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Monodisperse Droplet Generators as PotentialAtomizers for Spray Drying TechnologyWinston Duo Wu a , Kamlesh C. Patel a , Samuel Rogers a & Xiao Dong Chen aa Biotechnology and Food Engineering Group, Chemical Engineering Department, MonashUniversity, Melbourne, Victoria, AustraliaVersion of record first published: 05 Dec 2007.

To cite this article: Winston Duo Wu , Kamlesh C. Patel , Samuel Rogers & Xiao Dong Chen (2007): Monodisperse DropletGenerators as Potential Atomizers for Spray Drying Technology, Drying Technology: An International Journal, 25:12, 1907-1916

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Monodisperse Droplet Generators as Potential Atomizersfor Spray Drying Technology

Winston Duo Wu, Kamlesh C. Patel, Samuel Rogers, and Xiao Dong ChenBiotechnology and Food Engineering Group, Chemical Engineering Department, Monash University,Melbourne, Victoria, Australia

Spray drying is a widely used unit operation for manufacturingpowders of different characteristics. Many technological improve-ments have been made in the last few decades, and a great amountof research work is still being conducted. Recently, the hybrid spray-drying technique was reported, which combines the ink-jet tech-nology with spray drying in order to produce particles of uniformcharacteristics.[1–4] The monodisperse droplet generators (MDGs)have certain features that make them attractive for use as atomizersin spray-drying operations. In this article, a comprehensive intro-duction to a variety of monodisperse droplet generators is provided.The advantages and limitations of these generators are discussed.The fundamental equations for predictions of the average dropletsize are summarized. The discussion given in this study is aimedat the development of a hybrid technology that marries a suitableMDG with the existing spray-dryer technique (yet with some mod-ifications) to manufacture the product with uniform characteristicscontaining micro- to nanosized particles.

Keywords Atomizer; Droplet generators; Droplet size distri-bution; Identical particles; Monodisperse spray;Spray drying

INTRODUCTION

Spray drying is a mature industrial operation, widelyused for manufacture of high-quality particulate materialsin many industrial sectors ranging from food to dairy,chemicals, pharmaceuticals, and ceramics. Spray dryer pro-ductivity ranges from as little as a few hundred grams toseveral tons of powder per hour per dryer. The fundamen-tal principle in lab-scale and industrial-scale dryers is thesame, although the purpose of powder manufacturingand the quality of the powder produced during lab-scaledrying and industrial-scale drying can be different.Remarkable research was conducted in the last four

decades to achieve high production rates, better qualityproduct, lower energy requirements, and economicaldesign of equipment.[5] Spray-drying technology is stillopen to both theoretical and practical research in orderto model various phenomena occurring during spray dry-ing, improve the product quality and the energy efficiency,minimize the wall deposition and the overall productloss, and design and test new engineered products.

Four main processing steps are involved with spray dry-ing: atomization, mixing of hot gas and spray, evaporationfrom droplets, and collection of the dried product.[6–8] Eachstep is carried out considering the chemical and physicalproperties of the feed, the dryer design, and the requisitequality of the final product. Atomization, which facilitatesthe droplet formation and determines the size and the sizedistribution of droplets, plays an important role in achiev-ing the desired characteristics of the final product. Thecommonly used atomizers in dairy and other food indus-tries are rotary atomizers, pressure nozzles, and pneumatic(two-fluid) nozzles.[6,7] Sonic atomizers are recentlyemployed to small-scale spray-drying plants.[8] Theseatomizers produce polydisperse sprays to generate largesurface area for favoring rapid heat and mass transfer.The higher feed-handling capacity of atomizers is realizedby offering the greater kinetic energy to the process ofdroplets generation, leading to uneven disintegration ofbulk liquid into a spray of individual droplets with a poly-size distribution. The typical ranges of droplet size duringspray drying with the rotary atomizers, pressure nozzles,two-fluid nozzles, and sonic atomizers are 1–600, 10–800,5–300, and 5–1000 mm, respectively.[8]

The polydispersity of the spray inside the drying cham-ber leads to complex droplet trajectories and collisions ofdroplets with other droplets and the dryer walls. Eachdroplet may follow a different route inside the dryingchamber and experience different temperature-time andhumidity-time profiles. This causes variability in the resi-dence time distribution for particles in the dryer. Withexisting spray-drying technology, each particle in the dried

Correspondence: Professor Xiao Dong Chen, Chair ofBiotechnology and Food Engineering Group, Department ofChemical Engineering, Monash University, Clayton Campus,Room 206, Building 36, Victoria 3800, Australia; E-mail: dong.chen@eng.monash.edu.au

Drying Technology, 25: 1907–1916, 2007

Copyright # 2007 Taylor & Francis Group, LLC

ISSN: 0737-3937 print/1532-2300 online

DOI: 10.1080/07373930701727176

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product may have a different shape, size, density, porosity,moisture content, and nutrient content.

