Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 6.2.4 - Effect of liquid properties and solid material on bubble-wall attachment
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Permanent Link: http://ufdc.ufl.edu/UF00102023/00150
 Material Information
Title: 6.2.4 - Effect of liquid properties and solid material on bubble-wall attachment Particle Bubble and Drop Dynamics
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Fujasová-Zedníková, M.
Vobecká, L.
Vejrazka, J.
Ruzicka, M.C.
Drahoš, J.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: bubble-wall collision
surfactant
bubble attachment
hydrodynamics
flotation
 Notes
Abstract: The contribution deals with the experimental study of bubble interactions with an immersed horizontal solid wall. The effect of surfactant concentration (limited to a specific nonionic surfactant, terpineol) and the effect of solid wall material is investigated. The adsorption dynamics of terpineol surfactant is studied and adsorption parameters are evaluated on the base of Langmuir isotherm. To study the effect of solid material, the cleaned glass is used as a hydrophilic surface and polyethylene (PE), polypropylene (PP) and Teflon are used as the surfaces with different degree of hydrophobicity. Two particular stages of interactions (collision/bouncing and attachment process) are observed by the means of high-speed camera. The hydrodynamics of bubble-wall collisions is not affected by the material of solids while it is strongly affected by the presence of surfactant. Even the small addition of the terpineol to the system, which does not change the liquid physical-chemical properties, significantly affects the bubble hydrodynamics, suppresses the rebound from the wall and the bubble deformations. The bubble attachment to the solid surface depends on both the liquid properties and the solid material. No bubble attachment is found in the case of hydrophilic (glass) surface while in the case of hydrophobic (plastics) surfaces, the time required for bubble attachment to the solid surface varies with bubble size, surfactant concentration and solid materials. For example the proper concentration of terpineol in the liquid significantly prolongs the attachment time of bubbles to the PP surface while the bubbles attach to the PE and Teflon surfaces more easily. This knowledge helps to understand the bubble attachment process and to determine the selectivity and the efficiency of plastics flotation.
General Note: The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows
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Bibliographic ID: UF00102023
Volume ID: VID00150
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: 624-Fujasova-Zednikova-ICMF2010.pdf

Full Text

7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010



Effect of liquid properties and solid material on bubble-wall attachment

Maria Fujasova-Zednikova, Lucie Vobecka, Jiri Vejrazka, Marek C. Ruzicka, Jii DrahoS

Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic
Rozvojova 2/135, Prague 6, 165 02, Czech Republic
zednikova@icpf.cas.cz


Keywords: bubble-wall collision, surfactant, bubble attachment, hydrodynamics, flotation




Abstract

The contribution deals with the experimental study of bubble interactions with an immersed horizontal solid wall. The effect
of surfactant concentration (limited to a specific nonionic surfactant, terpineol) and the effect of solid wall material is
investigated. The adsorption dynamics of terpineol surfactant is studied and adsorption parameters are evaluated on the base
of Langmuir isotherm. To study the effect of solid material, the cleaned glass is used as a hydrophilic surface and polyethylene
(PE), polypropylene (PP) and Teflon are used as the surfaces with different degree of hydrophobicity. Two particular stages of
interactions (collision/bouncing and attachment process) are observed by the means of high-speed camera. The
hydrodynamics of bubble-wall collisions is not affected by the material of solids while it is strongly affected by the presence
of surfactant. Even the small addition of the terpineol to the system, which does not change the liquid physical-chemical
properties, significantly affects the bubble hydrodynamics, suppresses the rebound from the wall and the bubble deformations.
The bubble attachment to the solid surface depends on both the liquid properties and the solid material. No bubble attachment
is found in the case of hydrophilic (glass) surface while in the case of hydrophobic (plastics) surfaces, the time required for
bubble attachment to the solid surface varies with bubble size, surfactant concentration and solid materials. For example the
proper concentration of terpineol in the liquid significantly prolongs the attachment time of bubbles to the PP surface while the
bubbles attach to the PE and Teflon surfaces more easily. This knowledge helps to understand the bubble attachment process
and to determine the selectivity and the efficiency of plastics flotation.


