Stability of jets in liquid-liquid systems

Experiments were carried out to study the stability of jets in immiscible liquid systems under conditions where the jet velocity relative to continuous phase was zero. The laminar breakup lengths and the diameter of drops formed from laminar jets are in good agreement with the stability analysis for stationary column while breakup data for jets injected into quiescent liquids disagree with it. An approximate solution for theoretical drop size is presented. The experiment also showed that the hydrodynamic resistance of continuous phase increases the growth rate of disturbances but does not affect the wave length.     

Breakup of non-newtonian emulsion jets

The breakup of non-Newtonian emulsion jets into drops was experimentally studied by ejecting both O/W and W/O emulsions vertically downward into stagnant air through nozzles. Breakup lengths of non-Newtonian emulsion jets were found to be almost equivalent to those of Newtonian jets. Experimental breakup data establish that the static surface tension of the oil phase can be used as the surface tension of W/O emulsion jets, whereas the dynamic surface tension of aqueous surfactant solutions is used as that of O/W emulsion jets. Diameters of drops formed from non-Newtonian emulsion jets are in good agreement with the prediction from the stability theory previously developed by the authors. When the rheological index in a power law model is appreciably smaller than unity and the Ohnesorge number is significantly large, however, drop sizes are larger than the prediction because of the profile relaxation in jets. The critical velocity of emulsion jets, either O/W or W/O emulsion, is significantly lower than that of homogeneous Newtonian jets.     

The effects of surface active agents on jet breakup in liquid-liquid systems

An experimental study has been made of the effects of anionic, cationic and nonionic surface active agents on jet length, jet contraction and drop size resulting from the formation and breakup of liquid jets in another immiscible liquid, while undergoing mass transfer. Correlations previously established for uncontaminated systems with various rates of mass transfer successfully fitted the present data, when used with the diminished value of interfacial tension caused by the surfactant. Variables included nozzle size, flow rate, and type and concentration of surfactant. Data were obtained from measurements on over 3,000 drops, plus jet lengths and jet diameters from 501 photographs.     

Breakup of jets in power law non-newtonianndashnewtonian liquid systems

Non-Newtonian effects on the breakup of jets into drops were experimentally studied for power law fluidNdashNewtonian systems under the condition of zero jet velocity relative to the continuous phase. While laminar breakup lengths of Newtonian jets in non-Newtonian liquids agree with the prediction from the stability analysis for Newtonian systems, non-Newtonian jets in Newtonian liquids are less stable than Newtonian jets. Experimental diameters of drops formed from jets in NewtonianNdashnon-Newtonian and in non-NewtonianNdashNewtonian systems are in good agreement with the prediction based on stability analysis for Newtonian systems.     

The formation and breakup of molten oxide jets under periodic excitation

The experiments on the capillary breakup of slag jets at high temperatures are presented in this article. The impact of external excitations on the disintegration process was investigated in a furnace with optical access filmed at frame rates up to 10,000 fps. A synthetic calcia‐alumina slag was used to form jets at different temperatures (1570–1660°C) and jet velocities (0.6–1.4 ms^?1). The impact of external vibration on the breakup was evident: for low jet velocities, the jet length decreased, the droplet size increased, satellite droplet formation was hindered, and a distinct “pumping mechanism” was observed. For jets with higher velocity, the jet length decreased by 30%, the droplet generation frequency increased from 20 to 250 droplets per second, the drop sizes were uniform, and satellite formation was also suppressed. In this case, the ideal case in which the volume of one wave instability forms one droplet was achieved. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3350–3361, 2014     

Evaluation of a New High‐Pressure Dispersion Unit (HPN) for Emulsification

Emulsification plays an important role in the formulation of lipophilic pharmaceutical agents. These substances are often included in the disperse phase of an oil‐in‐water emulsion. To reach a high bioavailability and a good long‐term stability, drop sizes much less than 1 micron are required. For the generation of such emulsions, energy densities of a quality which can only be reached in high‐pressure systems, are necessary. Actually available apparatus, such as high‐pressure homogenizers fitted with valves, microfluidizer or jet disperser, reach particle sizes of about 0.2 micron in continuous processes. It is indispensable to produce emulsions with smaller globules in order to receive a maximum of diversity in application. Therefore, dispersion units with a higher efficiency in drop breakup are needed. Especially in the case of parenterally administered medicament formulations an average particle size between 0.04 and 0.1 microns is requested which is up to now not reachable by continuous emulsification. In this study the drop breakup behavior of a new high‐pressure nozzle is investigated with the example of oil‐in‐water emulsions and compared to the breakup behavior of a state‐of‐the‐art nozzle and to available data published.     

