Workshop proceeding - final.pdf - Faculty of Information and ...

The electric potential field is governed by the Poisson’s equation as follows:
2
∇ ϕ = 0 ,
(9)
where, φ is the electric potential and ∇ is the gradient operator. The electric field is obtained by
calculating the derivative of the electric potential field as follows:
E r = −∇ϕ , (10)
where, E r 2
is the electric field that is a vector variable. Having this variable, the value of ∇E was
obtained throughout the field and substituted into Equation (1) to calculate the DEP force in different
locations of the device.
The simulations were conducted in a two-steps process. First, the geometry of the device was
created and then divided into small elements using the Gambit-2.3 software package (Fluent USA,
Lebanon, NH). Along the electrode edges, fine structured quadrilateral elements were applied,
enabling the control of the thickness of these elements. Selecting the proper thickness of these
elements was crucial to predict the sharp gradients of the electric field along the electrodes and to
assure the convergence of the simulation. In the interior regions of the system, unstructured
quadrilateral elements were applied due to their flexibility.
Next, the Fluent-6.3 software package (Fluent USA, Lebanon, NH) was applied to solve the
governing Equations (9)-(10). The software applied the finite volume method to discretise the
governing equations across each element. The software was developed to solve the differential Navier-
Stokes equations in fluidic systems and predict the velocity, pressure and other flow variables.
However, other differential equations can be solved using the UDS (user-defined scalars) module of
the software.
The boundary conditions applied were as follows: the electrodes on one side of the device were
set at 5 V while the electrodes on the opposite side are grounded. The bottom, top and side walls of the
device were regarded as insulators, where the gradient of the electric potential was zero.
Figures 5-A, B and C show the gradient of the electric field square along x, y and z directions.
Figures 5-A and B show that the field has a much higher gradient at the corners of electrode tips.
Figure 5-C shows that the field gradient is positive in region between the electrode tips and is negative
at electrode edges. Therefore, particles with a positive CM factor are trapped when they are close to
the electrode tips, and are strongly levitated when are located within the intermediate region.
The simulation results assert that the design of this DEP platform is appropriate for the separation
experiments. The region between electrodes has a weak and uniformly distributed electric field, which
is suitable for producing negative DEP while the tips of electrodes have a sharp electric field, which is
suitable for producing positive DEP. Hence spatial separation of the particles will occur if some of
them experience negative DEP and the others experience the positive DEP forces.
4. Experimental
The DEP platform was microfabricated using photolithography on glass substrates. The glass
substrate was first metallised (Electron-beam evaporation technique) using Cr and Au with the
thickness of 50 nm and 150 nm, respectively. The AZ 1512 photoresist was then spin coated at 3000
rpm (acceleration of 1000 rpm/sec) for 25 seconds, and this procedure was followed by soft baking for
20 minutes at 90 °C. A mask aligner (Suss MJB3) was used to expose the mask pattern on to the
photoresist. The photoresist was developed and then the metal film was etched using reactive ion
etching. After patterning the sample was cleaned using isopropyl alcohol and acetone and then dried
with nitrogen.
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