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Three-Dimensional Computer Animation of an Electrowetting on Dielectric Based Lab-on-a-Chip
by
Frankie Yau
A project report submitted in conformity with the requirements for the degree of Master of Engineering
Graduate Department of Mechanical and Industrial Engineering University of Toronto
© Copyright by Frankie Yau 2011
ii
Three-Dimensional Computer Animation of an Electrowetting on
Dielectric Based Lab-on-a-Chip
Frankie Yau
Master of Engineering
Graduate Department of Mechanical and Industrial Engineering University of Toronto
2011
Abstract
This project presents a 3D graphical visualization of an electrowetting-on-dielectric (EWOD)
device. EWOD is the process altering the surface tension of droplet on a dielectric surface via
the application of an electric field. EWOD is used to manipulate droplet in digital microfluidic
devices via the creation of a surface tension gradient. Fluid flow in digital microfluidic devices
have been visualized using techniques including micro particle image velocimetry, and direct
imaging. This project presents a 3D graphical visualization of the key physical phenomenon that
occurs during the creation, transportation, merging, mixing, filtering, and splitting of liquid
droplets within an EWOD device. The visualization was created using 3D graphics software as a
series of meshes, and then animated.
iii
Acknowledgments
I would like to take this opportunity to thank my supervisor, Professor Sullivan, for his guidance
and support. I appreciate his help in the writing of this report. I would also like to thank
Michael Schertzer for explaining the details and requirements for the model, and providing me
with the necessary feedback on the model. I thank Professor Ben Mrad for reviewing and
providing feedback on the model.
I also thank the members of the free software community behind the Blender project for
providing the software used in to develop the model and the detailed documentation for the
software.
iv
Table of Contents List of Tables v List of Figures vi 1. Introduction 1
1.1. Background 1 1.2. Description of EWOD device 2 1.3. Literature Review 2
2. Model 8
2.1. Overview 8 2.2. Construction 8 2.3. Animating 13
3. Results and Discussion 16
3.1. Overview 16 3.2. Droplet Generation 16 3.3. Droplet Transportation 17 3.4. Droplet Merging 18 3.5. Mixing 19 3.6. Droplet Filtering and Washing 19 3.7. Droplet Splitting 21
4. Conclusion 22 Bibliography 23
v
List of Tables
Table 2.1: Colour and Alpha Values 11
vi
List of Figures
Figure 1.1: Schematic of EWOD Actuator 2
Figure 1.2: Experimental Setup 3
Figure 1.3: 2D Velocity Field at mid-height of droplet (z = 148 µm) 4
Figure 1.4: Side and Top View of Actuator 4
Figure 1.5: Experimental Setup 5
Figure 1.6: Two-electrode mixing 6
Figure 1.7: Three-electrode mixing 6
Figure 1.8: Four-electrode mixing 7
Figure 2.1: Electrode Layout 9
Figure 2.2: Mesh for Reservoir Barrier 10
Figure 2.3: Mesh for Filer 11
Figure 2.4: Menu View 14
Figure 2.5: IPO Editor 14
Figure 2.6: Lattice Keys 14
Figure 3.1: Droplet Generation – Experimental 16
Figure 3.2: 3D Graphical Representation of Droplet Generation 17
Figure 3.3: Droplet Transportation 18
Figure 3.4: Droplet Merging 18
Figure 3.5: Droplet Filtering 19
Figure 3.6: Close Up of Filtering 20
Figure: 3.7 Droplet Splitting 21
Chapter 1Introduction
1.1 Background
Electrowetting is the process of altering the wetting properties of a liquid on a surface with an
electric field. Electrowetting on dielectric (EWOD) uses an insulating layer over the electrodes
to prevent electrolysis of the liquid [1]. Droplets are manipulated via the application of
asymmetric electric fields [2]. Processes including dispensing, transporting, mixing, reacting,
mixing, and sensing can be accomplished in an EWOD device [3]. The contact angle in an
EWOD device is described using the Lippmann-Young equation [1].
20 2
coscos Vdlv
v γεθθ += (1.1)
where ε is the permittivity of the dielectric, γlv is the liquid-vapour inter-facial tension, d is the
thickness of the dielectric layer, V is the applied voltage, θ0 and θv are the contact angles between
the liquid substrate at zero and non-zero voltages respectively.
1
1.2 Description of EWOD device
Figure 1.1: Schematic of EWOD Actuator
A representative EWOD device (figure 1.1) consists of a ground electrode covered by a layer of
hydrophobic material on the top plate, and an array of actuation electrodes covered by a
dielectric and a hydrophobic layer on the bottom plate. The actuation electrodes are individually
addressable and are activated to generate a surface tension gradient across the droplet to induce
motion.