Introduction

The gas-liquid-solid three-phase systems are found in
many industrial applications. An important one is the
separation of solid materials by flotation. This process is
based on the ability of some solids to remain attached to the
bubble surface. Particles of such a solid then create
agglomerates with bubbles and float to the liquid surface,
from which they can be easily separated. The flotation was
originally used for the separation of coal or mineral ore
from mined ore deposits. Owing to its simplicity and high
efficacy, the flotation is nowadays also used for separation
of oil sands, print inks in paper-recycling, waste water
treatment and for separation of various plastic materials in
their recycling process (Ityokumbul, 1999). At present, the
last process becomes relevant because of its importance for
plastics recycling. While the mechanism of bubble-particle
attachment in the case of mineral flotation is well described
(Dai, 2000; Nguyen & Schulze, 2004), the research of
plastics flotation began in 1970s and its mechanism is not
yet completely understood.
Alter, (2005) and Shen et al., (1999) reported the most
significant differences between the minerals and plastics
from the point of view of the principles of the flotation
process: (i) in the standard flotation, the particles are usually
much smaller than the bubbles, while in the case of plastics


flotation; the particles are of bigger size. (ii) the inorganic
minerals form particles with high-energy surface, whereas
plastics have low-energy surfaces. Inverted size proportions
and different surface properties of solids change the
mechanism of bubble-particle interaction. As the theoretical
models for predicting the flotation efficiency (e.g. Dai,
2000; Nguyen & Schulze, 2004) were developed for the
standard flotation, they cannot be used in the case of plastic
flotation. This need defines the reason of our investigations,
which is to improve the knowledge about the bubble-
particle interaction mechanism.
Over recent years, many experimental studies dealing
with the plastics flotation are available (e.g. Drelich, 1999;
Shen et al., 2001; Shen et al., 2002; Basafova et al., 2005;
Reddy et al., 2008; Burat et al., 2009, some other works are
reviewed by Shen et al., 1999). However, the works provide
mostly experimental results on the flotation yield as a
function of various parameters, but not a fundamental study
of interaction mechanics.
Several papers study the behaviour of a fluid particle
(bubble or drop) colliding with a horizontal surface and
provide the simplified models describing the hydrodynamic
aspect of the interactions (Tsao & Koch, 1997; Klaseboer et
al., 2001; Okumura et al., 2003; Legendre et al., 2005;
Biance et al., 2006; Zenit & Legendre, 2009). Tsao & Koch,
(1997) observe air bubbles in deionised water bouncing on a









Plexiglas wall experimentally and provide the energy
balance of bubble before and after the contact with the wall.
Klaseboer et al., (2001) present the simplified model of drop
approaching the hydrophilic surface. The model is based on
the balance of forces acting on the bubble and the no-slip
boundary condition at the drop interface is assumed. The
toluene drop in deionised water bouncing on the hydrophilic
wall was studied also by Legendre et al., (2005). The
authors present the model analogous to the damped
mechanical oscillator, which provide the prediction of
contact time of the drop with the wall and the restitution
coefficient (ratio of impact and rebound velocity). Okumura
et al., (2 "11) and Biance et al., (2006) studied the low
viscosity drop in air bouncing on hydrophobic surface.
Zenit & Legendre, (2009) observe the air bubbles in water
and glycerol solutions bouncing on glass and Plexiglass
wall and provide the scaling of the restitution coefficient.
The particular cases of the collision of bubbles with
various surfaces and in different liquids is experimentally
studied by Malysa et al., (2005); Krasowska & Malysa,
(2007a); Krasowska & Malysa, (2007b); Krasowska et al.,
(2 1. ') (all the papers are from the same research group).
These works provide the detailed qualitative description of
bubble collision and attachment processes and high-quality
reasoning of possible mechanisms. However, quantitative
characterisation of the processes is missing.
The aim of the present work is to study the effect of
surfactant concentration and the effect of solid surface
material on the two particular stages of the bubble-wall
interactions: (i) the first bubble bounce/collision with the
wall (including the bubble approach, first contact with the
wall and the possible rebound), (ii) the bubble attachment to
the surface (formation of the three-phase contact line). The
study is limited to a specific surfactant (terpineol), which is
a nonionic surfactant often used as a frothing agent in
flotation (Basafova et al., 2005). The adsorption dynamics
of terpineol surfactant is also investigated in order to find
static and dynamic parameters describing the dependence of
surface tension on the terpineol concentration. The
considered materials of the solid are cleaned glass as a
hydrophilic surface and polypropylene (PP), polyethylene
(PE) and Teflon as hydrophobic surfaces.


