biomimetics
Communication
Mimicking Dolphins to Produce Ring Bubbles
in Water
Philippe Lesage 1,2 , Mohammed Kemiha 1,3 , Souhila Poncin 1 , Noël Midoux 1,† and Huai Z. Li 1, *
1
2
3
*
†
Laboratoire Réactions et Génie des Procédés (CNRS, UMR 7274), Université de Lorraine, 1 rue Grandville,
BP 20451, 54001 Nancy Cedex, France; [email protected]
Current address: Solvay, 39500 Tavaux Cedex, France; [email protected]
Current address: Centre for European Studies, Jagiellonian University, 31131 Krakow, Poland;
[email protected]
Correspondence: [email protected]; Tel.: +33-3-83-17-51-09
Noël Midoux sadly passed away before this article was published.
Academic Editor: Josep Samitier
Received: 27 May 2016; Accepted: 30 August 2016; Published: 7 September 2016
Abstract: Several studies report that dolphins, either captive or wild, can expel air from their
blowhole to form ring bubbles. By means of an experimental setup consisting of an orifice coupled to
a computer-controlled solenoid valve to simulate the dolphin’s blowhole and a vessel as the lungs,
we examined the formation mechanism of a ring bubble under varying experimental conditions.
With a better record than the most talented dolphin, we show that two aspects were demonstrated as
essential to the successful generation of a ring bubble in water: the valve’s opening duration, and the
pressure inside the vessel. The present findings suggest that during ring bubble production, dolphins
are likely to anticipate their action by both adjusting a suitable air pressure inside their lungs and
controlling their muscular flap for an adequate opening timing of their blowhole. This could provide
some evidence in favour of suggested cetaceans’ self-control capacities.
Keywords: ring bubble; generation mechanisms; dolphin; mimicking; hydrodynamics
1. Introduction
Air-breathing cetaceans always generate bubbles when expelling air in both solitary and social
contexts. In particular, dolphins can release air from the blowhole, and the evacuated air rises to
the free surface in a ring-shaped bubble [1–4]. The production of stable ring bubbles is a typical
object manipulation behavior. There is some indication [5] in favour of a learning procedure to form
ring bubbles. During ring bubble production, mothers attentively observe infants’ primary attempts
at ring bubble generation and produce ring bubbles in their turn to provide an example for their
infants. Thus, the quality of generated ring bubbles by infants is progressively improved, thanks both
to this trial-and-error practice and the example from their mothers. This is a clear manifestation of
cognitive capacities to some extent [6–8]. Reiss [9] proposed that, although expelling air in varied
shapes through the blowhole, dolphins seem to have good control of both the shape and timing of the
generation of ring bubbles. More recently, Ridgway et al. [8] similarly proposed that beluga whales
(Delphinapterus leucas) mimicked human sounds by changing the nasal traction pressure and making
suitable muscular adjustment of the vibrating phonic lips while over-inflating their vestibular sacs.
Previously, ring bubbles were observed by Walters and Davidson [10] in the course of their
experimental investigation into the development of an initially spherical bubble of air in water.
Other studies have only considered the rise of a ring bubble in the fluid dynamics literature [11,12].
The present work aims at understanding the fundamental mechanism of ring bubble generation and
the corresponding levels of physical control mechanisms.
Biomimetics 2016, 1, 6; doi:10.3390/biomimetics1010006
www.mdpi.com/journal/biomimetics
2. Materials and Methods
The respiratory system of a dolphin is mainly composed of the blowhole and lungs. Unlike other
mammals who breathe through their nostrils and mouth, dolphins breathe through the blowhole that
Biomimetics
1, 6top of their head. The dolphin must contract the muscular flap to open the blowhole
2 of 7
is
centred2016,
at the
that is naturally closed. In our mimicking experiments (Figure 1a), ring bubble formation was
performed
in and
a transparent
2. Materials
Methods Plexiglas tank of square cross section (0.20 m × 0.20 m) and 1.0 m high.
The employed fluid was demineralized water at constant temperature (293 K). A 1 × 10−3 m3
The respiratory
system
dolphinthe
is mainly
composed
the blowhole
and lungs.
Unlike
other
pressurized
vessel was
usedof
to asimulate
dolphin's
lungs. Aofpressure
transducer
(Lucas,
TX, USA)
mammals
who
breathe
through
their
nostrils
and
mouth,
dolphins
breathe
through
the
blowhole
that
is
with an accuracy of 50 Pa indicated the air pressure inside the vessel. To mimic the blowhole, air
centredgeneration
at the top ofwas
their
head.
