Surface Tension of Asphalt using AFM

Appy Beemer, Troy Pauli, and Julie Miller

Pavement Performance Prediction SymposiumAdhesion and Cohesion in Asphalt PavementsJune 23-25, 2005Cheyenne, WY

Overview

Definition of terms AFM Background Contact Mechanics Theory Experimental Data Analysis of Data Conclusions

Definitions Surface Tension

Force from the bulk molecules on a surface line of a liquid

Surface Energy Work or energy required to create a unit of surface area of a

solid

Work of Cohesion Work required to separate a material from itself at an

arbitrary boundary

Work of Adhesion Work required to separate two dissimilar materials at their

interface

Munson, Bruce R., Donald F. Young, and Theodore H. Okiishi. Fundamentals of Fluid Mechanics, 3rd ed. John Wiley & Sons, Inc. New York: 1998, p 26-28.

Importance for asphalt

Asphalt surface tension gives: Cohesive properties Adhesive properties

Time and temperature susceptible

Cohesive strength property may need further investigation

Making the Measurement

Make an asphalt toluene solution Initial solution concentration ~0.167g/mL

Spin cast solution onto a glass microscope slide

Volume deposited to slide - 2.0L Spin rate - 600 to 800 rpm

Roto-FilmSolution Spin

Casting Device

Filmetrics Thin-film Measurement System

Storing Conditions

Use ~ 1.0 m films Keep samples in dry box Purge box with nitrogen gas Keep samples at room temperature

The Operation of a Scanning Probe Microscope

1. Red Laser2. Quad-Photo Detector3. Piezo-tube Scanner4. Micro-Cantilever

12

3

4 Z

YX

THERMAL STAGE Atomic Force MicroscopeQuesant Q-Scope 250

AFM Force Curve Measurements

BimorphCantilever

Sample

Z

Y

X

Z-distance (nm)

0 500 1000 1500 2000

Z

-

d

e

f

l

e

c

t

i

o

n

(

n

m

)

-600

-400

-200

0

200

400

600

Z-distance (nm)

0 500 1000 1500 2000

Z

-

d

e

f

l

e

c

t

i

o

n

(

n

m

)

-600

-400

-200

0

200

400

600

Z-distance (nm)

0 500 1000 1500 2000

Z

-

d

e

f

l

e

c

t

i

o

n

(

n

m

)

-600

-400

-200

0

200

400

600

Z-distance (nm)

0 500 1000 1500 2000

Z

-

d

e

f

l

e

c

t

i

o

n

(

n

m

)

-600

-400

-200

0

200

400

600

Z-distance (nm)

0 500 1000 1500 2000

Z

-

d

e

f

l

e

c

t

i

o

n

(

n

m

)

-600

-400

-200

0

200

400

600

Z-distance (nm)

0 500 1000 1500 2000

Z

-

d

e

f

l

e

c

t

i

o

n

(

n

m

)

-600

-400

-200

0

200

400

600

Z-distance (nm)

0 500 1000 1500 2000

Z

-

d

e

f

l

e

c

t

i

o

n

(

n

m

)

-600

-400

-200

0

200

400

600

12RW

F offpull

=n

Z-position, m0.0 0.5 1.0 1.5 2.0

F

o

r

c

e

,

n

N

-7500

-7000

-6500

-6000

-5500

-5000

ApproachRetract

Deflection to Force

Detectors measure cantilever deflection Spring Constant x Deflection = Force Interested in load and pull-off force

Load

Pull-off Force

Z-position, m0.0 0.5 1.0 1.5 2.0

F

o

r

c

e

,

n

N

-7500

-7000

-6500

-6000

-5500

-5000

-4500ApproachRetract

Zero-Load Curve

Z-position, m0.0 0.5 1.0 1.5 2.0

F

o

r

c

e

,

n

N

-7500

-7000

-6500

-6000

-5500

-5000

-4500ApproachRetract

Negative-Load Curve

R2

R1

121

1

)(

111

RRRR+=

R1

aH

a

R1

a = 2R/3

Viscoelastic Material

-12000

-10000

-8000

-6000

-4000

-2000

0-2.5 -1.5 -0.5 0.5 1.5 2.5

Z position, um

L

o

a

d

F

o

r

c

e

,

n

N

.

050

100150200250300

0 5 10 15 20 25 30

Distance, umH

e

i

g

h

t

,

n

m

Viscoelastic Material

-14000

-12000

-10000

-8000

-6000

-4000

-2000

0

2000

-2.5 -1.5 -0.5 0.5 1.5 2.5

Z Distance, um

L

o

a

d

F

o

r

c

e

,

n

N

050

100150

200250

300

0 5 10 15 20 25 30

Distance, um

H

e

i

g

h

t

,

n

m

RaE

34 3*Phertz =

Contact Mechanics Model of an Interface

Hertzian Contact between Rigid Surfaces

Shull, K. R., (Nov. 2004), shullgroup.northwestern.edu/pdfpublic/ref054.pdf

Frictionless, ideal contact

REPR a

DMT 12*

4 / 94lim

2

==

REPR a

JKR 12*

/33lim

2

==

Contact Mechanics Model of an Interface

At the JKR and DMT Limits

( ) +++=2

*3 363

43 GRGRPGRPERa

P 0

Contact Mechanics Model of an Interface

At the JKR and DMT Limits JKR

Adds load to the model Does not account for adhesion outside the contact area Large probe, very soft surface, high surface energy

DMT Adds friction as well as load to the model Accounts for adhesion outside the contact area

Wetting

Small probe, hard surface, low surface energy

Drift Procedure

Make contact with surface (low force)Allow several force curve cycles (save each)Decrease the z scan range (decrease the force)Save another series of force curvesRepeat until no contact

0

10

20

30

40

50

60

70

80

90

100

-200 0 200 400 600 800 1000 1200 1400

Load, P, nN

S

u

r

f

a

c

e

T

e

n

s

i

o

n

,

,

d

y

n

e

/

c

m

.

