Stream Of Droplets Research Articles

Planar laser-induced fluorescence imaging of the spatial vapor distribution around a monodisperse acetone droplet stream exposed to asymmetric radiant heating

The development of liquid fuelled microcombustors faces many challenges, one of which being high asymmetric heat flux across the combustion chamber. Typically, thin walls provide little resistance to convective heat transfer and therefore, allowing high heat loss rates. Insulating the walls results in high wall temperatures, which increases the likelihood that radiation plays an important role. Both of these effects have the potential to induce asymmetries and strong temperature gradients in the gas flow, relative to the more uniform environment of a conventional combustor. This investigation uses planar laser-induced fluorescence (PLIF) to reveal the spatial vapor distribution around a monodisperse acetone droplet stream that is exposed to asymmetric radiant heating. Droplets with diameters from 117 to 222 μm flow past a single-sided array of radiant heating elements to provide the asymmetric heating. A frequency-quadrupled Nd:YAG laser provides a 266 nm light sheet to excite the acetone vapor around the droplets, which are exposed to different experimental conditions by varying parameters such as the droplet diameter and temperature of the radiating elements for a fixed exposure time of the droplets in the heated region. A CCD camera captures the fluorescence of the excited acetone vapor molecules over a broadband wavelength range between 350 and 550 nm, to give the radial and axial vapor concentration around each droplet. After processing the PLIF images, we obtain contour plots of the spatial acetone vapor concentration around the droplets which depict asymmetric vapor distribution. The potential impacts on vaporization, combustion and pollutant formation are discussed.

Gas dynamic virtual nozzle for generation of microscopic droplet streams

As shown by Gañán-Calvo (1998 Phys. Rev. Lett. 80 285–8), a free liquid jet can be compressed in diameter through gas dynamic forces exerted by a coaxially co-flowing gas, obviating the need for a solid nozzle to form a microscopic liquid jet and thereby alleviating the clogging problems that plague conventional droplet sources of small diameter. We describe in this paper a novel form of droplet beam source based on this principle. The source is miniature, robust, dependable, easily fabricated, essentially immune to clogging and eminently suitable for delivery of microscopic liquid droplets, including hydrated biological samples, into vacuum for analysis using vacuum instrumentation. Monodisperse, single-file droplet streams are generated by triggering the device with a piezoelectric actuator.

Breakup of a supported drop of a viscous conducting liquid in a uniform electric field

Numerical computations and order-of-magnitude estimates are used to describe the time evolution of a drop of a very viscous liquid of finite electrical conductivity attached to a metallic plate which is suddenly subject to a uniform electric field. Under the action of the electric stresses induced at its surface, the drop elongates in the direction of the field, and charged droplets are emitted when the strength of the field is higher than a certain critical value. A stationary emission mode exists in which the attached drop develops a conical tip and a thin jet, with small droplets emitted from the end of the jet in a process that involves the formation of a long ligament. The flow rate and the electric current carried by the stream of droplets emitted in this mode are determined by the flow and the transfer of charge in the attached drop, in particular in a small region around its tip and in a leading stretch of the jet, where the solution is nearly stationary despite the transient character of the jet further downstream. A simplified analysis of the stationary regions is carried out to elucidate the effects of the physical properties of the liquid (electrical conductivity, permittivity, viscosity, and surface tension), the volume of the drop, and the strength of the applied field. For high electrical conductivities and applied fields well above its critical value, the electrical and viscous stresses are large compared to surface tension stresses, and their balance gives a flow rate proportional to the square of the applied field. The electric current is then that of a stationary electrified jet fed with this flow rate.

Directed Aerosol Writing of Ordered Silica Nanostructures on Arbitrary Surfaces with Self‐Assembling Inks

This paper reports the fabrication of micro- and macropatterns of ordered mesostructured silica on arbitrary flat and curved surfaces using a facile robot-directed aerosol printing process. Starting with a homogenous solution of soluble silica, ethanol, water, and surfactant as a self-assembling ink, a columnated stream of aerosol droplets is directed to the substrate surface. For deposition at room temperature droplet coalescence on the substrates and attendant solvent evaporation result in continuous, highly ordered mesophases. The pattern profiles are varied by changing any number of printing parameters such as material deposition rate, printing speed, and aerosol-head temperature. Increasing the aerosol temperature results in a decrease of the mesostructure ordering, since faster solvent evaporation and enhanced silica condensation at higher temperatures kinetically impede the molecular assembly process. This facile technique provides powerful control of the printed materials at both the nanoscale and microscale through chemical self-assembly and robotic engineering, respectively.

