|
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.
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
|
polypropylene GSA
(Lee et al. 2008)
|
~0
|
1.6
|
~30
|
yes
|
carbon nanotubes
(Qu et al. 2008)
|
20
|
100
|
0.8
|
no
|
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. |
|
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
|
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
|
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
|
|
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
|
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) |
Kim and Sitti
Applied Physics Letters
2006
|
polyurethane
|
3 MPa
|
0.07
|
0.07
|
NM
|
0.4
|
12
|
1.5
|
~300
|
Gorb et al
JRS Interface, 2006
|
PVS
|
3 MPa
|
~1
|
0.4
|
NM
|
7
|
2
|
2.9
|
~300
|
Sitti and Fearing
IEEE ICRA 2003
|
PDMS
|
0.5 MPa
|
NM
|
0.003
|
NM
|
100
|
0.025
|
0.1
|
~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 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
JAST 2007
|
polyurethane
|
3 MPa
|
0.3 cm
|
4 mN
|
8 mN
(3 N/m)
|
6 mN
(2 N/m)
|
1.5
|
Greiner, Campo, Arzt
Langmuir 2007
|
silicone rubber
|
2.6 MPa
|
0.25 cm
|
2 mN
|
1.1 mN
(0.5 N/m)
|
NM
|
0.5
|
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) |
|
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
|
(2) Lee et al
Nature, 2007
|
PDMS+
p(DMA-co-MEA)
|
0.5 MPa?
|
120
|
NM
|
1
|
0.12
|
NM
|
|