FORMATION OF SPHERICAL AGGREGATE FROM MICODROPLET CONTAINING

SUBMICRON INCLUSIONS

Mariusz WOŹNIAK*, Justice ARCHER, Gennadiy DERKACHOV, Daniel JAKUBCZYK,Tomasz WOJCIECHOWSKI, Krystyna KOLWAS, Maciej KOLWAS

Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland

*Corresponding author: mwozniak@ifpan.edu.pl

AbstractWe have investigated formation of ordered aggregates

of spherical nanosilica particles in evaporating droplets ofaqueous colloidal suspension. Droplets have been freelysuspended in the electrodynamic quadrupole trap. Slowdrying process and the successive states of the aggregateformation have been investigated using static light scatter-ing method. Modification made to the typical configura-tion of the electrodynamic quadrupole trap enables us todeposit the final (and the semi-final) structures and toinvestigate them with SEM methods.

1 IntroductionEvaporation process on a flat surface leading to forma-

tion of drop-deposited nanoparticle films and self-assembled monolayers of nano-crystals have been recentlywidely observed and discussed in the literature [1]. Bymeans of the aggregation phenomenon it is possible tocreate composites whose physical properties are deter-mined not only by their chemical composition but also bythe specific morphology. This new kind of structures,known as metamaterials, exhibit unique (optical) proper-ties not present in conventional materials.

However, much less is known about aggregation proc-ess in systems with spherical geometry such as dryingmicrodroplets of suspension (e.g. [2]). Few studies con-cerning shell-structured or densely-packed aggregates ofspheres appear in the literature (e.g. [3]) and even fewerhave reported as regular, as dense or as large aggregatesthat we can observe and model [4,5]. Similar self-assembled structures, but generated by means of a fastspray drying method, were investigated in our formercollaborative work [6].

In contrast to evaporation of a film of suspension lead-ing to nanoparticle films, evaporation process of levitatedmicrodroplets is unaffected by flat bulk surface. This par-ticular geometry enables production of truly 3D photonicmetamaterials and quasi-photonic crystals with sphericalor quasi-spherical symmetry. In order to produce anddiagnose this kind of objects it is necessary to build highlyspecialized equipment. These requirements are satisfied byelectrodynamic traps with climatic chambers and addi-tional control instruments built in our laboratory. To ana-lyse and measure liquid microdroplets and particles ag-gregating within them we use static light scattering meth-

ods. Our modifications and improvements made to thetypically used geometry of the electrodynamic quadrupoletrap gives us a possibility to deposit aggregates one-by-one on a flat silicon substrate and to investigate them addi-tionally with SEM.

2 Evaporation process and surface phenomenaAggregation in a micodroplet of suspension is caused

by evaporation process of liquid phase. The droplet evapo-ration process is driven by mass and heat transportthrough the droplet surface. A convenient expressions ofthe evaporation dynamics of a spherical droplet of pureliquid can be found [5,7]:

( ) ( )cc cc a a

cc a

da p T p Ta aa Ddt T T

(1)

where a is a droplet radius, pa and pcc are the vapourpressure near the droplet surface and far from the dropletrespectively, Ta and Tcc are the temperatures of the dropletsurface and the reservoir. D is the diffusion constant ofvapour in the ambient gas and /M R , where Mand are the molecular weight and density respectively, Ris the universal gas constant.

The surface activity Δσ, describing the change of sur-face tension with respect to the surface tension of a pureliquid σw can be expressed as:

0ln 12a

w as a

aa aa taT TDp T

(2)

It is worth noticing that using Eq. (2) we can find thetemporal evolution of the surface pressure Δσ(t) from thetemporal evolution of the droplet radius a(t).

