Nucleation, growth and coalescence
phenomena of air bubbles on quartz particles
in different aqueous solutions
C. Oliveira*1, R. T. Rodrigues2 and J. Rubio2
Understanding the generation and behaviour of air bubbles in the presence of different reagents
is important in the solid/liquid separation processes used to treat mineral and water and
wastewater by flocculation–flotation. The presence of large air bubbles in aerated flocs, which
have higher up-rising rates, can lead to the development of higher capacity units. The present
work evaluated the generation and behaviour of air bubbles on hydrophilic particles of quartz in
water and other aqueous solutions (10 mg L21 920SH non-ionic polymer, 30 mg L21 DF250
surfactant and 50 mg L21 Flotigan EDA amine). The operating parameters were the air bubble
growth time, contact angle, adhesion, attachment and detachment. The main results showed that
the air bubbles grew more readily in the presence of non-ionic polymers and after adhesion in
non-ionic polymer and water. The formation of large bubbles in aerated flocs in an effort to
develop industrial flocculation–flotation operations with higher throughputs units is discussed.
Keywords: Air bubbles, Flocculation, Flotation, Aerated flocs, Air bubble nucleation
Introduction
Understanding the behaviour of air bubbles in aqueous
solutions is important in many areas, such as mineral
processing, water and wastewater treatment and the
processing of food and carbonated beverages (sparkling
wines, beers, soft drinks, etc.), due to the interactions
between air bubbles and solid particles, oil droplets and/
or other bubbles.1–6
In general, gas bubbles suspended in aqueous solutions acquire a surface charge, and the charge density
and sign depend on the chemical properties of the
solution. The main mechanisms involved may include
the adsorption of ions, dissociation of ionic groups,
residual surfactants and charge separation.7,8 For this
reason, the interactions among ions, molecules and
organic compounds and air bubbles and focused on
applications in many processes have been extensively
studied and reported.9–18
Much work has also been conducted to investigate the
features and behaviour of air bubbles in the presence of
reactants for mineral processing,19–27 with the aim of
improving the efficiencies of the flotation processes.
The nucleation, growth and coalescence of air bubbles
during the flocculation-flotation treatment of wastewater
has received special attention with respect to the
1
Unilasalle, Mestrado em Avaliação de Impactos Ambientas, Av. Vitor
Barreto, 2288, Canoas, Rio Grande do Sul, Brazil
Universidade Federal do Rio Grande do Sul, Programa de PósGraduação em Engenharia de Minas, Metalúrgica e de Materiais,
Laboratório de Tecnologia Mineral e Ambiental, Brazil
2
*Corresponding author, email [email protected]
ß 2014 W. S. Maney & Son Ltd
Received 26 December 2013; accepted 18 March 2014
DOI 10.1179/1758897914Y.0000000011
formation of aerated flocs or aeroflocs, which are
aggregates that show high up-rising rates in flotation
cells. These high rates are likely due to the high
probability of air microbubbles adhering to or being
entrapped by flocs as they grow into larger bubbles due to
mechanisms of nucleation, growth and coalescence.28–34
Jones et al.35 identified four main mechanisms of
nucleation: classical homogeneous nucleation (type I),
classical heterogeneous nucleation (type II), pseudoclassical nucleation (type III) and non-classical nucleation (type IV). Type I involves the nucleation of gas
inside a homogenous liquid solution, in the absence of
pre-existing gas cavities in the oversaturated system.
Therefore, the saturation level for nucleation is high.
Moreover, the bubbles produced rise to the surface of
the liquid, and new bubbles are not likely to form at the
same location as the first formation.
A type II system initially lacks pre-existing gas cavities
in the liquid or at the surface of the container. Usually,
this phenomenon also requires high levels of saturation.
In this case, the saturated system is suddenly transformed
into an oversaturated one (e.g. via the sudden reduction
of pressure during the generation of air microbubbles in
air dissolved flotation), which results in classical nucleation. In classical nucleation, the bubbles can form on a
flat surface (e.g. the nozzles), in a hollow on the walls of
the container or on the surfaces of particles in suspension.