The dolphin
must contract
themade
muscular
flap to open
the blowhole
that
bubble
done
through
a submerged
cylinder
of a polyvinyl
chloride
(PVC) pipe
is
naturally
closed.
In
our
mimicking
experiments
(Figure
1a),
ring
bubble
formation
was
performed
with an inner diameter of 2 mm, a wall thickness of 2 mm, and a length of 50 mm, at the bottom
in a transparent
Plexiglas
square
cross section (0.20 m ×
0.20 m) and 1.0valve
m high.
The employed
section.
The orifice
had atank
flatofexit.
A computer-controlled
micro-solenoid
of high
precision
−
3
3
fluid
was
demineralized
water
at
constant
temperature
(293
K).
A
1
×
10
m
pressurized
vessel
was
(Sirai, Bussero, Italy), mounted between the vessel and the orifice to mimic the dolphin’s muscular
used
to
simulate
the
dolphin’s
lungs.
A
pressure
transducer
(Lucas,
TX,
USA)
with
an
accuracy
of
action, permitted bubbles to be injected with a desired opening duration [13]. Different opening
50
Pa
indicated
the
air
pressure
inside
the
vessel.
To
mimic
the
blowhole,
air
bubble
generation
was
duration and vessel pressures produced different bubble volumes. Cylinders with other inner
done through
a submerged
made
of a polyvinyl
(PVC)smaller
pipe with
inner diameter
of
diameters
were
also tested:cylinder
1 mm gave
similar
results aschloride
2 mm, with
ringanbubbles;
however,
2
mm,
a
wall
thickness
of
2
mm,
and
a
length
of
50
mm,
at
the
bottom
section.
The
orifice
had
a
flat
exit.
the gravitational backflow of water in greater diameters tended to produce stable ring bubbles.
A computer-controlled micro-solenoid valve of high precision (Sirai, Bussero, Italy), mounted between
the vessel
and the orifice
mimicPhantom
the dolphin’s
action,
permitted
bubbles (Visions
to be injected
with
A high-speed
video to
camera
v711 muscular
ranging from
20 to
10,000 frames/s
Research,
a
desired
opening
duration
[13].
Different
opening
duration
and
vessel
pressures
produced
different
Del Mar, CA, USA) was used to visualize the formation of a ring bubble at the orifice. Flow velocity
bubblearound
volumes.
Cylinders
otherwere
innerquantified
diametersusing
were particle
also tested:
1 mm
gave similar
results
as
fields
a ring
bubble with
in liquid
image
velocimetry
(PIV,
Dantec
2
mm,
with
smaller
ring
bubbles;
however,
the
gravitational
backflow
of
water
in
greater
diameters
Dynamics, Denmark). Laser illumination sheets were generated by means of two pulsed
tended to produce stable
ringaluminium
bubbles.
neodymium-doped
yttrium
garnet (Nd:YAG) lasers (Dantec Dynamics, Skovlunde,
A
high-speed
video
camera
Phantom
v711 the
ranging
from
20 to
frames/s
Research,
Denmark) arranged side-by-side and crossing
vertical
axis
of 10,000
the ring
bubble.(Visions
Unlike the
highDel
Mar,
CA,
USA)
was
used
to
visualize
the
formation
of
a
ring
bubble
at
the
orifice.
Flow
speed camera, the PIV device had a limited measure rate of 25 frames per second. Fluorescent
velocity fields
around
ring(Dantec
bubble Dynamics)
in liquid were
using
particle
imageparticles.
velocimetry
Rhodamine
B beads
of 5a µm
werequantified
added to the
liquid
as seeding
An
(PIV,
Dantec
Dynamics,
Denmark).
Laser
illumination
sheets
were
generated
by
means
of
two
pulsed
orange filter placed in front of the camera avoided the reflection of the lasers on the bubbles and
neodymium-doped
yttrium
aluminiumlight
garnet
(Nd:YAG)
(Dantecplaced
Dynamics,
Skovlunde,
allowed
only the passage
of fluorescent
of the
particles.lasers
The camera,
orthogonal
to the
Denmark)
and crossing
the
vertical
axispeak
of theofring
bubble.
