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ILASS Americas, 24th Annual Conference on Liquid Atomization and Spray Systems, San Antonio, TX, May 2012
Generation of thermocavitation bubbles within a droplet
J.P Padilla-Martínez1, Darren Banks
2, J.C. Ramírez-San-Juan
1, R. Ramos-García
1, G. Aguilar
2,*
1 Departamento de Óptica, Instituto Nacional de Astrofísica, Óptica y Electrónica.
72000 Puebla, Pue. México 2 Department of Mechanical Engineering, University of California Riverside,
Riverside, CA 92521
Abstract
High-speed video imaging was used to study the dynamic behavior of cavitation bubbles induced by a continuous
wave (CW) laser into highly absorbing water droplets containing copper nitrate (CuNO4). The droplet lays horizon-
tally on a glass surface and the laser beam (λ=975 nm) propagates vertically from underneath, across the glass and
into the droplet. This beam is focused z=400 µm above the glass-liquid interface in order to produce the largest
bubble possible. The laser-induced bubble is always in contact with the substrate taking a half-hemisphere shape; it
reaches its maximum radius (Rmax ~ 1mm) and collapses rapidly thereafter, displaying a toroidal shape in the final
stage of the collapse. As a consequence of the bubble collapse, a liquid jet is produced at the droplet free interface
emerging out at velocities of ~ 3 m/s for this case, which allows the formation of secondary droplets due to the
breakup of the jet. The dynamics of cavitation bubbles in confined geometries (drops) and the liquid jet formation
are composed of multiple and interconnected hydro- and thermodynamic events that occur inside and outside the
droplet which we are currently exploring.
*Corresponding author: [email protected]
ILASS Americas, 24th Annual Conference on Liquid Atomization and Spray Systems, San Antonio, TX, May 2012
Introduction
Cavitation bubble collapse within droplets has been
investigated experimentally in recent years [1-9]. This
phenomenon is rich in terms of the multiple and inter-
connected hydro- and thermodynamic events that occur
inside and outside the droplet. Also, its full understand-
ing is crucial to predict and eventually control the many
practical applications that could benefit from the effects
produced by cavitation bubble collapse. For example,
erosion on surfaces is associated with liquid jets and
shock waves emitted by collapsing cavitation bubbles
10-11; in the biomedical area, thermocavitation could
be used to ablate soft-matter surfaces (tissue) for more
effective drug delivery [12]; and for the generation of
finer sprays to improve combustion efficiency [5]. So
far, the principal mechanisms of optical cavitation with-
in droplets have been the use of expensive pulsed
Nd:YAG lasers [1–5] and CO2 lasers [6-8]. Other non
optical methods have also been explored for cavitation
within a droplet, for example, spark discharge between
two thin electrodes (8-1000 mJ discharge energy) in a
microgravity environment. This mechanism of inducing
cavitation works through the electrical breakdown of
water molecules (Obreschkow et al. [9]) and requires
relatively high energy discharges.
The present paper reports on the collapse of bubbles
formed within a droplet through the use of a low power
continuous wavelength (CW) laser, offering a cheaper
option compared with the cavitation mechanisms men-
tioned above. The working fluid is a saturated solution
of copper nitrate (CuNO4) dissolved in water, which
has a high absorption coefficient (α =135 cm-1
) at the
operating wavelength (λ=975 nm). Using high speed
video (at frame rates up to 105 fps), we show the gener-
ation of a vapor bubble within a droplet by thermocavi-
tation [11-13], followed by a liquid jet emerging from
the droplet produced by the shock wave generated im-
mediately after the bubble collapses.
Experimental set up
A hemispherical droplet (5µL volume) confined by a
thin circular plastic washer of 5 mm in diameter and ~
100 µm depth rests on a glass microscope slide (1 mm
thickness). The CW laser beam (λ=975 nm) was fo-
cused (with a microscope objective sitting under the
glass slide) slightly above the glass-liquid interface.
The objective could be displaced vertically in order to
change the focal point (z) inside the droplet.
Figure 1. Experimental set up for the generation and
analysis of vapor bubbles produced by thermocavitation
within a droplet.