After its formation, an air bubble can grow and detach
itself from a solid surface and rise to the surface of the
liquid. However, in this case the bubble leaves behind a
gas cavity (i.e. is not fully separated) that allows the
generation of new bubbles at this point (pseudo-classicaltype III or type IV-non-classical nucleation).
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Nucleation, growth and coalescence phenomena of air bubbles on quartz particles in different aqueous solutions
Pseudo-classical nucleation requires pre-existing gas
cavities (on the surface of the container, in the liquid or
at the particle surface in suspension), which establishes a
finite energy barrier that must be overcome. Conversely,
the non-classical nucleation mechanism (type IV) lacks
this energy barrier due to the pre-existing gas cavities
(generated by type II or III nucleation), which provide a
stable resource for the nucleation of bubbles leading to a
production cycle of bubbles that gradually ceases over
time as the supersaturation levels decrease.
The bubble is free to grow when the nucleation
process is complete, and the growth rate is influenced by
a number of factors, such as the rate of diffusion at the
interface of the bubble, inertia, viscosity and the surface
tension of the liquid.35 Although the factors that govern
the growth of bubbles have not been elucidated in the
literature, molecular diffusion is considered to be the
factor that eventually stops growth. Thus, a generally
accepted theoretical description of the growth of a
bubble requires equations of continuity, motion, conservation of species in diffusion and heat transfer.
Furthermore, two or more bubbles can approach each
other, touch and coalesce during growth, via breaking of
the film (lamella) between neighbouring (touching)
bubbles, which results in the formation of a single large
bubble.36
Because dissolved air flotation generates air microbubbles in the size range of 30–70 mm (Ref. 26), nucleation, growth and coalescence mechanisms are very
important to the generation of aerated flocs in flocculation–flotation processes. Specifically, these mechanisms
can significantly increase the number density and growth
rate of air bubbles via air diffusion and coalescence into
the structure of the flocs, which increases the air volume
of bubbles and so maximises bubble size and promotes
higher up-rising rates.31,32
Previous studies31 have demonstrated that large
bubbles resulted in flocs with high up-rising rates. In
addition, it has been shown14 that polymer additives
interact with the air bubble surfaces, leading to new
questions about the effects of these interactions on the
behaviour of air bubbles, especially for air microbubbles
formed in dissolved air flotation processes that may
evolve into large bubbles in the structure of flocs. Thus,
the present work was conducted to investigate the
formation and behaviour of air bubbles in the presence
of a non-ionic polymer that demonstrated a high
tendency to form aerated flocs with large bubbles.
Results obtained in the absence and presence of conventional flotation reactants (amine and surfactant) that
prevent the coalescence are compared to evaluate the
potential of this polymer to favour such mechanisms.
liquid phases. All solutions were prepared with deionised water. The non-ionic polymer was chosen because
it has shown better results in the formation of aerated
flocs.31 Furthermore, the amine and surfactant were
chosen to increase the hydrophobicity of the hydrophilic
surface and maintain the stability of the froth, respectively, because they have been widely used in mineral
processing and the methods are well established in the
literature.23,25,37 The surface tensions of the liquid
phases were measured in a tensiometer (Kruss model
8451), and the results are given in Table 1.
Methodology
Experimental apparatus
The experimental apparatus developed especially for
these studies essentially consisted of a system that
saturated water with air in a flat acrylic cell and a
stereomicroscope (Zeiss Stemi SV11) coupled to a digital
camera (Sony Mavica MVD500). The cross-section of the
flat cell was rectangular with the following approximate
dimensions: 1?5 cm deep, 28 cm high and 4 cm long. The
centre of this cell included an orifice attached to glass
tubing (0?1 cm in diameter and 7?5 cm long) containing a
quartz particle adhered at the end. Furthermore, the flat
cell contained two orifices designed for the inlet and outlet
flow of aqueous solutions, which allowed the liquid phase
to flow through to the particle. A white light source
provided illumination. A steel saturation vessel was
equipped with an internal container made of glass
(40 cm high, 10 cm in diameter and 0?5 cm thick; 0?7 L
effective capacity). Figure 1 shows the scheme and details
of the experimental rig.
