Adhesion and Detachment Characteristics of “Soft” Adhesive Systems: from pressure sensitiveAdhesive Systems: from pressure-sensitive
adhesive tapes to gecko hairy foot pads
Boxin Zhao, Ph.D.
Assistant ProfessorWaterloo Institute for Nanotechnology, Department of Chemical
Engineering , University of Waterloo, Canada
IPR 2009 Symposium, May 1 IP
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Adhesion is a Fundamental Phenomenon in Naturein Nature
Spider Web
St Croix US Virgin IslandsSt Croix, US Virgin Islands
Gecko climbing on bamboo surfaces Dew drops adhering to a spider web
2
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Adhesion is also Essential to Engineering and Future InnovationsEngineering and Future Innovations
AdhesiveAdhesivepads 1mm
Paper – fiber networkAdhesive tapes & labels
20 �m
10nm
StickyBot, Stanford Mico/nano particlesPolymer adhesive shield
3
Univ, 2006
Polymers are Good AdhesivesIP
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Molecular Adhesion is Universal
Intermolecular attractive Interaction(Van der Waals forces)(Van der Waals forces)
Unit area
1
< 10nm
2
1
2St Croix, US Virgin Islands
2
Two smooth surfaces leap intoJohannes Diderik van der WaalsThe Nobel Prize in Physics 1910
Two smooth surfaces leap intocontact at nanometer (10-9m)distance
4Human hair ~ 100 micrometer in diameterIP
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Practical Adhesion is Complex
Cracks
Surface deformation in detachment
Surface roughness reduces adhesion
5The adhesion and detachment mechanisms matter
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Outline
Recent research
Gecko adhesive
PSA tape
Viscoelastic thinThe adhesives/paper interactions
Gecko adhesivesystem
Viscoelastic thincoating films
F t R h
Biomimetic or Bio-inspired Adhesion and Smart
Future Research
6
AdhesivesIP
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Overall Objectives
� To identify and characterize the behaviors of “soft” (synthetic and biological) adhesive surfaces and associated micromechanical propertiesassociated micromechanical properties
� To develop new concepts, approaches and techniques to tune adhesion and make smart adhesives .
Today: to highlight key research findings
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Pressure-sensitivePressure sensitiveAdhesives/Paper Interactions
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What are Pressure-sensitive Adhesives ?
PSAs are materials which adhere under a li htlight pressure.
PSAs are polymeric and have a property called viscoelastic.called viscoelastic.
They behave like liquid in bonding while fracture like solid in debonding.
C
OH
O
Face material
CH2 CH
C
O
O
CH2CH CH2CH
C
O
O
~50�m
Face material
Adhesive Coatinglayers
~ 50�m
Acrylic PSA
R
O
R
9
Backing material (Release / bonding Agent)IP
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PSAs Used in Papermaking
Performance requirements:
(1) Instant adhesion(2) Strong joint strength for
survival in further processingat ~ 60km/hr
(3) Repulpable in recyclingSplicing tape
Occasional Failure costs millions $$$
Research questions:
What are the fracture mechanisms?How to make stronger adhesive bonds?
10Flying-splice in papermaking mills
bonds?IP
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Peeling Adhesion Analysis
• Easy to perform• Providing information on both
paper and adhesive tapepaper and adhesive tape
• It involves complex mechanical effects– Peeling angle– Bending curvature
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Wheel – Peeling Tester
Constant peeling angle� = 90�
Video Camera
PSA TapeP
F
h l t ti
Paper
d
wheel rotating
W fi d d 0 i t f
12
Wheel We fixed d = 0 in most of our measurementsIP
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Peel Forces and Interfacial Phenomena
Tape
Interfacial failure
Typical peeling curves Tape Adhesivefibrils
yp p g
0 5
0.6400mm/min100mm/min
Peak Force
PaperPaper
0.3
0.4
0.5
rce,
N/m
m
Interfaicial FailureInterfacial Failure
Paper failure
0.1
0.2
Peel
For
Paper Failure
Mixed Failure
p
00 10 20 30 40 50
Peel Distance, mmDelamination
13
Delamination
Fibers do not break in delamination.
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Both Adhesion Forces and Failure Modes are Functions of Velocity
In logarithmic scales
are Functions of Velocityor
ce
Contact pressure & timeSurface energy & roughness …
Identified a critical velocity, Vc for the transition of failure
d
eelin
g Fo
Paper failure – interfiber bonding strength
modes
Established the linkbetween paper tape
Max
. P
PSA cohesive failureVc
between paper, tapeproperties, and adhesion performance.