The wide droplet size distribution increases the energyrequirement as well as the particle size distribution in thefinal product. Control over droplet characteristics duringatomization is somewhat restricted. This is objectionablein certain industries where a uniformity of particles is cru-cial; for example, functional food products, pharmaceuti-cals, and catalysts.[9] The accuracy of the simulation workand the reproducibility of the lab=pilot-scale validationresearch are also negatively influenced by the size distri-bution of droplets and the polydispersity of the spray.[10]

Small and uniform particles have recently becomerequisite in many applications, such as high-efficacy drugmicrocapsules, nutraceuticals, functional food ingredients(e.g., flavors, antimicrobials, preservatives, vitamins, andantioxidants), catalysts, etc. Some products in biologicaland pharmaceutical industries require highly sphericalparticles for their final applications. For instance, sphericaland uniform particles are highly suitable for the colloidaldrug delivery and dry formulation vehicles because theyprovide a practical means of maximizing the efficacy of thedrugs and controlling their release kinetics.[11] Nano- tomicron-sized spherical particles are widely used as signalreporters (or probes) to detect biomolecules in DNA assay,immunoassay, and cell bioimaging.[12] Nanoparticulate lyco-pene and carotenoids are becoming commercially availablein the market as food ingredients. Weiss et al.[13] conciselyreviewed existing and future applications of nanoparticlesin food and biotechnology fields. Micron- and nanosizedspherical particles are likely to offer several advantages suchas higher bioavailability, greater effective exposure area, andenhanced efficacy and safety during utilization compared tothat of their traditionally manufactured counterparts.

One way to restrict the nonuniformity of the product isto have droplets with the negligible size distribution andsimple, yet predictable trajectories. Recently, Patel andChen[1–3] have published a series of reports on a smart dry-ing technique, which utilizes a single stream dryingapproach. They introduced an ink-jet device, one type ofmonodisperse droplet generator (MDG), to a conventionallab-scale spray dryer in order to produce particles with uni-form characteristics. This approach seemed to offer goodcontrol over the droplet trajectory, size distribution, andcollisions. This lab curiosity of Patel and Chen[1–3] maybe further extended to manufacture ‘‘smart’’ particles at aproduction rate that allows commercialization for applica-tions such as development and testing of new products anddrying of heat-sensitive materials. In this article, severalMDGs are discussed that may be incorporated to thespray-drying technology as atomizers in order to produceparticles with uniform characteristics. Emphasis is givenon reducing the size of monodisperse droplets and increas-ing feed-handling capacity while using MDGs as atomizers.

The technology of monodisperse droplet generation wasfirst developed to improve the resolution in ink-jet applica-tions such as ink-jet printers and ink-jet recorders.[14]

Theoretical research commenced more than one centuryago, although in the last three decades the application ofmicron-sized monodisperse droplets has extended to abroad and increasing range, such as chemical deposition,[15]

microcapsules production,[16] DNA arraying,[17] polymerelectronics manufacture, controllable drug release, andindustrial printing.[18,19] The MDG is the key componentfor generating monodisperse micron-sized droplets. Anattractive characteristic of MDGs is their ability to pro-duce a single stream of droplets or a monodisperse spraywhile keeping control of the droplet trajectory. Line dis-pensing of droplets is possible and commonly used in officeink-jet printers. The concept of line dispensing was foundto be very useful for production of uniform lactose parti-cles using the single stream drying approach developedby Patel and Chen.[1–3] Another attractive feature ofMDGs is that they can be easily grouped in one mechanicalassembly to use them for bulk production of droplets; forinstance, ink-jet cartridges and multiple-nozzles arrays.

If the monodisperse droplets can be generated for spraydrying, particles can be manufactured with a narrow sizedistribution. In addition, uniform size and simple trajec-tory can make the droplet drying process more analyzableand controllable and reduce the interparticle and particle-wall collisions that result in wall deposition, corrosion, pro-duct loss, and energy wastage. If the flow pattern of the hotgaseous medium could be also practically simplified forindividual droplets, production of same characteristicparticles would be expected. To fully take advantage ofthe MDG, the feed-handling capacity must be extensivelyenhanced. Although there are still some concerns withthe advancement for drying of monodisperse sprays, theapplication of MDG has a great potential to improve thecontrollability of the droplet drying process and dramati-cally promote the overall quality of the spray-driedproduct. It is expected that the MDGs combined withspray-drying technology will provide an alternative topractical-scale productions with feed-handling capacitiesof at least 1 L per hour.

