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Juan David Gutierrez-Franco

Mechanical Engineering

Allan Hancock College

Mentor: Meysam R. Barmi

Advisor: Prof. Carl Meinhart

Mechanical Engineering

1


Why is microfluidics

important

• Reduction of laboratory size, analysis

time, and sample needed

• Micro Total Analysis System (μTAS)

• Lab-on-a-Chip (LOC)

• Microarrays

• Portable and easily controllable

devices for chemical and biological

applications

Lab-on-a-Chip (LOC)

http://gigaomized-greendemo.blogspot.com/2011/02/lab-onchip-what-is-this.html

2cm

10cm

Microarray

http://i.i.com.com/cnwk.1d/i/ne/p/2

007/fluidigm-007_550x367.jpg 2


The Big Picture

• Mixing chemicals to perform

experiments

Evaporation of solvent

Mixing

Evaporation

3


Goals of the Project

Closed System

• To understand the physics of

electrowetting on open and

closed systems

• To create and test open

surface electrowetting

devices

Open System

• To lower required voltage to

move droplet

• To control the evaporation

rate of the droplet

Evaporation

4


Lab-on-a-Chip mixing chemicals

2.5 mm

Lab-on-a-Chip performing chemical reactions

http://www.chem.utoronto.ca/staff/WHEELER/html/Main.htm

5


Electrowetting

Droplets containing different chemicals need to be

mixed. The movement is done with electrowetting.

6


Research Method: Electrowetting

• Apply voltage to

droplets on chips to

achieve

electrowetting

without electrolysis.

• Try different solution

concentrations,

dielectrics, voltages

and frequencies.

1.5 cm

2.5 cm

7


Research Method: Electrowetting

1M KCl solution 4V 1kHz gold electrode

8


Research Method: Electrowetting

65.0

Contact Angle Change vs. Voltage

f=100 Hz 2.0 M KCl

55.0

Contact Angle Change (°)

45.0

35.0

25.0

15.0

5.0

Electrowetting

Electrolysis

Saturation

-5.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0

Voltage (V)

Saturation is seen once the contact angle does not

change while increasing voltage.

9


Higher molarity solutions show

better response to voltage at 1 kHz

Concentration Electrowetting Electrolysis

1 M KCl 4 V 6 V

0.1 M KCl 24 V 16 V

0.01 M KCl 120 V 90 V

• 1 M KCl solution showed the lower voltage needed to cause

electrowetting.

• As molarity dropped, electrolysis occurred sooner than

electrowetting.

10


Effect of Molarity on

Electrowetting and Electrolysis

Molarity

2.00

1.75

1.50

1.25

1.00

0.75

0.50

0.25

0.00

Molarity vs. Voltage 300 Hz

Electrowetting

Electrolysis

0.0 5.0 10.0

Voltage (V)

KCl solution on gold electrode with no dielectric.

Molarity

2.00

1.75

1.50

1.25

1.00

0.75

0.50

0.25

0.00

Molarity vs. Voltage 1 kHz

Electrowetting

Electrolysis

0.0 5.0 10.0 15.0

Voltage (V)

• As molarity and frequency drop, electrolysis occurs

sooner.

11


Molarity Effect on Contact Angle

Change

Contact Angle Change ( )

Contact Angle Change vs. Voltage

f=300 Hz

60.0

50.0

40.0

0.1 M

30.0

0.5 M

20.0

1.0 M

10.0

2.0 M

0.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Voltage (V)

Contact Angle Change ( )

60.0

50.0

40.0

30.0

20.0

10.0

Contact Angle Change vs. Voltage

f=1000 Hz

0.1 M

0.5 M

1.0 M

2.0 M

0.0

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Voltage (V)

• Actuation voltage is same for all cases.

• Saturation is reached sooner by the higher molarity

solutions.

12


Evaporation

Once the droplets are mixed, the solvent needs to be

evaporated.

13


Research Method: Evaporation

• Made of two

layers: Ti/Pt

(200/2500Å)

• Resistance is

measured

Resistive Temperature Detector (RTD)

• Resistance relates

linearly to

temperature

14


Calibration of RTD

• Fabrication can alter properties of

the RTD

• Calibration is needed for each chip

• Readings of resistance at known

temperature used for calibration of

RTD.

Resistance (Ω)

Calibration RTD Sample 1

41.5

41.0

40.5

40.0

39.5

39.0

38.5

296.0 301.0 306.0 311.0

Temperature (K)

Resistivity (nΩ*m)

160.0

155.0

150.0

145.0

R = 0.1532T - 6.6023 140.0

Average Resistivity RTD

295.0 300.0 305.0 310.0 315.0

Temperature (K)

15


Evaporation Time

• Evaporation rate was measured and plotted vs. temperature

Evaporation Rate (nL/s)

5.0

4.0

3.0

2.0

1.0

Evaporation Rate vs Temperature

0.0

305.0 310.0 315.0 320.0 325.0 330.0

Temperature (K)

• As temperature increases, the rate of evaporation of droplets

increases.

16


Summary


17


Future Plans

• Find dielectric that decreases voltage needed for

electrowetting and prevents electrolysis.

• Find way to dewet a droplet after electrowetting occurs.

• Control the evaporation rate of the droplet using a peltier

heater and the reading of the resistance of the RTD.

• Combination of electrowetting and evaporation devices.

18


Acknowledgments

• INSET Program organizers

• Jens-Uwe Kuhn

• Dr. Nick Arnold

• Prof. Megan Valentine

• Arica Lubin

• Microfluidics Lab

• Prof. Carl Meinhart

• Meysam R. Barmi

• Irvin Martinez

19


Thank you for your

attention

Summer 2011

20


Definitions

• Electrowetting: modification of the properties of the droplet’s

surface by applying electricity.

• Modifies contact angle and surface tension.

• Surface from hydrophobic to hydrophilic

• Microfluidics: use of small volumes of fluid to perform tasks

(reactions, movement, mixing)

• Free Surface: system where the droplet is in contact with the air.

• Hydrophobic: repels water

• Hydrophilic: attracts water

22


Contact Angle

• Distance at which the

liquid (droplet) and

vapor (air) interface

meets a solid surface.

• If the angle is:

• >90 surface is

hydrophilic


Electrolysis

• KCl is present in solution as

ions

• Once voltage is applied,

positive ions (K+) go to the

negative electrode and

negative ions (Cl-) go to the

positive electrode

• K gains an electron

• Forms potassium atoms

• Cl loses an electron

• Forms chlorine atoms

24


Electrolysis happening

25


Residue seen on chips

Desired residue

after electrowetting

Residue when

electrolysis occurs

(7V)

Residue when

electrolysis occurs

(100V)

26


Electrowetting Effect

Before

After

27


Before

After

28


Change in diameter of droplet after

electrowetting

~4mm

~5mm

Before

After

29


RTD Cross-Section

• Two layers

• 200 Å Titanium

• Acts as adhesive

• 2500 Å Platinum

30


Peltier Cooler

• Used to cool or heat chips.

• Positive voltage on red cable

cools the top surface and heats

the bottom surface

• Negative voltage on red cable

heats the top surface and cools

the bottom surface

31


Evaporation Rate Curve


32

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