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Comparison of Fibrillar Adhesives (to glass)

(please email if you have updates to this data)
NM= not measured

There has been much confusion in the press recently about the performance of various ``gecko-like'' products. One point of confusion is the comparison of nano-scale contacts, typically done with an atomic force microscope, to large patch areas. This is an apples-to-oranges comparison, as sophisticated hierarchical structures are used in the gecko to get adhesion over large areas, and any material in a nanoscopic contact will have a huge pressure. (Pressure is force divided by area, so as area goes to zero, the pressure tends to infinity.) Another source of confusion is the nature of what makes a sticky material. Soft rubbers and the soft materials used in pressure sensitive adhesives such as Scotch tape are naturally sticky because they can make conformal contacts with hard surfaces, as well as having appropriate viscosity properties. Soft sticky materials can make strong attachments,  and by making fibers and stalks of soft material, patches with interesting adhesive properties can be made. 
A second point is the attachment and detachment of a gecko adhesive. A gecko adhesive does not require being pressed into a surface- the fibers engage by being dragged parllel to the surface with minimal normal force. A gecko adhesive exhibits ``frictional adhesion'' where the fibers push off the surface if there is not a force parallel to the surface, giving automatic release.

Comparison Tables:
Hard polymer Soft Polymer Carbon Nanotube


Product
normal stress
(N/sq.cm.)
(perp. pulloff)
shear stress
(N/sq.cm.)
shear adhesion
coefficient (1)
frictional
 adhesion? (2)
gecko lamella
(Lee et al. 2008)
~0
54
>5
yes
HDPE GSA
(Gillies et al. 2011)
~0
30
>10
yes
polypropylene GSA
(Lee et al. 2008)
~0
1.6
~30
yes
PUA inclined nanofibers
(Jeong et al. 2009)
~5
26
~70
yes (?)
polypropylene GSA
angled fibers
Lee et al. APL 2008
~0
4.5
~45
yes
carbon nanotubes
(Qu et al. 2008)
20
100
0.8
no
carbon nanotubes
(Maeno and Nakayama APL 2009)
?
45
0.9
?

Notes:
1) Shear adhesion coefficient is the ratio of pull-off shear stress to normal preload stress. A high adhesion coefficient means only a light contact will be needed to engage the adhesive.
2) Frictional adhesion is the key property where the normal pulloff force goes to zero when shear force is removed. In this way, the gecko can remove its foot with zero detachment force.
Macroscopic Patches
Common tests for measuring gecko adhesive performance include the peel test, the normal pulloff test, and shear adhesion strength. pulloff, shear, and peel tests

Hard Polymer Arrays

Product
Material
Modulus
90 degree
 Peel
(N/m)
Pull-off
(N)
Shear
(N)
Area
sq. mm.
Normal
preload
(N/sq.cm)
adhesion
coeff.
Effective
Modulus
(kPa)
Natural Gecko, Autumn et al
Nature 2000
beta keratin
2 GPa
~0
1
10
100
nil (<0.01)
(shear engaged)
8-16
100
Gecko Lamellar prep, Lee et al
JRS Interface 2008
beta keratin
1.5 GPa
NM
~0
0.27
0.5
<0.05
>5
100
Davies et al.
Int. J. Adh. Adh. 2008
polyimide
on SEM tape
2 GPa
NM
0.53
~0
196
0.25
2
NM

Geim
Nature Materials 2003
polyimide
on Scotch tape
3 GPa?
NM
3
NM
100
50
0.06
3000
(w/o buckling)
Lee et al.
JRS Interface 2008
polypropylene

1 GPa
< 0.1 N/cm
~0
4 N
240
0.05
~30
(shear)
~200
Lee et al.
APL 2008
polyproylene
1 GPa
<0.1
N/cm
~0
9N
200
<0.1
~45
(shear)
~200
Schubert et al.
J. Adhesion Sci. Tech 2007
polypropylene

1 GPa
< 0.1 N/cm
~0
1 N
1000
0.05
~2
(shear)
~200
Kustandi et al.
Adv. Funct. Mat. 2007
parylene
2.8 GPa
NM
0.7
NM
100
1
0.7
NM
Jeong et al.
Coll. Surf. 2008
PMMA
2.8 GPa
NM
~0
3
100
<1
3
(shear)
NM

Carbon Nanotube Arrays

Product Material Modulus 90 degree
 Peel
(N/m)
Pull-off
(N)
Shear
(N)
Area
sq. mm.
Normal
preload
(N/sq.cm)
adhesion
coeff.
Effective
Modulus
(kPa)
Ge et al
PNAS 2007
carbon nanotube
on Scotch tape
1000 GPa
2-5
0.8
1-6
16
25-50
<.1
~200
Qu and Dai
Advanced Materials 2007
carbon nanotube
on Si
1000 GPa
NM
~5
~2.5
16
125
<.2
NM
Qu et al.
Science Oct. 2008
carbon nanotube
on Si
1000 GPa
NM
~3
~16
16
125
<0.1
NM
Maeno and Nakayama APL 2009 multiwall carbon nanotube on
polypropylene
1000 GPa
NM
NM
45
100
50
0.9
NM
Zhao et al
J. Vac. Sci. Tech. 2006
carbon nanotube
on silicon
1000 GPa
0.08
0.5
0.6
8
>500
<.01
~200

