1. Contact Angle Measurements and Surface Energy
Ashley Tracey
Graduate Student
Department of Materials Science and Engineering
University of Washington θ

2. Outline Motivation Techniques and methodology
Contact angle and surface energy theory
Application to research

3. Motivation
Water contact angle can determine whether a surface is hydrophobic or hydrophilic
Contact angle can detect differences in substrates
Can calculate surface energies from contact angle measurements

4. Instrument Bench-top device (VCA Optima video goniometer)
Multiple fluids
Surface Analyst
Handheld device (Brighton Surface Analyst)
Water

5. Goniometer Methodology
Contact angle of a 1μL drop of fluid is measured – side view
Determine average contact angle from 10 drops
For surface energy calculations using Owens-Wendt model, need average contact angle for at least 2 fluids
Hydrophobic surface
Hydrophilic surface
Complete wetting when θ approaches zero

6. Surface Energy Calculated from Contact Angle
Three parameters influence the shape a drop takes on a solid surface
γsl : solid-liquid surface energy
γsv : solid-vapor surface energy
γlv : liquid-vapor surface energy γ sl s v lv
solid
liquid
vapor θ
These three values and contact angle are related by Young’s equation:
γsl = γsv -γlv cosθ (1)

7. Young’s Equation γsl = γsv -γlv cosθ (1)
Liquid-vapor surface energy and contact angle can be determined empirically
Need to develop model to determine other values

8. Calculating Surface Energy cont.
Many models developed to calculate solid-liquid and solid-vapor surface energies
Neuman Method (1 component)
Fox-Zisman Method (1 component)
Fowkes Method (2 components)
Owens-Wendt Method (2 components)
Van-Oss-Chaudhury-Good Method (3 components) -

9. Calculating Surface Energy
Berthelot assumed (late 1800s):
Wsl is the interfacial work of adhesion
Wss is the work of cohesion of a solid
Wll is the work of cohesion of a liquid
The work of cohesion: Wcoh = 2γ, thus (2) becomes:
Wsl = √(WssWll) (2)
Wsl = √(γssγll) (3)

10. Calculating Surface Energy cont.
The Dupre equation for the work of adhesion for two dissimilar materials:
Equating (3) and (4):
Equation (5) is the starting point for deriving equations to determine γsv
Wsl = γsv + γlv – γsl (4)
γsl = γsv + γlv – 2√(γsvγlv) (5)

11. Owens-Wendt Model Takes the surface energy to be composed of two parts
Dispersive component (d)
Van der Waals interactions
Polar component (p)
polar interactions, hydrogen bonding, acid-base interactions
γ = γp + γd (6)
Using equation (6), equation (5) becomes:
γsl = γsv + γlv – 2√(γsvp γlvp) – 2√(γsvd γlvd) (7)

12. Owens-Wendt Model cont.
Equating Young’s equation (1) and Owens-Wendt’s equation (7) and some algebra yields:
Equation (8) takes the form y = mx + b, where:
y = γlv(cosθ + 1)/2√γlvd
x = √(γlvp/γlvd)
m = √γsvp POLAR COMPONENT
b = √γsvd DISPERSIVE COMPONENT
(8)

13. Owens-Wendt Model cont.
Equation (8) has two unknowns (γsvp and γsvd)
We need the contact angle of at least two fluids, so we can plot x vs. y (Klaeble plot) and extrapolate m and b to calculate γsvp and γsvd

14. Owens-Wendt Spontaneous Wetting
Substituting θ= 0° for spontaneous wetting and γlv = γlvp + γlvd into equation (8) and some algebra:
Rearranging (10):
Equation (11) takes form of ax2 + bx + c = 0, can solve for roots using quadratic formula:
γlvp + γlvd = √(γsvp γlvp) + √(γsvd γlvd) (10)
1(√γlvd)2 - √(γsvd) √(γlvd) + [γlvp - √(γsvp) √( γlvp) = 0 (11)
ax2 + bx + c
= 0
x = [-b ± √(b2 – 4ac)]/(2a)

15. Owens-Wendt Wettability Envelopes
Spontaneous Wetting

16. Contact Angle Measurements in Research
Potential Surface Preparation Inspection Method for Composite Bonding

17. Motivation Adhesive bonds are highly desired in the aerospace industry
Eliminate local stress concentrations present at the sites of fasteners
Reduce part count and weight  increase efficiency
However, adhesive bonds are VERY sensitive to surface preparation
Want to be able to inspect a surface prior to bonding to ensure proper surface preparation
CONTACT ANGLE MEASUREMENTS
Voids
Contaminants

