axisymmetric capillary jets. These models postulate that, for slender deformations of the free surface, the radial profile of the axial velocity can be approximated as uniform (viscous slice, averaged, and Cosserat models) or parabolic (parabolic model). In classical works on spatial stability analysis with 1D models, considerable misinterpretation was generated about the modes yielded by each model. The already existing physical analysis of three-dimensional (3D) axisymmetric spatial modes enables us to relate these 1D spatial modes to the exact 3D counterparts. To do so, we address the surface stimulation problem, which can be treated as linear, by considering the effect of normal and tangential stresses to perturb the jet. A Green's function for a spatially local stimulation having a harmonic time dependence provides the general formalism to describe any time-periodic stimulation. The Green's function of this signaling problem is known to be a superposition of the spatial modes, but in fact these modes are of fundamental nature, i.e., not restricted to the surface stimulation problem. The smallness of the wave number associated with each mode is the criterion to validate or invalidate the 1D approaches. The proposed axial-velocity profiles (planar or parabolic) also have a remarkable influence on the outcomes of each 1D model. We also compare with the classical 3D results for (i) conditions for absolute instability, and (ii) the amplitude of the unstable mode resulting from both normal and tangential surface stress stimulation. Incidentally, as a previous task, we need to re-deduce 1D models in order to include eventual stresses of various possible origins (electrohydrodynamic, thermocapillary, etc.) applied on the free surface, which were not considered in the previous general formulations.

The production of viscoelastic capillary jets with gaseous flow focusing

The production of viscoelastic capillary jets with gaseous flow focusing is studied experimentally. In this technique, the liquid is injected at a constant flow rate through a feeding capillary located in front of the discharge orifice. A gas stream coflows with the jet across the orifice driven by a constant pressure drop. The gas stream sucks and drags the liquid, reducing the jet’s diameter well below the orifice diameter. Because of the rheological nature of the liquid, this focusing phenomenon differs from the Newtonian one in several regards. For given values of the polymer concentration, the injected flow rate, and the applied pressure drop, there is an interval of the capillary-to-orifice distance for which the jetting regime is reached. Outside that interval, the jet either suffers from the pull-out instability or breaks up before reaching the discharge orifice. Significant free surface oscillations can be observed in most of the jetting realizations. This oscillatory behavior is caused by a transient die swell effect which continuously appears right at the capillary exit. Ejection interrupts because the jet bulges to such an extent that the free surface pinches. Because of the stabilizing effect of the polymeric contribution to the axial stress, micrometer filaments with lengths up to 1 cm and Weber numbers on the order of 10−4 can be produced in front of the discharge orifice. The shear viscous stresses exerted on the emitted jet by the gas stream beyond the discharge orifice prevent the macromolecule recoiling. The resulting extensional viscosity inhibits the break-up process, and thus very long jets are produced.

Breakup of capillary jets with different disturbances

The disturbance on a capillary jet can be imposed by radius modulation, velocity modulation, or jet vibration. The objective of the study is to understand the equivalence between the three types of disturbances. Theoretical analysis based on the Bernoulli equation for unsteady flows is conducted. It is found that a radius-modulated disturbance is equivalent to a velocity-modulated disturbance with the same wave number if the non-dimensional amplitude of the radius disturbance is 1.5 times that of the velocity disturbance. This is validated by carrying out numerical simulation based on velocity modulation and comparing with the linear theory based on radius modulation. It is also revealed that disturbance generated by a vibrating nozzle with small amplitude is equivalent to velocity disturbance. The non-dimensional amplitude of the equivalent velocity disturbance is a function of non-dimensional vibration amplitude and vibration wave number. The wave number of the velocity disturbance is shown to be twice of the vibration wave number. Validated by experimental observation, if the vibration wave number is less than 0.5, each nozzle vibration cycle generates two droplets. If the vibration wave number is between 0.5 and 1, each vibration cycle generates one droplet.

Generation of micro-sized PDMS particles by a flow focusing technique for biomicrofluidics applications.

