Surface Tension of Asphalt using AFM
Appy Beemer, Troy Pauli, and Julie Miller
Pavement Performance Prediction Symposium
Adhesion and Cohesion in Asphalt Pavements
June 23-25, 2005
Cheyenne, 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 - 2.0μL
• Spin rate - 600 to 800 rpm
Roto-Film™
Solution Spin
Casting Device
Filmetrics™ Thin-film
Measurement System
Storing Conditions
•
•
•
•
Use ~ 1.0 μm films
Keep samples in dry box
Purge box with nitrogen gas
Keep samples at room temperature
The Operation of a Scanning Probe
Microscope
2
1. Red Laser
2. Quad-Photo Detector
3. Piezo-tube Scanner
4. Micro-Cantilever
1
Z
4
Y
X
3
THERMAL STAGE Atomic Force Microscope
Quesant Q-Scope™ 250
AFM Force Curve Measurements
Bimorph
Cantilever
Z
Sample
Y
X
600
ΔZ-deflection (nm)
400
200
0
-200
-400
-600
0
500
1000
Z-distance (nm)
1500
2000
600
ΔZ-deflection (nm)
400
200
0
-200
-400
-600
0
500
1000
Z-distance (nm)
1500
2000
600
ΔZ-deflection (nm)
400
200
0
-200
-400
-600
0
500
1000
Z-distance (nm)
1500
2000
600
ΔZ-deflection (nm)
400
200
0
-200
-400
-600
0
500
1000
Z-distance (nm)
1500
2000
600
ΔZ-deflection (nm)
400
200
0
-200
-400
-600
0
500
1000
Z-distance (nm)
1500
2000
600
ΔZ-deflection (nm)
400
200
0
-200
-400
-600
0
500
1000
Z-distance (nm)
1500
2000
600
ΔZ-deflection (nm)
400
200
0
− Fpull − off
-200
n πR
-400
= W12
-600
0
500
1000
Z-distance (nm)
1500
2000
Deflection to Force
Approach
Retract
-5000
Load
Force, nN
-5500
-6000
Pull-off Force
-6500
-7000
-7500
0.0
0.5
1.0
1.5
Z-position, μm
• Detectors measure cantilever deflection
• Spring Constant x Deflection = Force
• Interested in load and pull-off force
2.0
Zero-Load Curve
-4500
Approach
Retract
-5000
Force, nN
-5500
-6000
-6500
-7000
-7500
0.0
0.5
1.0
Z-position, μm
1.5
2.0
Negative-Load Curve
-4500
Approach
Retract
-5000
Force, nN
-5500
-6000
-6500
-7000
-7500
0.0
0.5
1.0
Z-position, μm
1.5
2.0
R1
1
R
R2
=
1
R1
+
1
( R2 → ∞ )
→
1
R1
R1
δ
aH
a
R1
δ
a = δ2R/3ζ
Viscoelastic Material
-2.5
-1.5
-0.5
0.5
1.5
2.5
-2000
-4000
-6000
300
-8000
-10000
-12000
Z position, um
250
Height, nm
Load Force, nN .
0
200
150
100
50
0
0
5
10
15
Distance, um
20
25
30
Viscoelastic Material
2000
-2.5
-1.5
-0.5
-2000
0.5
1.5
2.5
-4000
300
-6000
-8000
-10000
-12000
-14000
Z Distance, um
250
Height, nm
Load Force, nN
0
200
150
100
50
0
0
5
10
15
Distance, um
20
25
30
Contact Mechanics Model of an
Interface
Hertzian Contact between Rigid Surfaces
Shull, K. R., (Nov. 2004), shullgroup.northwestern.edu/pdfpublic/ref054.pdf
Phertz
4 E *a 3
=
3R
•Frictionless, ideal contact
Contact Mechanics Model of an
Interface
At the JKR and DMT Limits
2⎤
3R ⎡
(
)
a =
P + 3πGR + 6πGRP + 3πGR ⎥
* ⎢
⎦
4E ⎣
3
P → 0
PJKR =
PDMT
lim ζE * = 3πγ 12 R
ζ → δ 2 R /3a
= lim
ζ → 4δ 2 R / 9 a
ζE * = 4πγ 12 R
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
Surface Tension,γ, dyne/cm .
