5th Micro and Nano Flows Conference
Milan, Italy, 11-14 September 2016

Velocity measurements in an Omega-micromixer using Stereo-MicroPIV
F. Gökhan ERGİN 1,** Corresponding author: Tel.: +45 4457 8173; Fax: +45 4457 8001; Email: gokhan.ergin@dantecdynamics.com
1 Product Manager, Dantec Dynamics, Skovlunde, DK
Keywords: MicroPIV, Stereo MicroPIV, Image calibration, omega micromixer

Mixing is often enhanced in the presence of threedimensional (3D) flow structures such as vortices. In
micro scales the initiation of such 3D microstructures isoften suppressed because of the low Reynolds number
inherent to micro scale flows. Passive, omega-shaped
serpentine micromixers have the potential to induce
three- dimensional rotating flow structures around sharp
corners and thereby enhance mixing efficiency.
Experiments are performed on the flow through an omega
micromixer in order to check the presence of micro scalevortices. The microchannel features a simple Omega
shape with several 90-degree corners (Fig. 1). The
channel height and width is 1mm, which is rather large
compared to what can be achieved with current
micro-manufacturing techniques. This is intentional inorder to perform measurements at different channel
depths and construct the time-averaged 3D flow field in
the future campaigns. In the current study results from asingle measurement plane (center-plane) is reported.

Figure 2 Stereoscopic image calibration kit

Figure 3 View of the checkerboard calibration target.
Each square edge dimension is 100µm.

Figure 1 Omega-micromixer and its dimensions in mm
Stereoscopic MicroPIV technique [1,2] has received
increased attention recently, as it can provide thenecessary multiple-component velocity information in
micromixers in micron scale resolution. Experiments are
performed to measure all three velocity components inthe middle plane of the microchannel, at the sharp corners.
The experimental setup includes a Leica fluorescencestereomicroscope, 2x PlanApochromatic common main
objective, a custom-built micro image calibration kit (Fig.
2), laser and LED illumination, and a checkerboard
calibration plate (Fig. 3). The selected microscope andobjective configuration can produce a stereoscopic
half-angle of ~23 degrees in water (full angles 46degrees), and this means that the uncertainty of theoff-axis velocity component is expected as 2.4 times thatof the in axis velocity components [3].

Stereoscopic image calibration in a microchannel issimply difficult, if not impossible. In order to achieve an
accurate representation of the object space in 3D, adedicated image calibration kit is used, which consists ofa checkerboard calibration target, a calibration pool and a
microstage to traverse the target in the calibration pool.
The cover glass of the calibration pool and of themicrochannel are the same material simulating similar
optical conditions during calibration and measurements.
The calibration target includes two checkerboard patterns
for 1mm and 0.5mm object size. The larger pattern isused to match the channel dimension. Carefully placed
circular markers allow an unambiguous definition of theplanar coordinate axes. During calibration, a pulsed
mono- chromatic LED device is used for illumination,
which proves useful in obtaining calibration images withgood contrast. The calibration is performed (i) by imaging
the checkerboard pattern in known positions within thedepth of field (ii) by computing mapping function for the3D space using a 3rd order polynomial function used by
Soloff et al [4] and (iii) by performing calibration
refinement. The depth of field is adjusted by closing theaperture of the zoom optics of the stereomicroscope.

5th Micro and Nano Flows Conference
Milan, Italy, 11-14 September 2016

A depth calibration is also performed for accurate
positioning of the measurement plane in the microchannel.
The depth calibration, stereoscopic calibration andexperiments are performed at 5x system magnification,
which resulted in a Field of View of 3mm x 3mm. During
depth calibration and measurements the aperture wasfully open producing a 16µm field depth (important fordepth calibration) and 99µm correlation depth (importantfor experiments). During calibration the iris diaphragm
was only 2/3 open, producing a field depth of 32µm. Fivecalibration images were used at h=400µm, 450µm,500µm, 550µm and 600µm. The average reprojection
error for left and right cameras were found as <0.2pixels.

REFERENCES[1] Lindken, R., Westerweel, J., & Wieneke, B. (2006).
Stereoscopic
microparticle
imagevelocimetry. Experiments in Fluids, 41(2), 161-171.

A high precision syringe pump with a linear step
resolution of 12 nm is used to deliver the distilled water
seeded with 1µm-diameter Nile-red fluorescent particles.
The flow rate accuracy and reproducibility are given as±0.35% and ±0.05% respectively. The syringe pump candrive two syringes simultaneously and the flow rate isadjustable between 1.6 pl/min to 300 ml/min using
different syringe sizes. High quality airtight glass
syringes are used to generate a smooth flow during themeasurements. A flow rate of 15ml/min produced an
expected average velocity of 250mm/s. (Re = 250)

[4] Soloff, S.M., Adrian R.J., & Liu Z.-C. (1997)
Distortion compensation for generalized stereoscopic
particle image velocimetry. Meas. Sci. Tech. inFluids, 8, 1441-1454.

During measurements, pulsed illumination at 532nm froma 60mJ/pulse dual-cavity Nd:YAG laser is delivered tothe microscope using a liquid light guide and imaging isperformed using two FlowSenseEO 4Mpix PIV cameras.
Planar three component velocity measurements arecomputed by combining the 2D2C velocity field
information from each camera, and refined image
calibration information. Contrast of raw particle images
was enhanced by performing a background subtraction
using minimum pixel value found in the ensemble. An
ensemble masking technique is also applied.
It was challenging to find intricate flow details when thecomplicated flow field was observed using the 2D viewer.
3D views of the results indicate that micro-vortex systems
are present close to the corners of the omega-micromixer(Fig. 4). These small-scale vortices are only recognizable
using a 3D viewer and from a particular angle.

Figure 4 A vortex observed close to one of the sharp
corners in the simple omega-micromixer. Colors indicate
the magnitude of the velocity vector.

[2] Brede, M., Witte, M., & Leder, A. (2006)
Stereo-Micro PIV measurements of the threedimensional separated flow in the wake of a backward
facing step. Proc. 13th Int Symp on Applications ofLaser Techniques to Fluid Mechanics.
[3] Prasad, A.K., Stereoscopic Particle Image Velocimetry.
(2000) Experiments in Fluids, 29, 103-116.