Phenomena of nucleation, growth and coalescence of air
bubbles
To study the phenomena of the nucleation, growth and
coalescence of air bubbles, a volume of the solution
(containing one reagent) was inserted into a container of
aqueous solution (4, Fig. 1), and the flat cell was filmed.
A volume (0?5 L) of the same aqueous solution was
inserted into the glass container (0?7 L) of the saturator
vessel (2, Fig. 1), which received injections of filtered
compressed air for liquid saturation at 3 atm for 20 min.
The air injection was stopped subsequently stopped, and
a relief valve was opened to reduce the pressure of the
liquid to 1 atm. Thus, the supersaturated liquid was
transferred to the flat cell via the inlet point (5; Fig. 1).
As such, this liquid flow experienced a downward
trajectory within the flat cell, passing through the
particle, which was located in the centre of the cell (in
detail, Fig. 1), and emerging at the point (6, Fig. 1). This
procedure prevented air bubbles that formed due to the
flow of supersaturated liquid through the pipes from
reaching the quartz particle, which made it possible
exclusively to view the nucleation phenomena, i.e. the
emergence of air bubbles in gas cavities on the surfaces
of quartz particles. Furthermore, the saturator vessel
Experimental methods
Materials and reagents
Quartz (rock crystal) mineral particles were chosen due
to their high purity and because they were easily
obtained. These particles were carefully manually
broken in a Gral, and two particles (approximately
0?5 cm long) were chosen for the studies.
Water and aqueous solutions of 10 mg L21 non-ionic
polymer (920SH, Floerger), 30 mg L21 froth (Dowfroth
– DF250, Dow Chemical Company) and 50 mg L21
alquilamide (EDA Flotigam, Clariant) were used as the
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Table 1 Surface tension of liquid phases used in studies
NOS
Liquid phase
Surface tension/mN m–1
Water
Non-ionic polymer (10 mg L21)
Froth (30 mg L21)
Amine (50 mg L21)
72.7¡0.04
70.2¡0.18
57.9¡0.40
44.3¡0.38
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Oliveira et al.
Nucleation, growth and coalescence phenomena of air bubbles on quartz particles in different aqueous solutions
1 Scheme of experimental rig used in studies of nucleation, growth and coalescence of air bubbles on quartz particles
(in detail): (1) saturator vessel; (2) stereomicroscope coupled to digital camera; (3) flat cell; (4) container of aqueous
solutions; (5; 6) inlet and outlet flow; (7) white light source; (8) monitor for image reproduction
was located above the flat cell in this case to promote
drainage via gravity.
In these studies, two different quartz particles were
selected, and the experimental procedure described
above was conducted five times for each particle.
The quartz particles and materials were carefully
cleaned prior to all studies using an ultrasonic bath for
30 min with a solution of hydrochloric acid (10%HCl),
followed by immersion in a sulphocromic solution for
1 h and abundant rinsing with deionised water.
After the supersaturated liquid had flowed past the
particle, the resultant nucleation (emergence of air
bubbles in gas cavities on the surface of quartz particles)
(Fig. 2) was recorded on video (image resolution of
6406480 pixel) for a period of 5 min. The captured
videos were then divided in frames with intervals of 1 s.
All images were calibrated with a standard grid scale with
a 612 magnification. The imaging treatment was carried
out with Image Tool software electronic tools (version
3?0, UTHSCSA-University of Texas Health Science
Center in San Antonio, USA). The bubble diameter was
measured, and the values obtained from the five studies
were plotted as a function of time (according to each
frame analysed), which yielded a variety of times to
represent the trend in the growth of air bubbles. The trend
line was adjusted using a linear regression to represent the
2 Photomicrographs showing progress of captured images to study nucleation, growth and coalescence phenomena of
air bubbles on quartz particles in liquid phase (water without reagent)
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Oliveira and Rubio.14 The layer of adsorbed polymer is
believed to encourage the migration of dissolved gas to the
gas/liquid (bubble/solution) interface since the presence of
hydrophobic groups eases the mass transfer for the gas
phase (air bubble). The equations of the trend lines (Fig. 3)
for these conditions were as follows:
DF250 surfactant y~2:6432xz82:235 (R2 ~0:9761)
920SH non
ionic polymer y~2:458xz68:545
(R2 ~0:9999)
water y~1:9946xz81:325 (R2 ~1)
3 Diameter of air bubbles nucleated on quartz particle surfaces immersed in different liquid phases as function of
time: [polymer]: 10 mg L21; [surfactant]: 30 mg L21;
[amine]: 50 mg L21.