3 ExperimentOur experimental setup consists of a quadrupole elec-

trodynamic trap with 4 linear electrodes in a verticalalignment. The electrodes provide alternating (AC) electricfield in quadruple configuration. Separate annular elec-trodes around the vertical ones provide static (DC) fieldbalancing the particles weight. The trap is kept in a smallthermostatic chamber, which allows us to control parame-ters of the internal atmosphere by choosing ambient gasand adjusting temperature (from -40 up to +50°C). Figure 1shows schematic drawing of the experimental setup. Forgraphic purposes climatic chamber is not shown.

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Figure 1. Schematic drawing of the experimental setup.

Droplets are injected by on-demand injector andcharged by charge separation in the external field. Twocoaxial, counter propagating lasers are used simultane-ously for droplet illumination: green (532 nm) p-polarizedand red (654 nm). The entirely defocused image is used forscatterometry. The focused image is used for droplet posi-tion stabilization with a PID type loop. The temporal evo-lution of the droplets is obtained with the Mie ScatteringLookup Table Method [8]. It is based on the fitting of thecomplete Mie theory predictions (stored in the lookuptable) to the experimentally obtained scattering patterns.Our method provides accuracy of ±10 nm [8].

Additionally, it is worth noticing that vertical alignmentof the quadrupole trap allows us to progressively reduceDC field without changes of the AC field constrainingdroplet (aggregate) horizontal position and thus soft-landing of the aggregating structure on the silicon sub-strate at the bottom part of the trap. It is also possible toshift the substrate inside the trap up to the stabilizationpoint. Deposition allows us to further analyse aggregateswith microscopic methods (SEM, TEM, AFM).

4 Results and discussionThe experiment was conducted at the temperature of

288.2 K, the initial humidity of 94% and the atmosphericpressure of 1006 hPa. We used aqueous suspension of SiO2

spheres with diameter of 450 nm (MP-4540, Nissan Chemi-cal Industries). The suspension obtained from the manu-facturers was diluted with ultrapure water (Milli-Q Plus,Millipore) in 250:1 proportion resulting in the initial vol-ume concentration of nanoparticles ∼0.1%. Stabilizingagent introduced by the manufacturer was not removed.

Figure 2 shows experimental evolution of the dropletradius a t obtained with the Mie Scattering Lookup TableMethod as well as the numerical results of our evaporationmodel for pure water and water with solid inclusions(fraction of ∼0.1%).

0 2 4 6 8 10 12 14 16 18 200

2

4

6

8

10

12

Rad

ius

[m

]

Time [s]

Droplet evolution: Experimental results

Numerical model of: pure water water with solid inclusions

Figure 2. Evolution of the droplet of aqueous suspension of 450nm silica spheres (experimental and numerical results) andnumerical results for droplet containing pure water.

Using Eq. (2) we can express the surface pressure as afunction of the droplet radius .a Figure 3 shows a together with visualisation of the selected stages of

our simulation of the evolution of an evaporating dropletof nanosilica suspension (droplet size scaled for clarity) [4].

Figure 3. Surface pressure isotherm together with visualisationsof the selected stages of our simulations [4] of the evolution of adroplet of suspension (droplet size scaled freely for clarity).

We have observed and also modelled different thermo-dynamic states of the surface layer of inclusions, occurringduring evaporation of liquid with silica inclusions. Theyhave been separated in Figure 3 with vertical, doted lines.

For relatively large droplet (a > 7.9 μm) when the frac-tion of inclusions is smaller than ~3%, there is a surface gasof inclusions on the droplet surface (Figure 3 (a)). Subse-quently gas of inclusions is becoming denser (Figure 3 (b)).For 4.25 > a > 5.75 μm we observe the liquid-expandedphase (Figure 3 (c)). The surface is covered with large butdilute aggregates. Further evaporation of water com-presses the surface structures while the number of inclu-

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sions on the surface is further increasing. Additionally, thisstage was investigated with SEM after deposition of thedroplet with aggregating structure on the silicon surface(Figure 4 (a)). Due to the significant fraction of water de-posited structure was not stable and disintegrated formingthin layer of silica nanoparticles.