Soft Polymer Fiber Arrays


Product Material Modulus 90 degree
 Peel
(N/m)
Pull-off
(N)
Shear
(N)
Area
sq. mm.
Normal
preload
(N/sq.cm)
adhesion
coeff.
Effective
Modulus
(kPa)
Sameoto and Menon
JMM 2009
PDMS
(Sylgard 184)
1.8 MPa
NM
9mN
NM
1
2
5
~400
Yoon et al.
Nano Today 2009
polyurthane acrylate + Pt
19.8 MPa
NM
NM
31
100
0.3
100
NM
Jeong et al.
PNAS 2009
polyurethane acrylate
19.8 MPa
NM
15
78
300
0.3
70
~26
Kim and Sitti
Applied Physics Letters 2006
polyurethane
3 MPa
0.07
0.07
NM
0.4
12
1.5
~300
Murphy, Aksak, Sitti, Small 2008
polyurethane
3 MPa
NM
5
10
100
NM
NM
~20
Gorb et al
JRS Interface, 2006
PVS
3 MPa
~1
0.4
NM
7
2
2.9
~300
Parness et al.
JRSI 2009
PDMS
1.75 MPa
NM
0.5
1.7
100
0.25
2.1
~50(?)
Santos et al.
J. Adh. Sci. Tech. 2007
polyurethane
0.3 MPa
NM
1
~1
390
0.25
4-13
~100(?)
Sitti and Fearing
IEEE ICRA 2003
PDMS
0.5 MPa
NM
0.003
NM
100
0.025
0.1
~100
Davies et al.
Int. J. Adh. Adh. 2008
PDMS
0.6 MPa
NM
1.2 N
NM
123
0.25
40
~100
Ge et al
PNAS 2007
Scotch brand tape
0.1 MPa?
NM
NM
6
16
NM
NM
<100
Shan et al
IEEE Nano/Micro 2006
PDMS
2.5 MPa
NM
2.01
NM
100
NM
NM
~240




Spherical Probe Tests
load-drag-pull
Load, drag, pull test
load-pull test
Spherical indentation test Load then Pull off
Product                               
Material
Modulus
Probe Radius
(R)
Preload
Load-Pull
Normal Force
and
F/R (N/m)        
Load-Drag-Pull
Normal Force
and
F/R (N/m)
Adhesion Coefficient
Schubert et al
JRS Interface  2008
polypropylene 1 GPa
5 cm
2 mN
< 0.05 mN
(< 0.001  N/m)
0.8 mN
(0.016 N/m)
0.4
Northen and Turner
Current Apl. Phys 2006
organorods
~1 GPa
0.3 cm
12 mN
0.45 mN
(0.15 N/m)
NM
0.04
Kim and Sitti               
APL 2006
polyurethane
3 MPa
0.3 cm
22 mN
65 mN
(20 N/m)
NM
3
Murphy et al.
App. Mat. and Int. 2009
polyurethane
3 MPa
1.2 cm
256 mN
600 mN
(50 N/m)
NM
2.3
Murphy et al
JAST 2007
polyurethane
3 MPa
0.3 cm
4 mN
8 mN
(3 N/m)
6 mN
(2 N/m)
1.5
Greiner, del Campo, Arzt
Langmuir 2007
silicone rubber
2.6 MPa
0.25 cm
2 mN
1.1 mN
(0.5 N/m)
NM
0.5
Greiner, Arzt, del Campo
Adv. Materials 2009
silicone rubber
1.9 MPa
0.25 cm
0.5 mN
0.07 mN
(0.03 N/m)
NM
0.14


Nanoscopic Patches
These are measurements of very small numbers of contacts over a very small area. The results at the nanoscale can not be directly extrapolated to performance of macroscopic patches. In many cases an atomic force microscope is used to make contact with one to several fibers.
Notes:
(1) For natural gecko, we are assuming ``frictional adhesion''  mode, where a shear force is necessary for the pulloff force to be non-zero.
(See Fig. 4 of Autumn et al Nature 2000).  The shear and normal is estimated by assuming 100 to 1000 spatulae are in contact. Each spatula is assumed to be 200x200 nm.

(2) Haeshin Lee, Bruce P. Lee & Phillip B. Messersmith, ``A reversible wet dry adhesive inspired by mussels and geckos, '' Nature,448, 338-341 (19 July 2007)
AFM probe test
Product
Material Modulus
Pull-off
(nN)
Shear
(nN)
Area
sq. um.
Normal
stress
MPa
Shear
stress
MPa
(1) Natural Gecko,
Autumn et al
Nature 2000
beta keratin
2 GPa
20-200
35-350 nN
0.04
0.5-5
0.9-9
Sitti and Fearing
IEEE Nano 2002
polyester
0.85 GPa
280
NM
9
0.03
NM
Sitti and Fearing
IEEE Nano 2002
PDMS
0.5 MPa
200
NM
9
0.02
NM
Yurdumakan et al.
Chemical Comm. 2005

carbon
nanotube
1000 GPa 10
NM
0.0006
16
NM
(natural gecko)
Huber et al.
PNAS 2005
beta keratin
2 GPa
20
NM
0.04
0.5
NM
(2) Lee et al
Nature, 2007
PDMS+
p(DMA-co-MEA)
0.5 MPa?
120
NM
1
0.12
NM



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