For these experiments, we fixed the laser power to
275mW (the maximum power available from our laser
system) and the maximum bubble radius was controlled
by changing the focal point [13]. It was found that plac-
ing the laser focus z= 400 μm above the glass-liquid
interface led to the largest cavitation bubbles of ~ 1mm
radius. In order to record the formation and evolution of
the bubble inside the droplet, the droplet was illuminat-
ed perpendicular to the laser beam with diffuse back-
lighting from a white light source to project the bub-
ble´s shadow on a high speed video camera (Phantom
V7, Version: 9.1), which recorded images of up to 105
fps. The CW laser beam was blocked by a shutter,
which was connected together with the high speed
camera to a synchronization trigger, so the camera
initiated the recording when the shutter opened.
Results and discussion
Figure 2 shows the formation and evolution of a single
cavitation bubble created within a droplet of 5 µL vol-
ume. A vapor bubble appeared ~82 ms after the laser
began to irradiate the droplet (Fig. 2.A). The bubble had
a hemispherical shape and it grew until it reached its
maximum radius (Rmax ~ 1mm) in ~129 µs (Fig. 2.D),
causing a protrusion at the top of the droplet due to the
vapor expansion. When the bubble started to collapse,
the protrusion ceased growing (Fig. 2.E-F). The vapor
bubble took a toroidal ring shape during the final stage
of the collapse (Fig. 2.F), caused by the pressure near
the pole of the bubble, which is presumably higher than
the pressure on the sides during that stage of the col-
lapse, producing a negative curvature over the bubble
surface [14]. Lauterborn et. al. [10], using high speed
photography and holography, showed that once the
toroidal ring collapses, it is divided into several sepa-
rate collapsing parts, exactly as it occurred in our study,
where we observe a cloud around the toroidal ring (Fig.
2.G). The shock wave produced at the moment of the
collapse, produces a crown around the base of the pro-
trusion (Fig. 2 G-H), which emerges out the droplet at
velocities about 3 m/s, forming a characteristic jet-and-
crown shape, similar to that generated by droplet im-
pact on liquid pools [15-16]. The Figures marked with
lowercase letters (2 a-h) are topside views of the corre-
sponding uppercase ones shown above.
The ejected jet-and-crown outside the droplet allows
the formation of a liquid column of ~0.5 mm in diame-
ter, which propagates upward until it separates (Fig. 3);
forming small droplets (it is not shown due to the frame
size). This narrow column is found to reach velocities
of 3 m/s. In comparison, past work using pulsed lasers
to induce cavitation have measured velocities of 100 to
250 m/s for microjets of 5-50 µm in diameter. Besides
the potential use of jet formation for fire/fiber coating,
ink-jet printing and secondary atomization, this liquid
column has also found some potential use as a tempo-
rary liquid waveguide, as was shown by Nicolas Bertin
et. al [17] recently.
Figure 2. Sequences of images of formation and col-
lapse of vapor bubble into a small droplet. The images
with uppercase letters are when the camera was placed
almost perpendicular to the laser beam (Fig. 1) and
lowercase letters the camera was angled vertically
about 45 degrees. The exposure time is 41 µs and the
time between each frame is 2 µs.
Figure 3. Liquid column formed after the bubble col-
lapse.
Conclusions
Using a CW laser focused in a highly absorbent solu-
tion we showed that it is possible to generate vapor
bubbles within a quiescent droplet by thermocavitation,
with the added advantage of a very simple and inexpen-
sive experimental setup relative to other methods used
for the same purpose. Furthermore, the mechanism of
bubble formation is different from that induced by
pulses lasers or electrical discharger inside the drop,
and the liquid jet velocity (~3 m/s) is much smaller
compared with the velocities reported with pulsed laser
systems (see Ref. 5). However, the jet´s base diameter
in our case is much larger (~0.5 mm). The control and
stability of these liquid columns and droplet formation
due to the breakup of the column could be potentially
implemented in many applications, such as liquid jet
polishing of optics, wire and fiber coating, ink-jet print-
ing and temporary liquid waveguides.
Acknowledgements
The authors acknowledge financial support from
CONACyT-Mexico and Omnibus Academic Senate
Research Grant, UCR. Also a special thanks to Josue
Lopez for laboratory assistance.