Nomenclature

c terpineol concentration (mol L-1)
CAM added mass coefficient (-)
D surfactant diffusivity (m2s-1)
Db equivalent bubble diameter (m)
KL adsorption equilibrium constant for the
Langmuir isotherm (m3 mol'1)
R gas constant (J mol-'K 1)
Rb bubble radius (m)
t time (s)
t, adhesion time (s)
tc impact instant (s)
tr rebound instant (s)
tTpc instant of three-phase-contact line formation (s)
T temperature (K)
U instantaneous bubble velocity (m s-')
Uc bubble impact velocity (m s-')
U, bubble rebound velocity (m s-')


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

Ut bubble terminal velocity without the presence
of the wall (m s-')
y vertical bubble position (m)

Greek letters
p fitting parameters (-)
e restitution coefficient = UJUt(-)
r surfactant concentration at air/water interface
per unit area (mol m 2)
Fm maximum surface concentration of the surfactant
per unit area (mol m 2)
Pu liquid viscosity (Pa.s)
p liquid density (kg m-3)
0- surface tension (N m-1)
0-o surface tension of pure water (N m-1)

Dimensionless numbers
Ca capillary number = Up / a
Re Reynolds number = UtD&p /I
St modified Stokes number = UtDbCAp / 9,p


Experimental Facility

The measurements of bubble-wall interaction are
carried out in a rectangular cell of 110x110x260 mm in size
(the experimental setup is illustrated in Figure 1). In the cell
bottom, there is placed a device with a moveable capillary,
which enables the "on demand" production of a single
bubble with defined size (described in detail by Vejrazka et
al., 2008). The capillary of inner diameter 250 gm is able to
produce the bubbles with the size ranging from 0.4 to 1.9
mm. The bubbles impinge on an immersed horizontal solid
surface located 20 mm above the capillary tip.


Figure 1. Experimental setup.


To study the surfactant effect on bubble-wall
interactions, the four liquids are used: deionised water
(distilled and additionally filtered through activated carbon
and ion-exchange resin, final conductivity 1.1 pS/cm) and
the three aqueous solutions of terpineol with the
concentrations of 0.1x10-3, 0.3x103 and 1.0x103 mol/L.
The adsorption parameters are measured for the terpienol
concentration ranging from 1.3 x10-8 to 1.4 xl0-2 mol/L.
The surface tension under static conditions are
measured on the tensiometer Krtiss Kll using the du
Noiiying ring method. The measurement of surface tension
on the age of the bubble interface is carried out using the









bubble tensiometer Kriss BP100. The surface tension is
determined from the pressure inside the bubble. Viscosity is
measured by Ubbelohde viscosimeter and density by
balancing immersed solid body of known volume (Kriss
Kll). During the liquid properties measurements, the
temperature was kept on the value of 20C. The properties
of the liquids used for measurements of bubble-wall
interactions are given in Table 1.

