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Proceeding of 14th International Congress of Metrology in Paris 22-25 June 2009
METHODOLOGY FOR METROLOGY OF WETTABILITY VERSUS
ROUGHNESS OF ENGINEERING SURFACES
K.J. Kubiak* 1, T.G. Mathia2, M.C.T. Wilson1
1iETSI - Institute of Engineering Thermofluids Surfaces and Interfaces, School of Mechanical Engineering - University of
Leeds, Leeds LS2 9JT,United Kingdom
2LTDS - Laboratoire de Tribologie et Dynamique des Systèmes, CNRS UMR 5513, Ecole Centrale de Lyon, 36 Avenue
Guy de Collongue, 69134 Ecully,- France
krzysztof@kubiak.co.uk
Abstract
Both dynamic and static contact angles of liquid
drops are studied by a real-time imaging analysis system.
Many popular and widely used engineering materials like
aluminium, titanium, steel and copper alloys, ceramic
and plastic have been tested and the wettability of these
materials is reported. The measured values show that the
apparent contact angles depend strongly not only on the
solid and liquid properties, but also that the surface
roughness plays a significant role. The most important
2D and 3D topographical parameters, which influence
the wettability measurement, are selected by statistical
covariance analysis. The dynamics of wetting is explored
on different surface morphologies and the influence of
topography is elucidated.
Resume
Les angles de contact tant dynamiques que
statiques de gouttes liquides en contact avec des surfaces
réales usinés différemment sont mesurés en temps réel
grâce à un système d'analyse entièrement informatisé.
Plusieurs matériaux largement utilisé dans la construction
mécanique comme les alliages d'aluminium, de titane, de
fer, de cuivre, de céramiques et de polymères sont
caractérisés sur le plan de leurs mouillabilités issues du
même procède de finition par abrasion à deux corps avec
des abrasifs de tailles dont morphologies différentes. Ces
morphologies sont mesurées et évaluées par la quasi-
totalité des paramètres topographique de rugosité.
Les valeurs mesurées montrent que les angles de contact
apparents dépendent fortement pas seulement des
propriétés solides et liquides, mais aussi que la
morphologie de surface joue un rôle significatif. Les
2ème et 3ème paramètres topographiques les plus
importants, qui influencent la mesure de la mouillabilité,
sont identifiés par l'analyse de covariance statistique est
élucidée dans le contexte de la morphologie des surfaces.
Keywords : Surface roughness, Contact angle, Wetting,
Aluminium, Titanium, Steel, Ceramic, Plastic.
Introduction
A number of industrial processes like
lubrication, adhesion, printing, coating, spray quenching,
soldering, brazing, etc. essentially involve spreading and
wetting processes. Wetting and wettability can be defined
as the tendency for a liquid to spread on a solid substrate
[1]. From bibliographical analysis it can be noted that
there is a great number of scientific works on molecularly
smooth or modelled "simply rough" highly hydrophobic
surfaces but little work has been done on wettability and
spreading phenomena of real engineering surfaces
(Figure 1), very distant from idealistic examples.
However, some studies of the influence of surface
roughness on wetting have been conducted for water [2,
3] and oil [1] liquids, but only on metallic surfaces. In the
present paper the influence of roughness on initial and
static contact angles has been studied for wide range of
engineering surfaces (Figure 1): aluminium alloy
AA7064, titanium alloy Ti-6Al-4V, steel AISI 8630,
copper alloy UNS C17000, ceramic [46% silicon (SiO2),
17% magnesium (MgO), 16% aluminium (Al2O3), 10%
potassium (K2O), 7% boron (B2O3), 4% fluorine (F)],
and plastic poly-methylmethacrylate (PMMA). Many
industrial applications implement traditional or high-
technology manufacturing of engineering surfaces
usually for very specific applications. The most common
engineering materials in a great number of end user
functions require well-defined surface characteristics to
monitor properties related to optical reflexion, painting,
coating, sealing, friction, anti-corrosion, adhesion,
lubrication, wear resistance, sliding, electrical contact
resistance, biocompatibility, etc. [4, 5] There is a lack of
simple and reliable methodology for simultaneous
investigation of relationships between the nature of
materials, surface topography and wettability / spreading
properties. The first step in this field is one of the
principal aims of this work.