Flotigan EDA amine y~1:8052xz6:6302 (R2 ~0:9922):
In addition, the data showed that the bubble size
difference increased slightly as a function of time (as
reflected in the coefficients for the different solutions). For
example, the air bubble was 281 mm in size in water and
317 mm in size in the polymer solution after 100 s of
growth; after 200 s of growth, the sizes were 482 and
565 mm respectively. A similar effect was observed for the
polymer and amine solution, in which the bubble sizes were
212 and 381 mm after 100 and 200 s of growth respectively.
Likewise, surfactant and amine molecules can hydrophobically adsorb on the surfaces of the air bubbles,
which again results in a layer of adsorbed molecules at the
liquid/gas interface. However, the high hydrophobicity of
amines may result in the formation of a thick layer on the
bubble surface; this generates a type of film (barrier) that
hinders the passage of air to the gas/liquid interface and
slows growth of the bubbles over time. For example, this
phenomenon promotes an approximate 1?5-fold difference in bubble size compared with nucleation and growth
in water.
Although the presence of surfactant molecules significantly reduces the surface tension of the liquid, the
results showed that this parameter did not influence the
rate of growth of air bubbles in the presence of the
surfactant DF250. However, this result requires further
examination because the presence of DF250 surfactant
molecules clearly reduced the emergence of air bubbles at
gas cavities on the surface, which prevented the collection
of a large amount of data that was available for the other
liquid phases (water, polymer and amine). This phenomenon may be better investigated using other types and
concentrations of surfactants following optimisations of
the apparatus to enable longer study times. In addition,
complementary studies to evaluate the bubble detachment times in water (4?5 min) and in surfactant solution
(2?5 min) showed that bubble formation by detachment
was favoured in the presence of the reagent, which in
effect represents a decrease in the time required for bubble
nucleation. This phenomenon may be associated with the
significant reduction in the surface tension of the liquid
combined with the hydrophilicity of the quartz surface.
For these conditions, the affinity between the bubble and
particle is low and the physical properties of the liquid
promote air transport, which decreases the time of
attachment of the air bubble on the particle surface.
Figure 4 shows comparative images of the growth and
detachment of air bubbles from quartz particles in water
and in the DF250 surfactant solution (note the different
timescales in the two images on the right).
dispersion of the values. In addition, the bubble detachment times in water and DF250 surfactant solution were
determined to complement the data of bubble growth
times for these conditions.
Additionally, some of the captured images were
selected and subsequently analysed using the Drop
Shape Analysis software (Kruss, v1?7101 DSA3) to
determine the contact angle between the air bubble,
quartz particle surface and liquid phases.
Phenomena of adhesion and growth of air bubbles
The adhesion and growth behaviour of air microbubbles
on quartz particle was studied in order to simulate the
potential conditions for the growth and coalescence of
bubbles as suggested in studies of the generation of
aerated flocs.14,28,30–32
In this case, a flow of air microbubbles was generated
via the depressurisation of saturated water (at 3 atm) in
an orifice plate (2 mm in diameter) located inside the
output valve of the saturator vessel. These air bubbles
were injected into the inlet point (6, Fig. 1) of the flat cell,
allowing them to rise and throng the central region where
the quartz particle was located. Thus, the adhesion and
growth phenomena of bubbles on the particle surface
were observed. Videos were recorded using the same
settings described above, but with 69?6 magnification. A
qualitative and comparative imaging analysis was performed using the liquid phases.
Results and discussion
Nucleation, growth and coalescence of air
bubbles
The present studies have observed and recorded air bubble
nucleation on hydrophilic particles of quartz in the presence
of various liquid phases. Plots of the diameters of air
bubbles nucleated on quartz surfaces immersed in water
and aqueous solutions (non-ionic polymer, amide and
surfactant) as a function of time are shown in Fig. 3.