Table 1. Physical properties of the solutions used for the
measurements of bubble-wall interactions
Sol n deionised terpineol terpineol terpineol
water 0.1 mM 0.3 mM 1.0 mM

density 998 998 998 998
(kgm-3)
viscosity
viscosity 1.002 1.000 1.000 1.000
(mPa.s)
surface
tension 0.0722 0.0716 0.0701 0.0643
(N m-l1)

The glass cleaned by sulfochromic mixture,
polypropylene (PP), polyethylene (PE) and Teflon are used
as the solid materials. The surface wettability is
characterized by the three-phase contact angle, which is
evaluated from the image of a liquid drop laying on the
particular surface (Figure 2). The contact angle is
recognized from the drop reflection in the solid surface and
the measured values are given in Table 2.


water glass


water Teflon


-1.0



-1.1



-1.2


;ample of contact angle evaluation.


Table 2. Contact angles for considered liquids and solids

Solid Contact angle (o)
material water terpineol terpineol terpineol
0.1 mM 0.3 mM 1.0 mM

glass 53 53 54 54
PP 88 91 86 81

PE 91 85 89 81

Teflon 103 100 94 92

The bubble-wall interactions are recorded by a
high-speed camera Photron Fastcam SA1.1 with a
macroscopic lens (5400 fps, 1024x1024 pix, 0.0098
mm/pix). The sequence of images is treated in order to get
the bubble boundary. Assuming the bubble axial symmetry,
the centre-of-mass position, volume and surface area of the
bubble are evaluated for each image. The bubble size Db is
equivalent to the diameter of a sphere with the same volume.
The instantaneous bubble velocity is obtained by
differentiating the centre-of-mass position obtained from
each image. An example of evolution of the bubble's


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

centre-of-mass position and velocity is shown in Figure 3
(zero at y/Rb axis represents the position of solid surface and
negative sign indicates the bubble under the wall). The
bubble terminal velocity Ut, characterising the steady state
bubble rise velocity without the presence of solid surface, is
determined by averaging the velocity of the bubble far
before touching the wall.
Figure 3 demonstrates also the determination of bubble
impact and rebound instants. They are defined as the
moments when the distance between the centre-of-mass of
the bubble and the surface is equal to the equivalent bubble
radius Rb (in Fig. 3, it is depicted by the points where the
normalized bubble position blue circles crosses the
horizontal line with ordinate -1.0). Nevertheless, it should
be noted that the impact and rebound instants cannot be
defined exactly, as the bubble and the wall are always
separated by a liquid film (and hence no real contact exist)
until its attachment. The impact and rebound velocities, Uc
and U,, are found as velocities corresponding to the impact
and rebound moments (the curve with red diamonds at the
time t, and t, in Fig. 3). Different definitions of these
instants would lead to the different values of Uc and U,.
From the recorded movies, the instant of
three-phase-contact line formation is also evaluated (tpc). It
occurs long time after disappearance of all the visible
bubble movement and it is revealed by a sudden increase of
the bubble surface area. The adhesion time (the time needed
bubble to attach to the solid surface) is than defined as
t, = tTc tc.


-1.3 1 T -I -1
0.00 0.01 tc 0.02 0.03
t[s]
Figure 3. Time dependence of normalized bubble position
and velocity. Explanation of impact and rebound moments
evaluation. Deionised water, Db = 1.05 mm.


Results and Discussion

Adsorption of terpineol at the air/water interface
The surfactant added to the water has the ability to
adsorb to the air/water interface and changes its properties.
The amount of surfactant adsorbed per unit area of
liquid-gas interface (T) can be determined indirectly from
the surface tension measurements. A plot of surface tension
as a function of surfactant concentration in liquid phase is
generally used to describe the adsorption equilibrium at the
interface (Rosen, 1978).
The surfactant used in this work, terpineol, is a
non-ionic surfactant often used as a frothing agent in the
flotation process. Basaiova et al., (2005) and DeWitt &