An experimental investigation on the breakup of surfactant-laden non-Newtonian jets

The combined effect of polymers and soluble surfactants on the dynamics of jet breakup, and especially on satellite drop formation, was experimentally investigated. Xanthan gum and Carbopol^® 934 NF were dissolved in water with Sodium Dodecyl Sulfate as the surfactant. Controlled disturbances were imposed at the laminar jet interface using a piezoelectric vibrating nozzle with breakup dynamics recorded using a high-speed camera. Drop and ligament diameters were measured from the digital images. The focus of the work was investigating how bulk and interfacial properties of the prepared fluids influenced ligament and drop evolution. It was found that if the proper concentration of surfactant (close to the critical micelle concentration, CMC) was selected, and if the flow time scales were large enough, Marangoni interfacial stresses may lead to an increase in satellite drop size as previously reported for breakup simulations of shear-thinning jets covered with insoluble surfactant. It was also experimentally confirmed that the introduction of surfactant contributes to a delay in jet breakup.     

对置液柱撞击液膜破裂特征

采用高速摄像仪对两液柱撞击产生液膜的破裂过程进行了实验研究。分析了撞击液膜的破裂过程及表面波产生和传播过程,考察了射流直径、喷嘴间距和射流Weber数（We）对撞击液膜破裂的影响;定量分析了液膜表面波频率的变化及液膜破裂后的粒径分布情况。研究结果表明,液膜表面波传播频率随We的增大而增大,并沿液膜径向方向逐渐减小;随着射流We的增加,液膜边缘的液滴脱落频率增加;当We>1000时,液膜表面产生大量液滴团,且液滴团对液膜破裂具有促进作用;液柱撞击液膜发生破裂后90%以上的量纲1液滴粒径分布在0～1之间。     

ONE-DIMENSIONAL SIMULATION OF THE BREAKUP OF CAPILLARY JETS OF CONDUCTING LIQUIDS. APPLICATION TO E.H.D. SPRAYING

Nonlinear breakup of charged liquid jets is numerically analyzed in this work in the limit of a very small electrical Strouhal number T_e/T_b≪1 (i.e. negligible charge relaxation effects, applicable to highly conducting liquids), where T_e is the electric relaxation time of charges, and T_b is the breakup time in a Lagrangian framework following the liquid jet at its average axial velocity. The influence of the electrical Bond’s number and viscosity on (i) the capillary Rayleigh’s most probable breakup length, (ii) the breakup time, (iii) the volume of the satellite, and (iv) the charge of both main drop and satellite, are analyzed. The model is related to the microjet break-up phenomena in the electrospraying of liquids in steady cone-jet mode, and its range of applicability to those particular problems discussed. Previous experimental results [Mutoh et al., 1979, Convergence and disintegration of liquid jets induced by an electrostatic field. J. Appl. Phys. 50, 3174–3179; Clopeau and Prunet-Foch, 1989, Electrostatic spraying of liquids in cone-jet mode. J. Electrostatics 22, 135–159.] support our numerical finding that the influence of the electrical Bond’s number on Rayleigh’s length is small within the usual parametrical limits of stability of a steady Taylor cone-jet at atmospheric pressure.     

Analysis of Liquid Jet Breakup in One‐ and Two‐Phase Flows

The phenomenon of breakup of a jet into drops has been applied mainly to separation technologies in the chemical, pharmaceutical, and metallurgical industries. The paper deals with the experimental analysis directed at the breakup of polymer solutions flowing through an orifice nozzle. The analysis of the breakup and atomization of a liquid jet by a high‐speed gas jet is presented. Additionally, non‐Newtonian effects on the breakup of the liquid jet into drops were studied using the microphotography method. In the experiments, various aqueous solutions of polyacrylamide were used. The polymer solutions studied were power‐law fluids. Analysis of the photographs of the jet breakup showed that the length of the jets depends on the liquid and gas flow rates and on the concentration of the polymer used. High‐molecular‐weight polymers added to a solvent lead to changes in the rheological properties of the liquid and the breakup length of the jet.     