It can be seen that the air bubbles were more likely to
grow in the presence of a polymer solution than in the
absence of this reactant (in water), indicating that
microbubbles from dissolved air flotation may be encouraged to grow and coalesce by such additions, to generate
large bubbles such as those found in aerated flocs, as has
been described previously.30,31 This trend may be due to
the effects of adsorption of the polymeric macromolecules
on the surfaces of the air bubbles, as demonstrated by
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Nucleation, growth and coalescence phenomena of air bubbles on quartz particles in different aqueous solutions
4 Photomicrographs showing nucleation onto quartz surface followed by detachment of air bubbles: a, b water, c, d
30 mg L21 DF250 surfactant solution
It was also possible to observe the important phenomenon of the coalescence of air bubbles. The bubbles
gained mass during the nucleation (i.e. before detachment), and thus increased in diameter, which increased
the probability of bubbles approaching each other and
coalescing on the particle. The combination of these two
phenomena (nucleation and coalescence) is important
and may result in the generation of large bubbles within
the structures of aerated flocs, which promotes high uprising rates for these aggregates. Figures 5 and 6 show
images captured during the nucleation–coalescence in the
absence and presence of non-ionic polymer.
5 Photomicrographs showing nucleation phenomena followed by coalescence of air bubbles onto quartz particle
immersed in water (without reagent)
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Nucleation, growth and coalescence phenomena of air bubbles on quartz particles in different aqueous solutions
6 Photomicrographs showing nucleation phenomena followed by coalescence of air bubbles onto quartz particle
immersed in 10 mg L21 920SH non-ionic polymer solution
angle between these three phases (Fig. 7). These results
clearly showed that the contact angle (10u) was lowest
between the air bubble (hydrophobic gaseous phase) and
the hydrophilic particles in water because of the absence
The measurements of contact angle between the air
bubble, quartz particle surface and liquid phase indicated
that molecules adsorbed from solution onto the particle
surface and/or air bubbles tended to decrease the contact
7 Photomicrographs showing contact angles between air bubbles, quartz particle surface and liquid phases. [920SH
non-ionic polymer]: 10 mg L21; [DF250 surfactant]: 30 mg L21; [amine]: 50 mg L21
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Nucleation, growth and coalescence phenomena of air bubbles on quartz particles in different aqueous solutions
8 Photomicrographs showing adhesion, growth and coalescence of air bubbles on quartz surfaces immersed in water
(without reagent)
of absorption on the particle surface. The contact angle
slightly increased in the presence of surfactant and
polymer molecules, probably due to the adsorption of
these molecules, whose hydrophobic groups affect to
some extent the hydrophilicity of the solid phase. This
effect was most pronounced when the liquid phase
contained amine molecules, which are highly hydrophobic and show a high adsorption capacity on silicate
surfaces. This high adsorption increases the hydrophobicity of those surfaces which had affinity for the gaseous
phase (air bubble), resulting in a significant increase in the
contact angle (47u) between the three phases.
Phenomena of adhesion and growth of air
microbubbles
The studies of the adhesion, followed by growth, of the
air microbubbles on the quartz particle showed a
dependence of the behaviour of the bubbles on the liquid
phase that agreed with the results presented in Fig. 1.
The images showed that the growth of air bubbles was
more pronounced when the liquids were water and nonionic polymeric solution. This finding corresponds with
the finding that coalescence readily followed nucleation in
both of these liquid phases. This behaviour of the air
bubbles is attributed to the high values of surface tension
of these liquids (72?7 and 70?2 mN m21). Figures 8 and 9
show adhesion followed by rapid growth of the air
microbubbles as a result of the nucleation–coalescence
phenomena.
In contrast, when the liquid phase consisted of DF250
surfactant solution, growth was slightly less rapid, but the
amount of coalescence significantly decreased and it was
often difficult to observe coalescence at all (Fig. 10). This
observation is attributed to the decrease in the surface
tension (57?9 mN m21) due to the presence of the
surfactant molecules. This result is in agreement with
several previous studies,38–40 which have shown that the
presence of reagents adsorbed at the air/liquid interface
significantly influenced the coalescence of bubbles by
modifying the nucleation and coalescence rates. In this
context, Laskowski et al.40 showed that froth (surfactants)
reduced the size of the bubbles and prevented their
coalescence. Thus, an increase in the concentration of these
reagents reduced the degree of coalescence until a critical
concentration of coalescence (CCC) point was reached, at
which the mechanism was completely eliminated.
