Powder Technology 189 (2009) 263-269

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Powder Technology

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Influence of mixing characteristics for water encapsulation by self-assemblinghydrophobic silica nanoparticles

Laurent Forny, Khashayar Saleh ⁎, Isabelle Pezron, Ljepsa Komunjer, Pierre GuigonUniversité de Technologie de Compiègne, Département de Génie Chimique, Laboratoire CNRS – UMR 6067 Génie des Procédés Industriels, B.P. 20529, 60205 Compiègne Cedex, France

⁎ Corresponding author. Tel.: +33 3 44 23 52 74; fax:E-mail address: khashayar.saleh@utc.fr (K. Saleh).

0032-5910/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.powtec.2008.04.030

A B S T R A C T

A R T I C L E I N F O

Available online 24 April 2008

Water encapsulation using

Keywords:EncapsulationMixingFumed silicaHydrophobicity

silica nanoparticles was assessed using two different types of single step mixingprocesses. The influential mixing characteristics have been determined. Direct mixing at high rotationalspeed requires high shear and vigorous stirring properties. Progressive water atomisation using gentlemixing process requires high atomisation pressure and rapid surface refreshing of the mixed material.Mechanisms of powder formation were also proposed. Encapsulation of micrometric water droplets in shell-like structure is respectively obtained by either progressive size reduction of macroscopic particulates ordirect coating of pre-formed microscopic droplets. These mechanisms resulting from the interactionsbetween a solid particle and a liquid highly depend on parameters such as particle's hydrophobicity, surfacetension or kinetic energy.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Most processes used to encapsulate active principles in a dry staterequiremultiple-stage operations including usually the formulation ofan intermediate emulsion (e.g. [1]). As an example, dry emulsions arebased on the preparation of an Oil/Water emulsion containing watersoluble polymers [2,3]. These emulsions are spray dried to createindividual droplets which, after water phase evaporation, become dryparticles composed of a polymeric network containing oil dropletswith a maximum ratio between 60 and 65% by weight. Recent deve-lopments in encapsulation processes also focus on solid particles as anencapsulation agent to create new capsules properties [4–6]. Colloi-dosomes are capsules formed by the auto-association of polystyrenebeads at the interface of a W/O emulsion. Solid bridges are created toreinforce these capsules by partially sintering the beads [6]. Usuallytypical encapsulation processes are quite difficult to implement, allowlimited encapsulating ratio and require the use of an organic solvent.

This study focuses on a single step mixing process allowing theencapsulation of up to 98% by weight of water in a powder composedof silica nanoparticles. The encapsulated product is usually called “drywater” because it has similar flow properties as dry powders despitelarge water ratio. This process was first described in 1964 in a patentpublished by Degussa [7]. Solid phase was highly hydrophobic fumedsilica and mixing process was high shear mixing process represented

+33 3 44 23 19 80.

l rights reserved.

by a blender. At this time and despite promising properties thisinnovation did not find real successes in industrial applications. Sincemid 90's the interest for such product has been renewed throughcosmetic applications [8–10] and some scientific studies [4,10–13].

In an article published in 2007, we studied the structure of drywater [13]. In fact, particles are composed of micrometric waterdroplets surrounded by auto-assembled silica particles. This shell-likestructure isolates each individual droplet preventing them fromcoalescence (Fig. 1). We also observed that conditions leading to theformation of drywater strongly depend onmixing processes as well ashydrophobicity of fumed silica particles. While highly hydrophobicparticles may be used in high shear mixer, low hydrophobic particlesneed low shear mixing processes. During this study, we will focus onmixing processes characteristics in order to define critical parametersand to propose specific mechanisms of dry water formation. Thisinformation should help the scale-up of the process to pre-industrialor industrial unit.

2. Materials and methods

2.1. Fumed silica

Fumed silica were supplied by Degussa (Germany) from Aerosil®

range. General characteristics were described in a previous study [13].Three grades were used besides R812S and R972. R202 is highly hy-drophobic fumed silica aftertreated with a silicone polymer. R711 is afumed silica of low hydrophobicity aftertreated with methacrylsilane.300 is untreated fumed silica which spontaneously sinks in water.Finally, according to Degussa's methanol wettability [14] we can

Fig. 1. Typical structure of dry water particulates observed under ESEM after water removal and gold metallization [13].