Numerical simulation of jet breakup due to amplitude-modulated (A-M) disturbance

In order to characterize the mechanics of jet breakup, the finite volume formulations were employed to solve the Navier-Stokes equations and continuity equation of jet. The volume of fluid(VOF) method was used to track the free surface of jet. The spray process of the molten Pb 63Sn 37 alloy was simulated based on the mathematical model by means of FLUENT code. The configuration of jets generated in different disturbance ratios and modulation ratios was obtained. The theoretical results show that the droplets merge together by the number of disturbance ratio N, which agrees with the corresponding picture captured in the experiment. In addition, the droplet streams broken at non-optimal frequency are also uniform according to simulation results, which proves that the A-M disturbance can increase the width of the uniform droplet generating frequency.

Experimental and computational studies of liquid aerosol evaporation

The phenomenon of liquid aerosol evaporation was investigated both experimentally and computationally in this project. A vibrating orifice aerosol generator (VOAG) was utilised to generate a monodispersed stream of liquid droplets of a desired initial size. Three types of liquids, ethanol, hexane and water, of distinct volatilities were used. The droplets were allowed to travel through air at ambient temperature and pressure. The change in diameter of the droplets as evaporation occurs was measured dynamically using a variety of experimental techniques. Global sizing velocimetry (GSV) was applied to visualize the motion and change in diameter of droplets in real time so as to select an optimal set of operating conditions under which coalescence of droplets did not occur. A phase Doppler interferometer (PDI) was used to measure the Sauter mean diameter and axial velocity of droplets at various distances from the nozzle of the VOAG. The size and velocity profiles obtained were compared with calculations carried out using computational fluid dynamics (CFD) coupled with two different theoretical liquid evaporation models. It was observed that both liquid evaporation models produced similar droplets size profiles for ethanol and water, but a significant difference in the predicted sizes was observed for hexane. In general, a good qualitative agreement between the experimental and computational results was obtained. This represents the first report of a study involving direct comparisons of dynamic measurements of the size evolution of evaporating liquid aerosols with numerical predictions obtained from CFD calculations.

Experimental and mechanistic description of merging and bouncing in head-on binary droplet collision

The dynamics of head-on collision between two identical droplets was experimentally and computationally investigated, with emphasis on the transitions from merging to bouncing and to merging again, as the collision Weber number was increased. Experimentally the stroboscopically illuminated microphotographic images of two colliding droplet streams, generated through the ink-jet printing technique, were acquired with adequate temporal resolution of the collision event such that the instant at which the droplets merged for both soft and hard collisions was identified. Using this empirical information as an input, the simulated collision images were found to agree well with the experimental observations and allowed investigation of the collision flow field including the energy budget and the fundamental differences between the soft and hard collisions that lead to merging. It is further shown that the merging instant can be computationally assessed through the use of an augmented van der Waals force to effect merging through rupturing of the surfaces, with the associated Hamaker constant empirically but consistently extracted from the experimental observations.

On-Chip Single-Copy Real-Time Reverse-Transcription PCR in Isolated Picoliter Droplets

The first lab-on-chip system for picoliter droplet generation and RNA isolation, followed by reverse transcription, and PCR amplification with real-time fluorescence detection in the trapped droplets has been developed. The system utilized a shearing T-junction in a fused-silica device to generate a stream of monodisperse picoliter-scale droplets that were isolated from the microfluidic channel walls and each other by the oil-phase carrier. An off-chip valving system stopped the droplets on-chip, allowing thermal cycling for reverse transcription and subsequent PCR amplification without droplet motion. This combination of the established real-time reverse transcription-PCR assay with digital microfluidics is ideal for isolating single-copy RNA and virions from a complex environment and will be useful in viral discovery and gene-profiling applications.