Polydimethylsiloxane (PDMS), due to its remarkable properties, is one of the most widely used polymers in many industrial and medical applications. In this work, a technique based on a flow focusing technique is used to produce PDMS spherical particles with sizes of a few microns. PDMS precursor is injected through a hypodermic needle to form a film/reservoir over the needle's outer surface. This film flows towards the needle tip until a liquid ligament is steadily ejected thanks to the action of a coflowing viscous liquid stream. The outcome is a capillary jet which breaks up into PDMS precursor droplets due to the growth of capillary waves producing a micrometer emulsion. The PDMS liquid droplets in the solution are thermally cured into solid microparticles. The size distribution of the particles is analyzed before and after curing, showing an acceptable degree of monodispersity. The PDMS liquid droplets suffer shrinkage while curing. These microparticles can be used in very varied technological fields, such as biomedicine, biotechnology, pharmacy, and industrial engineering.

キャピラリージェット法を用いた液体金属の動的表面張力の測定

キャピラリージェット法を用いた液体金属の動的表面張力の測定

Stabilization mechanisms in the evolution of thin liquid-films

In recent years, there has been great interest in using control theory to alter the stability regimes of fluid systems. A flow property is measured at a point and relayed back to a control that alters a condition that opposes the instability, thereby postponing its onset. Here, we discuss an alternative to postponing and even eliminating instabilities without the need for measuring properties or designing control strategies: a shear flow imposed upon a system produces an interfacial viscous-capillary wave which, in the nonlinear regime, is capable of postponing or even eliminating the incipient instability. The literature shows several examples, whereby Rayleigh break-up of capillary jets is eliminated, van der Waals dry-out of a film is removed and thermocapillary instability is avoided by the application of a suitable surface shear or an imposed fluid flow. The stabilization mechanism is closely linked to the behaviour of the lower-order terms governing the evolution of the liquid–gas interface profile, providing an estimate for the time scales and shear strength involved. Our intention here is to develop a unified theoretical framework for the study of a large number of thin liquid-film configurations and related systems.

Measurement of the dispersion relation of a convectively unstable capillary jet under confinement

The dispersion relation of a confined capillary jet with negligible inertia is measured by Fourier analysis of the jet radius. The real part of the dispersion relation, obtained through the spatiotemporal Fourier spectrum, demonstrates that the phase velocity is independent of the perturbation frequency, at least in the accessible range of frequencies. The imaginary part of the dispersion relation, i.e., the spatial growth rate, is frequency dependent. To measure this dependence, an external forcing is used to amplify modes with different frequencies. The spatial growth rate is then obtained from the exponential growth of the corresponding Fourier mode. The phase velocity and the spatial growth rate are found to increase and decrease, respectively, with the capillary number and to depend only weakly on the degree of confinement. These observations are consistent with a spatial instability that convects the perturbations downstream with velocity proportional to the capillary number. The frequency of the dominant mode in the absence of forcing, on the contrary, depends mainly on the degree of confinement, i.e., on the jet diameter, and only weakly on the capillary number, which is consistent with the mode selection of the Rayleigh-Plateau instability. Experimental measurements are compared to predicted dispersion relations reported in the literature, and good qualitative agreement is observed.

Observation and Manipulation of a Capillary Jet in a Centrifuge-Based Droplet Shooting Device

We report observation and manipulation of a capillary jet under ultra-high centrifugal gravity in a proposed capillary-based fluidic device for the synthesis of microparticles in a centrifugal tube called Centrifuge-Based Droplet Shooting Device (CDSD). Using a high-speed camera, we directly observed the dripping to jetting transition of a viscous capillary jet of water and Sodium alginate solution generated from a glass capillary-orifice of a diameter of O (100) m under centrifugal gravity ranging from 190 to 450 g. A non-dimensional analysis shows that the mechanism of the dripping-jetting transition in the CDSD may follow that previously reported for a dripping faucet under standard gravity. We also fabricated calcium alginate microparticles by gelating droplets of sodium alginate solution obtained in the break-up of the capillary jet in the jetting regime and demonstrated fabrication of microbeads-on-a-string structures. We confirmed that the jetting regime of the capillary jet could be used to fabricate smaller particles than that of the dripping regime. The results show that the CDSD would be a more useful device to fabricate various polymeric structures and understand the physics of fluid jets under ultra-high gravity.

Convective-to-absolute instability transition in a viscoelastic capillary jet subject to unrelaxed axial elastic tension.