264 L. Forny et al. / Powder Technology 189 (2009) 263–269

classify these different fumed silica in increasing hydrophobicity orderas follows:

300bR711bR972bR812SbR202

Primary particle size goes from 7 to 16 nm, BET specific surface areafrom 100 to 300 m2 g−1 and tapped density from 50 to 60 g L−1 [15].

2.2. High shear mixing processes

At laboratory scale, dry water was produced in a simple way bymeansof a commercial blender (PowerblendMX2050,Braun,Germany).The glass bowl has an intermediate shape between a cylinder and atriangle. Its average diameter is around0.012m. Itwas loadedwith 4 goffumed silica and 96 g of deionised water. Rotational speed was set to1000, 8000, 12,000 and 18,000 rpm using an external potentiometer(Prolabo, France) and an optical hand held-tachometer (Rotaro, Rhein-Tacho, Germany). The mixing impeller is composed of 4 knife blades in ashape of a cross (Table 1). The ends of the blades are vertically curved.The diameter of the impeller was 0.055 m. Mixing time was set to 30 s.

A standard mixing vessel was also implemented in order to evaluatethe energy delivered to the mixed material. This energy could beassessed by measuring real torque but it would require specific mixerfitted with high precision torque meter. Characterisation of a mixingprocess depends on different parameters [16–18]. Some belong to themixed material such as density ρ, viscosity η and surface tension γ.Some are kinematical or dynamic characteristics such as rotationalspeed N, attraction of the gravity g and power P delivered to the mixedmaterial. Finally many parameters are geometric dimensions such asdiameter of the tank D, diameter of the propeller d, height of the mixedmaterial H, relative position of the propeller Y as well as number ofbaffles and their relative size (Fig. 2).

Reynolds number NRe which corresponds to the ratio of inertialforces to viscous forces was evaluated using water properties assum-ing that predominant mixing material is liquid phase:

NRe ¼ Nd2qg

ð1Þ

The Power number Np is analogous to a friction factor or a dragcoefficient relating the resistance forces to the inertial forces as:

Np ¼ PqN3d5

� �ð2Þ

In unbaffled tank and for Reynolds number above 300, Eq. (2) isaltered into the following expression to take into account vortexformation [16]:

/ ¼ PqN3d5

� �g

N2d

� � a� log NReb

� �ð3Þ

where a and b are constants depending on geometric dimensions.For Reynolds number below 300 or in baffled tank, Φ simply

corresponds to Np. Fig. 2.b shows typical variation ofΦ parameter as afunction of Reynolds number for common impellers placed in stan-dard mixing conditions.

Our experimental device was composed of an overhead stirrerhaving a maximal rotational speed N=6000 rpm (Eurostar powercontrol-visc 6000, Ika, Germany), differentmixing impellers of diameterd=0.05 m and an unbaffled plexiglas tank of diameter D=0.15 m. Thesedimensions verify classical standardised configuration (Fig. 2.a) whichwas fully approached by mixing 2.5 kg of water with 100 g of R812Sassuming that predominant mixing material is liquid phase. Corre-sponding water ratio was 96% by weight. Both A100 impeller (three-blade marine propeller) generating axial flow and R100 impeller(turbine composed of six flat blades mounted on a horizontal disk)generating radialflowwere used. Theywere supplied by Lightnin (USA).We also used helical propeller (three helical blades with radial squaretermination) having intermediate flow properties and knife bladesimpeller (two horizontal knife blades) creating essentially high shearforces. Both were supplied by Bioblock (France). According to classicalestimation, turbulent flow is achieved when NReN105 for axial flowpropellers and NReN104 for radial flow propellers [18]. In this study,turbulent flow was obtained for a rotational speed over 3000 rpm asNRe=1.2×105. Corresponding values of power numbers Φ (Table 2) aswell as a and b parameters were obtained from the literature [15]. In ourexperimental conditions, a=2.1 and b=18 for A100 propeller; a=1 andb=40 for R100 turbine. From Eq. (3), it was then possible to evaluatethe energy P delivered to the mixed material as:

P ¼ /g

N2d

� � logNRe�ab

� �qN3d5 ð4Þ

Experimental protocol consisted in determining minimal mixingtimes required for complete water encapsulation at 4000, 4500 and5000 rpm using the different mixing impellers. These durations for allthe water disappear were evaluated by visual observations. They weretherefore submitted to the operator's judgement and experimentalerrors may be quite important. Each experiment was repeated threetimes. The standard error on minimal mixing time was evaluated

Table 1Main geometric characteristics of both high shear and low shear mixing processes

