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

R 2009

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

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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 )

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