The convective-to-absolute instability transition in an Oldroyd-B capillary jet subject to unrelaxed axial stress is examined theoretically. There is a critical Weber number below which the jet is absolutely unstable under axisymmetric perturbations. We analyze the dependence of this critical parameter with respect to the Reynolds and Deborah numbers, as well as the unrelaxed axial stress. For small Deborah numbers, the unrelaxed stress destabilizes the viscoelastic jet, increasing the critical Weber number for which the convective-to-absolute instability transition takes place. If the Deborah number takes higher values, then the transitional Weber number decreases as the unrelaxed stress increases until two solution branches cross each other. The dominant branch for large axial stress leads to a threshold of this quantity above which the viscoelastic jet becomes absolutely unstable independently of the Weber number. The threshold depends on neither the Reynolds nor the Deborah number for sufficiently large values of these parameters.

Plasma Ignition Response for LOVA Gun Propellant at Low Loading Densities

The ignition of LOVA gun propellants at low loading densities is of interest for future gun systems with insensitive modular charges. The ignition system in conventional propellant systems is traditionally optimized for high loading densities. There is a need to be able to control the ignition for the envelop of the system, to reduce unburnt propellant at low loading densities and, at the same time, avoid pressure waves at high loading densities. Plasma ignition is interesting for this application, with the possibility to vary ignition energy and with the plasma plume penetrating deep into the propellant bed to achieve uniform ignition. This paper presents results from plasma ignition experiments in a 45-mm gun system, using NL008, an RDX-based CAB-LOVA propellant. The plasma igniter is a capillary jet type, driven by a pulse-forming network. A zero-dimensional model for the plasma igniter has been developed and will be discussed in the context of the experiments.

DGLSM based study of temporal instability and formation of satellite drop in a capillary jet breakup

Temporal instability of a cylindrical capillary jet is analyzed numerically for different liquid Reynolds numbers (Re=1,10,100),disturbance wave numbers (0<k≤1), and ratio of viscosity of continuous and dispersed phase (η=0.01,0.1,1). Simulations are done using an in-house code based on dual grid Level Set Method. Properties as well as physical interpretations of level set functions are used for characterization of temporal evolution and breakup of a liquid jet, the growth rate of disturbance, the breakup time and its location, and the diameter of primary as well as satellite drop. An excellent agreement of the numerical results with the published analytical as well as experimental results is presented. For higher viscosity ratio system (η=1), satellite drops are generated owing to multiple breakup sequences around the neck region of a highly deformed ligament. The breakup mechanism is self-repeating in the sense that every pinch off is always associated with the formation of a neck, the neck undergoes pinch off, and the process repeats. With increasing Re, for all the values of wave numbers, it is found that the growth rate increases; whereas, breakup time decreases. With an increase in the viscosity ratio, the growth rate decreases and breakup time increases accompanied by marginal change in the drop size. Contribution of the total volume of the jet to the satellite (primary) drop decreases (increases) with increasing k, decreasing Re and decreasing η. The present work leads to a better understanding of mechanism of capillary jet breakup and subsequent formation of main, satellite and sub-satellite drops, due to temporal growth of surface perturbation.

Experimental and linear analysis for the instability of non‐Newtonian liquid jets issuing from a pressurized vibrating nozzle

The laminar capillary breakup of viscoelastic jets to produce polymeric microcapsules is analyzed experimentally and theoretically. The phenomenon is based on subjecting a capillary jet to controlled disturbances so that it eventually breaks up forming individual droplets. A dispersion relation from a temporal linear analysis to describe and predict the system behavior that includes the Oldroyd‐B constitutive equation to take into account the viscoelasticity of the liquid is obtained. Dispersion curves relating growth rate and wavenumber of the perturbed jets are compared with experimental conditions and the chosen mathematical approach is found that fairly describes the system. The obtained dispersion relation eases the study of the effect of viscosity, elasticity, through relaxation times, and flow rate in the system. The approach allows finding the best conditions to obtain homogeneous droplets and describes the system qualitatively. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2070–2078, 2015

Atmospheric Plasma Jet-Enhanced Anodization and Nanoparticle Synthesis

Atmospheric-pressure discharge in the tubular capillary plasma jet was studied to reveal the possibility to accelerate anodization of aluminum foils and fabricate alumina nanoparticles in liquid acids. Two different positions of the atmospheric pressure plasma jet relatively to the acid electrolyte and aluminum electrode were studied. Whereas at larger distances, only slight oxidation was obtained, the nanoporous surface and alumina nanoparticles were produced at closer (3-5 mm) distances. The mode with arcing (sparking) resulted in the film of alumina nanoparticles. The obtained results could be useful for the cheap and convenient synthesis of nanoparticles and nanostructured surfaces for various applications, including medical, biological, energy conversion, and nanoelectronic devices.

