Manipulation of Microbeads using DCAC Electrical Fields

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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.

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Forces Involved!!

• Deterministic forces
• Dielectrophoretic
• Hydrodynamic
• Sedimentation
• Random
• Brownian

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Dielectrophoretic Force

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

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Electrohydrodynamic Forces

• Natural Convection
• Density variation
• Coulomb
• Charge gradients
• Dielectric
• Permittivity gradients

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Brownian Force

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

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Procedure
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Procedure

• First method cover slide
• Second method Droplet and probes
• Third method Wafer flooding

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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

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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
• Dont need to worry about contact
• Evaporation is much slower

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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

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Results and Discussion

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

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• 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.

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Problems

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

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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.
•  

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• 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.