Unlikeintensity.
the high-speed
laser
sheet,arranged
took twoside-by-side
successive images,
each at
the
intensity
the laser
impulse
These
camera,
the
PIV
device
had
a
limited
measure
rate
of
25
frames
per
second.
Fluorescent
Rhodamine
images were divided into several thousand small interrogation areas of 32 × 32 pixels in size. A crossB beads of 5was
µmthen
(Dantec
Dynamics)
were
to the liquid
as seedingareas.
particles.
orange filter
correlation
carried
out on the
twoadded
corresponding
interrogation
This An
correlation
then
placed
in
front
of
the
camera
avoided
the
reflection
of
the
lasers
on
the
bubbles
and
allowed
only
the
computed both the liquid flow field and the rise velocity of the ring bubble.
passage of fluorescent light of the particles. The camera, placed orthogonal to the laser sheet, took
two successive
eachsequence
at the intensity
peak of theof
laser
impulse
These
images were
The typical,images,
complete
of the generation
a stable
ringintensity.
bubble in
water—from
the
divided
into
several
thousand
small
interrogation
areas
of
32
×
32
pixels
in
size.
A
cross-correlation
formation of a ring bubble near the injection orifice to its progressive rise towards the free surface—
was
then in
carried
out
theVideo
two corresponding
areas.
This
correlation
then computed
is
shown
Figure
1bon
and
S1. Not havinginterrogation
the same scale
as the
dolphin’s
respiratory
system,
bothexperimental
the liquid flow
fieldaims
andmainly
the riseatvelocity
of the
ring
bubble. mechanism of ring bubbles.
our
setup
mimicking
the
generation
(a)
(b)
t=2160 ms
t=1080 ms
t=840 ms
t=600 ms
t=240 ms
Water
t=120 ms
P
To PC
Air
t= 0 ms
Vessel
Figure 1. (a) Experimental setup to mimic the generation of ring bubbles by dolphins. (b) A complete
sequence of the generation and rise of a stable ring bubble in water (relative pressure of 0.28 bar
and opening duration of 0.030 s). The dolphin’s respiratory system reprinted [14]. P, pressure;
PC, personal computer.
Biomimetics 2016, 1, 6
Biomimetics 2016, 1, 6
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3 of 7
The typical,
complete setup
sequence
of the
thegeneration
generation
of abubbles
stableby
ring
bubble
water—from
Figure
1. (a) Experimental
to mimic
of ring
dolphins.
(b)in
A complete
the formation
ring bubble
nearof the
injection
orifice
its (relative
progressive
riseoftowards
the free
sequence of of
theageneration
and rise
a stable
ring bubble
in to
water
pressure
0.28 bar and
openingshown
duration
of 0.0301bs).and
TheVideo
dolphin’s
respiratory
system
reprinted
P, pressure;
PC,
surface—is
in Figure
S1. Not
having the
same
scale as[14].
the dolphin’s
respiratory
personal
computer.
system,
our experimental
setup aims mainly at mimicking the generation mechanism of ring bubbles.
3. Results
Results
3.
The generation
a ring
bubble
is clearly
shown shown
by the high-speed
camera recordings
The
generationmechanism
mechanismof of
a ring
bubble
is clearly
by the high-speed
camera
with a frontwith
viewa(Figure
2a), during
its2a),
transient
formation
regime
and a topregime
view (Figure
the
recordings
front view
(Figure
during
its transient
formation
and a2b),
topfor
view
rise period
reaching
a stable
ring
shape. In
particular,
key In
moment
in ring
formation
(Figure
2b), after
for the
rise period
after
reaching
a stable
ring the
shape.
particular,
thebubble
key moment
in
is the
rupture
of the central
air coreof
due
a liquid
asdue
illustrated
by the
PIV
flow measurement
ring
bubble
formation
is the rupture
thetocentral
air jet
core
to a liquid
jet as
illustrated
by the PIV
(Figure
3). In fact, on
detaching
from
the bubble
moves
withthe
highbubble
inertia,moves
but after
passing
flow
measurement
(Figure
3). In
fact,the
onorifice,
detaching
from the
orifice,
with
high
throughbut
a short
the bubble
front slows
down
to thefront
viscous
drag
of the
water.
The
liquid
inertia,
afterdistance,
passing through
a short
distance,
thedue
bubble
slows
down
due
to the
viscous
inertia
to the The
sudden
injection
a gas
bubble
caninjection
form a central
the
drag
of due
the water.
liquid
inertia of
due
to the
sudden
of a gastongue
bubblethat
canpushes
form a into
central
bubble
from
below.