Makens, (1932) provided the experimental data of surface
tension of terpineol solutions in a small range of
concentrations; however no adsorption parameters were
evaluated. Figure 4 gives the surface tension in dependence
on terpineol concentration in the wide range, which can be
used for the evaluation of adsorption equilibrium. Using the
Gibbs adsorption equation and proper shape of isotherm
F(c), the surface tension as a function of terpineol bulk
concentration can be derived (Rosen, 1978; Chang &
Franses, 1995). The most commonly used non-linear
isotherm is the Langmuir isotherm
KLc
S=F KLc (1)
1+KLc
and the corresponding function a(c) is obtained in the from
(Chang & Franses, 1995):
r a = RTF, In( + KLc) (2)
where ao is the surface tension of pure water, FT is the
maximum surface concentration and KL is the Langmuir
equilibrium adsorption constant. The best fit of
experimental data in Figure 4 gives the adsorption
equilibrium parameters for terpineol solutions
m = 4.68x10-6 mol m2 and KL = 1.23 m3 mol1.
The knowledge of surfactant adsorption equilibrium is
important, however not sufficient for description of
dynamic surface tension behaviour, when a new interface is
created. As the adsorption mechanism is complex, the


80


70


E 60
Z
E
50
b

40


10-8 10-7 10-s 10-5 10-4 10-
c [mol L3]
Figure 4. Surface tension in dependence
concentration.


10-2 10-1


on terpineol


-2.96ms -2.22ms -1.48ms -0.74ms t= Oms 0.74ms






deionised water


4- W-


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

available adsorption models assume various simplifications
to be able to evaluate the dynamic adsorption parameters. In
this work, we tried to roughly estimate the surfactant
diffusivity in the bulk solution using the approach of
short-time and long-time approximation. The detailed
description of the evaluation technique is available in
Chang & Franses, (1995) (see the paragraph 5.2
"Approximate approaches" in their paper). The technique
assumes the diffusion-controlled adsorption model with the
Henry isotherm for short-time approximation and with the
Langmuir isotherm for long-time approximation. The
estimated terpineol diffusivity is D=4.51x10 "1 ms.
Using the value of diffusivity with the mass balance
equations and Langmuir isotherm, it can be evaluated the
dynamic surface tension as a function of the age of
gas-liquid interface. Figure 5 gives the comparison of this
function with the experimental data for three chosen
terpineol concentrations. The significant difference between
the calculated and experimental data indicates that it is not
sufficient to describe the adsorption of terpineol to the
gas-liquid interface by diffusion-controlled model and other
sophisticated model is necessary to use. We plan to continue
our investigation in this line in the future.


E 65
z
E
60
b


55 F


S I 111111 1|1 1111 experimental data I
111111 111111 calculated data
I I I 111111
-- f ----T-r -IfTtIl----1 -T T1t113
I I 1 I I I I
S I I l 11111 i m 1 1111 1 I
--__i-? ' ^ - 1 - i-i -- -i- 4 i-i ---i-a-i-a-L-l4
l ,11, ,1111
IN 1 11111111 1 111111 1 11111
L i r lll :) I l ll** *""*"* "T '
I I I I I I l I II I I I II I I
1OSS JI IIII II 111111 111 1111111
I 11111 lll111 i i 1 i i ll11 1
I 1111 11111 I 1111

1 111111 1 111111 1 111111 1 11111


50
10-2 10-1 100 101 102
surface age [s]
Figure 5. Dynamic surface tension in dependence on the
surface age. Red circles: c = 0.662x10-3 mol/L; Blue circles:
c = 1.775x103 mol/L; Green circles: c = 3.952x10-3 mol/L.
Calculated data assumes diffusion-controlled adsorption
with the terpineol diffusivity D = 4.51x10 1 ms.


1.48ms 2.22ms 2.96ms
fr -- et-et.-


f l f l:II-IJ=* .--- = -


terpineol 1.0x10-3 mol/L

Figure 6. Sequence of bubble-wall bouncing in deionised water and in most concentrated a-terpineol solution. Bubble size is
Db = 1.05 mm.


