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Dielectrophoretic On-chip Manipulation and Assembly of
Nanoparticles, Microparticles and Droplets
Orlin D. Velev Department of Chemical Engineering
North Carolina State University
Ketan H. Bhatt, Shalini Gupta, Brian G. Prevo
Simon O. Lumsdon, Eric W. Kaler
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On-chip colloidal engineering
Microelectronics
Electrical (photonic)signals
Electrical (photonic)signals
IBM
Electrical signals
Synthesis of micro- and nanostructured materials ?
?
Electrical signals or pressure
Liquid manipulation, reactions and analysis
Nanogen
Microfluidics
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Dielectrophoretic force acting on particles in planar electrode gap
0.1-2x104 HzGrounded
FDEP
231 )(Re2 rmsDEP ERwKF ∇= επ
r
The Clausius – Mossotti function K may have complex frequency behavior
( )( ) ( )222
12
1221
12
12
123
2Re
MWMW wK
τσστσεσε
εεεε
++−
++−
=12
12
2σσεετ
++
=MW
Maxwell-Wagner charge relaxation time
• Positive dielectrophoresis: K > 0. Particles are attracted to electric field intensity maxima.
• Negative dielectrophoresis: K < 0. Particles are repelled.
K < 0
K > 0
0E∇
E0
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Phenomenology of FDEP
Clausius-Mossotti function12
12
2εεεε
+−
=K
♦ FDEP is proportional to particle volume.
♦ FDEP is proportional to the dielectric permittivity of the medium, ε2.
♦ The DEP force vector is directed along the electric field gradient, which, in general, is not parallel to the electric field vector
23102 ERKFDEP ∇= εεπ
r
K < 0
K > 0
0E∇
E
♦ FDEP depends upon the magnitude and sign of the Clausius – Mossotti function:
Positive dielectrophoresis: K > 0 (or ε2 > ε1). Particles are attracted to electric field intensity maxima.
Negative dielectrophoresis: K < 0 (or ε2 < ε1). Particles are attracted to electric field intensity minima and repelled from maxima.
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Advantages of using alternating (AC) field
♣ Avoid electrophoresis and electroosmosis
♣ Avoid electrolysis
♣ Works with any particles
♣ Use the frequency dependence of FDEP
FDEP
+-+
++ -
-
-FEL
Gas
A couple of important electrode geometries (K > 0)
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Example: Dielectrophoretic behavior of latex microspheres
Low dielectric permittivity ε2Increased conductivity σ2 (counterion atmosphere)
⎪⎪
⎩
⎪⎪
⎨
⎧
>><+−
<<>+−
→
102
102
Re
12
12
12
12
MW
MW
wfor
wfor
K
τεε
εε
τσσ
σσ
• DC conduction governs low-frequency DEP attraction
• Dielectric polarization governs high-frequency DEP repulsion
T. Müller et al., J. Phys. D: Appl. Phys.29, 340 (1996).
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Second field-induced force: Particle chaining
Chains
Interactions between induced dipoles along the direction of the field
Fchain is always attractive. Proportional to K2
Weaker dependence on particle size than the direct dielectrophoretic force.
EFchain
222(max) EKRCFchain επ−=3103 <<C
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Summary:Electrophoretic + Dielectrophoretic
interactions on chip
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Overview – objects for on-chip manipulation
AC Voltage
Grounded
FDEP
5 - 10 nm
500 - 1000 nm
500 - 1000 µm
Nanoparticles
Microspheres
Droplets (from particle suspensions)
5 - 10 µmLive cells
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Dielectrophoretic assembly of microwires from gold nanoparticles
Suspension of nanoparticles 10-25 nm
High concentration along the field gradient
Planar electrode
Depleted area behind the wire
Speed of growth ≤ 50 µm/s
500 µm
Theoretical gradient strengthExperimental image
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Dielectrophoretic assembly of conductive microwires from metallic nanoparticles in
suspension
Hermanson, Lumsdon, Kaler and Velev, Science, 294, 1082 (2001).
