Manipulation of Microbeads using DC/AC Electrical Fields

By,Michael ScharrerNitin SharmaNeil Krishnan

What is Dielectrophoresis?

Moment of Polarizable particles under the action of AC electrical field.

Forces Involved!! Deterministic forces

Dielectrophoretic Hydrodynamic Sedimentation

Random Brownian

Dielectrophoretic Force Gradient in electrical Field Volume of the particles Polarizability of the particles

Frequency of AC signal Negative and Positive

Dielectrophoresis

Electrohydrodynamic Forces Natural Convection

Density variation Coulomb

Charge gradients Dielectric

Permittivity gradients

Brownian Force

Gives a Gaussian Probability distribution to the particle.

Higher the time scale of observation more is the deterministic movement observed.

Procedure

Various configurations of electrodes available for creating electric field

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

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

• Problems

• Evaporation

• Contact

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

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

Results and Discussion Positive DEP was

achieved once by Carmen and Changhong.

Result could not be repeated.

Conditions used:

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.

Problems

Complicated set-up (focusing, establishing contact, applying cover slide)

Lack of control over experimental variables (conductivity, voltage)

“Noise’ from excessive motion of beads

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. 

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.