Supplementary Material
On the droplet velocity and electrode lifetime of digital
microfluidics: voltage actuation techniques and
comparison
Cheng Dong, a† Tianlan Chen, a† Jie Gao, a Yanwei Jia, a Pui-In Mak, a*
Mang-I Vai, a and Rui P. Martins a,b
a
State-Key Laboratory of Analog and Mixed-Signal VLSI and FST-ECE, University of Macau,
Avenida Padre Tomás Pereira, Taipa, Macao, China.
b
on leave from Instituto Superior Técnico, University of Lisbon, Portugal
†
Equal Contribution
E-mail: pimak@umac.mo
Video Data Processing
Video files captured by a high-speed digital camera record the droplet movements
across two electrodes. Thus, a 3-sec length video (at 1200 f/s or 400 f/s) is enough to
record the whole droplet dynamics. Resolution of the video is 72.77 pixels per millimeter.
Processing of these videos follows the following steps:
Step 1: Each frame of the videos separate into individual image files. Since videos are
recorded under a sample rate of 1200 frames/sec, more than 3000 images can be
obtained from one video file.
Step 2: An image processing software Image J is used to analysis all pictures. First of all,
color pictures are transformed to 8-bit grayscale images. Then pixels with gray level
between 48 and 76 are extracted to enhance the droplet and suppress the background
noise. The size of droplet in the images is ~8570 pixels under our experiments settings,
graphs with a size between 1000 and 10,000 pixels and circularity between 0.5 and 1.0
is abstracted. The so obtained shape is considered to be the droplet. Center of mass of
this droplet is calculated automatically by Image J and recorded for the following
processes. A graphical illustration of above procedures is shown in Fig. S1.
Step 3: For one video, over 3000 center of mass of droplet is obtained and put into
Matlab for calculation of the following parameters:
1. Displacement: Since in our experiment droplet is moved horizontally, the x-axis
position of the center of mass of droplet is defined as displacement.
2. Velocity: Velocity is first differential of displacement.
3. Start and End Points: Since the displacement curve is in the shape of flat, raising and
flat again. We take 50 points from the flat region and calculate their mean value and
standard deviation. The start point of movement is defined as the point and its
consecutive 10 points exceeds a threshold, which equals the sum of mean value and
5 times standard deviation. The definition of end point of movement is all the same as
start point except finding it from the end of the displacement curve.
4. Average velocity: it equals the displacement difference between the start and end
points divided the time between these two points.
5. Lowest velocity: The local minimum on the velocity curve around the middle of the
start and end points should be the point that the droplet move across the gap
between two electrodes. The lowest velocity is defined as the median value of the
velocity of 20 points nearby this local minimum.
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(a)
(b)
Area: 8579
Centroid: (61,60)
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(c)
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Area: 8560
Centroid: (133,60)
Area: 8544
Centroid: (206,60)
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Area: 8540
Centroid: (132,60)
Area: 8547
Centroid: (122,60)
Area: 8567
Centroid: (105,60)
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Area: 8576
Centroid: (84,60)
Area: 8579
Centroid: (66,60)
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Area: 8549
Centroid: (163,60)
Area: 8575
Centroid: (210,60)
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Area: 8569
Centroid: (186,59)
Area: 8589
Centroid: (210,60)
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(d)
Fig. S1 Illustration of off-line image processing. (a) Each frame of the videos is separated into individual image files.
(b) Color pictures are transformed to 8-bit grayscale images. (c) Pixels with gray level between 48 and 76 (marked in
red) are extracted, in order that the shape of the droplet can be enhanced and background noise can be suppressed.
(d) Graphs with a size between 1000 and 10,000 pixels and circularity between 0.5 and 1.0 are abstracted as droplet.
Area and center of mass of this droplet is calculated.