High magnification, 8X speed
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Microwire structure by SEMBulk Surface
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Quantification of microwire growth rate
Assembly rate is not a function of field intensity EDiffusion controlled processBulk growth faster due to larger diffusion volume
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Modeling and simulation of microwire assembly
100 V
ε = 80
ε = 4
Finite element electrostatic calculations using conformal triangles mesh (TriComp package)
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Simulation of the kinetics of microwire assembly
Realistically reproduces the dynamics and features observed in real wire growth
Bhatt and Velev, Langmuir, 20, 467 (2004)
Experimental image – bulk wires
Bulk wires
Simulation
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Simulation proves the role of low-ε substrate and initial conditions
Experimental image – surface wiresSimulation
Simulation of the kinetics of assembly
Surface wires
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Control of wire branching and position
Straight unbranched wire through the bulk (high viscosity)
Parallel arrays on surface(high frequency, low intensity)
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Growing wires would spontaneously complete the circuit through the object
Predicting wire assembly in the presence of conductive object in the liquid
Wire assembly should begin at the object even though it is not connected
Model Experiment
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Summary – Microwire assemblyWe have learned to control
• Wire type – bulk or surface• Assembly pattern – single
straight or massively parallel• Growth direction & interfacing
We can simulate and predict The kinetic assembly processThe growth pattern and direction
Ready for nanotech applications♦ Bioelectronic interfacing♦ Chemical and biological sensors♦ Structures with anisotropic thermal and electrical properties
Bhatt and Velev, Langmuir, 20, 467 (2004)
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Overview – objects for on-chip manipulation
AC Voltage
Grounded
FDEP
5 - 10 nm
500 - 1000 nm
500 - 1000 µm
Nanoparticles
Microspheres
Droplets (from particle suspensions)
5 - 10 µmLive cells
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Photonic crystals via microsphere assembly …
… have been made quickly, but without long range orientation
… can be made exactly the way we want them, but slow and expensively
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Overview of geometries used in electrical field driven assembly
Holgado et al., 1999, Rogachet al., 2000, etc.
S. Fraden, A. Blaaderen, 2001
Trau, Savilleand Aksay, 1995, Gong and Marr, 2001, etc.
Presented here
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Dielectrophoretic assembly of electrically tunable photonic crystals
Experiment schematics
Latex or silica500 - 1000 nm
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Dynamics of the DEP controlled crystallization:Laser diffraction
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Stages of the 2D crystallization: Microscopy and diffraction
Fourier transform Diffraction pattern
5 s
15 s
Optical micrograph
• Single domain cm-scale crystals with specific orientation • Switchable 2D phase transitions
Lumsdon et al., Appl. Phys. Lett., 82, 949 (2003).
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Quantitative measurements via the 2D crystal diffraction pattern
0.7 µm diameter latex1.0 µm diameter latex
1.4 µm diameter latex
θλ
sincnh =
Corrected for the refractive index of the composite media
Corrected for refraction on exiting the cell
( ) 2/122 )1( wp
oc nn φφ
λλ−+
=
measair
cell
nn θθ sinsin =
von Laue equation for 2D point scatterers
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Effect of electrolyte concentration on distance between particle surfaces
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.0000001 0.000001 0.00001 0.0001 0.001 0.01
[NaCl] / M
Spac
ing
betw
een
part
icle
sur
face
s / µ
m
0.7 µm1.0 µm1.4 µm 1/κ
No electrolyte added
• Precise simple measurements of interactions in particle ensembles
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Scaling approximation
Controlling the crystallization:Effect of field and frequency
• Crystallization threshold not a strong function of particle size
50
100
150
200
250
300
100 1000 10000 100000
Frequency / Hz
ri E i
/ V
cm-1
μm
Disordered particles
2D crystal
2,
22
1 )( chainiichain
T
ErconstKC
F==
−ωεπ
Data for particles of three sizes: 0.7, 1 and 1.4 μm
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Assembly of 2D crystals is prevented above 25 % glycerol
Controlling the crystallization – effect of glycerolIn
tens
ity /
V cm
-1
Volume % glycerol
100
125
150
175
200
0 10 20 30 40 50 60
2D crystals
Disordered
1D Chains
• Separations, study of colloidal self-organization fundamentals
Lumsdon, Kaler and Velev, Langmuir, 20, 2108 (2004).
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DEP assembly of cell chains for biosensors
5 μm yeast cells
50-60 V200 Hz
DEP chaining of 0.1 % yeast cell solution in 10 μM PBS
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Cell-particle dipole interactions as a function of voltage and frequency
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Summary – 1D and 2D arrays by dielectrophoresis
• Rapid and simple assembly technique• Extremely large crystals with specific orientation without
microfabricated templates• Model for combination of chaining and dielectrophoresis supported by
direct observation and diffraction• Can measure electrostatic interactions in particle ensembles• Electrically tunable photonic devices demonstrated• Can be applied to making cell-nanoparticle biocomposites
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FDEP
Dielectrophoretic on-chip manipulation of suspended droplets
… can we manipulate droplets and why?