-i-Trnmr-Fi-rImmrr- l mrr-Trrtm t r-r nmi-t-Trrrm

-444414414- I 14-- 44- 4-4444h-44- 1mu-- 4 444I444 4--.14W



0 experimental data 111111 I 1111111 I III II111
I 1111111 angm uir 111111 11111 1 I 11111111 II 111111 1 I 11111
II 1111 I 4.68x 1 1111111 11111o I 1111111 I 1111111 I 111111
11111111 1111111 11111111 11111111 I 11111111 I 11111111 I 111111
I l fl I 11111111 1 IIIIIIII IU I I 1l111111 11111111 111111
- 1111111 1 11111111 1 11111
I 11111111 I I111111 11111111 11111111 I II I 11111111 I 111111
I11111111 I 1111111 11111111 11111111 II 1 I111111111 I111111
I 11111111 I I 1111111I 11111111 11111111 I Ill I 1111111 111111
- 11 I I IIIII I I I I IIIII I I II III
11111111 1111111 11111111 4 11111111 11111111 --I IJ II 111111
SIIIIIII = 4.68x10 1 mol m 111111 1 I1111111 I1IIIIII I IIIII

IIIIII 11111111 I PIII I IIIIIII


I IIIII3 I I I I IIII I I I I IIIII I I I I III
I I III I I I I I II I I I I III I I I I









Effect of a-terpineol and solid material on the
bubble-wall bouncing
The addition of terpineol to the system decreases the
bubble rise velocity, limits the bubble deformation and
suppresses the rebound from the wall. Figure 6 shows a
record of the collision in deionised water and in the most
concentrated terpineol solution. In pure water, the bubble
deforms from its initial shape before touching the wall.
When the bubble deformation is maximal (t = 0), the bubble
is almost motionless. The bubble then visibly rebounds
from the wall to a distance larger than its diameter. Several
rebounds are observed in deionised water and their number
depends on the bubble size. Contrary in the case of terpineol
solution (1.0x10-' mol/L), no bubble rebound is observed
(Fig. 6).
The normalized impact velocities in terpineol solutions
and in deionised water are compared in Figure 7. No
significant difference between data for water and terpineol
solutions is observed for smaller bubbles of spherical shape
(Re < 180). For bigger bubbles (Re > 180), the normalized
impact velocity in surfactant solutions is systematically
lower by about 10% than in deionised water, but the
difference is probably an artefact of the rather arbitrary
definition of the impact combined with a different bubble
shape. Figure 7 gives also the predictions of impact velocity
for deformable fluid particle with free-slip condition at the
particle interface (Fujasova-Zednikova et al., 2009) and for
spherical fluid particle with no-slip condition at the
interface (Legendre et al., 2005). The difference between
the two curves could roughly approximate the possible
difference between the data for mobile and immobile
interface. The similarity between the data obtained for
terpineol and the curve developed by Fujasova-Zednikova
et al., (2' ') indicates that the liquid flow around the bubble
displaces the surfactant molecules to the rear of the bubble,
and the interface at the bubble front remains mobile even in
the a-terpineol solutions. This observation is consistent with
the concept of stagnant cap model (Savic, 1953; Sadhal &
Johnson, 1983; Cuenot et al., 1997; also reviewed in Cuenot
et al., 1997), which assumes a change of free-slip boundary
condition at the bubble front to a no-slip boundary condition
at the bubble rear.


0.8

0.6 -

0.4

0.2


0 100 200 300 400 500 600
Re [-]
Figure 7. Normalized bubble impact velocity in
dependence on Reynolds number.

While the normalized impact velocity is insensitive to
the presence of a-terpineol in liquid, the rebound is strongly
affected by the surfactant. Figure 8 shows the restitution


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

coefficient (defined as the ratio of rebound velocity and
bubble terminal velocity, e = Ur / U). It is observed that e
decreases with increasing terpineol concentration, and again
no rebound is observed in the case of most concentrated
solution regardless on bubble size (zero values of restitution
coefficient). This suggests that in surfactant solutions, a
large portion of energy is dissipated by different processes
than in pure liquids. The possible mechanism of additional
energy loss can be associated with the bubble shape
deformation during the collision, which induces the
adsorption/desorption of surfactant on the bubble. The
additional energy dissipation can be caused by either the
surface viscosity of the interface covered by surfactant or by
the Marangoni stresses around the deforming bubble.