References
1. J. C. Carls and J. R. Brock, “Laser-induced
breakout and detonation waves in droplets. II.
Model,” J. Opt. Am. B 8, 329–336, 1991.
2. J.B. Zheng, W.F. Hsieh, S. Chen, and R. K. Chang,
“Laser induced breakout and detonation waves in
droplets. I. experiments,” J. Opt. Am. B 8, 319–
328, 1991.
3. Etienne Robert, Jacques Lettry, Mohamed Farhat,
Peter A. Monkewitz, and François Avellan, “Cavi-
tation bubble behavior inside a liquid jet,” Physics
of Fluids 19, 067106, 2007.
4. Lammert Heijnen, Pedro Antonio Quinto-Su, Xue
Zhao, and Claus Dieter Ohl, “Cavitation within a
droplet,” Physics of Fluids 21, 091102, 2009.
5. S. T. Thoroddsen, K. Takehara, T. G. Etoh, and
C.D. Ohl, “Spray and microjets produced by focus-
ing a laser pulse into a hemispherical drop,” Phys-
ics of Fluids 21, 112101, 2009.
6. J. H. Eickmans, W.S. Hsieh, and R. K. Chang,
“Laser-induced explosion of H2O droplets spatially
resolved spectra,” Opt. Lett. 12, 22–24, 1987.
7. W.F. Hsieh, J.B. Zheng, C. F. Wood, B. T. Chu,
and R. K. Chang, “Propagation velocity of laser-
induced plasma inside and outside a transparent
droplet,” Opt. Lett. 12, 576–578, 1987.
8. Ronald G. Pinnick, Abhijit Biswas, Robert L. Arm-
strong, S. Gerard Jennings, J. David Pendleton, and
Gilbert Fernandez, “Micron-sized droplets irradiat-
ed with a pulsed CO2 laser: measurement of explo-
sion and breakdown thresholds,” Applied Optics
29, 1990.
9. D. Obreschkow, P. Kobel, N. Dorsaz, A. de Bosset,
C. Nicollier, and M. Farhat, “Cavitation bubble dy-
namics inside liquid drops in microgravity,” Phys.
Rev. Lett. 97, 094502, 2006.
10. A. Phillip and W. Lauterborn, “Cavitation erosion
by single laser-produced bubbles,” J. Fluid Mech.
361, 75-116,1988
11. J.C. Ramirez-San-Juan, J.P. Padilla-Martinez, P.
Zaca-Moran, and R. Ramos-Garcia, “Micro-hole
drilling in thin films with cw low power lasers”,
Optical Materials Express 1, 598–604, 2011
12. J.P Padilla-Martínez, J.C. Ramírez-San-Juan, R.
Ramos-García, Feng Sun and G. Aguilar, “Ther-
mocavitation as a tool for stratum corneum Perme-
ation”, ILASS Americas, 23rd Annual Conference
on Liquid Atomization and Spray Systems.
13. Ramirez-San-Juan J.C., Rodriguez-Aboytes E.,
Martinez-Canton A.E., Baldovino-Pantaleon O.,
Robledo-Martinez A., Korneev N. And Ramos-
Garcia R., "Time-Resolved Analysis Of Cavitation
Induced By CW Lasers In Absorbing Liquids",
Opt. Express 18, 8735-8743 (2010).
14. A. Shima and K. Nakajima, “The collapse of non-
hemispherical bubble attached at solid wall” 3. Flu-
id Mech. 80, 369-391, 1977
15. A.L. Yarin, “Drop impact dynamics: splashing,
spreading, receding, bouncing”, Annu. Rev. Fluid
Mech. 38,159-192;2006
16. S.L. Manzello, J.C. Yang, “An experimental study
of a water droplet impinging on a liquid surface”,
Experiments in Fluids 32.580–589; Springer-
Verlag 2002
17. Nicolas Bertin, Regis, Wunenburger, Etienne Bras-
selet, and Jean-Pierre Delville, “Liquid-Column
Sustainment Driven by Acoustic Wave Guiding,”
Phys. Rev. Lett. 105, 164501;2010.