1.3 Literature Review
An investigation of the flow within a droplet in a parallel plate EWOD device using micro
particle image velocimetry was performed by Lu et al. [4]. Their experimental setup consisted
of a micro-PIV system as shown in Figure 1.2.
2
Figure 1.2: Experimental Setup [4].
The fluid used was deionized water seeded with 2 µm diameter, Nile red coloured, fluorescent
polystyrene microbeads (particle density: 2×107 particles/mL). A YAG laser was used to make
the microbeads fluoresce, while the long-pass filter ensures that the camera only captures only
the fluorescence.
Based on the experimental data, Lu et al. [4] determined that flow of liquid within the droplet is
symmetrical about the axis of motion (x-axis).
3
Figure 1.3: 2D Velocity Field at mid-height of droplet (z = 148 µm) [4]
The flow of fluid at the mid-height is depicted in both streamlines and velocity vectors in Figure
1.3.
Paik et al. visualized mixing of a fluorescent and non-fluorescent droplet within an
electrowetting device [5]. The experiment moved the fluorescent droplet on the left towards the
non-fluorescent droplet on the right until the droplets coalesce (Figure 1.4).
Figure 1.4: Side and Top View of Actuator [5].
4
The coalesced droplet was repeatedly moved left and right between two to four times while the
mixing process was observed. The droplets were illuminated using 490 nm (blue) light to trigger
fluorescence, and the mixing process was observed from both the top and sides using cameras
with long pass filters (>510 nm) to ensure only the fluorescence was captured.
Figure 1.5: Experimental Setup [5]:
The results of two, three, and four-electrode mixing at 8 Hz are depicted in Figures 1.6 to 1.8.
5
Figure 1.6: Two-electrode mixing [5]
Figure 1.7: Three-electrode mixing [5]
6
Figure 1.8: Four-electrode mixing [5]
The flow in two-electrode mixing (Figure 1.6) was reversible [5], implying laminar flow. In
addition, mixing occurs only at the interface between the fluorescent and non-fluorescent fluid
which indicts that the mixing is driven by diffusion rather than turbulence as expected. The
photo taken at t = 1.303s in Figure 1.7 (bottom row, middle image), and the photo taken at t =
1.871s in Figure 1.8 (bottom row, middle image) indicate the presence of internal circulation.
7
Chapter 2Model
2.1 Overview
A three-dimensional visualization of Michael Schertzer’s device was created in order to illustrate
key physical phenomenon. The processes visualized are generating, moving, merging, splitting,
mixing, reacting, filtering, and washing of droplets.
The visualization was generated from a three-dimensional model created using Blender 2.6.2
(http://www.blender.org/). The model scales are defined in terms of arbitrary units used in
Blender, Blender Units (BU). The dimensions along the x-y plane are to scale while the
dimensions in the z direction has been exaggerated for improved visualization. The devices
consists of six reagent reservoirs, an array of indexable electrodes, and a mechanical filter.
2.2 Construction
A three-dimensional model of the EWOD device was built using Blender 2.6.2. The substrates,
and dielectric layer were represented using a series of planes which were scaled to a horizontal
plane of 40 BU × 30 BU and extruded to a thickness of 2 BU and 0.3 BU respectively. The pivot
points were centred using the “Center New” function which moves the pivot point of an object to
coincide with its centre. The pivot point is a point separate from the vertices about which an
object pivots when rotated. The bottom plate was 0.3 BU below the X-Y (horizontal) plane.
The dielectric layer was directly on top of the bottom plate. The top plate was placed with its
bottom face at 2 BU above the X-Y plane.
8
A small electrode was created by extruding a plane of 2 BU × 2 BU to a thickness of 0.1 BU.
The electrode was then place on top of the bottom plate. The remaining small electrodes were
created as linked duplicates of the original electrode. Linked duplicates are objects which share
the same mesh. The electrodes underneath the reservoirs were created using a similar method.
The electrodes were arranged according to Figure 2.1. The electrode traces were then created by
scaling a plane down to 0.6 × 0.6 BU and then extruding it to the required shape.
Figure 2.1: Electrode Layout
In order to represent electrode activation, a series of area lamps, which illuminate a defined
region, were used to selectively illuminate the electrodes. A 2 × 2 BU area lamp was placed
under each small electrode to cause the electrode to glow when the lamp is on. A series of 1 × 1
BU area lamps where placed under the large electrodes. The series of lamps was arranged such
9
that the large electrode except the area overlapping with a small electrode would glow.
The barriers around the reservoirs were created as a series of linked duplicate. An object was
created by editing a plane into the shape shown in Figure 2.2 and extruding it to a height of 2.3
BU.