Infrared laser-assisted desorption electrospray ionization mass spectrometry

We have used an infrared laser for desorption of material and ionization by interaction with electrosprayed solvent. Infrared laser-assisted desorption electrospray ionization (IR LADESI) mass spectrometry was used for the direct analysis of water-containing samples under ambient conditions. An ion trap mass spectrometer was modified to include a pulsed Er:YAG laser at 2.94 microm wavelength coupled into a germanium oxide optical fiber for desorption at atmospheric pressure and a nanoelectrospray source for ionization. Analytes in aqueous solution were placed on a stainless steel target and irradiated with the pulsed IR laser. Material desorbed and ablated from the target was ionized by a continuous stream of charged droplets from the electrosprayed solvent. Peptide and protein samples analyzed using this method yield mass spectra similar to those obtained by conventional electrospray. Blood and urine were analyzed without sample pretreatment to demonstrate the capability of IR LADESI for direct analysis of biological fluids. Pharmaceutical products were also directly analyzed. Finally, the role of water as a matrix in the IR LADESI process is discussed.

Microdroplets: A sea of applications?

The exploitation of microdroplets produced within microfluidic environments has recently emerged as a new and exciting technological platform for applications within the chemical and biological sciences. Interest in microfluidic systems has been stimulated by a range of fundamental features that accompany system miniaturization. Such features include the ability to process and handle small volumes of fluid, improved analytical performance when compared to macroscale analogues, reduced instrumental footprints, low unit cost, facile integration of functional components and the exploitation of atypical fluid dynamics to control molecules in both time and space. Moreover, microfluidic systems that generate and utilize a stream of sub-nanolitre droplets dispersed within an immiscible continuous phase have the added advantage of allowing ultra-high throughput experimentation and being able to mimic conditions similar to that of a single cell (in terms of volume, pH, and salt concentration) thereby compartmentalizing biological and chemical reactions. This review provides an overview of methods for generating, controlling and manipulating droplets. Furthermore, we discuss key fields of use in which such systems may make a significant impact, with particular emphasis on novel applications in the biological and physical sciences.

Resonance-based light scattering techniques for investigation of microdroplet processes

An elastic or a Raman scattering intensity versus size parameter spectrum from a droplet shows a series of resonances. Each resonance contains a unique relation between the size and the refractive index of the scattering droplet, and the resonances from homogeneous droplets behave significantly differently than the resonances from layered droplets. These characteristics can be used to analyze observed resonances, and to determine the size and the refractive index of a homogeneous droplet or the inner and outer radii and the core and shell refractive indices of a layered droplet. We show that many microdroplet processes can be studied by applying resonance-based light scattering techniques to single droplets suspended in an electrodynamic balance and to highly monodisperse droplets in linear arrays. This paper focuses on deciphering internal composition distributions in microdroplets that develop due to fast physical or chemical processes. Experiments were conducted on linear streams of droplets that were generated by a modified vibrating orifice aerosol generator from a solution of non-volatile dibutyl phthalate dissolved in volatile freon. The residence time of the droplets in the gas phase was altered by varying the distance between the droplet generator and a laser beam that illuminates the droplets. The variation of the frequency of the droplet generator causes the droplet size to change in a prescribed manner, and thus, the scattering intensity as a function of the frequency shows a series of resonances due to the variation of the size. We have analyzed the resonance peak frequencies to obtain the size and the composition distribution inside the droplets as functions of time. During evaporation, the resonances of non-uniform composition droplets shift differently from those of uniform composition droplets. Specifically, lower order resonances from non-uniform droplets shift significantly more than higher order resonances. This is the basis for the determination of the size along with the composition distribution from the observed resonances. The experimental data show that various resonances observed in the Raman scattering spectra shift differently with time, as predicted by the theory. The size and composition distribution results obtained from the analysis of resonances show behavior that is expected from a droplet evaporation model.

Simulation and experiment research of the uniform drolet spray process

It is difficult to adjust the process parameters of material increase manufacturing based on droplet spray and generally these experiments are hard to implement.For solving this problem,a flow modal of uniform droplet spray based on volume of fluid(VOF) two phase flow model is developed.The pattern of droplet stream,the pressure field and the velocity field in the droplet spay process are studied by using the numerical simulation method.The inner low of uniform droplets formation is revealed and the optimal frequency of uniform droplet is obtained.On the basis of the simulation results,a uniform droplet spray device is built and equipped with a corresponding high speed camera.The jet pattern,droplet spray process and jet velocity are studied experimentally. The results show that the jet velocity depends on the jet pressure,which presents a periodical change,and uniformity of droplet stream depends on disturbance frequency and amplitude.The simulation results agree well with the experimental results,which proves the feasibility of the flow modal.The researches find a useful method for simulating the jet flow with different parameters, and lay the theoretical foundation for the application of material increase manufacturing based on droplet spray.