This research article is intended to review the governingtheory of droplet formation and existing methods of pro-ducing monodisperse droplets. Several commercial MDGsbeing utilized in various fields are also discussed for twocriteria: reducing the size of monodisperse droplets andenhancing the feed-handling capacity of MDGs. The dry-ing-relevant issues are also discussed with respect to thedisintegration of a liquid jet into individual droplets, stab-ility of droplets during drying, and other limitations ofthe technique with the view of developing a combinedmonodisperse droplet generation technique and spray-drying technology.

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PRINCIPLES OF DROPLETS GENERATION

Droplets are formed when the balance of forces on a liquidjet breaks. There are normally two groups of forces influen-cing the process of droplet formation; the first group belongsto motive forces (i.e., gravitational force, inertial force,electrostatic force, mechanical force, and aerodynamic force)and another group is for drag forces (i.e., surface tensionforce and viscous force). For different systems, the types offorces involved can be varied to facilitate droplet formation.However, four kinds of forces, which are gravitational force,inertial force, surface tension force, and viscous force, alwaysexist. To analyze the force balance on the droplets duringmonodisperse droplet generation, certain dimensionlessnumbers are used, which are presented in Table 1.

For the simplest case, which refers to the state of ‘‘drip-ping,’’ the gravity force and the surface tension force on thedroplet have to reach the equilibrium state for disinte-gration of the liquid jet into individual droplets. In a math-ematical form, this process can be described as:

q � g p6

�dd3 ¼ p � d0r) �dd ¼ 6d0r

qg

� �1=3

ð1Þ

Apparently, the dripping mechanism is concerned withlarge droplets and relatively low velocity conditions. Whena liquid jet issues from a nozzle, the equilibrium betweenthe surface tension and gravity cannot be maintained,and thus the dripping state gives way to a jetting state. Thistransition is described by the Weber number. By extendinga Taylor’s model, Clanet and Lasheras[20] obtained a criti-cal value for Weber number at which the transition fromthe dripping state to the jetting state occurs:

Wec ¼ 4Bo0

Bo1þ KBo0Bo� 1þ KBo0Boð Þ2�1

� �1=2� �

ð2Þ

where Bo and Bo0 are Bond number based on the insideand outside diameter of the nozzle at the orifice, respec-tively, and K is constant (which is 0.37 for the case of awater jet issuing in stagnant air). Both gravity and inertiaeffects are included by introducing the Bond number intothe equation, which was shown to be in good agreementwith experimental results.

For a continuous breakup of a non-viscous liquid jetwith adequate surface tension and low relative velocity,Rayleigh[21] obtained the following:

�dd ¼ 1:89d0 ð3Þ

This means that the average droplet diameter �dd is roughlytwice the liquid jet diameter d0, which may be considered asequivalent to the orifice diameter. If the disturbance to thejet is amplified by additional influences, such as electricalforce or mechanical force, the droplet size may be reduced.The Rayleigh theory represents the upper bound of actualsize of droplets broken up from a liquid jet. This has beenwidely accepted in subsequent developments to be a firstapproximation for developing mathematical theories toillustrate the droplet disintegration mechanism for manydroplet generators including some current commercial ones(Inotech, Inc., Switzerland).

To take the viscosity of the liquid into account, a moregeneralized Weber theory can be used for low-velocitylaminar jet breakup scenarios. The Weber theory was orig-inally extended from the Rayleigh theory. The optimumdisturbing wavelength (kop) and the average droplet diam-eter based on the consideration of surface energy to breakup the jet into droplets were described as:

kop ¼ffiffiffi2p

p 1þ 3lffiffiffiffiffiffiffiffiffiffiqrd0

p� �

ð4Þ

�dd ¼ 1:5kopd20

� �1=3 ð5Þ

When the viscosity and=or velocity of a liquid jet arefurther increased, the governing mechanism of jet breakupcan be classified into four primary regimes—Rayleigh jetbreakup zone, first wind-induced breakup zone, secondwind-induced breakup zone, and atomization zone. Thisclassification, schematically shown in Fig. 1, is based onthe relative importance of forces involved in the system,such as viscous force, surface tension force, inertial force,aerodynamic force, etc.[22] A combination of Ohnesorgenumber and Reynolds number is the most commonlyaccepted criterion of this classification for the steady ejec-tion of a liquid jet through a single, circular orifice intothe quiescent air. Reis et al.[23] studied the piezoelectric sin-gle-nozzle droplet generator and concluded that the nor-malized drop volume ejected was shown to be a functionof the Ohnesorge number at the orifice.

