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: gaguilar@engr.ucr.edu

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.

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