Mixing in microchannels

We study experimentally the fluidic mixing at the microscale. The active micromixer is composed of one main channel where the fluids are injected and three pairs of side channels. Mixing is achieved in a laminar flow by perturbing the main flow with transverse impinging jets from secondary side channels. This stretches and folds the layers in the flow stream causing chaotic advection, thereby increasing mixing. The current mixer is a silicon-etcheddevice with a glass cover slip anodically bonded on top to hermetically seal the chip. The main channel is 200 microns wide, 100 microns deep and 1300 microns long.
Experiments are performed with either the first, the two first or all three side channels activated. The flow is pressure driven in the side channels using a specially developed oscillating syringe pump and is controlled using a software/hardware Labview. The working fluids injected in the main channel consist of a fluorescent aqueous solution and deonized water. The time evolution of the flow is observed using an epi-fluorescent microscope and a YAG laser. The flow is characterized by micro-PIV measurements and visualizations. Mixing is quantified using the Mixing Variance Coefficient function.

We optimized mixing regarding the frequency and amplitude of oscillation in the side channels. We are able to achieve a very good mixing (97%) of two fluids using one channel pair within 10ms and within a distance of 200 microns. When two or three side channels are activated, the mixing is more robust.
We are fabricating a new generation of the micro mixer embedded pumps. We are in the process of fabrication and testing.

To see our latest results, click here.

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Particles separation and micro mixing

We are doing experiments on particles separation using AC electrokinetics properties. Applying an appropriate AC electric field to the suspension of particles changes the charge distribution at the interface between the particles and the electrolyte. The charge distribution depends on the difference of conductivity and/or permittivity of the particles and the electrolyte. Subject to a non-uniform electric field, the particles undergo a force that moves them. The phenomenon is called dielectrophoresis (DEP).

We present experiments on dielectrophoretic (DEP) separation and trapping performed in a titanium-based micro channel linear electrode array. The micro channel was fabricated in collaboration with Y. Zhang from N. MacDonald’s group. The device consists of an array of 24 electrodes sitting on the bottom of 200 microns wide, 30 microns deep and 6 millimeters long titanium channel. The electrodes are 20 microns wide with a pitch of 40 microns. The channel is versatile and biocompatible. The device is designed to allow multi-frequency DEP (p-DEP and n-DEP) in contrast with most of the previous, single-frequency designs.

We experimentally demonstrated the ability to separate fluorescent polystyrene particles based or their size. More experiments are in process to study the separation for:
- biological samples
- different electrolyte conductivities
- optimization

The device also allows the use of traveling waves to move particles using non-homogeneities in electric-field phase-driven DEP. The idea is to separate the particle and move them in specific location. The first experiments are in process.

By tuning the multi-frequency signal, we have also shown the ability of producing strong micro mixing. Once the particles are trapped, we have shown experimentally and theoretically that a small perturbation can strongly destabilized the flow.

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Nano particles concentration

Using the titanium device, we have experimentally shown the ability to concentrate Nanometer sized particles. We use the combination of the flow motion generated by AC electric field and the DEP force. We have then shown an increase of 25-30% in concentration of 10nm quantum dots and 15nm DNA in regions few microns in size.

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Polymerase Chain Reaction

Polymerase Chain Reaction (PCR) becomes more and more important for the detection of diseases and other biological study. We are fabricating and testing a new device enabling fast PCR for lab-on-chip application.

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Experimental study of a stretched vortex filament's dynamic

A vorticity filament has been extracted from its turbulent background in order to study its behavior.
Two different set-ups have been used. The first amplifies the vorticity of a laminar boundary layer flow, whereas the second amplifies the vorticity contained between two corotating disks. In boths cases, the stretching that strongly enhanced the vorticity is localized. A vortex created in this way modelizes a vorticity filament.
In the first experiment, the control parameters give access to two configurations :
- a configuration where the vortex is permanent
- a configuration where it explodes into turbulent spots
In the first configuration, the azimuthal velocity field has been characterized according to the stretching. The diameter and the circulation of the vortex have been analysed as a function of the stretching. These measurements have led to a proposal for a new stretched vortex model that takes into account the location of the stretching. This model solves the radial velocity divergence problem that appears in Burgers’ vortex, for example.
We have also shown that the location of axial velocity in the vortex core plays an important role in energy dissipation. The dissipation term related to the radial gradient of the axial velocity is dominant in the stretching range studied in our experiment.
We have shown that the instability developing around the vortex axis is a centrifugal type instability that produces Taylor rolls.
In the configuration in which the vortex explodes, the explosion frequency has been characterized. Turbulent spot measurements have been started.

The vortex produced between two rotating disks is more intense than in the channel. We have produced phase diagrams and conducted a systematic study of the circulation with regards to stretching, vortex length and disk rotation speed..

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Curriculum Vitae   (English version)     (French version)

PhD Thesis   Thesis