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 1 School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore 639798 2 Institute of High Performance Computing, A*STAR, Singapore 138632 3 School 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 U p 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, 2013 978-1-4673-5655-8/13/$31.00 ©2013 IEEE 213

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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

214

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.

215

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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[2] R. E. A. Arndt, “Caviation in fluid machinery and

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Palmer, K. S. Phillips, N. L. Allbritton and V.

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[4] P. Prentice, A. Cuschieri, K. Dholakia, M. Prausnitz

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[5] P. A. Quinto-Su, H.-H. Lai, H. H. Yoon, C. E. Sims,

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CONTACT *A. Q. Liu, Tel: +65-6790 4336; Fax: +65 6790 3318;

E-mail: [email protected]

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