Parent Droplet Research Articles

Puffing and micro-explosion of diesel–biodiesel–ethanol blends

The puffing and micro-explosion of a single burning droplet comprised of neat diesel, rapeseed methyl ester (RME); binary fuel mixtures of diesel–ethanol, diesel–RME, RME–ethanol; and ternary microemulsion of these fuel blends at various compositions have been studied using high speed backlight imaging method. Fuel droplet was suspended on the tip of a 130μm gauge thermocouple and it was ignited using a glow plug heater. Based on the temporal variation of droplet projected area, the characteristics of fuel droplets studied were classified into smooth burning, puffing and explosion. A ternary plot has been proposed to identify the mixture composition of the blends that can result in smooth burning, puffing and explosion. Micro-explosion phenomenon was observed in the ternary blends with ethanol percentages between 10% and 40%. Secondary droplets resulted from the puffing and explosion of suspended parent droplet were observed to undergo further explosion. The time scales associated with complete disintegration of secondary droplets are found to be comparable to the mixing and the chemical reaction time scales of sprays in diesel engines.

Consideration of low viscous droplet breakage in the framework of the wide energy spectrum and the multiple fragments

An improved model for low viscous droplet breakage has been developed. Unlike the previous work that considered the inertia subrange and adopted the assumption of binary breakage, this work considered the breakage of droplets in the framework of the multiple fragments and the wide energy spectrum (i.e., including the dissipation range, the inertia subrange, and the energy containing range simultaneously). The previous interactions between the droplet and the surrounding fluids have been considered through introducing the interaction forces. The effect of the surface deformation and oscillation resulting from these interactions on the constraints of multiple breakages has been accounted for. These factors have been neglected in the existing models. The wide energy spectrum distribution was found to have an important effect on the nonmonotone evolution of breakage frequency with increasing parent droplet size. The cumulative volume fractions predicted by this work showed a better agreement with the experimental data. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2147–2168, 2015

Exploring the mechanism of salt-induced signal suppression in protein electrospray mass spectrometry using experiments and molecular dynamics simulations.

Protein analyses by electrospray ionization (ESI) mass spectrometry can suffer from interferences caused by nonvolatile salts. The mechanistic basis of this effect remains to be fully investigated. In the current work we explore the behavior of proteins under native and denaturing conditions in the presence of NaCl, CsCl, and tetrabutyl ammonium chloride (NBu4Cl). All three salts interfere with the formation of "clean" [M + zH](z+) protein ions by progressively deteriorating spectral S/N ratios. We propose that salt interferences can be dissected into two independent aspects, i.e., (i) peak splitting by adduct formation and (ii) protein ion suppression. NaCl degrades the spectral quality by forming heterogeneous [M + zH + n(Na - H) + m(Cl + H)](z+) ions, while the integrated protein ion intensity remains surprisingly robust. Conversely, NBu4Cl does not cause any adduction, while dramatically reducing the protein ion yield. These findings demonstrate that adduct formation and protein ion suppression are indeed unrelated effects that may occur independently of one another. Other salts, such as CsCl, can give rise to a combination of the two scenarios. Molecular dynamics simulations of water droplets charged with either Na(+) or NBu4(+) provide insights into the mechanism underlying the observed effects. Na(+) containing droplets evolve relatively close to the Rayleigh limit (z/z(R) ≈ 0.74), whereas the z/z(R) values of NBu4(+) charged droplets are considerably lower (∼0.59). This difference is due to the high surface affinity of NBu4(+), which facilitates charge ejection from the droplet. We propose that the low z/z(R) values encountered in the presence of NBu4(+) suppress the Rayleigh fission of parent droplets in the ESI plume, thereby reducing the yield of progeny droplets that represent the precursors of gaseous protein ions. In addition, the rate of solvent evaporation is reduced in the presence of NBu4(+). Both of these factors lower the protein signal intensity. NaCl does not interfere with droplet fission, such that protein ions continue to form with high yield—albeit in heavily adducted form. Our findings expand on earlier proposals of charge competition as a key factor during the ESI process for salt-contaminated solutions.

