Gecko Adhesion

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Gecko Adhesion
Interfacial Phenomena April 24th, 2007

• ZiQiu Tong
• Premal Trivedi
• Jennifer Tullman

2
Outline

• Overview
• Capillary vs. van der Waals Forces
• Aspects to control
• Methods for fabricating
• Improvements
• Applications

3
Outline

• Overview
• Capillary vs. van der Waals Forces
• Aspects to control
• Methods for fabricating
• Improvements
• Applications

4
Overview

• Use geckos as a model to design synthetic
  adhesives
• Determine forces that govern adhesion
• Design synthetic adhesive to mimic whatever is
  causing this adhesion

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Types of Geckos

• Leopard Gecko

Tokay Gecko
Geico Gecko
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Different Gecko Foot Designs
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Structure of Gecko Feet
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Structure of Gecko feet - Setae
Full PNAS (2006)
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Structure of Gecko feet - Setae

• Setae occur in uniform arrays
• Length 110 µm
• Diameter 5 µm
• Density 14,400 per mm2
• Single seta can generate up to 200 µN of Force

Hansen PNAS (2006)
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Structure of Gecko feet- Spatulae
Full PNAS (2006)
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Structure of Gecko feet- Spatulae

• Each seta branches into 100-1000 spatulae
• Spatula Length
• 2 um
• Tip Radius
• 100 nm

Full PNAS (2006)
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Mechanism of Adhesion

• Proposed Mechanisms
• Friction
• Reject Friction forces too low
• Electrostatics
• Reject Use anti-static gun but adhesion
  remains
• Interlocking
• Reject Equally adhesive on smooth surfaces
• Suction
• Reject Adhesion remains even in vacuum
• Capillary Action
• Van der Waals

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Mechanism of Adhesion

• Proposed Mechanisms
• Friction
• Reject Friction forces too low
• Electrostatics
• Reject Use anti-static gun but adhesion
  remains
• Interlocking
• Reject Equally adhesive on smooth surfaces
• Suction
• Reject Adhesion remains even in vacuum
• Capillary Action
• Van der Waals

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Mechanism of Adhesion

• Proposed Mechanisms
• Friction
• Reject Friction forces too low
• Electrostatics
• Reject Use anti-static gun but adhesion
  remains
• Interlocking
• Reject Equally adhesive on smooth surfaces
• Suction
• Reject Adhesion remains even in vacuum
• Capillary Action
• Van der Waals

15
Mechanism of Adhesion

• Proposed Mechanisms
• Friction
• Reject Friction forces too low
• Electrostatics
• Reject Use anti-static gun but adhesion
  remains
• Interlocking
• Reject Equally adhesive on smooth surfaces
• Suction
• Reject Adhesion remains even in vacuum
• Capillary Action
• Van der Waals

16
Mechanism of Adhesion

• Proposed Mechanisms
• Friction
• Reject Friction forces too low
• Electrostatics
• Reject Use anti-static gun but adhesion
  remains
• Interlocking
• Reject Equally adhesive on smooth surfaces
• Suction
• Reject Adhesion remains even in vacuum
• Capillary Action
• Van der Waals

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Outline

• Overview
• Capillary vs. van der Waals Forces
• Aspects to control
• Methods for fabricating
• Improvements
• Applications

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Capillary vs. Van Der Waals Forces

• Capillary Adhesion
• Sand castle effect
• Depends on contact angle, type of surface
  (hydrophilicity)

• Van Der Waals
• Depends on polarization of molecules into
  dipoles
• Work in polar/nonpolar because dielectric is
  probabilistic
• Depends on distance
• ? A / (24pe2)

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Capillary Forces

• Changed relative humidity, measured adhesion
  force
• Also changed contact angle
• Capillary Action (Thin Film Adsorption) requires
  hydrophillic (not hydrophobic) substrate
• -30o hydrophilic
• -110o hydrophobic

AFM cantilever data
Samper Biophysical Journal (2005)
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Capillary Forces

• Capillary Action (Thin Film Adsorption) requires
  hydrophillic (not hydrophobic) substrate