1.0

0.8

0o.6

0.4

0.2

0.0


Ca / st [-]
Figure 8. Restitution coefficient in dependence on ratio
Ca / St for various solid materials and concentrations of
a-terpineol. Abbreviation Z&L (2'" "'' corresponds to Zenit
and Legendre, (2i 1 'r.


The experimental data of restitution coefficient are well
described by an expression obtained by Zenit & Legendre,
2009
S=exp(-fCa/St) (3)
where / is an empirical parameter. The authors determined
the value / = 30 from the best fit of their experimental data
(air bubbles in water and glycerine solutions bouncing on
the glass and Plexiglass 1 A.i! Comparison of our
experimental data with the equation (3) gives good
agreement for deionised water only. The influence of
terpineol presence is not captured by this equation and the
parameter / varies with the surfactant concentration. This
indicates that the ratio of Capillary and modified Stokes
number is not the sufficient term describing the bubble
collision process and the knowledge of other properties
seems necessary for correct description of bouncing. This
should include parameters characterizing the kinetics of
surfactant adsorption and desorption, and/or the apparent
interfacial viscosity.
The effect of solid material on the bubble bouncing
process is apparent from the Figure 8. In pure water, the
rebound does not depend on the material of the wall and
only a slight dependence is found in terpineol solutions.
This confirms the expectation that while the bubble remains
separated from the wall by a liquid film, it does not feel the
character of the solid surface.


e I






a-terpineol 0.1x10 molIL
S a-terpineol 0.3xlO mollL
I I
- l--f-------- a-terpineol 1.0x10 mollL
I 1 - Legendre et al. (2005)
/ -- Fujasova-Zednikova et al. (2009)






7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


-6.10ms t= Oms 0.74ms 1.48ms 2.22ms 2.96ms 3.70ms


IV..lqyrw


12.4ms


II


Figure 9. Sequence of bubble attachment process to the solid surface. PE surface, deionised water, Db = 1.05 mm.


0.10


0.05


u.uu
0.6
0.6


1.0 1.2
Db [mm]


0.20 A
-A-pp C) A
---- PEI I
0.15 --- Teflon _____---- ---



Am



0.00I I I
I I






0.6 0.8 1.0 1.2 1.4 1.6 1.8
..... II-..... --- ---...-------- ...---





Db [mm]
Figure 10. Adhesion time in dependence on bubble size for
0.3x103 mol/L, d) a-terpineol 1.Ox10 mol/L.


Effect of solid material and a-terpineol on the
bubble-wall attachment
After disappearance of all the visible bubble movement,
the bubble remains suspended below the wall and preserves
it spherical shape for a while. In the case of hydrophobic
surfaces, the bubble then attaches suddenly to the solid and
a three-phase contact line is formed. An example of a
bubble attachment process to the PE surface is illustrated in
Figure 9.
Before attachment, the liquid in the film separating the
bubble from the wall is squeezed. When the film becomes
very thin, its drainage rate starts to be controlled by the
intermolecular forces acting between the molecules of the
solid and liquid (see e.g. the concept of disjoining pressure,
Oron et al., 1997; de Gennes et al., 2004). In the case of
hydrophilic surfaces, these forces stabilize the film and the
bubble hence never attaches to the solid. Oppositely in the
case of hydrophobic surfaces, the film becomes unstable
and when its thickness is below a critical value, the
intermolecular force breaks it and the bubble attaches to the


0.10


0.05


0.8 1.0


0.8 1.0


0.10


0.05


u.uu
0.6


1.2 1.4 1.6 1.8
Db [mm]