TABLE 1Dimensionless number governing the process of dropletsgeneration, where q is the density; r0 is the characteristic

length, l is the viscosity of the fluid, v is the characteristicvelocity, and r is the coefficient of surface tension of the

fluid

Bond number(Bo)

qgr20

r

Gravity=surfacetension force

Capillary number (Ca)lv

rViscous force=surface

tension forceReynolds number (Re)

qgrv

lInertial force=viscous

forceWeber number (We)

qr0v2

rInertial force=surface

tension forceOhnesorge number (Oh)

l

ðqr0rÞ1=2Viscous force=surface

tension force

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With the higher velocity of the liquid jet, the effect ofaerodynamic forces becomes increasingly important, whilethe relative influence of surface tension force is lessened.The breakup of droplets from the jet may become unstableleading to formation of droplets of smaller and polydistrib-uted size. To get monodisperse droplets, the liquid para-meters such as density, viscosity, and surface tension ofthe liquid and the velocity of the liquid jet that make upof Reynolds number and Ohnesorge number must be con-trolled within the Rayleigh jet breakup zone.

The size of droplets generated by MDG can be theoreti-cally predicted and can be reduced by introducing distur-bances during the droplet generation process. BothOhnesorge number and Reynolds number of droplets,however, should be limited to a certain range to obtaindroplets of uniform size, although this may come at theexpense of the production capacity. For a single nozzleproducing uniform monodisperse droplets, the capacity islimited by the small volume and velocity of droplets. Moredetailed theories on disintegration of a liquid jet for dropletgeneration can be obtained from Goedde and Yuen,[24]

Eggers,[25,26] and Lin and Reitz.[27]

METHODS OF DROPLET GENERATION

Based on some knowledge discussed above, the recentlydeveloped droplet producing methods aim to generate dro-plets to be as small, monodisperse, reproducible, and con-trollable as possible. Reducing the size of monodispersedroplets and enhancing the production rate are the mainissues for the application of MDG to spray-drying tech-nology and will be discussed here. To ensure convenientand clear discussion, in the part of reducing droplet size,the existing droplet formation methods were classified,from the theoretical models to the practical technologies,based on various mechanisms and corresponding forcesinvolved in the droplet formation process. As far asthe feed-handling capacity is concerned, application of a

multiple-nozzle plate and controlling liquid jet parameterscan be promising tools.

Size Reduction of Monodisperse Droplets

For achieving the droplets of smaller size, the mostdirect way is to decrease the orifice diameter of the nozzle.However, this concept is limited by the higher costs and theincreased risk of clogging during operation. Anotheroption is applying more frequent disturbances to the liquidjet, which is limited by the higher power consumption.Keeping the Ohnesorge number and Reynolds numberwithin the Rayleigh jet breakup zone is a key idea to facili-tate reduction in the droplet size while maintaining mono-dispersity. Physical properties of the liquid to be jettedbecome important here to keep the Ohnesorge numberand Reynolds number within the Rayleigh jet breakupzone.

Hydrodynamic Droplet Generator (HDG)

Gravitational force and inertial force serve as drivingforces in this kind of droplet generator. The critical Webernumber (Wec) determines whether the state of liquid isdripping or jetting types. Under the small We, the fluiddynamics are in a dripping regime. Droplets grow to a sizeat which the gravitational force overcomes surface tensionand viscous forces. Ejection of a single droplet with a drip-ping state is schematically shown in Fig. 2a. The drippingmechanism relies on the gravitational force as the maindriving force; thus, the diameter of droplets ejected istended to be much larger than that of the nozzle. WithWe rising, the state of jetting replaces dripping, which isillustrated in Fig. 2b and Fig. 2c.[28] Accordingly, the effectof gravitational force as driving force partially gives way tothe effect of inertial force; thus, droplets can be ejectedbefore the size grows as big as in the dripping state. TheHDG with the droplet generation driven only by the gravi-tational force and the inertial force as motive forces is notable to stably form fine droplets with uniform size.

FIG. 1. Four different breakup regimes (Rayleigh jet breakup zone, first

wind-induced breakup zone, second wind-induced breakup zone, and

atomization zone) governed by different droplets breakup mechanisms.[22]

FIG. 2. Different states of jet perturbation with different Weber

numbers. (a) We ¼ 0.063, (b) We ¼ 1.73, and (c) We ¼ 2.3.[28]

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Hence, more disturbances should be involved during theprocess of droplets generation.