Full PNAS (2002)
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Van Der Waals

• Measured force
• Created model based on Johnson Kendall Roberts
  theory of adhesion
• Calculated size of tip
• Compared to actual size of tips

• F (3/2)pRW
• Wexpt 50 mJ/m2
• Fexpt 40 µN/seta
• Calculated R 0.13 µm

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Capillary vs. Van Der Waals Forces

• Capillary
• Used dead gecko
• Used spiney-tailed house gecko
• AFM cantilever data

• Van Der Waals
• Used live gecko
• Used Tokay gecko
• Used two methods of measuring adhesion

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Mechanism of Adhesion

• Proposed Mechanisms
• Friction
• Reject Friction forces too low
• Electrostatics
• Reject Use anti-static gun but adhesion
  remains
• Interlocking
• Reject Equally adhesive on smooth surfaces
• Suction
• Reject Adhesion remains even in vacuum
• Capillary Action
• Reject Equally Adhesive on Hydrophillic and
  Hydrophobic Surfaces.
• Van der Waals

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Mechanism of Adhesion

• Proposed Mechanisms
• Friction
• Reject Friction forces too low
• Electrostatics
• Reject Use anti-static gun but adhesion
  remains
• Interlocking
• Reject Equally adhesive on smooth surfaces
• Suction
• Reject Adhesion remains even in vacuum
• Capillary Action
• Reject Equally Adhesive on Hydrophillic and
  Hydrophobic Surfaces.
• Van der Waals

25
Mechanism of Adhesion

• Proposed Mechanisms
• Friction
• Reject Friction forces too low
• Electrostatics
• Reject Use anti-static gun but adhesion
  remains
• Interlocking
• Reject Equally adhesive on smooth surfaces
• Suction
• Reject Adhesion remains even in vacuum
• Capillary Action
• Reject Equally Adhesive on Hydrophillic and
  Hydrophobic Surfaces.
• Van der Waals
• Accept Shear Force Predictions Closely Meet
  Observed Values

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Mechanism of Adhesion

• Proposed Mechanisms
• Friction
• Reject Friction forces too low
• Electrostatics
• Reject Use anti-static gun but adhesion
  remains
• Interlocking
• Reject Equally adhesive on smooth surfaces
• Suction
• Reject Adhesion remains even in vacuum
• Capillary Action
• Reject Equally Adhesive on Hydrophillic and
  Hydrophobic Surfaces.
• Van der Waals
• Accept Shear Force Predictions Closely Meet
  Observed Values

27
Outline

• Overview
• Capillary vs. van der Waals Forces
• Aspects to control
• Methods for fabricating
• Improvements
• Applications

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Van der Waals

• Van der Waals is weak but universal.
• significant over short range (lt10 nm)
• spatular tip geometry is important
• easier to study and mimic

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Roughness

• Low/medium/high RMS surface roughness
• Note full contact at high and low RMS, partial
  contact at medium
• 90 nm worst (size of tip (foot) of spatula
  100nm)

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Conformity to Roughness
Majidi Thesis (2004)
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Self Cleaning

• Setal arrangement promotes escape of contaminants
• Contaminant only affects adhesion locally

Majidi Thesis (2004)
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Self Cleaning
Hansen_PNAS_2006

• For self-cleaning NOT to occur (i.e. energetic
  equilibrium is achieved between wall, particle,
  and spatulae),
• N gt 26
• (Observation 1-10 spatulae engage onto dust
  particles)

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Directional Adhesion

• Setae are slightly curved
• Adhesion is directional
• 10X higher adhesion in forward direction

Majidi Thesis (2004)
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Why the Angle ?
Dashed 45 Degree Angle Solid No Angle
(Straight)

• Angled spatulae provide greater resistance to
  tensile stress on rough surfaces
• Greater access to substrate surface area due to
  angle.