Impeller design Impeller size Maximum mixing rate Tank design Tank capacity

Powerblendblender

0.055 m 18000 rpm Bowl 2 L

Standard mixing vessel

A100

0.05 m 6000 rpm Cylindrical 7 L

R100

Helical propeller

Knife blades

Laboratory andpre-industrial mixers

0.130 m 3000 rpm Hemispherical 5 L0.315 m 3000 rpm Hemispherical 60 L

Triaxe® 0.405 m 155 rpm Hemispherical 27 L0.475 m 155 rpm Hemispherical 50 L

Cement mixer – – 23 rpm Hemispherical 130 L

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within 5%. Furthermore properties of mixed material are chosen ar-bitrarily and are changing during the encapsulation process. Never-theless this method allows reliable relative comparison of mixingpropellers efficiencies.

Beside this standard mixing vessel, laboratory and pre-industrialmixing processes were implemented. Commercial mixers UMC5 andUMC60E were supplied by Stephan (Germany). UMC5 has a capacityof 5 L and UMC60E has a capacity of 60 L. Both are composed of a bowltank and are fitted with two knife blades impellers of 0.13 m and0.315 m in diameter respectively. Mixing process was achieved usingrespectively 2 kg and20 kgof liquidwaterwith 60 g and600 gof R812S.Maximum mixing rate was 3000 rpm. Mixing time was set to 5 min.

2.3. Low shear mixing processes combined with water atomisation

As proposed in our previous article [13], dry water was producedusing R972 in Triaxe® low shear planetarymixer (Hognon S.A.S., France).Triaxe's specificity consists in simultaneous rotation and gyration mo-tions of the impeller that reduce stagnant zones inside the tank. Twodifferent hemispherical tanks having 27 L and 50 L loading capacitieswere used. The respective tank's diameterswere 0.415 and 0.485m. Theimpellerwasfittedwith two comb and two full blades andwas adjustedto be driven close to thewall of the tank.Mixing ratewas set to 37min−1

for the gyration and between 10 and 155 min−1 for the rotation. Theatomisation system was composed of a single fluid spraying nozzleproducing constant size droplets of approximately 80 μm (WM 304,Delavan, UK). Water was fed in by a diaphragm pump (MB 37A, OBL,

Italy) having constant strokes rate of 115 rpm. The feeding pressure wasadjusted between 2 and 10 bars by changing the stroke of the pump.Correspondingvelocityof the outcomingdropletswas increasing aswellas atomisation flow rate going up from approximately 4 to 9 L h−1.Encapsulation trials were performed by atomising 2 kg of water over60 g of fumed silica placed under different mixing conditions.

According to a patent published by Aveka Co. [19], low shearmixerscharacterized by low kinetic energy from 5.10−5 to 1 kg.m2.sec−2 (forexample small cement mixers) could be also used to produce drywater. In our study, we used a cementmixer having a capacity of 130 L,a diameter of 0.7 m and a revolution rate of 23 rpm. Both single fluidand bi-fluid spraying were implemented. Water was fed in thespraying nozzle by a plunger metering pump (RB30A70, OBL, Italy)having constant strokes rate of 70 rpm. The feeding pressure wasadjusted between 2 and 10 bars by changing the stroke of the pump.Encapsulation trials were performed by atomising 2 kg of water over60 g of R972.

2.4. Size and shape distribution

Simultaneous measurement of particle size distribution and shapecharacteristics were performed using a Camsizer® (Horiba Jobin Yvon—

Retsch Technology). Among all available shape characteristics, wefocused essentially on sphericity factor. This device is based on digitalimage processing by means of two cameras allowing measurement inthe range from30 μmto30mm. Thepowder is automatically fed in froma vibrating channel through a feed guide so all the particles free fall in

Fig. 2. Configuration of a standardised agitated vessel (a). Typical variation ofcorresponding Φ parameter as a function of Reynolds number (b) [18].

266 L. Forny et al. / Powder Technology 189 (2009) 263–269

the focusing range of themeasurementfield. To improveflowbehaviourof the samples, the feeding channel was previously coated using anaerosol silicon lubricant. Analyses were performed within less than5 min to reduce sample disturbance fromwater evaporation. The num-ber of analysed particles was depending on the variations of instanta-neous size distribution. Typically a population of one million particleswas necessary to achieve a stable particle size distribution. Sphericityfactor was plotted as a function of the diameter of a sphere having anequivalent projected area as theparticle. Note that a sphericityequal to 1corresponds to a perfect sphere. Each sample was investigated threetimes on three different batches.