Nebulization of water/glycerol droplets generated by ZnO/Si surface acoustic wave devices

Efficient nebulization of liquid sessile droplets (water and water/glycerol mixtures) was investigated using standing waves generated using ZnO/Si surface acoustic wave (SAW) devices under different RF powers, frequencies and liquid viscosity (varied glycol concentrations in water). At such high RF powers, there are strong competitions between vertical jetting and nebulization. At lower SAW frequencies of 12.3 and 23.37 MHz, significant capillary waves and large satellite droplets were generated before nebulization could be observed. At frequencies between 23.37 and 37.2 MHz, spreading, displacement or occasionally jetting of the parent sessile droplet was frequently observed before a significant nebulization occurred. When the SAW frequencies were increased from 44.44 to 63.3 MHz, the minimum RF power to initiate droplet nebulization was found to increase significantly, and jetting of the parent droplet before nebulization became significant, although the average size of the nebulized particles and ejected satellite droplets appeared to decrease with an increase in frequency. With the increase of glycerol concentration in the test sessile droplets (or increase in liquid viscosity), nebulization became difficult due to the increased SAW damping rate inside the liquid. Acoustic heating effects were characterized to be insignificant and did not show apparent contributions to the nebulization process due to silicon substrate’s natural effect as an effective heat sink and the employment of a metallic holder beneath the ZnO/Si SAW device substrates.

Physics of puffing and microexplosion of emulsion fuel droplets

The physics of water-in-oil emulsion droplet microexplosion/puffing has been investigated using high-fidelity interface-capturing simulation. Varying the dispersed-phase (water) sub-droplet size/location and the initiation location of explosive boiling (bubble formation), the droplet breakup processes have been well revealed. The bubble growth leads to local and partial breakup of the parent oil droplet, i.e., puffing. The water sub-droplet size and location determine the after-puffing dynamics. The boiling surface of the water sub-droplet is unstable and evolves further. Finally, the sub-droplet is wrapped by boiled water vapor and detaches itself from the parent oil droplet. When the water sub-droplet is small, the detachment is quick, and the oil droplet breakup is limited. When it is large and initially located toward the parent droplet center, the droplet breakup is more extensive. For microexplosion triggered by the simultaneous growth of multiple separate bubbles, each explosion is local and independent initially, but their mutual interactions occur at a later stage. The degree of breakup can be larger due to interactions among multiple explosions. These findings suggest that controlling microexplosion/puffing is possible in a fuel spray, if the emulsion-fuel blend and the ambient flow conditions such as heating are properly designed. The current study also gives us an insight into modeling the puffing and microexplosion of emulsion droplets and sprays.

Numerical simulations of supersonic gas atomization of liquid metal droplets

Computational fluid dynamics simulations incorporating supersonic turbulent gas flow models and a droplet breakup model are performed to study supersonic gas atomization for producing micron-sized metal powder particles. Generally such atomization occurs in two stages: a primary breakup and a secondary breakup. Since the final droplet size is primarily determined by the secondary breakup, parent droplets of certain sizes (1 to 5 mm) typically resulting from the primary breakup are released at the corner of the nozzle and undergo the secondary breakup. A comparison of flow patterns with and without the introduction of a liquid melt clearly indicates that the mass loading effect is quite significant as a result of the gas–droplet interactions. The flow pattern change reasonably explains why the final droplets have a bimodal mass size distribution. The transient size changes of the droplets are well described by the behavior of the Weber number. The present results based on the 1 mm parent droplets best fit previous experimental results. Moreover, the effects of inlet gas pressure and temperature are investigated in an attempt to further reduce droplet size.

Towards a model of spray–canopy interactions: Interception, shatter, bounce and retention of droplets on horizontal leaves

Pesticides used in agricultural systems must be applied in economically viable and environmentally sensitive ways, and this often requires expensive field trials on spray deposition and retention by plant foliage. Computational models to describe whether a spray droplet sticks (adheres), bounces or shatters on impact, and if any rebounding parent or shatter daughter droplets are recaptured, would provide an estimate of spray retention and thereby act as a useful guide prior to any field trials.Parameter-driven interactive software has been implemented to enable the end-user to study and visualise droplet interception and impaction on a single, horizontal leaf. Living chenopodium, wheat and cotton leaves have been scanned to capture the surface topography and realistic virtual leaf surface models have been generated. Individual leaf models have then been subjected to virtual spray droplets and predictions made of droplet interception with the virtual plant leaf. Thereafter, the impaction behaviour of the droplets and the subsequent behaviour of any daughter droplets, up until re-capture, are simulated to give the predicted total spray retention by the leaf. A series of critical thresholds for the stick, bounce, and shatter elements in the impaction process have been developed for different combinations of formulation, droplet size and velocity, and leaf surface characteristics to provide this output.The results show that droplet properties, spray formulations and leaf surface characteristics all influence the predicted amount of spray retained on a horizontal leaf surface. Overall the predicted spray retention increases as formulation surface tension, static contact angle, droplet size and velocity decreases. Predicted retention on cotton is much higher than on chenopodium. The average predicted retention on a single horizontal leaf across all droplet size, velocity and formulations scenarios tested, is 18, 30 and 85% for chenopodium, wheat and cotton, respectively.