Majidi Thesis (2004)
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Peeling vs. Fracture
Majidi Thesis (2004)
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Peeling

• Macro Scale Peeling
• Micro Scale Peeling

Full PNAS (2006)
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Outline

• Overview
• Capillary vs. van der Waals Forces
• Aspects to control
• Methods of fabrication
• Improvements
• Applications

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Method of Mimicking

• Sitti and Fearing, March, 2003
• Nanorobotic Imprinting
• Geim, June 2003
• E-beam lithography and
• dry etching
• Kim, 2007
• UV nano embossing
• Yurdumakan, 2005
• Multiwalled carbon nanotubes

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Sitti - Nanorobotic Imprinting

• Silicone rubber
• Pull-off force measurement
• F1809nN
• FJKR185nN

Sitti J. Adhesion Sci. Technol (2003)
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Statistics
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Method of Mimicking

• Sitti and Fearing, March, 2003
• Nanorobotic Imprinting
• Geim, June 2003
• E-beam lithography and
• dry etching
• Kim, 2007
• UV nano embossing
• Yurdumakan, 2005
• Multiwalled carbon nanotubes

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Geim E Beam Lithography

• Produced high aspect ratio polyimide hairs
• Densely packed
• Too thin ? fall down
• Long, closely spaced ? clumping

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Statistics
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Method of Mimicking

• Sitti and Fearing, March, 2003
• Nanorobotic Imprinting
• Geim, June 2003
• E-beam lithography and
• dry etching
• Kim, 2007
• UV nano embossing
• Yurdumakan, 2005
• Multiwalled carbon nanotubes

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Kim - UV Nano Embossing

• simple and cost effective replication method
• Fabrication of Anodic Aluminum Oxide (AAO) master
  mold
• Electropolishing
• Two-step anodization

Kim et al, Microsyst Technol (2007)
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Statistics
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Method of Mimicking

• Sitti and Fearing, March, 2003
• Nanorobotic Imprinting
• Geim, June 2003
• E-beam lithography and
• dry etching
• Kim, 2007
• UV nano embossing
• Yurdumakan, 2005 Zhao, 2006
• Multiwalled carbon nanotubes

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Multiwalled carbon nanotubes (MWCNT)

• Photolithography/chemical vapor deposition
• Bend repeatedly without failure
• Extraordinary electrical and thermal conducting
  property

Yurdumakan et al, The Loyal Society of Chemistry,
(2005) Zhao et al, J. Vac. Sci. Technol. (2006)
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Statistics
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Outline

• Overview
• Capillary vs. van der Waals Forces
• Aspects to control
• Methods for fabricating
• Improvements
• Applications

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How far have we progressed?
Photolithography Dry etching
AFM Nanorobotic Imprinting
Ideal Gecko Adhesive
UV Nanoembossing
MWCNT
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Improvements
Majidi Thesis (2004)
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Suggested Improvements

• Surface defects/dirt particles (the self-cleaning
  issue)
• Solution need flexible lamella, multiple
  layered design
• Clumping
• Solution Quick treatment with PEG/ similar
  Thiol
• Surface Roughness
• Solution Must adjust spatula tip geometry
  specific to rough of expected substrate

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Outline

• Overview
• Capillary vs. van der Waals Forces
• Aspects to control
• Methods for fabricating
• Improvements
• Applications

55
Adhesive Tape

• Future wall-climbing and surgical robot feet
• Potential adhesives for microelectronics and
  space applications

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RiSE Project

• Collaboration between UC Berkeley and Stanford

Full PNAS (2006)
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Conclusions
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References

• Sitti et al, J. Adhesion Sci. Technol. Vol 17,
  No. 8, pp. 1055-1073 (2005)
• Geim et al, Nature materials, Vol. 2 (2003)
• Kim et al, Microsyst Technol (2007) 13 601-606
• Yurdumakan et al, The Loyal Society of Chemistry,
  2005, 3799-3801
• Zhao et al, J. Vac. Sci. Technol. (2006)
• Majidi, C. Masters Thesis (Prof. Ron Fearing
  lab at UC Berkeley) (2004)
• Full et al, PNAS (2002)
• Samper, VD. Biophysical Journal (2005)

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Questions?