The same surfactant effect was observed for the amine
solution, but growth was significantly reduced. This
agrees with the trends reported above and is tentatively
attributed to the formation of a layer of adsorbed
hydrophobic molecules on the surface of air bubbles
that hinders gas mass transfer at the gas/liquid interface.
Figure 11 clearly shows these effects: comparison with
results for the other solutions (water, polymer and
surfactant) indicates the reduced growth of air bubbles
on the particle surface.
The behaviour of air bubbles in the presence or absence
of these reactants may help to explain the formation of
aerated flocs observed and outlined in previous research
studies and patents, and in the operation of technologies
to promote high throughput in solid–liquid separation
operations via flocculation–flotation. Future studies are
required to fully elucidate the interactions between air
bubbles and hydrosoluble polymers, which may appear to
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Nucleation, growth and coalescence phenomena of air bubbles on quartz particles in different aqueous solutions
9 Photomicrographs showing adhesion, growth and coalescence of air bubbles on quartz surfaces immersed in
10 mg L21 920SH non-ionic polymer solution
10 Photomicrographs showing adhesion, light growth and coalescence of air bubbles on quartz surfaces immersed in
30 mg L21 DF250 surfactant solution
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Nucleation, growth and coalescence phenomena of air bubbles on quartz particles in different aqueous solutions
11 Photomicrographs showing adhesion and absence of growth and coalescence of air bubbles on quartz surfaces
immersed in a 50 mg L21 Flotigan EDA amine solution
improve rapid solid/liquid separation via advanced
flocculation–flotation processes. Thus, understanding
the behaviour of air bubbles in the presence of various
polymers should advance the study of the generation of
aerated flocs that have great potential in several fields,
including mining and metallurgical and physico–chemical
separation operations. Examples of these applications
include the use of specific polymers to remove with high
efficiency adsorbed pollutants on carrier flocs; the solid/
liquid separations of slimes that are difficult to flocculate,
such as clays; the separation of organic and hydrometallurgical solvents (emulsified); rapid separations via the
flotation of rich proteins in dispersions to improve oil
enhanced production for the reuse of water in thickener
overflows and treatment of water for reuse in filtered
mining pulps.
Conclusion
which allowed their growth and coalescence. However,
this transfer was reduced in the presence of highly
hydrophobic amine molecules, possibly due to the
formation of an adsorbed molecule layer (barrier), which
reduced the growth rates of air bubbles after nucleation.
Similarly, the coalescence of bubbles was suppressed in
the presence of the DF250 surfactant solution and amine
but was favoured in the 920SH non-ionic polymer, which
indicated that large bubbles were very likely to form in the
structures of aerated flocs to possibly influence the high
up-rising rates of these aggregates.
The present study contributes to a series of publications on the formation of aerated flocs, providing new
demonstrations and agreements to explain this important phenomenon, which promotes many improvements
in the efficiencies of several processes, mainly in the
flocculation–flotation units.
Acknowledgements
The present studies of the nucleation of air bubbles on
quartz particle surfaces have shown the growth of
bubbles to depend on the degree of hydrophilicity or
hydrophobicity of the interface, which can be modified
by the absence or presence of different reagents (920SH
non-ionic polymer, DF250 surfactant and Flotigan
EDA amine molecules).
Dynamic contact angle data at the air bubble/solid/
liquid interfaces showed increases in the values for
polymer, surfactant and amine solutions (14, 24 and 47u
respectively). Furthermore, the results indicated that the
adsorption of polymeric macromolecules at the air/liquid
interface facilitate gaseous mass transfer to the air bubble,
The authors would like to thank the Clariant, Dow
Chemical Company and Floerger corporations for
technical information and for providing the reagents
samples. The authors would also like to thank all of our
colleagues at the LTM and the Universidade Federal do
Rio Grande do Sul; FAPERGS; CNPq; Capes; Finep
and all institutions supporting research in Brazil.
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