When
the
bottom
interface
impinges
on
the
upper
one,
the
bubble’s
morphology
tongue that pushes into the bubble from below. When the bottom interface impinges on the upper
changes
to form amorphology
ring, causedchanges
by the penetration
of the
liquid
In addition,
vortex
sheet
one,
the bubble’s
to form a ring,
caused
by tongue.
the penetration
of thethe
liquid
tongue.
which
expands
at the sheet
edgeswhich
has the
same sense
circular
motion
the of
central
liquid
tongue
to
In
addition,
the vortex
expands
at the of
edges
has the
sameas
sense
circular
motion
as the
facilitate
the ring
formation.
central
liquid
tongue
to facilitate the ring formation.
(a)
t = 0 ms
t = 26.32 ms
t = 42.10 ms
Ring formation
t = 57.89 ms
t = 89.47 ms
t = 163.16 ms
(b)
t = 300 ms
t = 400 ms
t = 550 ms
t = 800 ms
t = 1050 ms
Figure
Figure 2.
2. (a)
(a) Front
Front view:
view: temporary
temporarysequence
sequenceof
ofthe
thesuccessful
successfulformation
formationof
ofaaring
ring bubble
bubble recorded
recorded
with
high-speed
camera
at
950
frames/s
with
relative
pressure
of
0.28
bar
and
opening
duration
with high-speed camera at 950 frames/s with relative pressure of 0.28 bar and opening durationof
of
0.030
initial stage
stage (cf.
(cf. time
timescale
scaleindicated)
indicated)before
beforereaching
reaching
a stable
ring;
Top
view
of
0.030 ss during
during its
its initial
a stable
ring;
(b)(b)
Top
view
of the
the
bubble
during
its rise
after
having
reached
its stable
shape.
ringring
bubble
during
its rise
after
having
reached
its stable
ringring
shape.
McCowan et al. [5] proposed a four-point scale to assess the quality of the ring bubbles
McCowan et al. [5] proposed a four-point scale to assess the quality of the ring bubbles
immediately after production during their observation with four juvenile dolphins: poor for broken
immediately after production during their observation with four juvenile dolphins: poor for broken
ring; fair for partial ring; good as a complete but wide ring; and finally excellent for complete and
ring; fair for partial ring; good as a complete but wide ring; and finally excellent for complete and tiny
tiny ring. In our study, a complete and wide ring bubble can always become tiny during the rise, as
ring. In our study, a complete and wide ring bubble can always become tiny during the rise, as also
also reported in the literature [11,12]. Thus, we propose the use of a three-point scale to assess the
reported in the literature [11,12]. Thus, we propose the use of a three-point scale to assess the quality
quality of ring bubbles by grouping the good and excellent categories (i.e., successful, partial and
of ring bubbles by grouping the good and excellent categories (i.e., successful, partial and poor) as
poor) as shown in Figure 4.
shown in Figure 4.
Biomimetics 2016, 1, 6
Biomimetics 2016,
2016, 1,
1, 66
Biomimetics
4 of 7
of 77
44 of
(a)
(a)
c)
(( c)
100
100
(b)
(b)
YY(mm)
(mm)
80
80
60
60
40
40
20
20
000
0
20
20
40
40
60
80
60
80
X (mm)
(mm)
X
100
100
120
120
Figure 3.
3. Complex
Complex flow
flow around
around aa ring
ring bubble
bubble at
at its
its critical
critical forming
forming moment
moment by
by means
means of
of the
the particle
particle
Figure
Complex
image
velocimetry
(PIV)
device
(relative
pressure
of
0.55
bar
and
opening
duration
of
0.015
s).
(a) Brut
Brut
image velocimetry (PIV) device (relative pressure
pressure of 0.55 bar and opening duration of 0.015 s). (a)
image with
withthe
thebubble
bubble
in
water
in the
the
presence
of seeding
seeding
fluorescent
particles
and abeam
a laser
lasercoming
beam
image
with
the
bubble
water
in
presence
of
fluorescent
particles
and
beam
inin
water
in the
presence
of seeding
fluorescent
particles
and a laser
coming
from
the
left.