Figure 2.2: Mesh for Reservoir Barrier
The “Center” function was applied to move the mesh such that its centre coincides with the
piviot point. The barriers were moved the required locations and rotated to the required
orientation.
The filter was created by extruding a grid with 16 and 3 vertices along the x and y directions
respectively. The grid was then edited yield the shape depicted in Figure 2.3, and extruded to a
thickness of 0.1 BU.
10
Figure 2.3: Mesh for Filer
The “Center” function was used to move the object such that its point is located at the centre.
The object was rotated to the required orientation and moved to the required location.
Objects which are rendered in any appearance other than the default must be assigned a material.
A material is set of colour and alpha values, and rendering options. The substrates, dielectric
layer, barriers, and filter were assigned separate materials while the electrodes and traces shared
the same material. The parts were assigned the colour and alpha (opacity) values listed in Table
2.1
Table 2.1: Colour and Alpha Values
Part Red Green Blue Alpha
Top Plate 1 1 1 0.2
Bottom Plate 1 1 1 0.3
Dielectric Layer 0.5 0.7 0.7 0.1
Electrodes and Traces 1 0.8 0 1
Barriers 1 1 1 0.3
Filter 0 1 0 1
11
A rectangular prism with dimensions of 6 × 6 × 2 units was created to represent the fluid in a
reservoir. Four linked duplicates were made to represent the fluid in the remaining reservoirs.
Each droplet was represented using a truncated UV-sphere which is a spherical mesh consisting
of m parallel rings, and n meridians. A UV-sphere with 64 meridians and 32 rings was created.
The sphere was scaled up, and its top and bottom were truncated. The top ends were flat,
circular surfaces. The other droplets are represented with duplicates of the truncated sphere. The
rectangular prism and each of the truncated UV-spheres were assigned a material of its own with
initial red, green, blue, and alpha values of 0, 0, 1, and 0.6 respectively.
A lattice was created around each of the droplets and the droplet was made a child object of the
lattice with the lattice deform option. A lattice is a mesh which has an initial shape of a cube
with a side length of 1 BU. A lattice lacks faces; therefore, does not appear in the rendered
images. In Blender, parent and child relationships can be established between two objects.
When a parent object's location, orientation, or size is changed, the child objects' location,
orientation, or size will follow but not vice versa. If the parent object is a lattice, the lattice
deform option is available which causes child objects to deform when the vertices of the lattice
are moved. This option was used in the animation in order to enable the droplets to deform
without having to directly edit their meshes.
The particles were represented with a series of icospheres which are spherical meshes built from
triangular faces (similar to a geodesic dome). An icosphere was created with default properties
and then scaled down to a diameter of 2.8 BU. Linked duplicates of the ionosphere were then
made and placed throughout the rectangular prism representing the first antibody. Additional
linked duplicates were created and made child objects to the droplet of antibody.
12
The droplets were placed in their starting positions and the model was animated.
2.3 Animating
The animation was created by generating a series of frames and then converting them into a
video file. The animation was created to run at 96 frames per second in order to allow it to be
run at a slower speed while remaining smooth.
Key frames were created manually while the remaining frames were generated automatically via
interpolation. Key frames are frames which contain information about the size, shape,
orientation, and/or location of objects.
For the animation, each droplet moved at 1 electrode per second. The translation of the droplets
were accomplished by inserting a location key before the droplet moves, advancing the frame
count by 96, moving the droplet to the next electrode, and then inserting another location key.
Subsequent translations are made by advancing the frames, moving the droplet, and then
inserting a location key. The activation of the electrodes were illustrated by turning on and off
the required lamps beneath the electrodes and then inserting keys for the brightness.
The changes in size of droplets during merging and splitting were done by clicking "Add Shape
Key" for the lattice corresponding to the droplet whose size will change (Figure 2.4).
13
Figure 2.4: Menu View
to create a spine in the IPO editor labelled "Basis" (Figure 2.5). The numbers on the x-axis
corresponds to the frame number while the numbers.
Figure 2.5: IPO Editor
The basis curve in the IPO editor was adjusted such that the curve covers all of the frames in the
animation to enable the lattice deformation to occur at any point in the animation. Then the
lattice for the droplet is then edited and lattice keys were inserted at the appropriate frames.
Figure 2.6: Lattice Keys
14
The coloured horizontal lines represent the lattice keys. The key is found on the frame
corresponding to the intersection between the key and the basis. The lattice at frames before the
first key and after the last key takes the shape specified in the first and last key respectively.
Between keys, the shape of the lattice is determined by interpolation.