Peeling Velocity
PSA cohesive failure
Surface energy is determined by surface
14Zhao, Pelton, Tappi, 2004Zhao, Anderson, Banks, Pelton, J. Adhesion Sci. Technol. 2003, 2004
ychemistry
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Using Tape-peeling as a Measure of Paper Surface StrengthPaper Surface Strength
The peeling method
Industrial standard method
This method cost less than industrial methods; it is adopted by the Australian Pulp and Paper Institute
15
Australian Pulp and Paper Institute.
Zhao and Pelton, Tappi, 2004
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Adding Polyelectrolytes (PE) to Tune Interfiber Adhesion Strength
Fiber surface
Interfiber Adhesion Strength
Polymerchain
Fiber surfacenegatively-charged
10nm
Adhesion force
Adsorption AdhesionWater removal
Interfiber-bonds
CH2
CH
CH
CH2
n
CH OH - C C
CH2 CH2
NH
CH3 CH3
N+O
O
CH2OH
HO OH
O
16Hydrophilic PE
to enhance adhesion Hydrophobic PEto reduce adhesion
Hydrophilic cellulosenegative charged
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Summary
• Adhesives are highly deformed and form fibrils.
• The adhesion forces increase and failure modes change as peeling velocity increases.
• The max adhesive/paper joint strength is determined by paper surface strength This finding resulted in a simplepaper surface strength. This finding resulted in a simpleapproach to measure paper surface strength.
I t fib dh i t th b t d b ddi• Interfiber adhesion strength can be tuned by addingpolyelectrolytes.
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Dynamic Adhesion and Fracture of Thin yCoating Films: Solid- and Liquid-like Failure
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Failure Mechanisms - Two Extreme ScenariosScenarios
Snapping of a liquid bridge Brittle fracture of solid
liquid
solidBehaviors of
“soft” materialsis far less
understood
.
19e.g. ice, viscosity of 1011Pa.Se.g. water, viscosity of 10-3 Pa. S
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Research Objectives
T id tif d h t i• To identify and characterize:
the differences between liquid and solid like– the differences between liquid- and solid-likefailure mechanisms of micro/nano thin films
– Molecular interaction, surface deformation and instabilities in adhesion and subsequent separationseparation
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Sugar Viscosity as a Function of TemperatureTemperature
Simples sugars are ideal materials to study the solid- and liquid-like behaviors
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Sugar Films Coated onto Mica Surfaces
AFM imaging of sugar surfaceMica surface
Amorphous sugar coating ~0 05µm
AFM imaging of sugar surface
Amorphous sugar coating ~0.05µm
Mica surface
Three typical experimental temperatures in N2 atmospherestemperatures in N2 atmospheres– Glassy state at 23�C– Viscoelastic state at 40�C– Viscous fluid state at 75�C
22
– Viscous fluid state at 75 CImage RMS = 0.537nm
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Using SFA and FECO to Study Adhesion Failure Mechanisms
2a
FSFA: Surface Forces Apparatus
FECO: “Fringes of Equal Chromatic Order” -
Failure Mechanisms
Microscopy imaging lightAdhesive contact
2a
Top view – Newton’s ring
thin film interference patterns
S“FECO” fringes
Spectrometer
2a 50nm
23Wavelength (Å)
SFA 2000
Tabor, Winterton, Israelachvili,1970
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Using the JKR-theory as An Analytical Tool
JKR - plot
Tool
2ain
Jump-in
eter
UnloadingJKR theory -
reversible process
eter
UnloadingJKR theory -
reversible process,a, 2
a
Steady-statepeeling
in
Con
tact
dia
m
Jump out
loading
JumpinC
onta
ct d
iam
Jump out
loading
Jumpin
�
2aout
Compressive load
out
Fad
in
Compressive load
out
Fad
in
, F
Jump out - anabrupt peeling
��RFad 3�Classic adhesion theory ( equilibrium and elastic system)
Compressive loadCompressive load
24Surface energy
system)
Johnson, Kendall, Roberts (1971) Proc R Soc London Ser A 324:301–313.