Droplet streams for serial crystallography of proteins

Serial diffraction of proteins requires an injection method to deliver protein molecules - preferably uncharged, fully hydrated, spatially oriented, and with high flux - into the crossed beams of an alignment laser and a focused probe beam of electrons or X-rays of typically only a few tens of microns diameter. The aim of this work has been to examine several potential droplet sources as to their suitability for this task. We compare Rayleigh droplet sources, electrospray sources, nebulizers and aerojet-focused droplet sources using time-resolved optical images of the droplet beams. Shrinkage of droplets by evaporation as a means of removing most of the water surrounding the proteins is discussed. Experimental measurements of droplet size, conformation of proteins after passing through a Rayleigh jet, and triboelectric charging are presented and conclusions are drawn about the source configuration for serial diffraction. First experimental X-ray diffraction patterns from Rayleigh droplet beams doped with 100nm gold balls are shown.

Research on lateral instability of the uniform-charged droplet stream during droplet-based freeform fabrication

A droplet initial displacement assumption was made and a dynamic model of the charged droplets was developed to understand the lateral instability of the uniform-charged droplet stream for freeform fabrication. The distribution of charged droplets depending on deposition distance was obtained based on this dynamic model after the droplet charge was calculated by the FEM. Then, a droplet charging and depositing apparatus is set up to generate the uniform-charged droplet stream. The droplet charge was measured and the droplet distributions at different distances were collected in the experiment. The results show that the theoretical radius of droplet distribution area agrees well with the mean experimental value at the deposition distance of 50 cm. The theoretical trend of droplet stream dispersion fits well with experimental result. The maximum accurate deposition distance within which the radius of distribution area is less than one droplet diameter is about 27 cm according to the prediction of droplet distribution, which provides the basis for further accurate metal droplet deposition.

On-Chip, Real-Time, Single-Copy Polymerase Chain Reaction in Picoliter Droplets

The first lab-on-chip system for picoliter droplet generation and PCR amplification with real-time fluorescence detection has performed PCR in isolated droplets at volumes 106 smaller than commercial real-time PCR instruments. The system utilized a shearing T-junction in a silicon device to generate a stream of monodisperse picoliter droplets that were isolated from the microfluidic channel walls and each other by the oil-phase carrier. An off-chip valving system stopped the droplets on-chip, allowing them to be thermally cycled through the PCR protocol without droplet motion. With this system, a 10-pL droplet, encapsulating less than one copy of viral genomic DNA through Poisson statistics, showed real-time PCR amplification curves with a cycle threshold of approximately 18, 20 cycles earlier than commercial instruments. This combination of the established real-time PCR assay with digital microfluidics is ideal for isolating single-copy nucleic acids in a complex environment.

Analytical performance of a venturi-assisted array of micromachined ultrasonic electrosprays coupled to ion trap mass spectrometry for the analysis of peptides and proteins.

The analytical characterization of a novel ion source for mass spectrometry named array of micromachined ultrasonic electrosprays (AMUSE) is presented here. This is a fundamentally different type of ion generation device, consisting of three major components: (1) a piezoelectric transducer that creates ultrasonic waves at one of the resonant frequencies of the sample-filled device, (2) an array of pyramidally shaped nozzles micromachined on a silicon wafer, and (3) a spacer which prevents contact between the array and transducer ensuring the transfer of acoustic energy to the sample. A high-pressure gradient generated at the apexes of the nozzle pyramids forces the periodic ejection of multiple droplet streams from the device. With this device, the processes of droplet formation and droplet charging are separated; hence, the limitations of conventional electrospray-type ion sources, including the need for high charging potentials and the addition of organic solvent to decrease surface tension, can be avoided. In this work, a Venturi device is coupled with AMUSE in order to increase desolvation, droplet focusing, and signal stability. Results show that ionization of model peptides and small tuning molecules is possible with dc charging potentials of 100 Vdc or less. Ionization in rf-only mode (without dc biasing) was also possible. It was observed that, when combined with AMUSE, the Venturi device provides a 10-fold gain in signal-to-noise ratio for 90% aqueous sample solutions. Further reduction in the diameter of the orifices of the micromachined arrays led to an additional signal gain of at least 3 orders of magnitude, a 2-10-fold gain in the signal-to-noise ratio and an improvement in signal stability from 47% to 8.5% RSD. The effectiveness of this device for the soft ionization of model proteins in aqueous media, such as cytochrome c, was also examined, yielding spectra with an average charge state of 8.8 when analyzed with a 100 Vdc charging potential. Ionization of model proteins was also possible in rf-only mode.