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Conventional microfluidics with channels
Permanently rigged “pipes”, specific design for pre-defined operations
More like Factory-on-a-chip than Lab-on-a-chipHandling dispersions and biological objects a problem
Caliper Technologies Anderson et al., Nucleic. Acids Res. 28:60
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Previous work on moving droplets by electric fields
The channels are gone, but the walls are still there
Contact angle hysteresis, surface fouling, precipitation or aggregation forbidden, transport of cells, particles and biomolecules problematic
Pollack et al. Appl. Phys. Lett. 77:1725 (2000).Cho et al. J. Microelectrom. S. 12:70-80 (2003).
Jones et al. J. Appl. Phys. 89:1441 (2001).Jones, J. Electrostat. 51:290-299 (2001).
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Dielectrophoretic chips with suspended microdroplets: Basic principle
Liquid – liquid chip system without walls or channels
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Calculated field intensities for the two equilibrium droplet positions
Droplet-chip geometry to scale.Finite element electrostatic calculations using conformal triangles mesh (TriComp package).
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Fluid chip function 1: Dielectrophoretic transport of multiple droplets in
series
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Dielectrophoretic manipulation: Droplet speed and field intensity
231 )(Re26 ERwKFVRF DEPhydr ∇==≈ επμπ
rr
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Suspended drop transport: Energy dissipation
• No energy dissipation is detected by current monitoring or drop observations
• Estimate for the energy required to move a 500 nL water droplet 1 cm at 2 mm/s:
≥ 1.4×10-4≥ 1.6×10-7≤ 9.4×10-10Energy required / J
1500001701Energy ratio
UnderestimateUnderestimateOverestimateType of estimation
• Circular channel of diameter 20 µm
• Poiseuille flow
• θAdvancing = 90 deg• θReceding = 80 deg
• No viscous dissipation
• Stokes sphere in bulk liquidAssumptions and
approximations
Viscous flow in microfluidic channel
Hemispherical droplet dragged on solid surface
Droplet moved in fluorinated oil
Extremely low energy needed for suspended droplet transport
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Fluid chip function: Mixing of two droplets at electrode track junctions
gold nanoparticles →
sulfate latex →
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Fluid chip function: Complex precipitated shells
Ca3(PO4)2precip. shell →
Unique possibilities for materials synthesis and encapsulation
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Fluid chip function: Mixing of two droplets of aqueous suspension and
encapsulation inside oil droplet
gold nanoparticles →
sulfate latex →
dodecane →
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100 �μ
The droplets float suspended on the surface of perfluorinated hydrocarbon oil
12
2
γρΔβ bg
=
Droplet shape depends on Bond number
For β ≥ 1 the droplets flatten
"Outside-in" templating: Advanced structured particles templated by surface tension
100 µm
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Droplet assembly Examples of the advanced structured obtained
Velev, Lenhoff and Kaler, Science, 287, 2240 (2000).
β → 0
Spherical Metal coated doughnuts
β > 0
Thoroidal “doughnuts” Half-magnetic
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Dielectrophoretic chips with microdroplets: Internal particle separations
1 min 7 min 11 min 18 min 50 min
Time
Thermal gradients due to evaporation lead to Marangoni effect and thermophoreticparticle separation on top
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Fluid chip parallelization: Simultaneous materials synthesis in multiple
on chip droplet “microreactors”
Advanced particleassembly in droplets
Massive parallelization possible
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Encapsulation inside oil droplet: Polymerized shells
Polymer laced with gold nanoparticles
Photopolymerized particles with hexanediol diacrylate (HDDA)
Encapsulated suspension droplets
All products can be encapsulated
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Summary: Liquid-Liquid Microfluidics
• Microfluidics without any walls or channels• Simple, inexpensive, flexible, great
experimentation tool• Fundamental effects yet to be explained:
droplet charging and internal polarization• Technological potential for
– Parallelization– Single cell/biomolecule transport– Materials synthesis by particle
assembly or precipitation– Precipitation and agglutination
microassays
Velev, Prevo and Bhatt, Nature, 426, 515 (2003)
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Thanks to:
• Peter Killpatrick• Jeffrey Millman• Daniel Kuncicky• David Woolard• Kevin Hermanson
NSF CHE-9876674 & NSF CAREERCamille and Henry Dreyfus foundationARO STIR
$
AC Voltage
Ground
FDEP
On-chip field driven assembly
• Efficient and controllable• Works with particles on any size• Engineered microfabrication• Allows interfacing colloid structures with electric microcircuits
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Acknowledgements
http://crystal.che.ncsu.edu
http://www.che.ncsu.edu/velevgroup/
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