Synthetic cell division system: Controlling equal vs. unequal divisions by design

Cell division is one of the most fundamental and evolutionarily conserved biological processes. Here, we report a synthetic system where we can control by design equal vs. unequal divisions. We synthesized a micro-scale inverse amphipathic droplet of which division is triggered by the increase of surface to volume ratio. Using this system, we succeeded in selectively inducing equal vs. unequal divisions of the droplet cells by adjusting the temperature or the viscosity of the solvent outside the droplet cell accordingly. Our synthetic division system may provide a platform for further development to a system where intracellular contents of the parent droplet cell could be divided into various ratios between the two daughter droplet cells to control their functions and fates.

Analysis of the Disintegration of Charged Droplets Employing Boundary Element Method and Particle Method

It is well known that an initially charged droplet disintegrates repeatedly into several tiny sibling droplets and one parent droplet if the charge exceeds a certain value defined by the Rayleigh Limit. Many researchers have experimentally verified this phenomenon in their works. However, it is difficult to estimate the number, charge and mass of the sibling droplets because the sizes of the droplets are generally very small. In this paper, a new coupled analysis method for the disintegration of dielectric charged droplets employing boundary element method (BEM) and particle method is proposed. The behavior of the droplet during disintegration is analyzed by means of this method.

Fragmentation of falling liquid droplets in bag breakup mode

Using direct numerical simulations, the fragmentation of falling liquid droplets in a quiescent media is studied. Three simulations with different Eötvös numbers were performed. An adaptive, volume of fluid (VOF) method based on octree meshing is used, providing a notable reduction of computational cost. Results are presented in three main parts; droplet deformation and bag formation, bag breakup mechanism, and variations in the number and size of fragments. It is clearly shown that the droplet initially deforms into a thin hollow bag. After that, due to growth of instabilities over the thin liquid sheet, several punctures are formed. The holes further grow, generating a web of ligaments. In this stage, the fragmentation domain consists of a liquid torus left above the bag, a number of ligaments and droplets, and the parent droplet core. The liquid torus, the core, and the unstable ligaments disintegrate further. The breakup processes continue until the hydro-dynamic and the surface tension forces reach an equilibrium condition for each fragment. The growth of fragment numbers during the atomization stage is described and the size distributions between the simulations are compared. The analysis performed here precisely demonstrates the mechanism of the bag breakup and depicts an accurate history for the small fragments generated in the secondary atomization process. The outcomes can be used for development of the secondary atomization models. Moreover, the results can be directly used for better understanding of rain drop atomization during precipitation, as well as water droplet atomization in cooling towers.

Crystallization-in-emulsion process of a melted organic compound: In situ optical monitoring and simultaneous droplet and particle size measurements

The crystallization-in-emulsion process allows the production of solid particles exhibiting specific features. Here, the batch crystallization process carried out by cooling a melted oil dispersed as an oil-in-water emulsion was studied. Two experimental set-ups allowing the in situ visualization of the nucleation and growth phenomena occurring in the dispersed liquid phase were developed. Observations in quiescent medium of motionless droplets having a diameter of few tens of micrometers showed that primary nucleation started on the inner surface of the droplets. The fast growth of the crystals consumed all the liquid contained within each droplet and was confined within each droplet by the oil–water interface. Solid polycrystalline particles similar in size to the parent droplets were produced. Dynamic tracking of the transient evolution of the size distributions of the two populations of droplets and solid particles during the cooling process in a stirred vessel was carried out using an in situ optical probe. It was shown that the droplets crystallized very progressively during cooling, starting with the largest droplets and ending with the smaller size droplets since the induction time of primary nucleation was dependent on droplet volume. In dilute conditions (1%wt% of dispersed phase) each droplet was converted into a single solid particle. Secondary nucleation based on inter-droplet collisions was not observed in these conditions.

