Some Experimental Investigations in Micro/Nano Flows
Xuwei WANG, Xu ZHENG, Qun YU and Zhanhua SILBER-LI*
State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics,
Chinese Academy of Sciences, Beijing 100190, lili@imech.ac.cn
The fluid flows in Microfluidics have
extended to the micro/nano scales
comparing with the conditional fluid
mechanics and introduced many interesting
flow phenomena. At same time, with the
experimental methods of fluid mechanics,
the micro/nano flow dynamics can be
understood in depth. In this talk, we
summarize the experimental investigations
currently carrying on in LNM with a
MicroPIV/PTV system.
1. Flow velocity measurements in
microchannels with microstructures
Highly integrated microfluidic system
is a prospective development trend. To
reduce the energy consuming for fluid
transportation is an important point in
microchip
design.
In
nature,
the
super-hydrophobicity of lotus leaf surface is
mainly due to the micro/nano structures on it.
Microstructured super-hydrophobic surface
with very low surface energy could decrease
flow drag in microfluidics.
Based on this principle, we fabricated
microstructured surface by soft-lithographic
technique (Fig.1) [1]. The contact angle (CA)
on this surface reaches 147. To understand
its effects under flow condition, we designed
the microstructures on one side wall of a
microchannel (20m x 8m, Fig.2). Under
the driven pressure p1kPa, we measured
the flow velocity profile in the microchannel.
The result shows that with certain design
microstructures, the measured velocity close
to the wall increases 50%. Furthermore, in
the experiments, we found the stability of
the gas-liquid interface in the inter-space of
the microstructures is also an interesting
problem.
Fig.1 The microstructured PDMS surface for
mimicking lotus leaf surface
Fig.2 The flow in the microchannel with
microstructures.
2 Flow observation in a hybrid
micro-/nano-channel
The hybrid channel has a variety of
applications owing to its highly efficient
enrichment [2,3]. However, its flow principle
is still unknown. Because of the difficulty in
direct-observation to an inner nanochannel,
the flow measurement at the crossed section
becomes very important. The previous
studies show that the concentration
polarization makes the connection flow
more complex [4,5]. We fabricated a hybrid
channel with two orthogonal microchannels
of PDMS (50m × 100m) and a
nanoporous
polycarbonate
nuclear
track-etched membrane (PCTE, 50nm)
between them. The working liquid is calcein
and borax buffer. Under external electric
potential 50V or 100V, the fluorescence ions
(negative charge) pass through the
nanoporous membrane and enrich at the
cathodic side of the membrane. It shows that
the rate of ionic enrichment has a non-linear
relation with electric field strength and is
near four times more than the classic
electro-osmotic flow velocity. We also
observed
the
propagation
of
the
concentration depletion in the microchannel
(Fig3).
Fig. 3 Propagation of the concentration
depletion in the microchannel
Fig. 4 instance vectors of nano particle
Brownian motion
3. Measurement of the Brownian motion
of nano particles
Brownian motion has been used to
measure physical constants (e.g. Avogadro
constant,
Boltzmann
constant)
and
temperature etc. Recently, It has been used
for the micro-rheology study of complex
fluids in cells[6]. With the development of
fluorescence microscopy and nano particle
technology, it is possible to use nano
particles as the bio-probe for a higher
accuracy measurement.
Instead of tracking the particle trajectories
by hand in MPT method, we programmed
software for the particle detection and
trajectory linking. The program posses the
functions of discriminating spurious spots
from real particle images, marking
movement vectors of particles in time t
(Fig.4). Considering the distribution of
particle diameters, the average measured
diffusion coefficient versus the number of
particles recorded is calculated. About 1600
particle's trajectories in statistical calculation
can reach an uncertainty of about 2%. The
diffusion coefficients of 50nm, 200nm and
500nm particles in di-water drops are
measured, 8~10nm diameter augment is
observed.
Acknowledgement: The authors gratefully thank
the support of this work by the National Natural
Science Foundation of China (10672172 and
10872203), the National Basic Research Program
(2007AC744701) and the Hi-Tech Research and
Development Program of China (2007AA04Z302).
References:
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Adv.in Micro,Nano Molecular System.2006
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Bohn et al., Sensors and Actuators A, 2003, 102
223-233.
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2004, 4:1099–1103
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Han. Pys. Rev. Lett.,2007, 99, 044501
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Hoffman, J.C. Crocker, A.R. Bausch and D.A.
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