Gas-focused liquid microjets from a slit

We report an experimental study on a new arrangement of flow focusing that uses a slit aperture. The slit can achieve similar flow chocking function as a classic circular aperture to established gas flow pressure drop. Fundamentally, the jetting mechanism and quantitative behavior of slit flow focusing are very similar to the classic flow focusing because of the singularity nature of the jet despite the drastically different aperture shapes. After re-scaling the jet shape, the axial coordinate with the hydraulic diameter of the slit, the experimental data follow closely with the universal jet profile derived in Gañán-Calvo, Ferrera, and Montanero [“Universal size and shape of viscous capillary jets: Application to gas-focused microjets,” J. Fluid Mech. 670, 427-438 (2011)] based on circular aperture. Practically, the slit configuration expands the possibility of manipulating the jets. The two thin plates that form the slit aperture can also be used as electrodes to apply electrohydrodynamic excitation to the jet for controlled jet breakup with narrower droplet size distribution. In addition, the modest throughput of a single flow focused jet can be increased by operating a linear array of jets focused and excited by the same slit.

Capillary plasma jet: A low volume plasma source for life science applications

In this letter, we present results from multispectroscopic analysis of protein films, after exposure to a peculiar plasma source, i.e., the capillary plasma jet. This plasma source is able to generate very small pulsed plasma volumes, in kilohertz range, with characteristic dimensions smaller than 1 mm. This leads to specific microscale generation and transport of all plasma species. Plasma diagnosis was realized using general electrical and optical methods. Depending on power level and exposure duration, this miniature plasma jet can induce controllable modifications to soft matter targets. Detailed discussions on protein film oxidation and chemical etching are supported by results from absorption, X-ray photoelectron spectroscopy, and microscopy techniques. Further exploitation of principles presented here may consolidate research interests involving plasmas in biotechnologies and plasma medicine, especially in patterning technologies, modified biomolecule arrays, and local chemical functionalization.

Periodic emission of droplets from an oscillating electrified meniscus of a low-viscosity, highly conductive liquid.

The generation of identical droplets of controllable size in the micrometer range is a problem of much interest owing to the numerous technological applications of such droplets. This work reports an investigation of the regime of periodic emission of droplets from an electrified oscillating meniscus of a liquid of low viscosity and high electrical conductivity attached to the end of a capillary tube, which may be used to produce droplets more than ten times smaller than the diameter of the tube. To attain this periodic microdripping regime, termed axial spray mode II by Juraschek and Röllgen [R. Juraschek and F. W. Röllgen, Int. J. Mass Spectrom. 177, 1 (1998)], liquid is continuously supplied through the tube at a given constant flow rate, while a dc voltage is applied between the tube and a nearby counter electrode. The resulting electric field induces a stress at the surface of the liquid that stretches the meniscus until, in certain ranges of voltage and flow rate, it develops a ligament that eventually detaches, forming a single droplet, in a process that repeats itself periodically. While it is being stretched, the ligament develops a conical tip that emits ultrafine droplets, but the total mass emitted is practically contained in the main droplet. In the parametrical domain studied, we find that the process depends on two main dimensionless parameters, the flow rate nondimensionalized with the diameter of the tube and the capillary time, q, and the electric Bond number B(E), which is a nondimensional measure of the square of the applied voltage. The meniscus oscillation frequency made nondimensional with the capillary time, f, is of order unity for very small flow rates and tends to decrease as the inverse of the square root of q for larger values of this parameter. The product of the meniscus mean volume times the oscillation frequency is nearly constant. The characteristic length and width of the liquid ligament immediately before its detachment approximately scale as powers of the flow rate and depend only weakly on the applied voltage. The diameter of the main droplets nondimensionalized with the diameter of the tube satisfies d(d)≈(6/π)(1/3)(q/f)(1/3), from mass conservation, while the electric charge of these droplets is about 1/4 of the Rayleigh charge. At the minimum flow rate compatible with the periodic regimen, the dimensionless diameter of the droplets is smaller than one-tenth, which presents a way to use electrohydrodynamic atomization to generate droplets of highly conducting liquids in the micron-size range, in marked contrast with the cone-jet electrospray whose typical droplet size is in the nanometric regime for these liquids. In contrast with other microdripping regimes where the mass is emitted upon the periodic formation of a narrow capillary jet, the present regime gives one single droplet per oscillation, except for the almost massless fine aerosol emitted in the form of an electrospray.