study of nano/micro jets generated by laser -induced bubbles...
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STUDY OF NANO/MICRO JETS GENERATED BY LASER-INDUCED
BUBBLES IN THIN FILMS S. Xiong
1, T. Tandiono
2, C. D. Ohl
3 and A. Q. Liu
1
1School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore 639798
2Institute of High Performance Computing, A*STAR, Singapore 138632
3School of Physical & Mathematical Sciences, Nanyang Technological University, Singapore 637371
ABSTRACT The paper presents a liquid jet in thin films (height
varies from micro to nanometers) and the study of its
dynamics under the effects of the thin film’s dimension and
viscosity. Furthermore, the penetrating jet velocity is
investigated in terms of the laser energy and the distance
between the laser focus and the targeted gas bubble
surface. In the microchannel, strong shear stress ruptures
part of the gas bubble and Rayleigh-Plateau instability
further shattered it into small bubbles. On the other hand,
in the nanochannel, the nanojets accelerate liquids with
thickness of hundreds of nanometers.
INTRODUCTION
Liquid jets generated by cavitation bubbles are
extensively investigated as a key role in the damaging
mechanism behind erosion of hydraulic machinery for
decades [1, 2]. Recently, with the prosperous ultrasonic
and laser technologies, the concentrated energy of the
liquid jets has been used in biomedical engineering, such
as mixing in enzyme-catalyzed reaction [3], membrane
disruption [4] and cell lysis [5].
The asymmetric collapse of a vapor bubble near a rigid
boundary is always companied with a high-speed liquid jet
directed towards the boundary [6, 7]. On the contrary, the
jet directed away from a free surface [8, 9]. It is been
demonstrated that the movement of the liquid jets depends
on the physical properties of the boundary [8-10]. Because
of the similarity to human tissue and cells, some studies
focus on the bubble growth and collapse near an elastic
material, e.g. polyacrylamide gel [11] and gelatin surface
[12]. These research results can be used to simulate the jets
penetrating the biomaterials. The jet generated by
bubble-bubble interaction also attracts attentions due to its
rich phenomena and its similar dynamics as jet close to an
elastic boundary. The mutual interaction between two laser
induced bubbles associated with jetting, fragmentation and
entanglement were addressed in a thin liquid layer [13].
The coupled oscillation of two bubbles could lead to
directional microjets which were used for single-cell
membrane poration [14]. However, the thoroughly
understanding of the dynamics and well control of all
paramenters still remains a significant challenge,
especially on confined small scale.
Given the trend of miniaturization, it is a broad
interesting subject of the bubble dynamics and liquid jets
on increasingly small scales, where the viscosity effect
eventually becomes important. In this paper, jetting is
studied in the thin films whose height is changed from
microscale to nanoscale. The dynamics are highly
depending on the thin film’s dimension and viscosity.
PHYSICAL MODEL Optical breakdown leads to plasma formation by the
absorption of a focused laser pulse in the liquid. Plasma
formation is accompanied with the generation of shock
wave and cavitation bubble expansion. To characterize the
shock wave propagation, we investigated the pressure
amplitude at the shock front. It is can be find that the
pressure amplitude decreases inversely with the distance
and proportional to the laser energy.
The pressure wave travels through the liquid and
reflects on the surface of the gas bubble. The reflections of
the pressure wave on the gas/liquid interface result in a
pressure gradient ∆P between the laser-induced bubble and
the surface of the gas bubble. After the liquid is set into
motion, the sudden change in the velocity is transmitted to
all fluid particles. Each fluid particle acquires a velocity Up
before reaching the gas bubble surface. As shown in Fig.
1(a), the jet focusing can be produced by the concave
surface between liquid and gas, which converges the liquid
towards the center of the curvature.
Illumination
Glass chip with
microchannels
Objective lens
Dichroic
mirror
Nd:YAG
pulse laser
Digital delay
generator
High speed
camera
(a)
(b)
Figure 1: Schematic illumination of (a) the jet
formation and (b) experimental setup.
