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.

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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.

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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.

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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).

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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].

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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.

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Figure 1.6: Two-electrode mixing [5]

Figure 1.7: Three-electrode mixing [5]

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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.

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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.

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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

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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.

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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

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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.

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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).

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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

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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.

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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].

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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.

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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

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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

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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.

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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.

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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.

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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.

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