1

electrowetting-driven

digital microfluidic devices

Frieder Mugele

University of Twente

Physics of Complex Fluids

Fahong Li, Adrian Staicu, Florent Malloggi, Rina Bakker, Jean-Christophe Baret

EW for droplet-based digital microfluidics

detachment / drop generation

droplet motion

drop merging

mixing

surface contamination

outline

wetting & electrowetting – some basics

principles of drop actuation

basics of EW-modeling

application-related fundamental issuesmixing in microfluidics

surface protection

conclusions & wish list

I: wetting & liquid microdroplets

50 µm

H. Gau et al. Science 1999

lvLpp lv

slsvY

cos

capillary equation Young equation

electrowetting: the switch on the wettability

conductive liquidinsulatorcounter electrode

UU

advancingreceding

high voltage:contact angle saturation

low voltage: parabolic behavior

20

2

1cos

))(cos(

Ud

U

lvY

electrowetting equation:

II: origin of electrowetting

dVEDAGi

ii

2

1

U

E

20

2UA

dA sl

r

i

ii

slsvsllvlv AUd

A

20

2

sleff

+

++

+ +++ + +

2)(20 rEplv

Maxwell stress:2)(

20 rEpel

modified capillary equation modified Young equation

principles of drop actuation

sld

el Adx

dU

dUxC

dx

dW

dx

dF 202

2)(

2

1

~ U

driving force:

how to make water run uphill with EW?

matrix chip

1mmITO glassoperating voltage: 70V @ 10kHzinsulator: teflon AF

matrix chip(10x10 electrode lines)

characteristics

drop volumes: 1nL … 1µL

possilbe fluids: broad spectrum ( table)

suitable liquids for EW

D. Chatterjee et al., Lab on a Chip 2006

characteristics

drop volumes: 1nL … 1µL

actuation voltage: few tens of volts

possilbe fluids: broad spectrum ( table)

drop speed & switching speed: O(cm/s) & tens of Hz

substrate materials: any insulator + hydrophobic

top coating (typically: Teflon AF)

• fAC=10 kHz

• fosc=17 Hz• glycerol + NaCl

solution in silicone oil

III: modelling EW-driven flow

500 µm

numerical calculations: volume of fluid

fexp= 24 Hz fnum= 34 Hzµ=80mPa s

volume of fluid calculationsexperiment

principle: contact angle variation + hydrodynamic response

attached state: = 65° detached state: = 155°

caveat: contact line dynamics !

IV a: EW-driven mixing in oscillating droplets

500 µmsalt water; fosc = 80 Hz; fAC= 10 kHz

PIV measurements(J. Westerweel & Ralph Lindken TU Delft)

flow visualization

drop oscillations speed up mixing 100 times

IV b: surface protection & oil entrapment

V ≈ cm/s

time

volt

age µm thick oil layers are entrapped

under moving drops entrapped film undergo instability

and break-up into droplets

conclusions

electrowetting is driven by the gain in electrostatic energy upon reducing the contact angle and/or moving the drop

dynamics: local contact angle variation + hydrodynamic response

EW is very reliable, reproducible, and broadly applicable

physical principles of EW are well understood

F. Mugele and J.-C. Baret

Electrowetting: from basics to applications

J. Phys. Condens. Matt. 17, R705-R774 (2005)

EW review article

issues to be fixed

droplet properties desired drop volumes ( electrode size)?

liquid properties (conductivity, chemical composition, surfactants)

device characteristics surface material requirements (Teflon AF)

AC – DC voltage ambient medium: oil vs. air? surface cleanliness / washing steps

reaction protocols volume vs. surface-bound reactions T-steps

detection techniques optical measurements integration of eletrical sensors (e.g. for conductivity)

drop splitting

CJ Kim et al.; UCLA

numerical simulations using diffuse interface model (Cahn-Hilliard equation)