Themoment
critical moment
moment
of ring
ring formation
formation
with
the penetration
penetration
of aatongue
liquid tongue
tongue
at
from
thefrom
left. the
Theleft.
critical
of ring formation
with thewith
penetration
of a liquid
at
the top
coming
The
critical
of
the
of
liquid
at
the
top
of
the
bubble
can
be
observed.
Both
(b)
the
flow
field
and
(c)
streamlines
identify
the
central,
of the
be observed.
Both (b)Both
the flow
field
andfield
(c) streamlines
identify the
central,
the
topbubble
of the can
bubble
can be observed.
(b) the
flow
and (c) streamlines
identify
thehowever
central,
however dissymmetrical
dissymmetrical
liquid jet
jet
contour
and edge
edge recirculation
recirculation
as the
the main
main
factors
affecting the
the
dissymmetrical
liquid jet contour
and
edge recirculation
as the main factors
affecting
the generation
of
however
liquid
contour
and
as
factors
affecting
of aa ring
ring structure.
structure.
ageneration
ring structure.
generation
of
(a)
(a)
(b)
(b)
c)
(( c)
Figure 4. Three-point
Three-point scale to
to assess the
the quality of
of ring bubbles
bubbles generated by
by our mimicking
mimicking setup
Figure
Figure 4.
4. Three-point scale
scale to assess
assess the quality
quality of ring
ring bubbles generated
generated by our
our mimicking setup
setup
shown
in
Figure
1a:
(a)
Successful;
(b)
Partial;
(c)
Poor.
shown
shown in
in Figure
Figure 1a:
1a: (a)
(a) Successful;
Successful; (b)
(b) Partial;
Partial; (c)
(c) Poor.
Poor.
The experimental
experimental data
data and
and the
the quantitative
quantitative observation
observation of
of four
four juvenile
juvenile dolphins
dolphins[5]
[5] are
are collected
collected
The
The experimental
data andAt
the
quantitative
observation
of four juvenile
dolphins
[5] are
collected
in
Tables
1
and
2,
respectively.
total,
we
repeated
400
experiments
with
a
time
interval
of
5
minutes
in Tables 1 and 2, respectively. At total, we repeated 400 experiments with a time interval of 5 minutes
in
Tables
1
and
2,
respectively.
At
total,
we
repeated
400
experiments
with
a
time
interval
of
5 min to
to
allow
the
stabilisation
of
the
water
flow
after
the
passage
of
a
bubble.
As
Supplementary
Materials,
to allow the stabilisation of the water flow after the passage of a bubble. As Supplementary Materials,
allow
the stabilisation
of the are
water
flow after
the passage
of a bubble.
As
Supplementary
high-speed
video recordings
recordings
provided
to illustrate
illustrate
the formation
formation
of both
both
successful and
andMaterials,
poor ring
ring
high-speed
video
are provided
to
the
of
successful
poor
high-speed
video
recordings
are
provided
to
illustrate
the
formation
of
both
successful
and
poor ring
bubbles
(Videos
S2
and
S3,
respectively).
bubbles (Videos S2 and S3, respectively).
bubbles
(Videos
S2 andthat
S3, by
respectively).
These
data show
show
means of
of aa well-controlled
well-controlled mimicking
mimicking setup,
setup, we
we can
can certainly
certainly surpass
surpass
These
data
that by
means
These
data show
that in
bythe
means
of a well-controlled
mimicking
setup,
wethe
canhighest
certainly
surpass
the
most
talented
dolphin
performance
of
ring
bubble
production
with
successful
the most talented dolphin in the performance of ring bubble production with the highest successful
the
talented
dolphin
the performance
ring bubble
with the mainly
highest due
successful
ratemost
of 52%.
52%.
However,
theinproduction
production
of ring
ringof
bubbles
is not
notproduction
always successful,
successful,
to the
the
rate
of
However,
the
of
bubbles
is
always
mainly due
to
rate
of
52%.
However,
the
production
of
ring
bubbles
is
not
always
successful,
mainly
due
to
the
dissymmetric flow
flow turbulence
turbulence and
and high
high instability
instability at
at the
the bubble
bubble interface,
interface, as
as shown
shown in
in Figures
Figures 22 and
and
dissymmetric
dissymmetric
flow turbulence
and
high
instability
at the
bubble
interface,
as shown
in Figures
2 and
3.
3.