Figure 4. SEM images of the deposited structures: (a) middlestage of the formation, (b) semi-final and (c) final (totally dried)stage of the aggregation/evaporation

For a = ~4.25 μm a phase transition to liquid-condensed is possibly observed. Further compressionleads to the surface liquid-solid transition for a = ~3.55 μm(Figure 3 (e)). This state can be possibly seen in Figure 4(b). It can be inferred from careful observation of the SEMimage that at the time of the deposition this structure wasstill containing a significant fraction of water, which totally

dried out after landing. Therefore, we can see circulartraces around the solid aggregate left by evaporating liq-uid. It can be noticed, that at this stage of aggregation, awell-organized structure, foreseen by our numericalmodel, appears on the aggregate surface. Subsequently,when the evaporation of the droplet continues, after therapid growth of the surface pressure, a collapse of thesurface layer takes place for a = ~3.45 μm (Figure 3 (f)). It isfollowed by the formation of a multilayered surface solidassociated with the significant decrease in the surfacepressure. The SEM image of the final structure observedafter the deposition can be seen in Figure 4 (c). A transitionfrom spherical symmetry to less regular structure can beidentified by comparing Figure 4 (b) with Figure 4 (c).

5 ConclusionWe have shown that analysing the evolution of the

droplet radius only, it is possible to derive enough infor-mation to determine surface states of inclusions of evapo-rating droplet of colloidal suspension. Our unique experi-mental setup gives us a possibility to deposit the aggregat-ing structure at the selected stage of the evolution andanalyse it with microscopic methods. Therefore, we wereable to compare our numerical models of evaporation andaggregation with experimental results obtained with staticlight scattering method and SEM analysis. We found verygood consistency between them.

However, application of SEM to the analysis of dy-namic processes is limited. Hence, we were able to observeonly to the final and the semi-final structures of dryingaggregates because the previous stages were not stableenough and they disintegrated after deposition. Moreover,further evaporation on the surface of the substrate alteredthe evolution path.

Future perspectives for our research includes detailedconsideration of the successive aggregation stages, utiliza-tion of various nanoparticles, as well as application ofadditional methods to “freeze” the evolution of the dropletat the earlier stages (e.g. by adding UV-active polymers).

6 References[1] Rabani E., Reichman D. R., Geissler P. L., Brus L. E., Drying-mediated self-assembly of nanoparticles, Nature, 426: 271-274 (2003).

[2] Lee S. Y., Gradon L., Janeczko S., Iskandar F., Okuyama K., Forma-tion of highly ordered nanostructures by drying micrometer colloidaldroplets, ACS Nano, 4 (8): 4717–4724 (2010).

[3] Lauga E.. Brenner M. P., Evaporation-driven assembly of colloidalparticles, Phys. Rev. Lett., 93: 238301 (2004).

[4] Derkachov G., Kolwas K., Jakubczyk D., Zientara M., Kolwas M.,Drying of a Microdroplet of Water Suspension of Nanoparticles: fromSurface Aggregates to Microcrystal, J. Phys. Chem. C, 112: 16919-16923(2008).

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[5] Jakubczyk D., Kolwas M., Derkachov G., Kolwas K., Surface statesof micro-droplet of suspension, J. Phys. Chem. C, 113 (24): 10598-10602(2009).

[6] Onofri F.R.A., Barbosa S., Toure O., Woźniak M., Grisolia C., Sizinghighly-ordered buckyball-shaped aggregates of colloidal nano-particlesby light extinction spectroscopy, J. Quant. Spectrosc. Radiat. Transfer,126: 160–168 (2013).

[7] Pruppacher H., Klett J., Microphysics of Clouds and Precipitation(Kluwer, The Netherlands, 1997).

[8] Jakubczyk D., Derkachov G., Kolwas M., Kolwas K., Combiningweighting and scatterometry: application to a levitated droplet ofsuspension. J. Quant. Spectrosc. Radiat. Transfer, 126: 99-104 (2013).

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