14 APPLICATION OF ELECTROWETTING PHENOMENA TO ENLARGE DROPS

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14 APPLICATION OF ELECTROWETTING PHENOMENA TO ENLARGE DROPS IN A CONTINUOUS FLOW ELECTRO-COALESCER DEVICE

14 APPLICATION OF ELECTROWETTING PHENOMENA TO ENLARGE DROPS IN A CONTINUOUS FLOW ELECTRO-COALESCER DEVICE A. Bandekar and G. G. Chase Current work and results Introduction 1200 Upstream Downstream after 20 mins 1200 Downstream after 40 mins 1000 600 400 25 20 250 VOLTAGE 200 100 150 0 50 15 100 50 0 10 RPM 20 0 0 Graph 1 Graph 2 150 Drope size in micrometers 0 600 400 Droplet count per volume Downstream after 40 mins Downstream after 60 mins 1000 800 600 400 200 40 60 80 100 150 200 0 0 120 140 160 Droplet size in micrometers 50 Graph 4. (b) Downstream after 20 mins 1200 20 50 Drop size in micrometers 0 180 200 Graph 4. (d) 0. 5 1 Distance from centre ( cm ) 800 0 200 Upstream 0 0 %wt/wt of PCM concentration 100 Graph 4. (a) Potential difference = 0 V 1400 10 5 1000 200 15 5 Downstream after 60 mins 200 Thickness (μm) at 1 cms from center 10 0 20 RPM-12 DIRECTION OF DROP MOVEMENT DIRECTION OF DROP 25 300 RPM-9 600 100 Dropl size in micrometers Graph (2) shows that when the polymer is mixed in concentration of 15% wt/wt best result is obtained. This is confirmed by thickness measurements which are minimum when coated so. Thickness of the coating was measured at various points over a period of time. Graph 3 shows the thickness is maximum at about 1 cm from the disk center. This may explain why drops near the edge tend to move towards the edge and drops near the center tend to move towards the center of the disk. On further analysis it was seen that the optimum period for drying a disc is two days. Electro-coalescer device Graphs 4. (a-d) show the drop size distributions of water droplets upstream and downstream at various times and applied potentials. In all the cases the gap distance between the discs was 0. 1 cm and the flow rate was 4 ml/sec, the step change was for 1 min. Step change for 1 minute means the potential was applied for 1 min and then turned off and it remained off for 1 minute and then again turned on and after staying on for 1 minute again turned off and so on. The only difference between the experiments shown in the plots was the applied electric potential (300, 350, 380, and 0 volts respectively). The average drop size upstream was ≈30 micrometers. Downstream the average drop sizes increased with potential showing 85, 110, and 120 microns respectively in graphs (a), (b) and (c). In graph (d) with no applied potential the drop size essentially did not change. Comparison when the discs have gradient coating and when the coating is uniform In case of disc having uniform coating the average drop size was ≈ 70 microns and in case of gradients coating it increased to ≈110 microns. The reason could be that, due to the gradient the contact time between the water droplet and surface increased , increasing its probability of coalescing with a high number of droplets. 1400 Upstream Downstream after 20 mins 1200 Downstream after 40 mins Downstream after 60 mins 1000 800 600 400 200 0 0 0 50 100 Drop size in micrometers 150 200 Graph 5. (a) Gradient coating 0 50 100 150 Drop size in micrometers Graph 5. (b) Uniform coating Current setup for running the experiments Figure 1 200 1. 5 Graph 3 (a) 150 Graph 4. (c) i. Drop count per volume 300 RPM-7. 2 800 400 200 Drop count per volume 350 Thickness of coating Thickness (μm) VOLTAGE 400 30 Voltage required to initiate electrowetting RPM-4. 8 Downstream after 40 mins 1200 Downstream after 40 mins Droplet count per volume Drop count per volume 800 Completed work and summary of results 30 Upstream Downstream after 20 mins 1000 0 400 Potential difference = 380 V 1400 Downstream after 60 mins 0 A technique for coating the disc in least possible time was developed. Also the disc was coated in such a way that a gradient coating was achieved. Different dielectric recipes were developed and tested for their performance. In each case the polymer was Polystyrene co-methyl methacrylate (PCM) and the solvent was toluene. The hydrophobic layer was Fluoropel. TM. . Another parameter that was tested was the rpm of the motor. The disc was rotated at different rpm while coating. While testing for the voltage required for droplet movement the size of water droplet was maintained constant at 5 microliters in each case. Graph (1) shows the relation between voltage required to move a water droplet and the rpm of the disc while its being coated. Potential difference = 350 V 1400 Downstream after 60 mins Drop count per volume The electrowetting effect is defined as “the change in solid-electrolyte contact angle due to an applied potential difference between the solid and the electrolyte”. If the electric potential slows droplet movements relative to ULSD flow or causes droplets to concentrate then coalescence can be enhanced. This phenomena can be used for coalescence of droplets and hence increase the effectiveness of liquid dispersion filtration. In this work stainless steel discs are coated by dielectric hydrophobic polymers. The coating is applied with a gradient in the coating thickness that cause drops to move in one particular direction to concentrate drops locally. Potential difference= 300 V 1400 (b) Figure 2 Here Figure 1 gives the schematic of the electro-coalescer device. Figure 2 shows (a) drops entering and passing through the slit between two disks with NO electric field. Figure 2 (b) with the electric field applied shows drops are attracted to the bottom disc surface and coalesce; enlarged drops exit the slit. Future research u Obtain the optimum parameters such as gap distance for running the electro-coalescer device. u Run the device in series with a hydrophobic filter to check whether it increases the filter efficiency. 200