Electro-Hydrodynamic Droplet Generator (EHDG)

The electro-hydrodynamic droplet generator (EHDG)uses electrostatic force in addition to gravitational forceand inertial force to accomplish droplet generation. Whenelectrostatic and surface tension forces balance each other,the meniscus becomes conical, forming a Taylor cone. Theapex of the cone ejects a thin jet, which subsequentlybreaks up into droplets with the size approximately closeto the jet size. The concept is schematically illustrated inFig. 3. The droplets are smaller than the orifice diameterand have a narrow size distribution. The EHDG with asingle nozzle was recently expanded to incorporate multiplejets in the system for achieving higher productioncapacity.[19,29] However, the specific requirements forliquid properties, the extreme complexity of electrostaticfield arrangement applied in multiple jets EHDGs, andthe relatively high cost to generate uniform droplets mayhamper the industrial application of EHDGs.

Mechano-Hydrodynamic Droplet Generator (MHDG)

The mechano-hydrodynamic droplet generator (MHDG)introduces additional mechanical force to the liquid jet toinfluence droplet breakup. This section will discuss twosuch resources of mechanical forces: shear force appliedby another fluid and vibration force generated by piezo-electric materials transducer (PZT).

There are a few droplet generation methods currentlybeing investigated in laboratory that use shear force tofacilitate disintegration of droplets from the bulk fluidor the liquid jet. Three such methods are depicted inFig. 4.[30–32] Figure 4a depicts the droplet generator having

a three-microchannel junction. The microchannels can bemachined on a silicon plate using a dry etching process.Xu and Nakajima[31] fabricated such a nozzle with inletchannels having 100 mm in width, which was tapered downto 50 mm near the junction of the three inlet channels. Thesimplified version of the three-microchannel junctiongeometry is a T-junction microfluidic device (TMD), whichis another droplet generator that uses the shear force. TheTMD can be fabricated on polymer plate such as a poly-methyl methacrylate (PMMA) plate by using an endmill.[32] The channel for disperse and continuous phasescan be of different diameters. A typical T-junction arrange-ment is schematically shown in Fig. 4b. Another devicewhere the shear force is used to produce a droplet is astraight-through microchannel (SMC), which is schemati-cally illustrated in Fig. 4c. This device can be fabricatedby deep reactive ion etching (DRIE) of the microelectro-mechanical system (MEMS).[30] They are mainly utilizedas emulsification devices by permitting different liquids toflow in a continuous phase channel and a dispersed phasechannel. The gas, such as air, can be replaced a dispersephase fluid in order to produce hollow droplets (bubbles)or particles. The flow rates of liquid in continuous phaseand dispersed phase play a determinant role in TMDand SMC. It was shown that monodisperse droplets of9–16 mm could be formed by controlling the flow rate ofboth continuous and disperse phase fluids.[30–32]

The size of monodisperse droplets formed by TMD andSMC could be 2–3 times bigger than that of the outletchannel. To reduce the droplet size further, Link et al.[33]

successfully developed a sequential T-junction microfluidicdevice (sTMD), schematically shown in Fig. 5, which couldpassively break up the liquid into small droplets with thesize comparable to the outlet channel width without

FIG. 3. A schematic diagram of a multiple jets EHDG. (a) A sectional view and (b) a full view. The homogenizer between the feed tube and nozzles

was placed to control the velocity of the bulk liquid to each nozzle.[19,29]

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changing the volume fraction of the dispersed phase. Thepotential coalescence phenomena among emulsion dro-plets, however, need to be addressed for the dryingpurpose.

PZT-driven MHDGs have been widely utilized in ink-jetprinting applications. There are two primary methods torun these kinds of droplet generators, continuous modeand drop-on-demand (DOD) mode. Both operating modescan produce fine and monodisperse droplets using mechan-ical disturbances originated by a voltage-induced PZT.With the continuous mode, it is possible to run the

generator at a speed of 100 KHz, but at the cost of higherpower consumption. The droplet diameter can be as bigas twice the orifice diameter. On the other hand, DODdroplet generators are able to efficiently eject dropletswith a frequency up to 2 KHz. Droplet generators in aDOD mode produce a single droplet for each voltagepulse. Droplets can be formed with sizes comparable tothe orifice size by controlling the waveform pattern usedto excite PZT. Formation of satellite droplet can beavoided by adjusting the pulse characteristics such aspulse amplitude, width, rise=fall times, and frequency. Acomprehensive review of PZT-driven devices was doneby Le.[14] As far as the spray-drying technology is con-cerned, a selection between a continuous mode and aDOD mode can be made based on the required pro-ductivity and the final size of the particles to be achieved.The droplet generator with a continuous mode is prefer-able over a DOD model for drying operations wherethe higher productivity is more important compared tothe smallness of the particle.