2.5. Microscopic observations

Water rich powder particulates were observed by means of anEnvironmental Scanning ElectronicMicroscope (XL30, Philips, Nether-lands). This equipment may be operated with a poor vacuum allowingthe imaging of wet systems. It was mentioned in our previous articlethat water phase should be sublimated in order to perform good

Table 2Minimum mixing times required for water encapsulation at different rotational speedusing different impellers placed in standard mixing conditions

Mixing impeller Φ Mixing rate Mixing time Power delivered Total energy

(rpm) (s) (W) (k J)

A100 ∼0.3 4000 960 16 15.34500 510 22 11.25000 390 29 11.3

Helical propeller – 4000 460 – –

4500 3305000 260

R100 1.1 4000 270 73 19.74500 220 101 22.25000 160 135 21.6

Evaluation of the power and the corresponding energy delivered to the mixed material.

observations [13]. Additional experiments revealed that initialstructure of the sample could be maintained by very progressivewater evaporation. For this purpose, the powder sample was depo-sited onto a small piece of self-adhesive tape that covered a Peltierthermoelectric device placed in the microscope and maintained at2.5 °C. The pressure was then slowly reduced down to 1.4 Torr pro-voking progressive water evaporation. Gold metallization was per-formed after water evaporation to improve picture resolution. Notethat smallest powder particulates are attached favourably to the Pel-tier device. The device was turned over to remove excess materialswhich were mainly larger particles.

3. Results and discussion

3.1. Influence of particles hydrophobicity and rotational speed in ablending process

Rotational speed of the blender played a key role in the transfor-mation of the mixed material. Below 1000 rpm both liquid and solidphase remained distinctly separated except usingAerosil 300which sankimmediately into the liquid phase. At 8000 rpm R711 and R972 res-pectively produced a viscous liquid and a homogenous mousse (Fig. 3).Finally, successful water encapsulation required highly hydrophobicR202 or R812S fumed silica as well as strong turbulent flow conditionsproduced by high rotational speed above 12000 rpm. The difference inbehaviour between fumedsilica is attributed toparticles hydrophobicity.Penetration of solid particles into water phase is enhanced by lowhydrophobicity and high shearing forces delivered by high rotationalspeed.

It was mentioned in our previous article that R972may produce drywater by decreasing water ratio down to 90% and mixing time to 10 s[13]. It was also noticed that additional mixing time of 20 s led to theformation of homogenous mousse. Investigations were performed onthis powder. Dry water having a water ratio of 90% was produced usingR812S or R972 in the commercial/Powerblend blender rotating at18000 rpm for 10 s. It was noticed that size distribution is more narrow

Fig. 3. Influence of fumed silica hydrophobicity from Aerosil® range (Degussa) on finalproduct quality obtained by blending 96 g of water and 4 g of solid particles at18,000 rpm for 30 s.

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for powder prepared with R812S (span=1.4) than for powder preparedwith R972 (span=2.5). The respective average size diameters of equi-valent spherical particles goes from dv(50)=180 μm to dv(50)=527μm.Fig. 4 shows that outlines of the particulates produced using R972 areirregular. This structure may be described as mousse fragments sur-rounded by free silica particles. Increasing mixing time induces themergingof these individual fragments and creates a continuousmousse.

3.2. Water encapsulation using standard mixing vessel

Encapsulation trials were performed using Aerosil R812S and dif-ferent impellers placed in standard mixing vessel. No transformationwas observed below 3000 rpm. This confirmed that successful powderformation requires turbulent flow conditions generating good stirringproperties. Power and the corresponding energy delivered to themixed material during water encapsulation were evaluated fordifferent rotational speeds over 3000 rpm (Table 1). Energy valuesindicate significant variation of efficiencies. While radial flow pro-peller R100 delivers between approximately 20 and 22 kJ for completewater encapsulation, A100 requires only around 12 kJ for the sametask. Helical propeller exhibits intermediate mixing time and probablydelivered less energy than R100 due to its favourable shape reducingresistive forces. Knife blades impeller was inefficient for all the testedmixing rates (up to 6000 rpm). This inefficiency is attributed to poorturbulent flow conditions which could be enhanced by higher rotatingspeed or larger impeller diameter. However knife blades impeller wasused to enhance mixing processes involving other impellers and alsoto improve fineness of the final product. For example, the processusing A100 was stopped after 4 min at 4000 rpm. At this time, themixed material presented partial encapsulation characteristics withmainly macroscopic particulates. The impeller was then replaced byknife blades and additional mixing time of 1 min at 1000 rpm wasperformed. Doing so, the quality of the final product was significantlyimproved and no trace of macroscopic particles was found anymore.Total energy delivered to the mixed material is decreased sincehorizontal shape of knife blades presents low resistance to the drivingof the impeller. Mixing time was reduced to 5 min in comparisonwithinitial 16 min using impeller A100 alone. These experiments provethat mixing process may be significantly optimized using appropriatemixing conditions. Powder formation requires high shear forces andvigorous stirring properties in the same time. Axial flow propellersallowing good stirring conditions but having low shearing propertieslengthening mixing time. On the other hand, radial flow propellercreating zones of high shear forces and good stirring conditionsthrough strong turbulent flow induces larger power consumption.Finally the most efficient impeller should be a polyvalent impeller

