Preload (mN) - Nano Ontario

Biomimetic Micro/Nano Structured Surfaces: Fabrication, Characterization, and Applications
H. Shahsavan, B. McDonald, Z. Pan, P. Patel and B. Zhao
Chemical Engineering and Waterloo Institute for Nanotechnology, University of Waterloo, ON, Canada
*email: [email protected]
Introduction
Characterization
Introduction
Outstanding interfacial properties of the natural and biological systems
have inspired the scientists to mimic these properties in synthetic
structures. Adhesive toe pads of gecko and superhydrophobic leaves of
lotus plant are the most renowned examples of such systems. Herein,
we report our recent progress in fabrication and characterization of
synthetic biomimetic structures. Gecko-inspired fibrillar interfaces were
fabricated and used to tailor adhesion, friction and wetting. Moreover,
the leaves of trembling aspen plant were used for fabrication of
synthetic random structures for the same purpose.
A micro-indenter machine equipped with motorized stage, bottom view
and side view microscopes was used for measurement of adhesion,
friction, and wetting.
Motorized
 Both local and global coefficient of friction of a materials decreases
through surface patterning.
 The preload determines the mode of contact and as a result the real
contact area between the mating surfaces.
 We only experienced laid and transient contact for friction test on
the rigid pillars. [4]
12
70
(b)
Light
Source
Indenter
probe
Camera
Friction
Coefficient
(mN)
Force
Friction of
(c)
Bottom View
PDMS Micro-pillars
Contact area
Lab View
Load Cell
(a)
Transient Laid Contact
FX
Conformal
(f)
Side View
(e)
Camera
(d)
Inverted
Microscope
4X
1060
50
8
40
630
20
4
10
20
0
2
Fx=1.73Fz+8.2 & R =0.99
70
Friction Force (mN)
Fz
Stage
60
2
Fx=1.73Fz+8.2 & R =0.99
50
40
30
20
2
Fx=0.80Fz+2.49 & R =0.99
100
0
0
5
10
15
20
Preload (mN)
0
25
2
Fx=0.80Fz+2.49 & R =0.99
5
5
10
10
15
15
2020 25
25
Preload (mN)
Preload (mN)
 As the asperities are remarkably softer and shorter for synthetic
leaf, conformal contact is achievable resulting a wide range of
friction control ability. [2]
(a) Gecko toe pad embroidered naturally by arrays of (b) setae which are terminated by (c) brushes
of spatula, ref1; (d) Trembling aspen leaf with (e & f) magnified surface asperities, from ref 2.
Fabrication
Adhesion
Negative Mold (Si-W)
PDMS Pillars
Positive Mold (SU-8)
PDMS Holes
Trmbling Aspen Leaf
Trembling Aspen Leaf
Negative Leaf Pattern
Friction Force (mN)
60
60
6
50
50
4
40
40
2
30
30
0
20
20
-2
100
100
-10
200
200
0
Bulged crack line
Hexadecane
Trans. Oil
70
70
50
50
60
40
40
50
30
40
20
30
30
2
Fx=1.73Fz2+8.2 & R =0.99
Fx=1.73Fz+8.2 & R =0.99
2
Fx=1.73Fz+8.2 & R =0.99
20
10
100
0
20
10
0
0
0
Synthetic Leaf
Preload 2 mN
10
Laid-Transient
Contact
100
200
300
400
500
600
F-PDMS Flat
F-PDMS Pillars
27⁰
67⁰
133⁰
42⁰
70⁰
125⁰
102⁰
107⁰
151⁰
Water
60
Friction Force (mN)
Friction Force (mN)
(a)Thin-film terminated structure from PDMS, Inking is the main fabrication
step (150m height and 50m diameter) adopted from ref 3, (b) epoxy
micropillars (25m height and 10m diameter) from ref 4, and (c) synthetic
leaf from PDMS from ref 2.
60
20
Flat PDMS
Friction tests were conducted on the rigid micro-pillars made from
epoxy. The results reviled the a remarkable decrease in friction force for
different preloads. Thus, surface patterning can be used for decrease in
friction force. This is also the case for patterning of soft materials. [4]
70
Synthetic Leaf
30
Contact angle measurement experiments showed:
I. Chemical surface modification slightly increases contact angle
II. Surfaces patterning increases the contact angle more pronouncedly
III. Chemical and topographical modification of the surface dramatically
increase the contact angle. This trend observed for different liquids
such as water, transmission oil, and hexadecane. [5]
Friction
Epoxy Pillars
40
Wetting
Displacement (m)
Film-ended Pillar
Conformal
Contact
Displacement (m)
30010 400
400 20 500
500 30600
600
300
Displacement(m)
(m)
Displacement
Preload 30 mN
50
0
10
10
-4
-600
00
-20
Flat PDMS
0
Crack trapping
Positive Mold (SU-8)
Circular contact line
Friction Force (mN)
Fabrication path
Negative Mold (Si-W)
 Introduction of the simple micro-pillars reduces the adhesion force
when the test is carried out by a spherical probe. By use of a flat
punch the adhesion strength of the fibrillar interfaces increase.
 The fibrils’ tip shape crucially determines the adhesion behaviour.
 Thin film-terminated micropillars increase the adhesion force and
energy remarkably due to crack trapping mechanism and enhanced
compliance. [3]
(mN)(mN)
Force Force
Friction
(mN)
Force
Friction
Soft-lithography technique has been utilized to fabricate micro/nanostructures. Negative master mold was micro-holes on Si-wafer,
positive master mold was SU-8 micropillars, and the leaf of the
trembling aspen was the master mold for synthetic leaves. Doublecasting technique with PDMS intermediate mold was used for pattern
transfer to epoxy and also PDMS. Inking technique was utilized to
change the tip shape of the micropillars. SAM coating of the PDMS
intermediate mold is necessary for double-casting fabrication
technique.
60
References
2
Fx=0.80Fz+2.49 & R =0.99
5
0
2
2
10
15
20
Fx=0.80F
&RR=0.99
=0.9925
F =0.80F
+2.49 &
z+2.49
x
z
Preload
(mN)
5
10
15
5
10
15
2020
25
25
Preload (mN)
Preload (mN)
We thank NSERC for financial supports.
1.
2.
3.
4.
5.
Autumn K, Liang YA, Hsieh ST, Zesch W, Chan WP, Kenny TW, et al., Nature, 2000; 405: 681-685.
McDonald B, Patel P, Zhao B, Chem Eng. Process Tech, 2013; 1:1012.
Shahsavan H and Zhao B, Soft Matter, 2012; 8: 8281-8284.
McDonald B, Shahsavan H, Zhao B, Macromolecular Materials and Engineering, 2013; in press.
Shahsavan H, Arunbabu D, Zhao B, Macromolecular Materials and Engineering, 2012; 297: 743760.