Effect of Change in Ratio of Electrode to TotalPitch Length in EWOD Based Microfluidic System
Abhilash PaneriDepartment of Mechanical Engineering
Birla Institute of Technology and Science
Pilani, India 333031
Email: [email protected]
N.N.SharmaDepartment of Mechanical Engineering
Birla Institute of Technology and Science
Pilani, India 333031
Email: [email protected]
Abstract—The present work investigates the effect of variationof length of electrode for a EWOD (electrowetting on dielectric)based microfluidic flow. The discussed model takes into accountthe effect of energy gradient on droplet movement. The voltageand pitch length are kept constant at 60 V and 200μm respec-tively. The parameter measured is the velocity of the droplet inthe direction along channel length.
Various configurations were simulated and the best config-uration was attributed to the one with the highest and stableenergy gradient. Simulations of droplet movement are done onCoventorWare. The increase in length of electrode has been foundto increase the velocity of droplet movement.
Index Terms—Microfluidics, Electrowetting on Dielectric(EWOD), Surface tension. Energy gradient
I. INTRODUCTION
The consequences of changing the thermal or electrical
energy in changing the surface tension, which eventually in-
duces fluid flow, were studied first time by Lippmann [3]. The
actuation of fluid flow in micro-sized channels by changing
the surface tension is an attractive option [4] and is gain-
ing importance in the developing technology of MEMS and
microfluidics. Electro capillary-based microfluidic actuation
schemes, in which the surface tension is modified by appli-
cation of an electrical potential, provide greater driving force
and consume less power than thermocapillary-based methods.
Electrocapillary effects have been used in variety of microflu-
idic actuation schemes, including continuous electrowetting
(CEW), electrowetting (EW) and Electro-wetting on dielectric
(EWOD)[1]. In CEW, a liquid metal flows continuously in
the channel containing an electrolyte under the influence of
the electric potential applied across the channel. The problem
with CEW is that it requires use of two liquids, which may
create fabrication complexities and may damage the sample.
In the EW scheme, the liquid movement in a channel is a
result of change in the surface tension of the liquid induced
by Electrical Double Layer (EDL) formed at the interface of
the channel, air, and the liquid when a voltage is applied to
the electrode. But, as the droplet is in direct contact with the
electrodes, electrolysis of water may occur at high actuating
voltages, which limits the maximum attainable actuating force
[7], unless some form of insulating layer is coated on the
electrodes to lessen the electrolytic effect.
Droplet flows in EWOD based microfluidic flow are dif-
ferent from continuous systems as they deal with individual
droplets instead of continuous liquid flow. Electrowetting-on-
dielectric (EWD) microfluidics is based on the actuation of
droplet volumes up to several microliters using the principle
of modulating the interfacial tension between a liquid and an
electrode coated with a dielectric layer [8]. In order to move
the droplet, electric field is applied to only one portion of the
droplet by establishing an electric field in the dielectric layer.
This creates an imbalance of interfacial tension, which forces
the droplet to move [9]. Droplets are usually sandwiched
between two parallel plates; the bottom being the chip surface,
which houses the electrode array and the top surface being
either a continuous ground plate or passive top plate.
EWOD based microfluidic flows are better in monitoring
the water flow and actuation of liquid flow. Because of which,
EWOD is applied to fields like drug delivery systems, medical
devices and diagnostics[11], etc. Droplet velocity in micro
channels involving EWOD based microfluidic flows are mainly
dependent upon the frequency with which the electrodes are
switched in the device[10]. More is the frequency of switching
electrodes, higher will be the velocity with which droplet under
actuation will travel.
The performance of an EWOD based microfluidic flow
depends on the size, shape and orientation of its electrodes.
Scientists around the world have tried many different shapes,
but the most extensively applied are square shaped electrodes
[1].
