Manipulation of Microbeads using DC/AC Electrical Fields

1. Manipulation of Microbeads using DC/AC Electrical Fields • By, Michael Scharrer Nitin Sharma Neil Krishnan

2. What is Dielectrophoresis? • Moment of Polarizable particles under the action of AC electrical field.

3. Forces Involved!! • Deterministic forces • Dielectrophoretic • Hydrodynamic • Sedimentation • Random • Brownian

4. Dielectrophoretic Force • Gradient in electrical Field • Volume of the particles • Polarizability of the particles • Frequency of AC signal • Negative and Positive Dielectrophoresis

5. Electrohydrodynamic Forces • Natural Convection • Density variation • Coulomb • Charge gradients • Dielectric • Permittivity gradients

6. Brownian Force • Gives a Gaussian Probability distribution to the particle. • Higher the time scale of observation more is the deterministic movement observed.

7. Various configurations of electrodes available for creating electric field Procedure

8. Procedure • First method – cover slide • Second method – Droplet and probes • Third method – Wafer flooding

9. First Method : Cover slide • Use Gold electrodes • Adjust probes to touch the contact pads of electrodes • Place a small droplet (2.5l) at the site of interest • Cover with a cover slide cut to appropriate size • Problems • Evaporation • Contact

10. Second Method : Droplet and Probes • Use probes as electrodes • Position probes to lie flat on cover slide • Place a large droplet (0.1 ml) at the site of interest • Advantage • Don’t need to worry about contact • Evaporation is much slower

11. Third method : Flooding wafer • Glass wafer with gold electrodes placed in a petri-dish • Petri-dish flooded with solution till wafer is immersed • Same as first procedure • Advantages • Evaporation effects are minimal • Beads are more stable • Disadvantages • Difficult to position probes • Difficult to see beads

12. Results and Discussion • Positive DEP was achieved once by Carmen and Changhong. • Result could not be repeated. • Conditions used:

13. After changing the procedure by using the probes directly as electrodes, we got some accumulation of beads on the probe tips. • Excessive motion of beads made results unreliable and unrepeatable.

14. Problems • Complicated set-up (focusing, establishing contact, applying cover slide) • Lack of control over experimental variables (conductivity, voltage) • “Noise’ from excessive motion of beads

15. Recommendations • Implement measuring the conductivity of the buffer/beads solution. • (A set-up to do this for small amounts of liquid probably exists on campus. We tried to locate the necessary equipment but were not successful in the given time.) • Fabricate a glass cover to constrain the liquid in the electrode region, prevent quick evaporation and excessive motion of the beads. • This work was started, but not finished in time. It should be straightforward using microscope slides and glue.

16. Integrating the electrical probes on the microscope stage: • This would allow the stage and viewing area to be moved after electrical contact has been established. • Redesign wafer to allow all electrodes to be contacted from the same pads: • This would allow the probes to quickly be brought into contact after the liquid has been applied and the microscope has been focused. • Improve the adhesion of the Au electrodes on the wafer. • Currently the electrodes have a tendency to peel off at higher voltages (~4V) which obviously limits the range of conditions that can be applied.