18. Surface Preparation Protect surface prior to bonding
Remove/prevent a weak boundary layer
Maximize adherend/adhesive intimate molecular contact
Ensure intrinsic adhesion forces across the interface yield required joint strength/service life
Generate a specific adherend surface topography
Assist in the adhesive hardening
Surface preparations for composites:
Peel ply (as tooled)
Abrasion (sanding or grit blasting)

19. Peel Ply Used for Structural Composite Bonds
Last layer on part before curing
Protects surface during handling
Removed just prior to bonding
Clean, chemically active surface
Precision Fabrics 60001
Polyester
PF 52006
Nylon
PF SRB
Siloxane Coated Polyester

20. Surface Energy to Probe Composite Surface
Why use surface energy to probe the surface preparation method applied to the composite for bonding?
One requirement of adhesion is the adhesive must wet the substrate
This is controlled by surface energy
Contact angle is influenced by surface preparation
Use this approach to probe surface to determine if we can detect contaminants or variations that correlate with bond quality

21. Materials Toray 3900/T800 unidirectional laminates
Precision Fabric Group polyester peel ply
Precision Fabric Group nylon peel ply
Precision Fabric Group SRB release ply
Autoclave cure of composites
Fluids used for contact angle analysis:
De-ionized water (DI water)
Ethylene Glycol (EG)
Glycerol (Gly)
Diiodomethane (DIM)
Formamide (Form)

22. Goniometer Methodology
Contact angle of a 1μL drop of fluid is measured – side view
Peel ply removed and CAs measured within 1 hour
All CAs viewed at 0°
4 fluids, 10 drops per fluid evaluated on each surface
Average CA and stdev calculated to determine surface energies and generate wettability envelopes 0°
30°
45°
60°
90°
Camera

23. Brighton Surface Analyst Methodology
Contact angle of a 1.38μL drop of water is measured – top view
CA is calculated by fitting the circumference to the volume of the drop
Average CA and stdev calculated for comparison to water CAs measured with goniometer
Note: rectangle in image is a reflection of light from camera

24. Peel Ply Texture Orientation
Effect of surface texture left in epoxy resin after peel ply removal
Why we measure CA at 0° w.r.t. the peel ply angle
Peel ply
Composite

25. Peel Ply Texture Orientation
Peel ply angle is defined as the angle at with the peel ply texture is oriented with respect to the goniometer camera 0°
30°
45°
60°
90°
Camera

26. Effect of Peel Ply Texture Orientation on Contact Angle
Peel ply orientation affects contact angle measurement
Contact angles measured at 0 and 90 degrees are greatest

27. Wettability Envelopes
Angles of 30 and 60 degrees produced similar wettability envelopes
Angles of 0 and 90 degrees produced similar wettability envelopes

28. Evaluation of Peel Ply Texture and its Affect on Contact Angle
Difference in contact angle at differing orientations of the substrate is due to the texture left in the resin upon peel ply removal
The fluids form non-circular drops on the substrate
Top down pictures from right to left of form., DI water, EG, and gly drops (note: white circles are a reflection of light from the microscope)

29. Peel Ply Texture Affects Drop Shape
The non-circular drops resulted in differing contact angle measurements at different orientations
45° view
0°/90° view
Substrate
Fluid drop
45° view °/90° view

30. Surface Preparation
Can water contact angle identify a difference between surface preparations performed on the same substrate?
Polyester peel ply
Nylon peel ply
SRB release ply

31. Effect of Surface Preparation Method on Water Contact Angle Measurement

32. Difference in Surfaces Confirmed by FTIR
polyester prepared
SRB prepared
nylon prepared
Polyester peaks
Siloxane peak

33. Peel Ply Contamination
Can contact angle be used to identify surface contamination?
Contaminants are detrimental to bonding
Example: siloxane contamination
Mix Solids Target Level
0% (control)
0.0001%
0.001%
0.01%
0.05%
0.1%
0.2%
0.3%
0.5% 1% 2%

34. Surface Energies

35. Wettability Envelopes

36. Conclusions
Contact angle measurements can be used to detect differences between surfaces
Hydrophobic vs hydrophilic
Contact angle measurements can be used to calculate surface energies
Research application: composite bonding
Peel ply contamination
Surface preparations

37. Acknowledgements FAA, JAMS, AMTAS Boeing Company
Paul Shelley
Precision Fabric Group
Richmond Aircraft Products
Airtech International
Prof. Mark Tuttle (UW)