Fig. 4. ESEM observations of particulates obtained by blending 10

producing axial and radial flow in the same time, as for example thehelical propeller.

3.3. Optimization of high shear processes at larger scale

Initial quality of the encapsulated powder produced at laboratoryand pre-industrial scales using Stephan mixers was very poor due tothe presence of largemacroscopic particulates. Increasingmixing timeup to 10min brought no real benefit. To improve powder quality usinglaboratory scale process, mixing motor was doped to reach 4000 rpmso shearing and turbulent flow properties were increased. Concerningpre-industrial process, stirring properties were improved by verticallyshifting knife blades on the mixing shaft of approximately 0.05 m.With this configuration up to 20 kg of dry water was produced in lessthan 5 min. From these experiments and the experience we acquiredusing standardmixing vessel, wemay propose that an efficient mixingprocess would be fitted with a centrally located axial flow propellerand off-center high shear propellers usually called choppers.

It may also be noticed that mixing time is not necessary pro-portional to the amount of mixed material. Preparation of 100 g ofhomogenous dry water using commercial blender required a mini-mum of about 10 s whereas less than 5 min was enough to produce250 times more material using optimized pre-industrial mixer. Thisdifference may be explained by the enlargement of surfaces efficientfor powder encapsulation as for example surfaces of knife blade.Concerning mixing rate, it may be noticed that increasing size of themixer reduces requirements in terms of rotational speed. Velocity atthe tip of the propellers is approximately the same between blendingprocess and pre-industrial mixer which allows maintaining similarReynolds number as well as similar high shear rates.

3.4. Characteristics of low shear mixing processes

Different mixing conditions had to be tested to produce dry waterusing the Triaxe. First remark concerns mixing rate. In smaller tank,rotational mixing rate had to be set around 150 rpm, below whichwater encapsulation was incomplete and most of liquid phase re-mained in the bottom of the tank. In larger tank, rotational mixing ratehad to be decreased to approximately 100 rpm, below which waterencapsulation was incomplete whereas above which mixing processproduced a mousse. Assuming that water phase has to be atomizedover free silica particles available for water encapsulation, incompletepowder formation is attributed to poor stirring conditions, i.e. whenthe mixing rate is set too low, when the mixing blades are driven toofar from thewall of the tank orwhen the configuration of the propellerdo not deliver enough stirring (for example, four comb blades).

g of Aerosil® R972 with 90 g of water at 18,000 rpm for 10 s.

Fig. 5. ESEM observation of particulates containing 97% of water prepared with Aerosil® R972 in low shear mixing process.

268 L. Forny et al. / Powder Technology 189 (2009) 263–269

Concerning mousse formation, as previously suggested this phenom-ena is attributed to high shear forces that force low hydrophobic silicato form a continuous network involving water phase. In the biggesttank, larger diameter of the propeller induced zones of high shearforces at the tip of the blades that destabilized the system.

Second remark concerns the feeding pressure. At constant efficientmixing rate, low feeding pressure (below approximately 8 bars) wasnot able to produce dry water. We may suggest that a minimal kineticenergy is required to achieve water droplets encapsulation. Below thislimit, solid particles would not attach to the droplets.

Microscopic observations of particulates produced with R972 inthe Triaxe reveal typical dry water shell-like structure (Fig. 5). Shapeanalysis were performed and results were compared with dry waterhaving same 97% water ratio produced using R812S in the blenderrotating at 18000 rpm for 30 s. Particulates produced using low shearmixing processes aremuchmore spherical than particulates producedin high shear mixing processes, especially for increasing particlediameter. Average sphericity factor goes from 0.81 to 0.61 (Fig. 6). Thisdifference in powder characteristics indicates that mechanisms ofpowder formation are not the same.