Under
the
higher
pressure
inside
the
vessel,
the
increase
of
the
opening
duration
intensifies
the
3. Under the higher pressure inside the vessel, the increase of the opening duration intensifies the
Under
the
higher
pressure
inside
the
vessel,
the
increase
of
the
opening
duration
intensifies
the
flow
flow turbulence
turbulence to
to make
make the
the ring
ring structure
structure unstable.
unstable. On
On the
the other
other hand,
hand, the
the impulse
impulse momentum
momentum
flow
turbulence
to
make
the
ring
structure
unstable.
On
the
other
hand,
the
impulse
momentum
quantity
of
quantity of
of the
the liquid
liquid tongue
tongue is
is not
not sufficient
sufficient to
to correctly
correctly penetrate
penetrate the
the air
air core
core centre
centre to
to form
form
regular
quantity
aa regular
the
liquid
tongue
is
not
sufficient
to
correctly
penetrate
the
air
core
centre
to
form
a
regular
ring
shape
ring shape
shape if
if the
the pressure
pressure is
is too
too small
small and
and the
the opening
opening duration
duration too
too short.
short. A
A suitable
suitable compromise
compromise
ring
if
the
pressure
is
too
small
and
the
opening
duration
too
short.
A
suitable
compromise
appears
to be
appears to
to be
be required
required between
between the
the injected
injected bubble
bubble volume
volume and
and the
the impulse
impulse momentum
momentum quantity.
quantity.
appears
required
between the
injected bubble
volume
impulse
momentum
quantity.
Our
experimental
Our experimental
experimental
investigations
suggest
thatand
the the
optimal
range
for both
both the
the
relative
pressure
inside
Our
investigations
suggest
that
the
optimal
range
for
relative
pressure
inside
the
vessel
and
the
opening
duration
is
0.28
to
0.55
bar
and
0.015
to
0.030
s,
respectively,
to
guarantee
the vessel and the opening duration is 0.28 to 0.55 bar and 0.015 to 0.030 s, respectively, to guarantee
Biomimetics 2016, 1, 6
5 of 7
investigations suggest that the optimal range for both the relative pressure inside the vessel and the
opening duration is 0.28 to 0.55 bar and 0.015 to 0.030 s, respectively, to guarantee a greater success
rate. The physical mechanism of the formation of a bubble ring described here has some similarities to
the expanding rings surrounding the mushroom-shaped cloud created by an above-ground nuclear
detonation, or by firing certain types of artillery through a fast opening based on the sudden turbulence
injection [11,12].
Table 1. Quality of ring bubble production according to the three-point scale (Figure 4) with our
mimicking setup operating under different conditions.
Relative Pressure
(bar)
Opening Duration
(s)
Bubble Volume
(×109 m3 )
Success
(%)
Partial
(%)
Poor
(%)
0.14
0.14
0.14
0.28
0.28
0.28
0.55
0.55
0.55
0.015
0.030
0.040
0.015
0.030
0.040
0.015
0.030
0.040
1.90
3.42
4.75
6.08
10.26
12.35
10.17
17.10
20.05
29.0
32.0
29.0
34.0
52.0
35.5
50.0
43.0
29.5
42.2
19.5
30.0
26.5
24.5
23.0
34.0
29.0
39.0
28.8
48.5
41.0
39.5
23.5
41.5
16.0
28.0
31.5
Table 2. Existing data of ring bubble production by juvenile dolphins in the literature [5].
Juvenile Dolphin
Tentative Number
Success (%)
Avalon
Brisbee
Liberty
Norman
88
13
20
30
31.8 *
30.8
33.3 *
6.7 *
* Some data were missing during observations [5].
Unlike a gas–gas system such as smoke rings, which are usually formed when a puff of smoke
is suddenly injected into clear air, the turbulence induced by the air in the ring bubble formation in
water as gas-liquid system should be more intensive to overcome the water drag that is much higher
than that opposed by the still air surrounding the outer parts of the puff for smoke rings. Due to the
significant difference of both density and viscosity between air and water, an excessive turbulence
during the initial stage of ring bubble generation (as shown in Figure 2), might explain why it is
difficult to achieve a systematically reliable success rate for either dolphins or our mimicking setup.