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Contact Behavior of Sugar Surfaces at 23�C Viscosity of 1014 Pa S
�
23 C, Viscosity of 1014 Pa.S
120
eter
(�
m)
Loading
Unloading�
40
80
Con
tact
dia
me
Jump out
Glucose
JKR theoryCrack initiation Propagation (<30ms)
2546 J /
0-50 -25 0 25 50
C i l d ( N)
C 23�C Contactarea
2546 mmJ /��
Compressive load (mN)Jump-out
Adhesion Hysteresis - time effects
25Brittle fracture/cracking
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Time Effects150
m)
(A)8
(B)
100
ct d
iam
eter
( �m
(a)
(b)
(c)
4
6 1�m/s
0.2�m/s
F/R
(N/m
)
0
50
75 50 25 0 25 50 75
Con
tac
Jump out
(c)
Glucose23�C
0
2 Glucose23�C
F
-75 -50 -25 0 25 50 75
Compressive load (mN)
Contact time: a < b < c
0 20 40 60 80 100Contact time (minutes)
Separation velocity
Glucose molecules rearrangements at the interface
26
rearrangements at the interface- a dynamic processAir
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Adhesion and Coalescence of Viscous Fluid Surfaces at 75� C(103Pa S)Fluid Surfaces at 75 C(103Pa.S)
• Coalescence occurs right after adhesive contact
• Contact area scales with time
160
80
120
men
iucs
( �m
) 2.0~ ta
tact
are
a (�
m)
0
40
Rad
ius
of
20µm
Rad
ius
of c
ont
27
00 200 400 600
Time (s)Transient surface patterns during fluid-fluid coalescence
Zeng, Zhao, Tian, Tirrell, Leal and Israelachvili, 2006
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Detachment of Viscous Sugar Surfaces
Viscous fingerings due to the Saffman-Taylor instability
Instability during the peeling of adhesive tape y y
~50nmd4000�
da
~200�m
urfa
ce
ive
tape
d
a
Gla
ss s
u
Adh
esi
20�m
F Frankel G M Whitesides On
28
F. Frankel, G. M. Whitesides, Onthe Surface of Things, 1997
Viscous fingerings consume a large amount of energy, giving a strong adhesive bond.
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Evolution of Interface Ripples/waves in DetachmentDetachment
Solid-likesharp tips
(high local stress)
CavitationsLiquid-likerounded fingers• video demo – a slow-down
process
This may be due to the lateral
rounded fingers(low local stress)
Co-existence of sharp tips and
29
This may be due to the lateralacceleration of fluid during its normal separation.
round fingers were observed for the first time, suggesting a unifying theory.
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SummarySummary
• The fracture of two adhered surface was manifested• The fracture of two adhered surface was manifestedby crack nucleation and propagation at one extreme and the snapping of a liquid bridge at the other
• The fracture of two adhered viscoelastic surfaces was manifested by rounded fingers
• Practical Implications– Cavitations and fingerings consume a large amount
of energy resulting in a strong adhesive bondof energy, resulting in a strong adhesive bond.– Adhesion can tuned by adjusting material viscosity.
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Understanding Gecko Adhesive System –System –
learn from nature
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Gecko – a Super Climber
Tokay gecko on walls
Marbled GeckoPhoto Courtesy Ben MoultonJohn Bokma @2007
House gecko on ceilings
Hawaiian GeckoPhoto Courtesy Ben Moulton
There are about 850 gecko species.
We focus on Tokay gecko,the largest species
32
Quincy Dein Photography St Croix, US Virgin Islands IP
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What is Known about Gecko AdhesionBA
Spatulae (- keratin nano-structures) behave like adhesive tape.
Adhesion via intermolecular van der Waal forces
100-1000 spatulae/setae
~14,400 setae/mm2
33
14,400 setae/mm
10 nm
100 nm
200 nm
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Recent Research and Challenges
Many research on the fibrillar surfaces (varied aspect-ratio, shape)
2�m Physical characterization of gecko attachment and detachment.
Design of ‘responsive’ surfaces for smartGeim, Nature, 2003
Design of responsive surfaces for smartadhesives and robotic applications
StickyBot,Stanford UnivStanford Univ,2006
34Kim, 2007
UCSB teamIP
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Gecko Attachment and Detachment- Peeling Mechanism
Gecko engages attachment
Peeling Mechanism
1000
Gecko engages attachmentat small pulling angles whiledetachment at large angles� F� F� FF
100
1000
rgy,
F/( �
b)
)cos1( ����
�bFWalking gecko on walls
1
10
0 40 80 120 160Peel
ing
Ene
)(
��
0.1
Peeling angle, �
35Detaching gecko foot
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Gecko Attachment and Detachment- Peeling Mechanism
Gecko engages attachment
Friction
Peeling Mechanism
1000
Gecko engages attachmentat small pulling angles whiledetachment at large angles� F� F� FF
n
Adhesion
100
1000
rgy,
F/( �
b)
)cos1( ����
�bFWalking gecko on walls
1
10
0 40 80 120 160Peel
ing
Ene
)(
��
0.1
Peeling angle, �
Surface features of th k t l
36Detaching gecko foot
the gecko setal arrays IP
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Gecko Foot Pad vs Adhesive TapeA
Soft polymerE ~ 105 Pa
Strong -keratin, E ~ 109 Pa E ~ 105 PaE ~ 109 Pa
Build-in micro/nano fibrillar structures Stress-induced adhesive fibrills
37Fibrillar structures consume a large amount energy in
detachment, resulting in high adhesion strength
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Contact Dynamics (Adhesion and Friction) MeasurementsMeasurements
G i i ReleasingGripping Releasing
Surface Forces ApparatusHigh Friction
Low
HighRepulsion
Gecko setal arrays are structurally anisotropic, exhibiting
HighAdhesion
LowFriction
y p , gstrong directional adhesion and friction properties.
