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ME 381 Term Project:
Dynamic Wettability Switching
by Surface Roughness Effect
Bo He, Hang Cheng and Hongzhou Jiang
Introduction
• Surface tension is the dominant force in sub
millimeter length range;
• Applications in microfluid handling
technique;
• Surface tension control by electric potential,
thermal gradient and optical means.
Example: electrowetting
• Principle:
Ref): M.G. Pollack, et.al, Applied
Physics Letter, vol.77 (11) 2000.
• Device:
Ref): J.Lee, et al, Sensor
& Actuator, 2001.
Wettability shift due to roughness
108.4
154.4
Flat surface
Roughened surface
Testing device
Droplet motion across different wettability regions
Interface
Interface
 flat rough 
Soft Lithography
• Soft Lithography was first developed by M. Whitesides in
Harvard in 1990s
• A non-photolithographic strategy based on self-assembly and
replica molding for carrying out micro- and nanofabrication.
• It provides a convenient, effective, and low-cost method for
the formation and manufacturing of micro- and nanostructures.
• Unlike conventional lithography, these techniques are able to
generate features on both curved and reflective substrates and
rapidly pattern large areas.
Soft Lithography Process
• In soft lithography, an elastomeric stamp with patterned relief structures
on its surface is used to generate patterns and structures with feature size
ranging form 30 nm to 100 mm.
• Elastomeric polydimethylsiloxane (PDMS) is most widely used. Other
materials include polyurethanes, polyimides, and cross linked phenol
formaldehyde polymers
•
•
•
•
•
Microcontact Printing (mCP)
Replica Molding (REM)
Micromolding in Capillaries (MIMIC).
Microtransfer Molding (mTM).
Solvent-assisted Microcontact Molding (SAMIM).
Microcontact Printing (mCP).
• An "ink" of alkanethiols is
spread on a patterned PDMS
stamp. The stamp is then
brought into contact with the
substrate. The thiol ink is
transferred to the substrate
where it forms a self-assembled
monolayer that can act as a
resist against etching, or as the
carrier for chemical/biological
functionality. Features as small
as 300 nm have been made in
this way.
•
Figure 1: Schematics of Microcontact printing
(mCP) process
http://www.sims.nrc.ca/ims/ ittb/2000-02e.html
Replica Molding (REM)
•
A PDMS stamp is cast against a conventionally patterned master. Polyurethane
is then molded against the secondary PDMS master. In this way, multiple
copies can be made without damaging the original master. The technique can
replicate features as small as 30 nm
http:// www.engr.washington.edu/~cam/CAMreplicamolding.html
Micromolding in Capillaries (MIMIC)
• Micromolding in Capillaries
(MIMIC). Continuous channels
are formed when a PDMS
stamp is brought into conformal
contact with a solid substrate.
Capillary action fills the
channels with a polymer
precursor. The polymer is cured
and the stamp is removed.
MIMIC is able to generate
features down to 1 µm in size
Figure 3: Schematics of Micromolding in
Capillaries (MIMIC).
http://www.engr.washington.edu/~cam/CAMmimic.html
Example: Microcontact printing (mCP) reveals
its application with micro fluidic networks (mFN) to
pattern substrates with proteins
(a) Fluorescence from a patterned
immunoglobulin G monolayer on
a glass slide created by mCP;
(b) AFM image of a small
stamped feature of antibodies on
a silicon wafer;
(c) A neuron and its axonal
outgrowth on affinity-stamped
axonin-1;
(d) Repetitive stamping of
different proteins onto the same
plastic substrate;
(e) Water condensation pattern on
micropatterned albumin forming
Figure 4: Microcontact printing (mCP) and microfluidic networks (mFN) droplets of ~2 mm in diameter;
are powerful techniques to pattern substrates with proteins. Examples of (f) Fluorescence micrograph of
different proteins patterned by
applications of these techniques
http://www.snf.ch/nfp/nfp36/progress/ bosshard.html
mFN
Limitations and Unsolved problems
• PDMS Deformation
PDMS shrinks upon curing and swells in a number of nonpolar solvents, which makes it difficult for high resolution
molding.
• Difficulty of Registration
the elasticity and thermal expansion of PDMS limit the
accuracy in registration across a large area and application
in multilayer fabrication
• Limited Aspect Ratio
The softness of an elastomer limits the aspect ratio of
microstructures in PDMS
Device fabrication
PDMS
SU8
(a)
(b)
(c)
PR
Thin PDMS
Si
(e)
PDMS
(f)
PDMS
PDMS
Si
Si
Top of pillar
(d)
(g)
Bottom substrate
PDMS
Air path
Rough pattern and thin PDMS
membrane
Device testing
• Membrane actuation by pneumatic means.
OFF
ON
Device testing
• Roughness switch.
Actuated
Released
Problems and future direction
• Penumatic cannot provide enough
membrane deflection;
• Addressable control: electrostatic actuation.
Top glass
Superhydrophobic
Medium hydrophobic
Finite Element Simulations
• Pneumatic Actuation Case
Objective: To diagnose the pneumatic
actuated chip.
Tools: ABAQUS and ANSYS.
• Electrical Actuation Case
Objective: To determine the applied
voltage.
Tools: ANSYS Multi-Physics Solver
• Summary
Pneumatic Actuation Case
• Modeling
Dimensions:
a = b = 25 µm
Thickness = 1 µm
Target z = 25
µm.
Boundary Conditions
Material Properties:
E = 0.75 MPa
v = 0.49
Pneumatic Actuation Case
• Solutions
1. ABAQUS-S4R reduced 4
node shell element.
2. Number of elements: 281
3. Nonlinear solution tag
4. ANSYS’s verification
Pneumatic Actuation Case
Electrical Actuation Case
• Modeling
Dimensios:
a = b = 2 µm
Thickness = 1 µm
Gap = Target z = 3.3 µm
Boundary conditions
Material Properties
Electrical Actuation Case
• Solutions
1. Sequentially Electrostatic-Structural coupled
solver
2. ANSYS Solid122 and Solid95 elements
3. Triangular meshing and brick meshing
4. Nonlinear geometric option
5. Time step increment
6. The closest z-displacement = 3.24 µm.
Electrical Actuation Case
Simulation summary (1)
• For the Pneumatic case, our simulation
results indicated fundamental limitations
of the device structure. The reason is
probably that the membrane above the
air path collapses first once the suction
is applied. This will block the path and
stop the further deflection of the
membrane. New design of pneumatic
actuation structure is needed to provide
enough membrane deflection.
Simulation summary (2)
• For electrostatic case, our simulation
predicted the appropriate voltage range. The
structure optimization can be performed in
future. The contact pair of the lower surface
of the film and the upper surface of the pillar
can be added to predict more accurate
results. The fillet radius would be determined
by the art of fabrication process. However,
larger fillet radius does provide less stress
concentration and less convergence problem
for FEM simulation.
Conclusion
• Surface tension actuation actuation
mechanism in micro fluid manipulation;
• Soft lithography;
• A membrane device fabrication and
pneumatic actuation;
• Finite Element Analysis simulation,
ABAQUS and ANSYS.
Acknowledgements
Thanks to Prof. Espinosa and TA Yong Zhu.
Questions
???