D
Laser bubble Gas bubble h
Pulse laser
MEMS 2013, Taipei, Taiwan, January 20 – 24, 2013978-1-4673-5655-8/13/$31.00 ©2013 IEEE 213
In the case of jetting caused by the reflection of a shock
wave from the gas/liquid interface, the initial velocity of
the liquid jet Ujo is twice the particle velocity Up. The
pressure wave strength ∆P can be estimated as
1
2joP cU (1)
where c is the speed of sound in the liquid, ρ is the density
of the liquid.
With the decreasing channel height, the effect of shear
stresses from the channel wall plays a more important role
on the jet dynamics. In a microchannel, due to the shear
stress, the gas near the wall moves much slower than that in
the center of the channel. This results in the rupture of a
part of the gas and a small bubble is formed. On the other
hand, in a nanochannel, the significantly stronger shear
stress fixes two sides of the bubble near the wall, and only a
very thin jet sheet can penetrate into the gas bubble and
rupture into two liquid columns. Besides, the jet strength is
weakening with the decreasing channel height.
EXPERIMENTAL RESUTLS Standard lithography and wet chemical etching
techniques were used for fabricating the
micro/nanostructure onto a 25 × 45-mm borosilicate glass
substrate with thickness of 0.7 mm. The channel height
varies from 5 µm to 550 nm. Inlets and outlets are drilled at
the ends of channels for injecting liquids. Another piece of
borosilicate glass was thermally bonded with the etched
substrates. The chip was fixed in a home-made chip holder
and connected to syringes in order to inject a
light-absorbing liquid into the channels.
As shown in Fig. 1(b), the optical setup to create a
laser-induced cavitation bubble includes a pulsed laser,
several optical components, and an inverted microscope.
The bubbles were generated by the Nd:YAG laser (Orion,
New Wave Research, Fremont, CA) with a single laser
pulse at the wavelength of 532 nm and a duration of
approximately 6 ns. The laser was focused through a
microscope objective (20×, NA = 0.50) into
the micro/nanochannel to vaporize the liquid. The images
were captured by a high-speed camera ((SA-1.1, Photron).
The bubble dynamics was recorded at 300000-500000
frame per second (fps). The exposure time for each frame
is 370 ns. By gradually shifting the triggering time of the
camera, the dynamical evolutions of the bubble are studied
in detail.
Figure 2 shows the temporal evolution of an initially
stationary gas bubble (B1) perturbed by a laser-induced
bubble (B2) in a 3.7-µm channel. A pressure wave is
created by vaporizing a small amount of liquid with a laser
pulse. The strong pressure gradient causes the gas bubble
deformation. The flow focusing leads to the formation of a
liquid axial jet. At the same time, part of the gas bubble is
split from the main body, and forms small bubbles in front
of the jet. In this experiment, the interaction between the
two bubbles happens in a short time. As shown in Fig. 3,
the jet has already reached the maximum length at t = 3 µs.
The growth time of the microjet is much shorter than that in
a capillary tube (i.e. 30 µs) [15]. Surface tension controlled
the jet retracting process. The remaining region inside the
reexpansion B1 is gray. It suggests that there is a thin liquid
Figure 3: Evolution of jet velocity in a microchannel.
Figure 4: Initial microjet velocity as a function of the
laser energy with different distances between the laser
focus and gas bubble surface.
Figure 2: Sequential snapshots showing the evolution of
jet and laser-induced bubble in a microchannel with h =
3.7 µm. Scale bar is 20 µm.
B2
B1
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film left on the glass surface inside B1 when the
rebounding gas pushes out most of the liquid. The
cavitation bubble ends its expansion in 1.5 µs, and then
collapses into a small remaining gas bubble within 20 µs.
However, we didn’t observe vigorous compression and
reexpansion of B2 during the expansion period of B1. That
means the pressure wave is damped very fast in the
microchannel.