A note on spark bubble drop-on-demand droplet generation: simulation and experiment

The fluid dynamics of the spark bubble-generated droplet is studied both experimentally and numerically. The emphasis is especially on the droplet behavior after pinch-off. Commercial inkjet printers often produce satellite droplets along with parent droplets which are not desirable from the viewpoint of printing efficiency. Furthermore, standard drop-on-demand droplet generators are normally restricted to the generation of droplets with the same size as the nozzle diameter. In the spark bubble droplet generation method, a spark-generated bubble induces droplet formation through a hole in a solid surface separating the liquid and air interfaces. Immediately after ignition occurs, a bubble forms and creates pressure waves as it expands and contracts in a nonsymmetrical fashion. These pressure waves, depending on the geometries of the bubble location, plate, and hole may cause a single droplet smaller than the plate aperture to form and break up. In this article, a combined numerical and experimental study has been conducted to investigate the droplet behavior created in this manner. A high-speed camera is utilized to capture the droplet formation process. The numerical simulations have been carried out using the boundary integral spatial solution coupled with the time integration, i.e., a mixed Eulerian–Lagrangian approach. There is reasonable agreement between the simulations and experiment.

The effect of confinement-induced shear on drop deformation and breakup in microfluidic extensional flows

Droplets of de-ionized water and four aqueous surfactant solutions were generated in oil using a microfluidic flow-focusing device. The morphological developments of the drops in extensional flow and confinement-induced shear flow at various extension rates were studied using a hyperbolic contraction. This novel approach to droplet deformation within a microfluidic device allowed the probing of droplets within a nearly uniform extensional flow. The focus of this work was to study the effect of confinement-induced shear on droplet deformation and breakup in extensional flows. Droplet deformation was found to increase with both increasing capillary number and increasing confinement, for a fixed viscosity ratio of λ=0.1, with the effect of the shear induced by confinement being quite dramatic. The addition of surfactant to the droplets resulted in the production of tails, which streamed from the rear of the droplets and produced daughter droplets much smaller than the parent droplet. In the partially confined limit, where the flow was purely extensional, a single tail was formed at the center of the droplets trailing edge. With enhanced confinement, shear effects from the wall became important, the droplets were observed to take on a bullet-like shape, and two tails formed at the trailing edge of the droplet. The critical value of the capillary number and confinement needed for the formation of tails varied with the surfactant used.

Effects of Fine Water Mist on a Confined Blast

A computational, multi-phase, model has been developed to study the interactions between water droplets and radial expansion of a gas cloud in a spherical chamber. Initial conditions for the gas cloud are specified based on chemical equilibrium calculations for the detonation of a high explosive (RDX). Mono-dispersed water droplets are injected at uniform concentration into the chamber prior to the expansion. A Lagrangian model is used to track the breakup of the parent drops near the shock front to form child drops, which are extremely small. The Navier–Stokes solutions show that the child droplets accumulate near the shock front and evaporate at 100 times higher rate than the parent droplets. Latent heat absorption is the dominant mechanism followed by the sensible heat absorption by the water vapor (and droplets), and momentum absorption from the high velocity gases by the child droplets. The simulations also show that the water vapor formed by the evaporation increases the gas density at the shock front. The increased density and reduced gas temperature (cooling) have opposite effects on the pressure at the shock front. This leads to only a modest suppression in the pressure. At realistic droplet concentrations (0.08 kg droplets/m3 of air), the water mist is shown to evaporate completely in a short time (2.42 ms) prior to shock reflection at the chamber wall mainly due to the breakup at the shock front. High concentrations of mist may be desirable, but are difficult to achieve in practice at the total flooding conditions.

Rapid enrichment of biomolecules using simultaneous liquid and particulate dielectrophoresis

Here we show that dielectrophoretic (DEP) liquid actuation can be used to dispense arrays of nanoliter-sized droplets loaded with biomolecules. Size-based enrichment of these biomolecules occurs rapidly and simultaneously with the droplet dispensing. The physical mechanism responsible for the effect is the positive DEP force directed toward the electrodes that is imposed by the non-uniform electric field during the very rapid DEP actuated flow before droplet formation. Experiments conducted with a suspension of lambda DNA (molecular weight: 31.5 x 10(3) kDa) and lectin protein (120 kDa) containing identical molar concentration shows separation of DNA and protein within the nanolitre sized droplets formed along the electrode. The density ratio of protein to DNA varies smoothly from 1 : 1 in the parent droplet to approximately 3 : 1, favoring the smaller sized protein in the daughter droplet dispensed furthest from the parent droplet, approximately 2.4 mm from the parent droplet. Experiments conducted with binary protein solutions containing identical molar concentrations of bovine serum albumin (66 kDa) and fibrinogen (340 kDa) reveal that enrichment is enhanced as the length of the electrodes is increased. The density ratio of BSA to fibrinogen varied from 1 : 1 in the parent droplet to approximately 1.97 : 1 at the last (tenth) droplet, located approximately 4.2 mm from the parent droplet. The entire process, consisting of droplet dispensing and particle separation, occurs in less than one second.

