Simultaneous 3D temperature and velocity field
measurements of micro-flow with laser-induced
fluorescence and micro-digital holographic particle
tracking velocimetry: Numerical study
Longchao Yao (姚龙超)
1
, Xuecheng Wu (吴学成)
1,
*, Jing Yang (阳 静)
1
,
Yingchun Wu (吴迎春)
1,2
, Xiang Gao (高 翔)
1
, Linghong Chen (陈玲红)
1
,
Gérard Gréhan
2
, and Kefa Cen (岑可法)
1
1
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
2
UMR 6614/CORIA, CNRS, Normandie Université, BP12 76801 Saint Etienne du Rouvray, France
*Corresponding author: wuxch@zju.edu.cn
Received January 28, 2015; accepted May 15, 2015; posted online June 12, 2015
We propose a method for simultaneous 3D temperature and velocity measurement of a micro-flow field. The 3D
temperature field is characterized with two-color laser-induced fluorescence particles which are tracked with
micro-digital holographic particle tracking velocimetry. A diffraction-based model is applied to analyze defo-
cused particles to determine the intensity ratio of two fluorescent dyes on the particle. The model is validated
with experimental images. As the result shows that the intensity ratio nearly remains unchanged with respect to
depth positions, defocused particles can be used as 3D temperature sensors. Numerical work is carried out to
check the method, and 3D temperature and velocity field in a 120 μm × 120 μm×80μm test volume are
retrieved.
OCIS codes: 280.6780, 280.7250, 050.1970.
doi: 10.3788/COL201513.072801.
Microfluidics has demonstrated its usefulness in various
fields of science and technology in recent decades
[1,2]
.Most
of the applications of microfluidics involve fundamental
principles of mass and heat transfer. Simultaneous mea-
surement of multiple parameters is important for mass
and heat transfer estimation
[3]
. There is an urgent need
for methods of measuring the temperature and velocity
distributions in the micro-flow field because of its dimen -
sion and complexity compared to the macro-scale flow.
Micro-digital holographic particle tracking velocimetry
(micro-DHPTV)
[4]
is one of the well-established and
accurate nonintrusive optical methods for sophisticated
micro-flow 3D velocity diagnostics. Some other methods
take advantage of a defocused particle image
[5,6]
to track
particles in three dimensions. In comparison, micro-
DHPTV works well in a wider depth position range with -
out depth calibration. While 3D velocity field is resolved,
3D temperature field measurement in micro-flow remains
a challenge. This is due to the lack of a temperature sensor
that is efficient at arbitrary depth positions. Commonly,
some nonintrusive methods such as laser-induced fluores-
cence (LIF)
[7,8]
or thermo-liquid crystal (TLC)
[9]
thermom-
etry are only efficient when the test area is in the focal
plane, rendering them 2D techniques. Recently, defocused
two-color LIF
[10]
and astigmatic TLC
[11]
imaging methods
have been developed to estimate the 3D temperature field.
Numerical analysis of defocused fluorescent particles
have also revealed new methods for 3D micro-flow diag-
nostics
[5,12]
. In this Letter, a method is proposed to measure
3D temperature and velocity at the same time with a
combination of two-color LIF and micro-DHPTV.
Numerical work is carried out to confirm the validation
of this method.
Figure
1 presents a detailed schematic of a typical
design of the integrated LIF and micro-DHPTV system.
A double pulsed laser ðλ ¼ 532 nmÞ is expanded to a
plane wave before illuminating the test section. The fluo-
rescent dyes, temperature-insensitive sulforhodamine 101
(SRh101) and temperature-sensitive rhodamine B (RhB),
on the tracer particles are excited. With the help of the
dichroic filters and band-pass filter, the fluorescence emit-
ting from SRh101 and RhB is separated and recorded by
CCD 1 ðλ > 660 nmÞ and CCD 2 ð560 nm < λ < 600 nmÞ,
respectively. At the same time, scattering light ðλ ¼
532 nmÞ from the particles interferes with the undisturbed
incident wave to form a hologram pair that is recorded
by CCD 3. Given that the particles are usually semi-
transparent polymeric spheres, we do not necessarily
use an epi-fluorescent microscope geometry.
In two-color LIF thermometry, the fluorescence inten-
sity of the temperature-sensitive dye is usually dependent
on temperature, while the temperature-insensitive dye
is used as the reference dye. The intensity ratio of the
two fluorescence emissions on a tracer particle is used
to determine the temperature and can be directly ex-
pressed as
[8]
I
RhB
I
SRh101
¼
I
in
C
RhB
ε
RhB
Q
RhB
I
in
C
SRh101
ε
SRh101
Q
SRh101
; (1)
COL 13(7), 072801(2015) CHINESE OPTICS LETTERS July 10, 2015
1671-7694/2015/072801(5) 072801-1 © 2015 Chinese Optics Letters