4. Discussion
As shown in Figures 2 and 3, as well as in the video recording included in the Supplementary
Materials, the flow turbulence and bubble’s interfacial instability render beyond the reach of modeling
and computation for a quantitative description. The measurements of the flow fields only in the water
phase (as shown in Figure 3) did not allow the formation of successful or unsuccessful ring bubbles
to be distinguished, as this is the very initial stage with excessive turbulence. Ideally, the flow fields
should be measured in the gas phase too but it is still impossible at present. Detailed descriptive data
are also missing in the literature with regard to the ring bubbles produced by dolphins. However,
it is clearly demonstrated that the successful generation of a stable ring bubble stems from the dual
action of suitable pressure inside the vessel (dolphin’s lungs) and the well-controlled opening duration
of the electronic valve (dolphin’s muscular action). Thus, dolphins do not blow ring bubbles at
random. Our experimental investigation suggests that the generation of stable ring bubbles may
need certain experience and forethought not only by juvenile but also older dolphins as reported
in the literature [4]. The present results points towards the assumption that dolphins monitor the
Biomimetics 2016, 1, 6
6 of 7
quality of generated ring bubbles and exert anticipatory planning of certain level prior to ring bubble
generation. The existing literature [6,7] suggests that dolphins do have a number of cognitive abilities,
which
might
Biomimetics
2016,be
1, 6consistent with our hypothesis. Certainly, the dolphins’ blowhole is larger than6the
of 7
PVC pipe (with an inner diameter of 2 mm) used in this work. The main objective of the present
study
wasof
tothe
investigate
the mechanism
of ring bubble
generation.ofThus,
scaling
plays a secondary
objective
present study
was to investigate
the mechanism
ring the
bubble
generation.
Thus, the
role
as shown
the similarity
between
bubbles
in water,
mushroom
clouds,
and smoke
rings.
scaling
plays aby
secondary
role as
shown ring
by the
similarity
between
ring bubbles
in water,
mushroom
In
particular,
a
rigid
pipe
with
a
larger
inner
diameter
would
induce
some
instabilities,
as
the
capillary
clouds, and smoke rings. In particular, a rigid pipe with a larger inner diameter would induce some
forces
can noas
longer
prevent forces
water from
entering
pipe.water from entering the pipe.
instabilities,
the capillary
can no
longer the
prevent
During
During the
the rise
rise of
of aa stable
stable ring
ring bubble,
bubble, the
the plane
plane of
of the
the ring
ring isis usually
usually perpendicular
perpendicular to
to the
the
direction
of
the
rising.
When
the
ring
bubble
moves
progressively
upwards,
the
rise
velocity
diminishes
direction of the rising. When the ring bubble moves progressively upwards, the rise velocity
with
the increase
ring diameter
and the
shrinkage
theshrinkage
cross-section
of the
air core. The
diminishes
with of
thethe
increase
of the ring
diameter
andofthe
of the
cross-section
of perfect
the air
symmetric
flow field
(Figure flow
5a) around
a stable5a)
ring
bubble
measured
the PIV
device reveals
the
core. The perfect
symmetric
field (Figure
around
a stable
ringby
bubble
measured
by the PIV
fluid
recirculation
structure
around
the
air
core
(Figure
5b).
device reveals the fluid recirculation structure around the air core (Figure 5b).
(a)
(b)
70
60
Y (mm)
50
40
30
20
10
10
20
30
40
50
60
70
80
90
100
X (mm)
Figure 5.
5. (a)
(a) Flow
Flow field
field and
and (b)
(b) streamlines
streamlines around
around aa stable
stable rising
rising ring
ring bubble
bubble at
at 0.45
0.45 m
m above
above the
the
Figure
orifice
with
relative
pressure
of
0.28
bar
and
opening
duration
of
0.030
s.
orifice with relative pressure of 0.28 bar and opening duration of 0.030 s.
5. Conclusions
5.
Conclusions
Ourstudy
studyprovides
provides
some
insight
intolevel
the oflevel
of that
control
that dolphins
may
on
Our
some
insight
into the
control
dolphins
may exercise
on exercise
themselves
themselves
as
well
as
on
their
surrounding
water
medium.
Clearly,
the
complex
mechanisms
behind
as well as on their surrounding water medium. Clearly, the complex mechanisms behind some of
some
of the apparently
trivial behaviors
of cetaceans
could
be understood
better
by mimicking
the
apparently
trivial behaviors
of cetaceans
could be
understood
better still
bystill
mimicking
them
them
under
well-controlled
operating
conditions.