表面张力变化对含气泡液体射流破裂的影响

使用高速相机研究了表面张力变化对含气泡液体射流破裂过程的影响。通过改变表面活性剂浓度获得了不同表面张力的液体射流。实验发现当液体射流速度保持不变时,减小液体表面张力会增加射流破裂长度。表面活性剂一方面降低了液体动态表面张力,减小了射流表面不稳定波的增长率,增大了射流破裂长度;另一方面表面活性剂在射流表面的非均匀分布会产生Marangoni应力,促使液体向射流变形区运动,从而推迟了射流破裂的发生,增大了射流破裂长度。通过理论分析得到了液体射流破裂长度表达式。发现射流内部气泡会显著缩短含表面活性剂射流的破裂长度。通过气泡扰动射流速度和吸附表面活性剂的分析,揭示了内部气泡对含表面活性剂射流破裂的影响规律。     

The Influence of the External Imprinted Flow Field on Capillary Instability Driven Jet Breakup

The breakup of a flowing liquid jet surrounded by a second flowing liquid is governed by capillary instabilities. These instabilities are caused by minor perturbations in the flow field that build up sinusoidal waves at the jet interface and eventually break it up. The phenomenon of such fatal wavelengths was described first by Lord Rayleigh and later extended by several authors to quiescent and flowing liquid‐liquid systems. The jet formation in a co‐flowing environment was only recently investigated. Originating from the forced breakup of liquid jets in air, this paper reports on the breakup mechanism of liquid jets surrounded by a fluid. By using different flow cells and consequently different flow profiles along the jet trajectory, the breakup mechanism given by the material and process parameters could be excited or suppressed. Two questions concerning droplet generation and the droplet size distribution are addressed: the suppression of the fatal wavelength in transient flow fields and the use of external flow fields to support the fatal wavelength.     

The effect of hydrodynamic parameters on the production of Pickering emulsions in a baffled stirred tank

Pickering emulsions are potential industrial scale alternatives to surfactant-based emulsions. The stability of Pickering emulsions depends on the physicochemical nature of the liquid–particle interface and the hydrodynamic conditions of the production process. This article investigates the effect of hydrodynamic conditions on the drop size of concentrated Pickering emulsions in baffled stirred tanks. Oil in water emulsions composed of silicon oil, water, and hydrophilic glass beads as stabilizing particles were produced. Two impellers were used at different sizes: Rushton turbine (RT) and pitched blade turbine. The effects of power per mass, Reynolds number, tip speed, and Weber number on the droplet sizes were studied. The energy dissipated around the impeller and the size of the impeller high shear zone were found to be critical to the emulsion droplet sizes. The breakup and droplet-particle contact mechanism of the RT was found to be more favorable for the production of the Pickering emulsions.     

Experimental study on drop formation in liquid-liquid fluidized bed

Drop formation in liquid-liquid fluidized bed was investigated experimentally. The normal water was injected via a fine-capillary spray nozzle into the co-flowing No. 25 transformer oil with jet directed upwards in a vertical fluidized bed. Experiments under a wide variety of conditions were conducted to investigate the instability dynamics of the jet, the size and size distribution of the drops. Details of drop formation, drop flow patterns and jet evolution were monitored in real-time by an ultra-high-speed digital CCD (charge couple device) camera. The Rosin-Rammler model was applied to characterize experimental drop size distributions. Final results demonstrate that drop formation in liquid-liquid system takes place on three absolutely different developing regimes: bubbling, laminar jetting and turbulent jetting, depending on the relative Reynolds number between the two phases. For different flow domains, dynamics of drop formation change significantly, involving mechanism of jet breakup, jet length pulsation, mean size and uniformity of the drops. The jet length fluctuates with time in variable and random amplitudes for a specified set of operated parameters. Good agreement is shown between the drop size and the Rosin-Rammler distribution function with the minimum correlation coefficient 0.9199. The mean drop diameter decreases all along with increasing jet flow rate. Especially after the relative Reynolds number exceeds a certain value about 3.5×10⁴, the jet disrupts intensely into multiple small drops with a diameter mainly ranging from 1.0 to and a more and more uniform size distribution. The turbulent jetting regime of drop formation is the most preferable to the dynamic ice slurry making system.     

A Photographic Study of Flash-Boiling Atomization

To gain an insight into the mechanisms of flash-boiling atomization, heated water was injected from a single-hole orifice into heated air (steady injections, liquid pressure 697 kPa, air pressure ambient, test temperatures from 300 to 426 K, orifice diameter 0.34 mm, length 1.37 mm). The breakup regime of interest in the study was that where the spray divergence starts at the nozzle exit. Short-duration backlit photographs and laser diffraction dropsize measurements showed that these flashing jets comprise an inner intact core which is surrounded by the diverging fine spray. These details about the spray structure are not visible in conventional photographs of flashing sprays that use scattered light illumination. The present results cast doubt on a previously proposed theory of flash-boiling atomization that attributes the divergence of the spray cone to the expansion processes that occur in an underex-panded compressible flow, since that theory implies that the liquid is already atomized upon leaving the nozzle. Instead, the photographs show that drops are expelled from the unbroken liquid jet starting at the nozzle exit (presumably by rapid vapor bubble growth within the jet). The core region remains intact for some distance downstream of the nozzle exit, and its breakup eventually produces relatively large drops. As the liquid temperature approaches boiling, the intact length and the core drop size decrease. Thus operation close to boiling is desirable for effective atomization. However, the nozzle mass flow rate decreases and practical difficulties are found (owing to “vapor-lock”) as the liquid is heated near boiling.     