Load-Unload Procedure

Increase z scan range to contact surfaceSave force curve at initial contactIncrease the z scan range (increase the load)Save force curveRepeat until at desired loadDecrease the z range (decrease the load) Save force curve Repeat until no contact

0

10

20

30

40

50

60

70

80

90

100

-200 0 200 400 600 800 1000 1200 1400

Load, P, nN

S

u

r

f

a

c

e

T

e

n

s

i

o

n

,

,

d

y

n

e

/

c

m

.

010

20

30

40

50

60

70

-200 0 200 400

Load, P, nN

S

u

r

f

a

c

e

T

e

n

s

i

o

n

,

,

d

y

n

e

/

c

m

.

AAB-1 Set

= 46.9 (Avg. 47.9)

AAD-1 Set

= 42.7 (Avg. 45.0)

0

10

20

30

40

50

60

70

80

90

100

110

-100 0 100 200 300 400 500

Load, P, nN

S

u

r

f

a

c

e

T

e

n

s

i

o

n

,

,

d

y

n

e

/

c

m

.

AAF-1 Set

= 48.4 (Avg. 47.9)

0

10

20

30

40

50

60

70

-200 -100 0 100 200 300 400

Load, P, nN

S

u

r

f

a

c

e

T

e

n

s

i

o

n

,

,

d

y

n

e

/

c

m

.

AAM-1 Set

= 50.3 (Avg. 49.9)

0

20

40

60

80

100

120

140

-200 0 200 400 600 800

Load, P, nN

S

u

r

f

a

c

e

T

e

n

s

i

o

n

,

,

d

y

n

e

/

c

m

.

010

20

30

40

50

60

-125 -100 -75 -50 -25 0 25 50

Load, P, nN

S

u

r

f

a

c

e

T

e

n

s

i

o

n

,

,

d

y

n

e

/

c

m

.

AAB

Average Surface Tensions

: 47.9 1.2

010

20

30

40

50

60

-125 -100 -75 -50 -25 0 25 50

Load, P, nN

S

u

r

f

a

c

e

T

e

n

s

i

o

n

,

,

d

y

n

e

/

c

m

.

AABAAD

Average Surface Tensions

: 47.9 1.2: 45.0 2.6

010

20

30

40

50

60

-125 -100 -75 -50 -25 0 25 50

Load, P, nN

S

u

r

f

a

c

e

T

e

n

s

i

o

n

,

,

d

y

n

e

/

c

m

.

AABAADAAF

Average Surface Tensions

: 47.9 1.2: 45.0 2.6: 47.7 2.2

010

20

30

40

50

60

-125 -100 -75 -50 -25 0 25 50

Load, P, nN

S

u

r

f

a

c

e

T

e

n

s

i

o

n

,

,

d

y

n

e

/

c

m

.

AAB

AAD

AAF

AAM

Average Surface Tensions

: 47.9 1.2: 45.0 2.6: 47.7 2.2: 49.9 1.6

010

20

30

40

50

60

-125 -100 -75 -50 -25 0 25 50

Load, P, nN

S

u

r

f

a

c

e

T

e

n

s

i

o

n

,

,

d

y

n

e

/

c

m

.

AAB

AAD

AAF

AAM

JKR

JKR/DMT

JKR/DMT

DMT

JKR DMT

Average Surface TensionsLeast Cohesive

Most Cohesive

Least Cohesive

Most Cohesive

Wc = 2

AAD-1 45.0 2.4AAF-1 47.7 2.2AAB-1 47.9 1.2AAM-1 49.9 1.6

AAD-1 Neat 40.9 1.0AAD-1 PAV 240 h 44.7 1.3AAD-1 PAV 480 h 43.7 1.4AAD-1 PPA & PAV 96 h 46.2 0.5AAD-1 PPA & PAV 184 h 47.0 0.7

Conclusions Neat asphalt adhesive properties:

Long-range, wetting forces Short-range, non-wetting forces

Neat asphalt cohesive properties Least cohesive asphalts (AAD) Moderately cohesive asphalts (AAB & AAF) Most cohesive asphalts (AAM)

Modified asphalt cohesive properties Least cohesive (Neat) Moderately cohesion (PAV) Most cohesive (PPA Modified)

Future Work

Explore other parameters Temperature Rate Aging Additives

Adhesion-cohesion balance

Continue to refine experimental procedures

ACKNOWLEDGEMENTS

FHWA for their Financial Support under Contract No. DTFH61-99C-00022

Questions?

OverviewDefinitionsImportance for asphaltMaking the MeasurementStoring ConditionsDeflection to ForceZero-Load CurveNegative-Load CurveViscoelastic MaterialViscoelastic MaterialContact Mechanics Model of an InterfaceAt the JKR and DMT LimitsDrift ProcedureLoad-Unload ProcedureAAB-1 SetAAD-1 SetAAF-1 SetAAM-1 SetAverage Surface TensionsAverage Surface TensionsAverage Surface TensionsAverage Surface TensionsAverage Surface TensionsConclusionsFuture WorkQuestions?