Microfluidic emulsification and sorting assisted preparation of monodisperse chitosan microparticles

A microfluidic device for generating monodisperse chitosan microparticles and separating the desired particle from smaller particles created as a byproduct of this process was described. The purpose of this study is to separate the satellite droplets from their parent droplets to enhance the size uniformity of the desired microparticles. A double T-junction design was first employed to control the emulsification and the separation, respectively. The results show that the size and gap of the parent droplets are tunable by adjusting the water and oil flow rates. A separation ratio of the satellite droplets of more than 99% was observed. The proposed microfluidic chip is capable of generating relatively uniform micro-droplets with well controllable diameter, and it has the added advantages of being a simple, low cost, and high throughput process. In the future this apparatus can be used to fabricate size-controlled monodisperse microparticles to act as drug carriers for biotechnology and biomedicine applications.

Implications of droplet breakup and formation of ultra fine mist in blast mitigation

Blast-induced droplet breakup producing ultra fine water mist process was examined in view of assessing its implications on blast mitigation. An earlier review proposed that droplet breakup process, amongst other implications, may weaken the shock due to breakup energy absorption. In this work, droplet breakup energies for water droplets have been determined from the surface energies of both parent and child droplets. A breakup energy of 18 J/kg was required to fragment a 0.5 mm parent droplet into 10,000 mono-dispersed child droplets. Compared to the vaporization energy of 2.25 MJ/kg, the droplet breakup energy was found not significant in weakening the shock. While the droplet deformation energy and curvature effects could increase the breakup energy, its overall contribution to the total energy extraction was not as significant as the latent heat of vaporization. Further, the analysis shows about 22-fold increase in surface area of the child droplets. The study revealed the surface-to-volume ratio of the ultra fine droplets and their vaporization timescale should be well positioned for shock energy extraction.

Adaptive collision meshing and satellite droplet formation in spray simulations

Improved numerical methods and physical models have been applied to droplet collision modeling. Numerically, an adaptive collision mesh method is developed to calculate collision rate. This method produces a collision mesh that is independent of the gas phase mesh and adaptively refined according to local parcel number density. An existing model describing the satellite droplet formation during the collision process is improved to reflect the experimental findings that the satellite droplets are much smaller than the parent droplets. The adaptive collision mesh and the improved satellite model have been used to simulate three impinging spray experiments. The model was able to qualitatively predict the occurrence of small satellite drops and bi-modal post-collision drop size distributions. The effect of the collision mesh and the satellite droplet model on a high-speed non-evaporating diesel spray is also assessed.

Theory of the Cooperative Evaporation of Volatile Droplets

A theoretical approach to the cooperative process of evaporation of volatile liquid droplet arrays is proposed which combines the equation of the liquid-gas interface with a Landau-type description. The spatial and temporal behaviors of the process are shown to be governed by the spontaneous symmetry breaking mechanism of a parent droplet configuration. The applicability of the approach is illustrated by the theoretical description of recent experiments in closed and open systems.

Microfluidic separation of satellite droplets as the basis of a monodispersed micron and submicron emulsification system

Emulsions are widely used to produce sol-gel, drugs, synthetic materials, and food products. Recent advancements in microfluidic droplet emulsion technology has enabled the precise sampling and processing of small volumes of fluids (picoliter to femtoliter) by the controlled viscous shearing in microchannels. However the generation of monodispersed droplets smaller than 1 microm without surfactants has been difficult to achieve. Normally, the generation of satellite droplets along with parent droplets is undesirable and makes it difficult to control volume and purity of samples in droplets. In this paper, however, several methods are presented to passively filter out satellite droplets from the generation of parent droplets and use these satellite droplets as the source for monodispersed production of submicron emulsions. A passive satellite droplet filtration system and a dynamic satellite droplet separation system are demonstrated. Satellite droplets are filtered from parent droplets with a two-layer channel geometry. This design allows the creation and collection of droplets that are less than 100 nm in diameter. In the dynamic separation system, satellite droplets of defined sizes can be selectively separated into different collecting zones. The separation of the satellite droplets into different collecting zones correlates with the cross channel position of the satellite droplets during the breakup of the liquid thread. The delay time for droplets to switch between the different alternating collecting zones is nominally 1 min and is proportional to the ratio of the oil shear flows. With our droplet generation system, monodispersed satellite droplets with an average radius of 2.23 +/- 0.11 microm, and bidispersed secondary and tertiary satellite droplets with radii of 1.55 +/- 0.07 microm and 372 +/- 46 nm respectively, have been dynamically separated and collected.