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16
N)
Gripping direction Releasing direction
To mimic gecko adhesive pads andfunctionalities,
12
s, F
±v (m
N
F+v Adhesive –frictionbehavior
Load -dependentfriction
anisotropiccurvedstructure isessential.
8
on fo
rces behavior
“JKR” Amontons friction law
0
4
Fric
tio
� = F-v / L-v = 0.25
0-8 -4 0 4 8
Normal force L (mN)L+v L-vAdhesion Repulsion
39
Zhao, Israelachvili, et al., Langmuir, 2007
Normal force, L±v (mN)IP
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Summary
Gecko foot pads behave like adhesive tape while its robust and responsive adhesion arises from the build-in pmicro/nano-sized fibrillar structures.
Many things are still unknown e g the formation ofMany things are still unknown, e.g., the formation ofgecko fibrils.
This suggests a new strategy to design and tuneThis suggests a new strategy to design and tuneadhesion by surface patterning.
A
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Current and Future Research
Biomimetic Adhesion and Smart (responsiveBiomimetic Adhesion and Smart (responsive,adaptable) Adhesive Devices
41
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Biomimetic studies for responsive and d t bl t i l
Nature Science Fabrication Application
adaptable materials
?
?
Responsive and adaptable to external , both chemical and mechanical, stresses
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NON-Responsive surface
Climbing Velcro man Tokay gecko on Climbing Velcro manVideo demo
walls
5mm
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Overall Research ObjectivesOverall Research Objectives
As future technological innovations gear towards g gminiaturizing machines and maximizing performance density, our challenges as engineers and scientists become our ability to build micro and nano machinesbecome our ability to build micro- and nano-machinesand understand phenomena at a scale we normally do not deal with.
Focus on polymeric materials for both mechanical and biological applicationsbiological applications.
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Biological Fibrillar Adhesive Structures
AFibrillar micro/nano structures is h t i ti f bi l i lcharacteristic of biological
attachment devices, which are natural Post-it Note
45(Materials Today, 2004)
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Fabrication of micro polymer pillarsFabrication of micro polymer pillars
(i) Microfabrication of silicon masters for molding
(ii) PDMS mold fabricated using silicon masters
(iii) Polymer micro-pillar structures fabricated using PDMS mold
• Key design factors: Number density, Aspect ratio, Mechanical strength, Surface chemistry
46In collaboration with Dr. Israelachvili and Dr. Turner group at UCSB
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Fabrication of micro polymer pillarsith “t il d” tiwith “tailored” properties
Water droplet
Micro-structured surface
In collaboration with Dr. Alex Penlidis and Dr. Neil McManus
47
S.-H. Zhu, N.T. McManus, C. Tzoganakis, A. Penlidis, 2007
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Fabricating Curved Structures g
Tilted PDMS mold
f
Gecko directional adhesion
48
Curved micro-pillarsTo mimic this properties, we fabricate curved pillars.
In collaboration with Dr. Turner group at UCSB and Dr. Pesika group at Tulane University
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FABRICATION OF BIOMIMETICSTRUCTURES AT SMALL SCALES FOR A
SummaryRESPONSIVE AND ADAPTABLE
MATERIALS APPLICATIONS
PROF BOXIN ZHAO
A
PROF. BOXIN ZHAOCHEMICAL ENGINEERING, U WATERLOO
Gecko-like micro/nano structured materials
Surface forces and micro/nano tirbological studies
Micro/nano porous hydrogels , artificial cartilage, and joint lubrication
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Acknowledgements
� UCSBDr Jacob Israelachvili
� McMaster UniversityDr Robert PeltonDr. Jacob Israelachvili
Dr. Noshir Pesika Dr. Hongbo Zeng Dr. Yu Tian
Dr. Robert PeltonDr. Shiping ZhuDr. John MacGregor Dr. Honglu Yu
Dr. Kenny RosenbergDr. Patricia McGuigganDr. Mathew Tirrell
gDr. An-chang Shi
Dr. Gary Leal
� CollaboratorsDr Kellar Autumn (Lewis & ClarkDr. Kellar Autumn (Lewis & Clark
College, Oregon, USA)Dr. Kim Turner (Mechanical
Department, UCSB )
50
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