Finite Element Simulations

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					    ME 381 Term Project:

Dynamic Wettability Switching
 by Surface Roughness Effect

Bo He, Hang Cheng and Hongzhou Jiang
• Surface tension is the dominant force in sub
  millimeter length range;
• Applications in microfluid handling
• Surface tension control by electric potential,
  thermal gradient and optical means.
          Example: electrowetting
• Principle:                         • Device:

Ref): M.G. Pollack,, Applied    Ref): J.Lee, et al, Sensor
Physics Letter, vol.77 (11) 2000.     & 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
           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

  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).
   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
                                                                         (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
applications of these techniques                                         different proteins patterned by bosshard.html                      mFN
Limitations and Unsolved problems
• PDMS Deformation
  PDMS shrinks upon curing and swells in a number of non-
  polar solvents, which makes it difficult for high resolution
• 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

                      SU8         PR          Thin PDMS
(a)        Si

(b)             (e)                    PDMS

(c)   Si
                (f)                    PDMS

                                          Top of pillar
(d)             (g)                       Bottom substrate

                            Air path
Rough pattern and thin PDMS
            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
            Tools: ANSYS Multi-Physics Solver
• Summary
 Pneumatic Actuation Case
• Modeling
                      a = b = 25 µm
                      Thickness = 1 µm
                      Target z = 25

               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
         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
 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.
• Surface tension actuation actuation
  mechanism in micro fluid manipulation;
• Soft lithography;
• A membrane device fabrication and
  pneumatic actuation;
• Finite Element Analysis simulation,

Thanks to Prof. Espinosa and TA Yong Zhu.