Two-dimensional modeling of the effects of insoluble surfactant on the breakup of a liquid filament

Breakup of surfactant-laden liquid jets has received increasing consideration during the last few years because of its diverse applications, but theoretical studies have been largely restricted to evolution equations based on one-dimensional flow assumptions. Here, a fully two-dimensional finite element algorithm was used to solve the set of equations describing the dynamics of a Newtonian liquid filament covered with an insoluble surfactant in order to provide a better understanding of the underlying physical principles governing the formation of satellite drops. Results indicate that for a viscous liquid jet, formation of satellite drops between main drops is favored by the addition of surfactants. This effect is lessened, and even eliminated, by either decreasing the surfactant strength or increasing the surfactant diffusivity. On the other hand, low-viscosity liquid jets form satellite drops regardless of the presence of surfactant, but the addition of surfactant can either reduce or increase the size of the satellite formed. Reduction of the size of the satellite drop is favored by the addition of weak surfactants, a result that is in agreement with previous one-dimensional flow analyses. Conversely, addition of strong surfactants of low surface diffusivity increases the size of the satellite drop formed due to Marangoni stress-induced reversal of the capillary flow. The detailed information provided by the two-dimensional model has enabled a better understanding of the competition between viscous, inertia and capillary forces during jet breakup, and of how the competition between them changes due to the presence of the surfactant. This understanding can help in the rational design of systems such as spray, atomization, and jet printing to prevent the formation of satellite drops.     

Stability of multiple emulsions under shear stress

Multiple liquid emulsions of the water in oil in water (W₁/O/W₂) type are used in a variety of consumer or technical applications, for instance in the encapsulation of certain active ingredients. The encapsulation process and release mechanisms of the inner phase of the carrier drops are important in order to properly process and formulate such liquid-liquid systems. In this work the stability and breakage of multiple W₁/O/W₂ emulsions under mechanical shear stress are investigated for emulsions with different surfactants and surfactant concentrations of the internal emulsion. Stressing the emulsions in a mechanical stirring process is compared to the membrane emulsification process. The membrane emulsification process results in higher encapsulation efficiencies than the stirring process. The emulsion droplets were subjected to shear stress below and above the critical capillary number for drop breakup. The results show that stable inner emulsions with sufficient surfactant concentrations increase the overall encapsulation efficiency for multiple emulsions subjected to shear stress, although the effect is not prominent. The depletion of the carrier oil droplets could be achieved for Ca numbers below the critical limit, reducing the encapsulation efficiency below 10 %. This shows that even a low shear stress can result in content release from the internal droplet phase. The experimental emulsion release study is supported by a numerical simulation of drop deformation and break-up under shear stress.     

Continued self-similar breakup of drops in viscous continuous phase in agitated vessels

Drop breakup in viscous liquids in agitated vessels occurs in elongational flow around impeller blade edges. The drop size distributions measured over extended periods for impellers of different sizes show that breakup process continues up to 15–20 h, before a steady state is reached. The size distributions evolve in a self-similar way till the steady state is reached. The scaled size distributions vary with impeller size and impeller speed, in contrast with the near universal scaling known for drop breakup in turbulent flows. The steady state size of the largest drop follows inverse scaling with impeller tip velocity. The breadth of the scaled size distributions also shows a monotonic relationship with impeller tip velocity only.     

VISCOUS JET BREAKUP: NONSINUSOIDAL DISTURBANCES

The surface tension-driven breakup of viscous jets is observed when the jet's exit velocity is modulated with a nonsinusoidal disturbance. The disturbance is generated by the addition of a sine wave and a harmonic. The merge direction, the merge time of potential satellite droplets, and the breakup length are controlled by the phase angle between the fundamental and harmonic component. The thin filament equation is modified to account for asymmetric disturbances and predicts the shape of the jet at breakup and the breakup time. A model is developed to estimate the merge time and direction of the potential satellite droplet.