Using cement mixer, bi-fluid spraying nozzle was inefficient. Airflow blew fluffy silica particles away from water droplets preventingthem from encapsulation. However, dry water was produced usingsingle fluid spraying nozzle. Spraying pressure as well as distancebetween spraying nozzle and mixed material was key parameter forsuccessful water encapsulation. For a distance of approximately 10 cm,spraying pressure had to be set around at least 8 bars. This observationconfirms previous remark about the kinetic energy of the droplets.

Fig. 6. Sphericity factor as a function of particle size diameters for dry water having 97% wmixing process (×).

3.5. Mechanisms of powder formation

All the encapsulation trials that were performed gave informationabout mechanisms of powder formation. Dry water particulates areformed by the auto-association of fumed silica particles aroundmicro-metric water droplets. Assuming that the role of the mixing process isto create these droplets and to assist their coating by solid particles, wemay propose the following interpretations.

For high shear mixing processes, vigorous stirring conditions splitbulk water into macroscopic droplets that are immediately isolatedfrom each other by spontaneous fumed silica particles coating. Thesemacroscopic droplets are then divided into microscopic particlesunder high shear forces. Average particles size is progressively de-creasing until most silica particles are involved in shell-like structureand stable microscopic size distribution is reached. Non sphericalshapes are attributed to mechanical entanglement between branchedsilica particles promoted by high shear forces. Increasing mixing time,changing mixing rate or decreasing water ratio has almost no in-fluence on size distributions, particles shapes or powder character-istics. This type of process is only applicable for highly hydrophobicparticles that may overcome strong high shear forces without any riskof mousse formation.

For low shear mixing processes, micrometric water droplets arepre-formed bymeans of an atomisation system. These droplets have tobe immediately coated by free silica particles to avoid any coalescencephenomena. Therefore surface of mixed material over which waterphase is atomized has to be continuously refreshed. Rotating bedmixers are especially well adapted because they do not need rapid

ater ratio produced using R972 in low shear mixing process (▲) or R812S in high shear

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stirring conditions which may create high shear forces capable ofdisturbing the system. However this condition is not sufficient and wenoticed that kinetic energy of the droplets or surface tension of theliquid play key roles in successful droplets coating. For example weproduced dry water using R812S in low shear mixing processes byadding 8% by weight of ethanol into the water phase which reducedsurface tension to approximately 50 mN m−1 at 20 °C. Also, the samesolution mixed with R812S in a high shear mixing process produced amousse. From these observations further investigations will beperformed to explain in details the way these parameters influencemechanisms of powder formation.

4. Conclusion

Mixing conditions required for water encapsulation using silicananoparticles were investigated in this study. In high shear processes,dry water is produced by progressive size reduction frommacroscopicto microscopic size distribution of water droplets coated with silicananoparticles. Strong stirring conditions initiate droplets formationwhile high shear characteristics perform size reduction. In low shearprocesses, same structure is obtained by spontaneous solid particlescoating of themicroscopicwater droplets produced by the atomisationsystem. This second mechanism appeared to be much more sensitiveto the operating conditions. Non-coating or in the contrary mousseformationphenomenawas frequently observed. Atomisationpressure,stirring conditions and distance between spraying nozzle and mixedmaterial had to be precisely controlled. Finally, dry water could beproduced at a pre-industrial scale using both mixing processes.

During this study we noticed that mechanisms of powder forma-tion highly depend on interfacial interactions between solid particlesand the liquid phase. Parameters such as particle's hydrophobicity,surface tension of the liquid or even kinetic energy at which particlesmeet the liquid surface have great influence. Theway these parametersinfluence powder formation will be discussed in a further study byevaluating the energy of adhesion of solid particles at water/air in-terface. We should then be able to explain more precisely the differ-ence of final product quality obtained with different fumed silicaparticles depending on their physico-chemical characteristics and theproperties of the mixing process.

Acknowledgements

This projectwasfinancially supported by the Pôle Régional Génie desProcédés (Région Picardie, France). ESEM experiments were performed

in the Service d'Analyses Physico-Chimiques (UTC, France) by FrédéricNadaud. Camsizer® was placed at disposal by Horiba-Jobin Yvon (Long-jumeau, France). We express our grateful thanks to James O'Donnell forits technical support.

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