It is interesting to observe that some small bubbles are
ruptured during the jetting, which has not been reported in
other bubble-bubble interaction or jetting studies. In a
relatively shallow channel, the viscosity effect becomes
dominant and the high shear stress hinders the movement
of gas near the channel walls. It results in the rapture of the
front part of the gas bubble. As shown in Fig. 2, the
raptured part shrunk into a cylindrical thread within 2 µs.
The Rayleigh-Plateau instability causes the breakup of a
long fluid cylinder into small bubbles [16].
Figure 3 shows the evolution of the jet velocity in the
microchannel. The jet tip travels from the gas bubble
surface into the center with an initial velocity of 35 m/s.
After reaching its maximum length, the liquid slowly
retracts from the gas bubble. The retracting jet velocity is
much slower. While the jet penetrating process is dominant
by inertia, the retracting process is controlled by the
surface tension and viscosity. Fig. 4 shows the initial jet
velocity as a function of the laser energy with different
distances between the laser focus and the bubble surface. It
is demonstrated that the jet velocity is increased by
decreasing the distance between the two bubbles, as well as
increasing the laser energy. As noted before, the
experimental results suggest that the velocity is
proportional to the pressure of the shock wave. In Fig. 4,
the jet velocity reaches 65 m/s. Calculated from Eqn. (1),
with ρ = 1×103 kg/m
3, c = 1497 m/s, the pressure strength
can be estimated as 11 MPa to 49 MPa when the intial jet
velocity is changed from 15 to 65 m/s.
Figure 5 shows that the jetting in the nanochannel ( h =
550 nm) is much milder due to the viscous effects. The jet
accelerating process is very short. As shown in the frame t
= 0, a thin layer of liquid film penetrates into the gas
bubble within 370 ns. Thereafter, liquid columns formed
from the breakup of the triangular liquid jet. The dynamics
after the first frame is slow. The gas/liquid interface moves
outwards due to the collapse of the nanobubble, while
surface tension further minimizes the surface area. The
liquid column gradually contracts into a droplet (18
femtoliter). The thickness of the thin liquid layer is
estimated as 160 ± 30 nm from conservation of mass. It
offers possibilities on precision fluid handling, nanodroplet
generation.
The nanojet velocity is also sensitive to the laser energy
and the distance. Fig. 6 shows the nanojet velocity as a
function of the distances between the laser focus and
bubble surface when the laser energy is 6 µJ. The power of
the jetting attenuates fast with increasing distance. The jet
velocity decreases from 32 m/s to 3 m/s while the distance
is only reduced 6 µm. The laser power has a much greater
effect when the distance is shorter as shown in Fig. 7.
When the laser energy changes from 7.5 µJ to 11.5 µJ, the
jet velocity increases by 140% for D = 9 µm, while only by
0
3.3 µs
6.7 µs
-2 µs
0
2 µs
4 µs
2 ms
10 µs
8 µs
6 µs
Gas Liquid
Laser
Bubble
Figure 5: Evolution of the nanojet and laser-induced
bubble in a nanochannel with h = 550 nm. Scale bar is 10
µm.
Figure 6: Nanojet velocity as a function of the distances
between the laser focus and bubble surface when the laser
energy is 6 µJ.
Figure 7: Nanojet velocity as a function of the laser
energy with different distances between the laser focus
and bubble surface.
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40% for D = 13 µm. No obvious effect can be observed
when the distance is increased to 20 µm.
CONCLUSIONS In conclusion, the laser-induced bubble causing
nano/microjets in thin films is studied. The dependency of
the jet velocity on the laser energy and inter-bubble
distance is demonstrated. Surface forces play important
roles in the confined regions. While part of the gas bubble
is split by the shear stress with the microjet, the nanojets
can only penetrate into the gas bubble as thin as 1/3 of the
channel height. This study provides insights in the
fundamental research on liquid jets within confined
environments and potential applications in biomedical
fields.
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CONTACT *A. Q. Liu, Tel: +65-6790 4336; Fax: +65 6790 3318;
E-mail: [email protected]
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