Based on the principle of the continuous mode dropletgeneration, Inotech, Inc., Switzerland, has developed anencapsulator system using a single or multiple nozzle thatallowed two different fluids to pass through a concentricnozzle. This kind of nozzle can process high viscous liquids(max. 300 mPa � s for 0.5-mm orifice diameter nozzle). Atypical setup of the encapsulator system is shown inFig. 6. This encapsulator with a single nozzle of 195-mmdiameter has been successfully utilized to capture highlyconcentrated cell suspensions in 250-mm hydrogel witha viscosity of 240 mPa � s and a production rate of

FIG. 4. MHDGs with shear force as driving force to form droplets. (a) Three-microchannel junction droplet generator.[31] (b) T-junction microfluidic

device (TMD).[32] (c) Straight-through microchannel (SMC).[30]

FIG. 5. Sequential T-junction microfluidic device (sTMD) where

droplets of the size comparable with the size of outlet channels are formed

by passive breakup.[33]

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36 mL=h.[34] This system can readily be tested to serve as anatomizer in spray dryers.

MDGs operating with a DOD mode have better con-trollability on droplet generation frequency and dropletsize and are also readily available in the market. MicroFabTechnologies, Inc. (Plano, TX) has fabricated a range ofdroplet generators that can be operated with a DOD modeto eject fluids of different characteristics and temperatures.The low productivity and high operating costs limit theiruse to the lab-scale applications.

The use of a laser-drilled hole as an orifice to formdroplets has recently attracted more attention with the cur-rent development of laser technology. Similar to the glasscapillary or the nozzle, the laser-drilled hole on a steel sub-strate can generate small droplets with the size comparableto the orifice size. The fluid ejection and disintegration ofdroplets from the orifice can be facilitated by a piezoelectri-cally driven actuator and an appropriate feed pressure con-troller. Alternatively, a silicon nozzle fabricated by DRIEis assembled as a microfluidic dispenser by SpectraTM,Inc. (USA). This kind of generator can produce dropletswith as small volume as 1 pL (<10 mm in diameter) froma 21.5-mm orifice diameter.

As previously mentioned, reducing the nozzle size to getsmaller droplets is not always practical. A waveformpattern, which is the core technology of these variousPZT driven MDGs, can also control the droplet size downfor a given nozzle of size ranging from 20 to 200 mm. Chenand Basaran[35] proposed a new pattern of waveform,called waveform-3, which enabled a reduction in dropletradius by more than a factor of 2 compared to the dropletsize generated through the same orifice with a PZTactivated by traditional waveform-1 and waveform-2.

In general, MDGs with both continuous and DOD modescan theoretically form fine and monodisperse droplets forspray drying, but practical applications cannot be realizeduntil the yield of droplets is greatly enhanced.

Feed-Handling Capacity Enhancement for MDGs

For a single nozzle, the parameters of liquid phase mustbe kept within the Rayleigh jet breakup zone or first wind-induced breakup zone to obtain stable monodisperse dro-plets. This puts constrains on allowable flow rate and limitsthe capacity of a single nozzle. To achieve higher through-puts from MDGs, the number of nozzles in the systemmust be dramatically increased. To create multiple jetmonodisperse droplet generators, both the manufacturingaspects and the control of liquid jet parameters should beconsidered.

Manufacturing of the Multiple-Nozzle Plate

The use of a multiple-nozzle plate permits multiplestreams of monodisperse droplets to be generated simul-taneously. For the successful operation, one has to ensurethat every nozzle in the system is as identically manufac-tured as possible. The size error distribution of nozzlesmanufactured using different methods was investigatedby SpectraTM, Inc. Compared to laser drilling methods,DRIE fabricated silicon nozzles (150 M-Class Jet Modules,SpectraTM, Inc., USA) had smaller errors regarding theirorifice size.