Effect of viscosity on droplet-droplet collision outcome: Experimental study and numerical comparison

The influence of viscosity on droplet-droplet collision behavior at ambient conditions was studied experimentally and numerically. N-decane, monoethyleneglycol (MEG), diethyleneglycol (DEG), and triethyleneglycol were used as liquid phase providing viscosities in the range from 0.9to48mPas. Collision Weber numbers ranged approximately from 10 to 420. A direct numerical simulation code, based on the volume-of-fluid concept, was used for the simulations. Experimentally, observations of two droplet streams using a modified stroboscopic technique (aliasing method) were used to investigate the whole range of impact parameters during one experimental run. The experimental method has previously been verified for the water/air system [C. Gotaas et al., Phys. Fluids 19, 102105 (2007)]. In the present work, it was tested and validated for the n-decane/air system. Measured data agree well with those published in the literature. Well-defined regions of stretching separation and coalescence were identified, while reflexive separation regions were not found by using a single sinusoidal disturbance. However, the onset of reflexive separation was identified for MEG and DEG using an amplitude modulation technique. The results show that the criteria for onset of reflexive separation for viscous fluids provided by Y. I. Jiang et al. [J. Fluid Mech. 234, 177 (1992)] are not valid. This is consistent with the results given by K. D. Willis and M. Orme [Exp. Fluids 34, 28 (2003)]. A new empirical correlation for the onset of reflexive separation for high viscosity fluids is presented. The borders between coalescing and stretching separation were shifted toward higher Weber numbers with increasing viscosity. The lack of occurrence of reflexive separation for the single sinusoidal disturbance (small droplets), as well as the stretching separation boundary shift, can be explained by dissipation of collision kinetic energy in viscous flows inside the merged droplet after collision. Results from numerical simulations for MEG, DEG, and TEG correlated well with experimental data for the same fluids.

Focusing capillary jets close to the continuum limit

The demand for techniques that can reliably deliver and control nanometre-scale volumes of liquid is a growing priority in biotechnology and medicine. Capillary jets are capable of supplying a steady stream of monodisperse liquid droplets. But because of the increasing forces and pressures needed to counteract surface tension for droplets of decreasing size, reaching the nanoscale with such an approach is difficult. One way of overcoming such limitations is to electrostatically focus a jet as it emerges from a capillary. Another, which we report here, is to focus such a jet by hydrodynamic means, a double flow-focusing arrangement that involves a manifold capillary that delivers a second immiscible fluid jet that envelopes and guides the jet from an inner capillary. Under the appropriate working conditions, this enables the generation of continuous steady capillary fluid jets down to submicrometre diameter—approaching the ultimate continuum limit, which is supported by a proposed theory.

Production of spherical and uniform‐sized particles using a laboratory ink‐jet spray dryer

AbstractThere is a need in the area of particle technology and powder manufacturing to produce particles with uniform characteristics for attaining stringent product quality. In this paper, a new concept of spray drying is proposed to assess the possibility of adapting the same concept to industrial spray drying operations for producing spherical and uniform‐sized particles. This concept is based on ink‐jet technology, where single streams of identical droplets can be achieved using ink‐jet devices. The ink‐jet device, similar to nozzles found in office ink‐jet printers, was introduced to spray drying operations as an innovative atomizer because of its attractive ability to produce monodisperse droplets with a precise control over droplet characteristics. In this paper, a new concept of single stream drying and some calculations on feasibility of a newly built Ink‐Jet Spray Dryer (IJSD) are discussed. The single stream drying technique could provide a promising tool for producing uniform particles and also for testing and designing new products. Copyright © 2007 Curtin University of Technology and John Wiley & Sons, Ltd.

Spontaneous Separation of Charged Grains

In 1867, Lord Kelvin described an experiment in which two streams of water droplets were connected so that each stream amplified the charge on the second stream [W. Thomson, Proc. R. Soc. London 16, 67 (1867)]. We present here a complementary effect in flowing grains that spontaneously separates similar and well-mixed grains into two charged streams of demixed grains. This effect has important consequences for industrial and natural processes.