1 electrowetting-driven digital microfluidic devices frieder mugele university of twente physics of...
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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
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)