After considering the individual nozzles, the wholearrangement of nozzles within one nozzle plate is anotherissue. The geometrical arrangement of nozzles on the plateis also very important for the stability of liquid jets anddroplets in the chamber. Nozzles can be placed in a circularplate using a square pattern, concentric circles, or concen-tric polygons. Fabrication of such a mechanical assemblywith a few hundred individual devices may be very complexand expensive. Brenn[36] designed a laser-drilled orificeplate with 62 holes in a common piezoceramic block toform multiple streams of monodisperse droplets with theflow rate up to 13.8 L=h. It was proposed that the mutualdistance between two orifices be at least 3 times the nozzlediameter to prevent coalescence among neighbor parallelstreams. Patel and Chen[1–3] proposed a design of a mul-tiple-orifice plate with either laser-drilled holes or individ-ual PZT-driven MDGs using a triangular arrangementon a circle plate in context to drying operations. Each holeor nozzle was surrounded by an air-dispensing holethrough which the hot air could be blown in a cocurrentfashion, as shown in Fig. 7a. In the current market,SpectraTM, Inc., has developed M-class print-head moduleswith a die consisting of 304 addressable jets individuallypowered by a PZT, as shown in Fig. 7b. Many arrange-ments are possible depending on the type of the dropletgeneration and the specific requirement.

FIG. 6. A schematic diagram of the encapsulator designed by Inotech, Inc.

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Controlling Liquid Jets

During formation of monodisperse droplets using amultiple-nozzle plate, the most important parameters tobe controlled are the ejecting velocity and disturbingfrequency of the liquid jet, which have a crucial impacton the final droplet size and the size distribution. The jetvelocity and disturbing frequency for all the nozzles shouldbe kept identical. A homogenizer consisting of baffles,beads, frits, or other obstructions can be set between thefluid feed reservoir and the inlet of nozzles to balance thepressure on the liquid for all the nozzles, thus keeping theirvelocities as identical as possible (see Fig. 3a).[29] The scale-up process can be more easily realized for PZT-drivenMDGs. Multiple dispensers with one PZT for each nozzlecan be assembled in a group and controlled individually.The multiple streams generator, similar to the M-classprint-head modules designed by SpectraTM, Inc., is cur-rently being utilized in the research fields of chemicaldeposition and biomaterials printing. As multiple jets aresimultaneously generated through individual nozzles,coalescing phenomena between droplets of mutual streamsor the same stream must be prevented. The droplet trajec-tory and the distance between two droplet streams, as wellas between two neighbor droplets within the same stream,should be well controlled, especially if the stream ofdroplets is traveling down into a chamber of moving gas.

DISCUSSION

Production of uniform particles in micron- and evennanosize is targeted by a wide range of practical fields,including spray drying. The throughput of current spraydryers can be over 20 tons per hour, although poor con-trollability over the particle size distribution and severalassociated problems restrict their application for manufac-turing of specialized products. The main reason for thelarge particle size distribution is the poor performance ofatomizers that generate polydisperse sprays from the bulk

liquid. The gas turbulence along with polydisperse dropletsinside the drying chamber can lead to undesirable agglom-eration, wall deposition, product loss, and energy wastageand can result in extreme complexity for analyzing andcontrolling the drying process. For minimizing these pro-blems with the existing spray-drying technology, the mono-disperse droplet generators can be a useful tool to producemonodisperse sprays of uniform droplets. However, thecurrent scale of production is likely to be sacrificed dueto the very small feed-handling capacity of the MDGs.The single-droplet generators are currently able to producea few milligrams of powder per hour. The throughput forMDGs may be elevated by using multiple MDGs in a sin-gle assembly or multiple MDGs working parallel in anindividual assembly. It is common to group several hun-dred ink-jet devices in a single mechanical-hydraulic plateto produce multiple streams of uniform droplets for print-ing applications. If an effective multiple-nozzle design isconstructed for the purpose of drying, then productioncapacities up to a few kilograms of powder per hour maybe achieved.

In the laboratory, drying of monodisperse droplets mayallow measurement of several droplet characteristics (e.g.,droplet size, size distribution, and trajectory) during pro-cessing.[1–3] This feature makes the drying process moreanalyzable for developing a suitable drying kinetics modeland also for scale-up or scale-down purposes. In addition,it is expected that the process optimization would be rela-tively easy with drying of monodisperse spray comparedto the traditional spray-drying process due to the higherreproducibility advantage with drying of monodispersedroplets. Application of MDGs as atomizers for spray dry-ing operations may also offer several advantages from pro-cess control to product quality, in particular, desirablespherical particles, negligible particle size distribution,and better predictions of process parameters and energyrequirements. Furthermore, the final product is expectedto be more efficiently collectable from the cyclones dueto uniform characteristics of the dried particles.

FIG. 7. Two designs of a multiple-nozzles plate for generating monodisperse sprays. (a) A circle multiple-nozzles plate with a triangular arrange-

ment.[1] (b) M-class print-head module with a die consisting of 304 addressable jets individually powered by a PZT (SpectraTM, Inc.).