In recent times, developments in μ-TAS (micro total anal-
ysis systems), biosensors [2] and LOC (Lab on chip) devices,
the implementation of EWOD based microchannels is gaining
importance . In the present work, an EWOD based microfluidic
flow has been simulated for a constant potential difference to
its electrodes of 60V [6] and then keeping the pitch (electrode-
gap pair) length at constant value of 200 μm. However, the
length of the electrode is varied and corresponding droplet
velocity is observed. The work is organized in 5 sections. In
the next section, model of EWOD based microfluidic flow is
developed. Third section presents the simulation including the
boundary condition, material used and the numerical results
obtained. Fourth section discusses the results and the final
section draws conclusions based on the simulation results.
2010 International Conference on Computer Applications and Industrial Electronics (ICCAIE 2010), December 5-7, 2010, Kuala Lumpur, Malaysia
978-1-4244-9055-4/10/$26.00 ©2010 IEEE 25
II. ENERGY BASED MODEL FOR ELECTROWETTING
INDUCED FLOWS
Jones [5] developed and discussed the energy-based model
for EWOD systems for vertical flow, according to which the
reason for the movement of the droplet is energy gradient. To
actuate droplets, the interfacial energy at an end of the droplet
is reduced by applying a voltage to an electrode at that end of
the droplet. The electric field-induced reduction in interfacial
energy causes the droplet to locally spread out. The resulting
change in contact angles sets up a pressure gradient, which
drives the droplet toward the actuated electrode. Flow can
also be analyzed from energy-minimization considerations,
according to which the droplet minimizes its surface energy
by transiting to the actuated electrode. The energy gradient
is thus the driving force behind EW-induced motion of the
droplet.
The energy minimization based approach is first explained
through the prediction of capillary rise or fall due to applied
voltage to the capillary walls. Fig. 1 shows a circular capillary
of radius R and length L with a coating of a dielectric material
on its inner wall. The thickness of the dielectric layer is tand its dielectric constant is k. Application of voltage across
the dielectric layer changes the capillary height. The rise in
fluid level(capillary height) is analyzed by estimating the total
system energy as a function of the capillary height h. The total
energy is the sum of the dielectric-liquid interfacial energy, the
dielectric-air interfacial energy and the potential energy of the
liquid column. The system energy when the liquid meniscus
is at a height h with an applied voltage V is
E(h) = 2πRh
(γSL
0 − kε0V2
2t
)+2πR(L−h)γSA
0+πR2ρlgh
2
2(1)
where γSL0 , γSA
0 are the Dielectric-Droplet and
Dielectric-Air interfacial energy respectively. ε0 is the permit-
tivity of vacuum, g is acceleration due to gravity, ρl is droplet
density, k is the dielectric constant.
Differentiating the above equation with respect to h, we
obtain
∂E
∂h= 2πR
(γSL
0 − kε0V2
2t− γSA
0
)+ πR2ρlgh (2)
Obtained above, is the expression of force (i.e. rate of
change of momentum), which when further simplified can be
written as,
d(mυ)
dτ= C1 + C2h (3)
where m is the mass and υ is the velocity of droplet .
Or,
υ = C1(τ) + C2(τ)h (4)
where C1 and C2 are constant terms
The model in (4) can be applied to the droplet actuation
on a Lab-on-chip device. The meniscus height is analogues
to the length of electrode under the droplet (Fig.2) and gis analogues to horizontal acceleration of the droplet. The
electrodes are switched on and off periodically, for droplet
to move. As shown in Fig.2, when the second electrode is
switched on, surface energy of the droplet over the activated
electrode decreases. As a result droplet moves under energy
gradient towards right to attain the minimum energy configu-
ration. Similarly, the next electrode is switched on and second
electrode is switched off, so the droplet keeps on moving under
the energy gradient. According to (4), more is the value of h;
more is the velocity of the droplet.
Fig. 1. Schematic of Electrocapillary rise model
Fig. 2. Electrowetting actuator cross-section. The black (centre) elctrode is‘on’, and the white electrodes are ‘off’
26
III. SIMULATION
A channel of length 1200 μm is considered with a constant
pitch of 200 μm. Substrate used is SILICON 100, on which
a dielectric material glass is used, with its k equal to 2.