100
90
80
70
60
50
•Make contact with surface (low force)
40
•Allow several force curve cycles (save each)
30
•Decrease the z scan range (decrease the force)
20
•Save another series of force curves
10
•Repeat until no contact
0
-200
0
200
400
600
800
Load, P, nN
1000
1200
1400
Load-Unload Procedure
Surface Tension,γ, dyne/cm .
100
90
80
70
60
•Increase z scan range to contact surface
50
•Save force curve at initial contact
•Increase the z scan range (increase the load)
40
•Save force curve
30
•Repeat until at desired load
20
•Decrease the z range (decrease the load)
10
•Save force curve
•Repeat until no contact
0
-200
0
200
400
600
800
Load, P, nN
1000
1200
1400
AAB-1 “Set”
Surface Tension,γ, dyne/cm .
70
60
50
40
30
20
γ = 46.9 (Avg. 47.9)
10
0
-200
0
200
Load, P, nN
400
AAD-1 “Set”
Surface Tension,γ, dyne/cm .
110
100
90
80
70
60
50
40
30
γ = 42.7 (Avg. 45.0)
20
10
0
-100
0
100
200
Load, P, nN
300
400
500
AAF-1 “Set”
Surface Tension,γ, dyne/cm .
70
60
50
40
30
20
γ = 48.4 (Avg. 47.9)
10
0
-200
-100
0
100
Load, P, nN
200
300
400
AAM-1 “Set”
Surface Tension,γ, dyne/cm .
140
120
100
80
60
40
γ = 50.3 (Avg. 49.9)
20
0
-200
0
200
400
Load, P, nN
600
800
Surface Tension,γ, dyne/cm .
Average Surface60Tensions
50
40
30
AAB γ : 47.9 ± 1.2
20
10
0
-125
-100
-75
-50
-25
Load, P, nN
0
25
50
Surface Tension,γ, dyne/cm .
Average Surface60Tensions
50
40
30
AAB γ: 47.9 ± 1.2
AAD γ: 45.0 ± 2.6
20
10
0
-125
-100
-75
-50
-25
Load, P, nN
0
25
50
Surface Tension,γ, dyne/cm .
Average Surface60Tensions
50
40
30
AAB γ: 47.9 ± 1.2
AAD γ: 45.0 ± 2.6
AAF γ: 47.7 ± 2.2
20
10
0
-125
-100
-75
-50
-25
Load, P, nN
0
25
50
Surface Tension,γ, dyne/cm .
Average Surface 60Tensions
50
40
30
AAB
γ: 47.9 ± 1.2
AAD
γ: 45.0 ± 2.6
AAF
γ: 47.7 ± 2.2
AAM
γ: 49.9 ± 1.6
20
10
0
-125
-100
-75
-50
-25
Load, P, nN
0
25
50
JKR/DMT
JKR
DMT
JKR/DMT
Surface Tension,γ, dyne/cm .
JKR
DMT
60
50
40
30
AAB
20
AAD
AAF
10
AAM
0
-125
-100
-75
-50
-25
Load, P, nN
0
25
50
Average Surface Tensions
AAD-1
AAF-1
AAB-1
AAM-1
45.0
47.7
47.9
49.9
± 2.4
± 2.2
± 1.2
± 1.6
AAD-1 Neat
AAD-1 PAV 240 h
AAD-1 PAV 480 h
AAD-1 PPA & PAV 96 h
AAD-1 PPA & PAV 184 h
Least Cohesive
Most Cohesive
40.9
44.7
43.7
46.2
47.0
± 1.0
± 1.3
± 1.4
± 0.5
± 0.7
Least Cohesive
Most Cohesive
Wc = 2γ
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. DTFH6199C-00022
Questions?