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Some theoretical and practical aspects of existingMDGs that may potentially be utilized as atomizers forspray-drying technology were reviewed here. Accordingto the governing theory on droplets formation, it was notedthat the controlled frequency and the amplitude of disturb-ance on a liquid jet can reduce the droplet size for thenozzle of a given orifice size. Monosized droplets generatedfrom a single MDG can be realized at the expense of pro-duction rate owing to the fine volume and the small dropletformation rate. The mechano-hydrodynamic droplet gen-erators (MHDGs) were found to be the most suitablecompared to HDGs and EHDGs to generate small anduniform droplets for spray-drying technique. The methodof generating a monodisperse spray has been extensivelyused for various printing technologies and is rapidly beingutilized for many other applications. The Biotechnologyand Food Engineering Group at Monash University,Australia, seemed to be the first to propose the applicationof PZT-driven MHDGs to spray dryers and have success-fully obtained spherical and uniformly sized lactose parti-cles using a newly developed lab-scale ink-jet spray dryer(IJSD).[1–3] The IJSD using multiple streams of dropletsis under development within the same group to achieve alarger throughput.

The success of this technology relies on the consistentperformance of the MDGs and the behavior of monodis-perse sprays in the drying chamber. There are certain con-cerns with applying the method of monodisperse dropletgeneration to the spray-drying technology. The commonlyencountered problem was the blockage of the nozzle duringoperation, especially for MDGs of small orifice diametersand for liquids having suspended particles of diametergreater than the critical diameter, which is generally 5%of the orifice diameter.[37] The blockage problem may bemore severe under drying conditions due to drying out ofthe liquid at the orifice. However, it may be overcome byfiltering the concentrated feed and offering a flow of low-temperature air around the nozzle tip.

It is essential to ensure the stability of monodispersedroplets inside the drying chamber for production of uni-form particles using MDGs. The stability of monodispersedroplets is affected by various atomization, drying, proces-sing, and design parameters such as feed pressure, feedproperties (viscosity and surface tension), pulse characteris-tics for driving PZT actuators, location of the device in thedrying chamber, position of the device with respect to theair distribution plate, location of the air dispersing holes,air droplets contacting methods, air flow patterns insidethe chamber, air properties and flow rates, geometry ofthe drying chamber, and position of the fluid-bed dryer(if any). Care must be taken to control the drop trajectoryinside the chamber for avoiding interparticle and particle-wall collisions.[38] One way to avoid collisions is to guaran-tee that particles are dried up to a safe moisture content

level before gaining random trajectories. Charging ofparticles could be another option to avoid collisions; how-ever, this would introduce some complexity and extra costs.

SUMMARY

In this review article, we have presented a need for pro-ducing spherical and identical particles and some MDGsthat can be applied as ‘‘smart’’ atomizers to the spray-drying technique. An attractive feature of these dropletgenerators is the ability to produce spherical droplets witha negligible size distribution and a controlled drop trajec-tory. MDGs as atomizers, if incorporated to the dryingchamber, uniform, spherical, and reproducible particles ormicrocapsules of diverse morphology and microstructuremay be generated. In particular, MDGs, especially PZT-driven MHDGs, seemed to have high potentials to serve asatomizers in spray dryers at a reasonable productioncapacity. Furthermore, the drying can be facilitated by incor-porating only a small volume dryer because the drying timewould be very small for droplets with the size in the order ofa few microns. The use of these atomizers may be limited atthis stage to the lab-scale only due to the very small pro-ductivity. There is a large scope of work that needs to becarried out to look at the detailed hardware and softwareaspects of the whole idea and the related lab=pilot-scalesetups. A detailed research work is under progresswithin the same research group at Monash University,Australia.

NOMENCLATURE

Bo Bond number (based on the inner diameter of theorifice)

Bo0 Bond number (based on the outer diameter of theorifice)

Ca Capillary number�dd Average diameter of droplets (m)d0 Diameter of the liquid jet (m)g Gravitational constant (9.8 m=s2)Oh Ohnesorge numberRe Reynolds numberr0 Characteristic length (m)v Velocity of the liquid jet (m=s)We Weber numberWec Critical Weber number

Greek Symbols

kop Optimum disturbing wavelength (m)l Viscosity of the liquid (mPa � s)q Density of the liquid (kg=m3)r Surface tension coefficient of the liquid (N �m�1)

ACKNOWLEDGEMENT

This is to acknowledge that the work is mainly sup-ported by ARC Discovery Grant DP0773688. The first

MONODISPERSE DROPLET GENERATORS AS POTENTIAL ATOMIZERS 1915

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author is also supported by an International Studentshipfunded by Department of Chemical Engineering, MonashUniversity.

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