Droplet liquid is taken as water with its viscosity 5.01e-09
Mpa.s, density 9.9982e-016 kg/μm3, and k value of 1000.
The simulations for this channel configuration are done on
Coventorware R©. A process model was designed and a 3D
model of the channel is generated and meshed (Fig.3). While
keeping the pitch constant, the ratio of electrode length to gap
is varied and change in droplet velocity due to this variation
is observed. The ratio of electrode length to gap length(pitch
ratio) between two adjacent electrodes simulated are 1:3, 3:5,
1:1, 11:9, 5:3, and 3:1.The results of the simulation are shown
in Table I. Voltage used on all the electrodes are 60 V.
Frequency of switching electrodes used is 1670 Hz [10].
Fig. 3. 3D model of the EWOD based microchannel
The snapshot of the simulation result done on
Coventorware R© for the pitch configuration 3:1 ( Fig.
4). In Fig. 4(a), the droplet shown is at the beginning of
the simulation. As soon as voltage is applied to the second
electrode, the droplet distorts under the action of the surface
tension forces and change in the droplet-dielectric contact
angle (Fig. 4(b)). The droplet moves towards the right end
of the channel (Fig .4(c)) and finally reaches the right end as
shown in Fig. 4(d).
IV. RESULTS
The maximum velocity of the liquid droplet was observed
for the electrode to gap ratio of 3:1( length of electrode =
150μm, length of gap = 50 μm). As shown in Table I, as
we increase the length of the electrode, keeping the total
pitch length constant at 200μm, the velocity of the droplet
also increases. Increase in velocity of droplet with increase in
electrode length is also obtained from (4).
V. CONCLUSIONS
The results we obtained after simulation are found to be
in agreement with the proposed theory, and in both cases,
the velocity of the droplet increases with increase in pitch
ratio. The experimental values of droplet velocity against pitch
TABLE IVELOCITY READINGS FOR DIFFERENT PITCH RATIOS
Pitch Ratio Velocity of Droplet(cm/s)
1:3 6.2
3:5 9.2
1:1 13.6
11:9 16.7
5:3 20.4
3:1 50.1
Fig. 4. Snapshot of the simulated microchannel
as simulated in Coventorware R©, and the values obtained
from (4) are plotted in Fig. 5. It is observed that the velocity
distribution with pitch ratio in (4) is linear, whereas that
obtained using Coventorware R© is non-linear, but both show
the trend of increase in velocity with increase in pitch ratio. At
very low and high values of the pitch ratio, the deviation of the
experimental values from the values obtained from (4) is less,
while for the intermediate ratios, large deviation is observed.
The linear distribution of velocity with h in (4) is debatable
and needs to be rectified as the adopted model in (4) do not
take into account the change in shape of the droplet, which
is significant in case of LOCs. For low values of pitch ratio,
the deformation of the droplet is low, as lesser part of it is
over the electrode, and so the deviation is low. For very high
values of pitch ratio, deformation of droplet takes place, but
it happens for a very short time, as the droplet experiences
very large force. As a result, the effect of change of shape
on velocity is negligible, and so the deviation is low for high
values of pitch ratio. So, the velocity values obtained from (4)
and from Coventorware R© show very low deviation for smaller
27
and higher values of pitch ratios, while the deviation is large
in the intermediate range, as shown in Fig. 5. The refinement
of model for inclusion of non-linearity due to non-uniform
shape of the droplet is being worked further.
Fig. 5. Average droplet linear velocity versus pitch ratio for both simulationand theoretical values
ACKNOWLEDGMENT
We would like to acknowledge Aeronautical Development
Agency (ADA), India for providing the MEMS Design Center
with software facility under the aegis of NPMASS (National
Program for MEMS and Smart Sensors)
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