1. Electrokinetic Microflows
ME 381R Lecture 22
Electrokinetic Microflows
Dr. Li Shi
Department of Mechanical Engineering
The University of Texas at Austin
Austin, TX 78712

2. IC Thermal Management Challenge
Here is the system layout of Prof Goodson’s project. We can see that the whole system was on one chip and there are several important components like the macro scale heat exchanger system. They are Micro channels in the evaporator region which sits on the chip, heat source; Condenser region and electrokinetic pump providing the driving force of the fluidic medium. Let’s talk about the EK pump first. EK pump controls flow by electrical potential across a porous medium, which generates a force that induces the liquid to flow. The electroosmotic flow (EOF) is generated in the charge double layer that forms in the first few nanometers of the liquid/dielectric interface. Solvated ions move under the influence of an applied external field, carrying the bulk liquid by viscous drag. The electroosmotically-driven flow rate, QEOF, is directly proportional to the applied voltage and the zeta potential of the porous pump medium. The maximum pressure generated, PMAX, is inversely proportional to the square of the pump medium's pore diameter. Therefore, by optimizing the pump medium's pore size and zeta potential, and controlling the applied voltage.
There is a company from Sandia lab research group focus on EK pump application. The animation can help us understand the physical phenomena behind that.
Courtesy: Prof. Ken Goodson, DARAPA Thermal Management Workshop

3. Electroosmotic Microchannel Cooling System
Electrokinetic (EK) pumps Animation:
Here is the system layout of Prof Goodson’s project. We can see that the whole system was on one chip and there are several important components like the macro scale heat exchanger system. They are Micro channels in the evaporator region which sits on the chip, heat source; Condenser region and electrokinetic pump providing the driving force of the fluidic medium. Let’s talk about the EK pump first. EK pump controls flow by electrical potential across a porous medium, which generates a force that induces the liquid to flow. The electroosmotic flow (EOF) is generated in the charge double layer that forms in the first few nanometers of the liquid/dielectric interface. Solvated ions move under the influence of an applied external field, carrying the bulk liquid by viscous drag. The electroosmotically-driven flow rate, QEOF, is directly proportional to the applied voltage and the zeta potential of the porous pump medium. The maximum pressure generated, PMAX, is inversely proportional to the square of the pump medium's pore diameter. Therefore, by optimizing the pump medium's pore size and zeta potential, and controlling the applied voltage.
There is a company from Sandia lab research group focus on EK pump application. The animation can help us understand the physical phenomena behind that.

4. Cooligy 150 W PC Prototype
Here is the system layout of Prof Goodson’s project. We can see that the whole system was on one chip and there are several important components like the macro scale heat exchanger system. They are Micro channels in the evaporator region which sits on the chip, heat source; Condenser region and electrokinetic pump providing the driving force of the fluidic medium. Let’s talk about the EK pump first. EK pump controls flow by electrical potential across a porous medium, which generates a force that induces the liquid to flow. The electroosmotic flow (EOF) is generated in the charge double layer that forms in the first few nanometers of the liquid/dielectric interface. Solvated ions move under the influence of an applied external field, carrying the bulk liquid by viscous drag. The electroosmotically-driven flow rate, QEOF, is directly proportional to the applied voltage and the zeta potential of the porous pump medium. The maximum pressure generated, PMAX, is inversely proportional to the square of the pump medium's pore diameter. Therefore, by optimizing the pump medium's pore size and zeta potential, and controlling the applied voltage.
There is a company from Sandia lab research group focus on EK pump application. The animation can help us understand the physical phenomena behind that.

5. Positive Dielectrophoresis (DEP)
 : permittivity
 : conductivity
: frequency
FDEP = 2  m r3 Re[ f CM(p*, m*)]E2rms
Clausius-Mossotti factor
i* =i + ji /
Re[ f CM(p*, m*)] > 0
Medium
Particle r F
DEP
Low E Field High E Field
The particle is attracted to the region of high E field

6. Positive Dielectrophoresis
Particles are attracted to the region of high E field
Courtesy:
P.R.C. Gascoyne
UT MDACC
Nanobelts can be trapped between an electrode pair with high yield using positive DEP with a 5 Vp-p, 1 MHz ac voltage.

7. Negative Dielectrophoresis
Courtesy:
P.R.C. Gascoyne
UT MDACC
Re[ f CM(p*, m*)] < 0 F
DEP
Low E Field High E Field
Particles are repelled to the region of low E field
We have developed a negative DEP focusing channel for micro-cytometry applications

8. Flow Cytometry
Measure size, shape, granularity, and fluorescence intensity of biological particles
Hydrodynamic Focusing Mechanism
Hydrodynamic
Focusing
Light Source
Cell
Sheath Flow
High Speed
Flow Chamber
Sample Flow
Fluorescence
Photo Detector
Sheath Flow:
-- Prevent cells from clogging
-- Focus cells within the focal point of the optical detection system

9. Motivation for a Micro Cytometer
Becton Dickinson FACSCalibur at ICMB UT Austin
Fluid Control & Optical System
Micro Cytometers:
-- Small, Portable
-- Inexpensive
Laser
-- Requiring small
sample volume
Flow
Chamber
-- Easy to operate
Waste
Reservoir
Sheath
Fluid Reservoir
Requiring a large reservoir for sheath fluid and waste
Sheath fluid must be kept free of dust and bacteria

10. Design of the DEP Focusing Channel
Particle
Electric Field Line
FNeg-DEP
Electrode
Top Wafer
Bottom Wafer
Elliptic
Channel
Electrode ring
Particle
When Re[fCM] < 0, negative DEP forces repel particles towards the center of the channel

11. Electric Field Simulation
a. E (V/m)
1.50105
1.32105
1.15105
9.75104
8.00104
6.25104
4.50104
2.75104
1.00104
Electrode
1.001015
5.621014
3.161014
1.781014
1.001014
b. -E2 (V2/m3)
7.5 V
The direction of the DEP force depends on the sign of Re[f]
FDEP = 2  m r3 Re[ f CM(p*, m*)] E2rms (Negative DEP: Re[f] <0)

12. Clausius-Mossotti Factor
Eukaryotic cells
Cytoplasm
Membrane
Frequency window
for negative DEP

13. Proof of Concept
250 mm wide channel