Although
our
experimental
device
needs
under well-controlled operating conditions. Although our experimental device needs consideration
consideration
alternatives
and improvements,
the optimal
ring bubble
of
alternatives of
and
improvements,
the optimal conditions
for conditions
ring bubblefor
generation
(asgeneration
shown in
(as
shown
in
Table
1)—pressure
ranging
from
0.28
to
0.55
bar
and
time
intervals
from
to
Table 1)—pressure ranging from 0.28 to 0.55 bar and time intervals from 0.015 to 0.030 s0.015
in our
0.030
s
in
our
work—correspond
quite
well
to
previously
reported
pressure
(0.27
to
0.66
bar)
and
work—correspond quite well to previously reported pressure (0.27 to 0.66 bar) and intervals between
intervals
between
to 0.5 s) frequencies
with fundamental
frequencies
200 tospeech
300 Hzmimicry
for human
bursts
(0.05
to 0.5 s)bursts
with (0.05
fundamental
of 200 to
300 Hz forofhuman
by
speech
mimicry
by
a
beluga
whale
[8].
It
is
worth
noting
that
all
that
is
known
is
that
air
is
ejected
a beluga whale [8]. It is worth noting that all that is known is that air is ejected from the dolphins’
from the dolphins’
Whether
thethe
lungs
or bursa
source
of theofair
is still a matter
of
blowhole.
Whether blowhole.
the lungs or
bursa are
source
of theare
airthe
is still
a matter
conjecture
without
conjecture
without
more
data
on
the
animal.
Further
investigation
would
be
required
for
the
analysis
more data on the animal. Further investigation would be required for the analysis of the anatomy of a
of the anatomy
of a dolphin
with
respect
to the ring bubble formation.
dolphin
with respect
to the ring
bubble
formation.
In
addition
to
the
biomimetic
aspect
as mentioned
above,
the successful
production
of ring
In addition to the biomimetic aspect as mentioned
above,
the successful
production
of ring bubbles
bubbles
in
water
or
other
liquids
could
bring
new
challenges
for
modeling
the
rising
morphological
in water or other liquids could bring new challenges for modeling the rising morphological evolution
evolution
classical
bubble
shapesThe
[15,16].
The ring
peculiar
ringstructure,
bubble structure,
along
eddy
of
classicalofbubble
shapes
[15,16].
peculiar
bubble
along with
thewith
eddythe
vortex,
vortex,
would
be
useful
to
intensify
the
gas–liquid
mass
transfer
[17]
encountered
in
numerous
would be useful to intensify the gas–liquid mass transfer [17] encountered in numerous industrial
industrial applications,
such as
harmful
gas
removal
and CO2 capture.
applications,
such as harmful
gas
removal
and
CO capture.
2
Supplementary Materials: The following are available online at www.mdpi.com/link, Video S1: Formation
Rise—A complete sequence of the generation and rise of a ring bubble in the tank recorded with an average
speed camera (relative pressure of 0.28 bar and opening duration of 0.030 s), Video S2: Successful Ring—The
formation in slow motion of a perfect ring bubble of successful quality recorded with a high-speed camera at
950 frames/s (relative pressure of 0.55 bar and opening duration of 0.015 s), Video S3: Poor Ring—The formation
in slow motion of a broken ring bubble of poor quality recorded with a high-speed camera at 950 frames/s
(relative pressure of 0.55 bar and opening duration of 0.015 s).
Biomimetics 2016, 1, 6
7 of 7
Supplementary Materials: The following are available online at http://www.mdpi.com/2313-7673/1/1/6/s1,
Video S1: Formation Rise—A complete sequence of the generation and rise of a ring bubble in the tank recorded
with an average speed camera (relative pressure of 0.28 bar and opening duration of 0.030 s), Video S2: Successful
Ring—The formation in slow motion of a perfect ring bubble of successful quality recorded with a high-speed
camera at 950 frames/s (relative pressure of 0.55 bar and opening duration of 0.015 s), Video S3: Poor Ring—The
formation in slow motion of a broken ring bubble of poor quality recorded with a high-speed camera at
950 frames/s (relative pressure of 0.55 bar and opening duration of 0.015 s).
Acknowledgments: The authors acknowledge financial support by the Université de Lorraine, France.
Author Contributions: Huai Z. Li conceived and designed the experiments; Huai Z. Li, Philippe Lesage,
Mohammed Kemiha, Souhila Poncin and Noël Midoux performed the experiments; Huai Z. Li,
Mohammed Kemiha, Souhila Poncin and Noël Midoux analyzed the data; Huai Z. Li wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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