15
Chapter 3Results and Discussion
3.1 Overview
An animation of the device was produced by rendering the three-dimensional model. The
animation depicts the generating, transporting, mixing, splitting, and filtering of droplets. Screen
shots of the animation are presented and discussed in subsequent sections.
3.2 Droplet Generation
Droplet generation is accomplished by pulling a “tongue” of the fluid of a controlled size from
the reservoir, and then splitting the tongue off. A result generated experimentally by Cho et al.
[6] is depicted in Figure 3.1.
Figure 3.1: Droplet Generation – Experimental [6].
16
A 3D graphical representation of the droplet generation process is shown in Figure 3.2.
Figure 3.2: 3D Graphical Representation of Droplet Generation
The visualization of the droplet generation process depicts the activation of the electrodes used
for creating a “tongue” of fluid, and the cutting of the “tongue” to yield a droplet.
3.3 Droplet Transportation
Droplets are transported in the device via activation of the indexable electrodes. To move a
droplet, the electrode adjacent to the droplet is activated in order the decrease the contact angle
of the portion of droplet in the vicinity of the electrode. This causes a surface tension gradient
which pulls the droplet towards the activated electrode.
17
Figure 3.3: Droplet Transportation
The visualization shows the droplet travelling towards the activated electrode.
3.4 Droplet Merging
Droplets are merged through transporting them towards the same electrode. The volume of the
final droplet is sum of the incoming droplets as dictated by conservation of mass and
incompressibility.
Figure 3.4: Droplet Merging
The visualization of the merger of two droplets shows the two droplets being pulled by the
18
activated electrode towards each other and the conservation of volume.
3.5 Mixing
The merged droplets are mixed being moved back and forth to generate internal circulation and
encourage diffusion mixing.
3.6 Droplet Filtering and Washing
The droplet is washed by passing washing fluid through it while over a filter to prevent the
suspended particles from being washed out. The process consists of moving the droplet towards
the filter. A droplet of washing fluid is transported towards the filter from the other side in order
the wet the filter and draw the droplet with suspended particles towards the filter.
Figure 3.5: Droplet Filtering
19
Figure 3.6: Close Up of Filtering
The visualization shows the small droplet of washing fluid being drawn towards the filter and
wetting it. The large droplet is then drawn towards the filter while the suspended particles
remain on one side.
20
3.7 Droplet Splitting
A droplet is split in two by activating two electrodes adjacent to the droplet. This creates forces
pulling on both ends of the droplet which divides it into two smaller droplets.
Figure: 3.7 Droplet Splitting
The visualization shows the activation of the two electrodes beside the filter resulting in the
droplet being pulled apart into two droplets.
21
Chapter 4Conclusion
The model presented here visualizes key physical phenomena in the EWOD device droplet
generation, movement, merging, mixing, filtering, washing, and splitting. The sequence of
electrode actuation was visualized along with the simultaneous droplet motion. For all functions
visualized, the important aspects of the physical processes were presented.
22
Bibliography
[1] J. H. Chang, D. Y. Choi, S. Han, and J. J. Pak, “Driving characteristics of the
electrowetting-on-dielectric device using atomic-layer-deposited aluminum oxide as the
dielectric,” Microfluidics and Nanofluidics, vol. 8, no. 2, pp. 269–273, 2010.
[2] M.J. Schertzer, R. Ben-Mrad, and P.E. Sullivan, “Using capacitance measurements in
EWOD devices to identify fluid composition and control droplet mixing,” Sensors and
Actuators B: Chemical, vol. 145, no. 1, pp. 340–347, 2010.
[3] M. G. Pollack, A. D. Shenderov, and R. B. Fair, “Electrowetting-based actuation of
droplets for integrated microfluidics,” Lab On A Chip, vol. 2, no. 2, pp. 96–101, 2002.
[4] H.-W. Lu, F. Bottausci, A. L. Bertozzi, C. D. Meinhart, and C.-J. Kim, “ PIV investigation
of 3-dimensional flow in drops actuated by EWOD,” in Journal of Micro Electro
Mechanical Systems, 2008, pp. 571–574.
[5] P. Paik, V. K. Pamula, M. G. Pollack, and R. B. Fair, “Electrowetting-based droplet
mixers for microfluidic systems,” Lab On A Chip, vol. 3, no. 1, pp. 28–33, 2003.
[6] S. K. Cho, H. Moon, and C.-J. Kim, “Creating, Transporting, Cutting, and Merging Li-
quid Droplets by Electrowetting-Based Actuation for Digital Microfluidic Circuits,”
Journal of Microelectromechanical Systems, vol. 12, no. 1, pp. 70–80, 2003.
23