1.2
Db [mm]


1.4 1.6 1.8


a) deionised water, b) a-terpineol 0. 1xl0- mol/L, c) a-terpineol




solid. In this work, the adhesion time t, is evaluated and it
characterises an indirect measure of the critical film
thickness.
It is observed that the bubble attaches only to the
hydrophobic surfaces (PP, PE, and Teflon). No bubble
attachment is observed in the case of bubble colliding with
the cleaned glass. The attachment process is affected by
both the surface material and the terpineol concentration.
Figures 10a-d show the adhesion time of bubble at
hydrophobic surfaces in all solutions used. In the case of PE
and Teflon, the adhesion time decreases with increasing
terpineol concentration (bubble attaches to these surfaces
more easily). Oppositely, t, values significantly increase in
the case of PP surface. The difference of t, values between
the PP surface and PE or Teflon surfaces increases also with
increasing concentration of terpineol. Such a results help
with the choice of conditions for froth flotation of PP, PE
and Teflon mixtures, where the PE and Teflon will be
preferably floatable while the PP particles will remain in the
vessel.


-A-- pp a)
-+- PE
- I- Teflon I

-------- -----~-----~---,^^-A--- --< -

- - - - - 4 ,--- - f ^-, ) +
- - - -t 47-


V t


u.uu00
1.4 1.6 1.8 0.6


--- PP b)
-------- PE-----
- I- Teflon I__ _ _ I__ _ _



-- .---.-----
I I




i i '


A l
I I I I
,/^

-- ,---- ---- ------ ---- -- ---




"l I" -- ------ l


I A A









As it could be concluded, the presence of terpineol and
the solid material strongly affect the bubble attachment to
the surface. It is remarkable that the attachment cannot be
characterized by the contact angle of the particular
three-phase system (Tab. 2). For example, the contact
angles are similar in the cases of PP and PE surface, while
the adhesion time differs significantly. This indicates that
the adhesion time can be another parameter characterising
the surface wettability. In addition, it could give more
information about the ability of bubbles to attach to the solid
surface than contact angle.

Conclusions

The paper presents the experimental study of the
interaction of bubbles with an immersed horizontal solid
surface. The investigation is focused on two particular
processes: the first bubble-wall collision and the bubble
attachment to the wall. The effect of the solid material and
the presence of specific surfactant (a-terpineol) are
investigated. The adsorption dynamics of terpineol
surfactant is studied and adsorption parameters for
Langmuir isotherm are evaluated.
The presence of terpineol in water decreases the bubble
velocity and also suppresses its rebound from the surface.
No bubble rebound is observed in the Ixl-3 mol/L solution
of terpineol. Data of the impact velocity indicates that the
bubble nose has mobile interface and the rebound
suppression is hence not caused by the modification of the
drainage of liquid film, which separates the bubble from the
solid surface. The data for restitution coefficient suggests
that if the surfactant is present, a large portion of energy is
dissipated during the rebound by different processes than
the ones occurring in pure liquids. The possible mechanism
of additional energy loss can be associated with the
adsorption/desorption of surfactant on the bubble interface
induced by the bubble deformation during the collision.
No significant effect of the solid surface material on the
collision process is observed. This confirms the expectation
that while the bubble remains separated from the wall by a
wetting film, it does not feel the character of the solid
surface.
The attachment of the bubble to the solid surface is
observed only in the case of hydrophobic surfaces (PP, PE,
Teflon). The adhesion time (time required for the
three-phase contact line formation) depends both on the
solid material and on the terpineol concentration. It is found
that in the presence of terpineol, the bubbles attach easily to
PE and Teflon surfaces while the trend is opposite in the
case of PP surface. Remarkably, the adhesion time cannot be
correlated with the contact angle of a particular three-phase
system because no proved relation between these quantities
is found. Increased knowledge about adhesion time
contributes to the better description of the attachment
mechanism and it can also be used to optimize operating
conditions in selective flotation of plastics.

Acknowledgements

The financial support of the Grant Agency of the
Academy of Sciences and the Grant Agency of the Czech
Republic is gratefully acknowledged. The project GAAV
No. KJB200720801 supported all the measurements of


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

collision and attachment process. The measurements of
density, viscosity and surface tension were made using the
equipment acquired in the frame of projects GAAV No.
IAA200720801 and GACR No. 104/07/1110.

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