Microfluidic flow sorting systems enable us to actuate the trajectory of the transported objects (cells, droplets and particles etc.) in inexpensive, efficient and reliable way. Several approaches have already been demonstrated, including deterministic lateral displacement
2.4. Attempt to use microfluidic sorters based on electrodes 55 [215], hydrodynamic filtration [216], dielectrophoresis [217], acoustophoresis [218] [219], optical force [220].
Many microfluidic sorting devices exploit dielectrophoretic forces that depend on the volume and dielectric contrast of the sorted objects and the continuous medium. Such sorting device functions as follows[221] : the droplets are evenly spaced and periodically injected into the device forming a train. They flow to the asymmetric ‘Y’ sorting junction, where they can take one of two paths. Droplets flow into the wider channel by default due to its lower hydraulic resistance. When a pulse of high-voltage alternating current (AC) is applied across the electrodes adjacent to the sorting junction, the droplets are deflected into the narrower channel of the junction by dielectrophoresis (see figure 2.6). One well established application based on this dielectrophoretic (DEP) sorting is the fluorescence-activated droplet sorting (FADS) [95] [221] where a laser detection is integrated which focus on the channel at the gap between two electrodes.
Figure 2.6: (a) A monodisperse emulsion is injected into the sorting device. The inset image shows the emulsion droplets being spaced-out with more carrier fluid. Scale bar, 1 mm. (b) Trajectories of droplets stream through the sorting junction. When an AC electric field is applied across the electrodes, the droplets are deflected into the narrow arm. In the absence of a field, the droplets flow into the wider arm owing to the lower hydraulic resistance (inset). Scale bar is 100µm. Figure adapt from [221].
For our application, we search to deflect periodically a small number of droplets into one channel as shown in figure 2.2 (a). Based on FADS, a simplified microfluidic sorting device is made by suppressing the fluorescence laser detection system since there is only one population of droplets generated with a flow focusing junction. An additional continuous phase inlet is located in the main channel helping to space the droplets when they enter the sorting channel. The design of our system is shown in the figure 2.7.
We make PDMS microfluidic devices using standard soft lithographic methods. We design electrodes as channels within the PDMS device located in the proximity of the asymmetric ‘Y’ sorting junction. In the conventional approach, metallic electrodes are fabricated by injecting liquid solder into microfluidic channels, and allowing the solder to cool and solidify. This method is named ‘microsolidics’ [222]. We have used this technique in the beginning of our study to elaborate the microfluidic droplet sorters. We also explore another alternative method to elaborate liquid electrodes by replacing the solder by highly concentrated electrolyte solution. Thought the Electrowell, a multiple channels accessory, we can apply a voltage to liquids injected in microfluidic chips. This device imposes a difference of potential at the terminals of a microfluidic circuit. Using liquid electrodes, although the electric field has not been fully characterized, this technique provides an easier and efficient method of the electrode fabrication.
We verified that this simplified microfluidic droplet sorter deflects efficiently the droplet of interest into the desired channel. Figure 2.8 shows that in the absence of external force, the droplet flows into the upper channel owing to the lower hydraulic resistance (Channel
Figure 2.7: Sketch of the design of microfluidic device integrated with electrodes channels close to the asymmetric ‘Y’ junction. Inset showing that Outlet 1 is the channel with higher hydrodynamic resistance and Outlet 2 with lower resistance (Channel widthupper
> Channel widthbottom). The droplets flow into this branch only in the presence of an electric field. Scale bar is 30µm.
widthupper >Channel widthbottom). Whereas if the electric field is turned on, the droplet is directed into the narrow channel by dielectrophoresis.
Figure 2.8: (a) When the electric field is switched off, droplet flows along the upper channel because of low flow resistance. (b) When the electric field is switched on, the droplet is driven in the the lower channel of the branch. Scale bar is 30 µm. The artificial added color is used to indicate the different states of electrodes.
We are able to program the number of cycles and the voltages applied to the electrodes to synchronize the arrival of droplets depending on flow rates, droplet size and droplet rate.
In the example in figure 2.9, we could drive periodically a short train of four droplets into the narrow channel. Cycles of 1 KHz single-ended square waves are sent to the electrodes over 2 seconds. The voltage across the electrodes is 3Vp−p which is amplified by a factor of 100 by a high-voltage amplifier(Trek). The cycle is repeated every 9 seconds.
To conclude, the electrode sorter provides an effective way to produce periodically a small number of droplet. However, there are some limits in this method. First of all, in order to evenly space the droplets (which is critical for periodically deflection), the train of droplets should be confined in the main sorting channel. This condition imposes that droplet size is determined by the geometrical parameters and there is not much room to tune the droplet size as we wish. Secondly, the synchronization between the arrival