Hydrophobic
surface Hydrophilic
surface
CeramicMetallic
highsolid surface free energylow
goodwettabilitypoor
goodadhesivenesspoor
lowcontact anglehigh
CeramicMetallic
highsolid surface free energylow
goodwettabilitypoor
goodadhesivenesspoor
lowcontact anglehigh
θθ
Steel AISI 8630 Ceramic
Images of measured surfaces
Material properties
Figure 1: Examples of measured contact angles and
comparison of surface material properties.
Proceeding of 14th International Congress of Metrology in Paris 22-25 June 2009
Experiments
The effect of solid surface roughness on the
wettability measurements is investigated by contact angle
measurements in the direction parallel to the surface
texture (Figure 3).
Tested materials
In order to evaluate the influence of material
properties on wetting phenomenon a wide range of
common engineering materials were selected:
1. Aluminium alloy AA7064,
2. Titanium alloy Ti-6Al-4V,
3. Steel AISI 8630,
4. Copper alloy UNS C17000,
5. Ceramic made from fluorphlogopite mica in a
borosilicate glass matrix, with chemical
composition: 46% silicon (SiO2), 17%
magnesium (MgO), 16% aluminium (Al2O3),
10% potassium (K2O),7% boron(B2O3), 4%
fluorine (F), (machinable glass ceramic).
6. Poly-methylmethacrylate (PMMA).
Material selection
Selection of these materials were based on the
different properties like electric conductivity, type of
material (metallic alloy, ceramic, polymer), mechanical
properties (ductile, brittle and semi-brittle), therefore the
material properties influence can be analysed. All the
selected materials are widely used in manufacturing
industry and easily accessible materials.
Surface preparation
Tested surfaces will be prepared by the abrasive
polishing process [6] which is the most reproducible
finishing process. Materials were cut into small cubes
(10mm x 10mm x 10mm), with one side polished to
produce a wide range of surface roughness Sz= 4 - 111
μm. These different values of surface roughness were
obtained by polishing on sandpapers with the following
grain grids: 80, 400, 600, 2500. Measured values of
surfaces roughness Szare presented in Table 1. All
specimens were prepared following the same polishing
procedure, however due to different material properties
the obtained roughnesses varied for different materials
[7]. Therefore, the statistical method of covariance
analysis will be used to analyse the results [8].
Table 1: Surface roughness (Sz) of prepared surfaces.
Materials Topographical characteristic of tested
surfaces Sz,μm
Process
1Process
2Process
3Process
4
Aluminium
alloy 6.6 7.8 9.8 46.9
Titanium
alloy 5.3 9.0 9.9 15.1
Steel alloy 4.0 6.6 12.8 54.8
Copper
alloy 8.0 9.8 16.4 43.9
Ceramic 35.1 58.0 50.4 111.0
PMMA 13.8 40.9 21.2 83.7
Examples of morphologies of prepared surfaces (Process
1 -4) are presented in Figure 2.
Process 1 (Sz= 6.6 μm)
0
1.8 mm
50
25
2.3 mm
μm
Process 2 (Sz= 7.8 μm)
0
1.8 mm
50
25
2.3 mm
μm
Process 3 (Sz= 9.8 μm)
0
1.8 mm
50
25
2.3 mm
μm
Process 4 (Sz= 46.9 μm)
0
1.8 mm
50
25
2.3 mm
μm
Figure 2: Examples of measured morphologies of tested
surfaces prepared by abrasive polishing (material
aluminium alloy AA7064).
Contact angle measurements
The contact angle between the water and tested
materials has been measured using a PG-X goniometer
with image resolution 640x480 pixels. This fully
automated apparatus with integrated pump, delivers
accurate droplets in steps of 0.5 µl and the built-in
camera captures a sequence of images to measure the
Proceeding of 14th International Congress of Metrology in Paris 22-25 June 2009
dynamic wetting or the static contact angle at
'equilibrium'. Principle of operation and position of
camera is presented in Figure 3. The drop volume was
taken within the range where the contact angle did not
change with the modification of the volume (4 ± 0.5 μl).
Such volume of water have another advantage: the
measured contact angles are almost identical for parallel
and perpendicular directions to the surface polishing
direction. Therefore, experimental analysis can be
restricted to parallel direction only. All surface before the
test have been ultrasonically cleaned with alcohol.
Measurement temperature was set at ambient temperature
(~22ºC). Initial contact angles were measured
immediately after the drop deposition at time t=0s, and
static contact angles in the equilibrium state were
measured after 20 s from deposition. After that time
water drops started to evaporate and the contact angles
decreased due to the so-calledcontact angle hysteresis. In
this study, only advancing contact angles will be
investigated.
Water
drop
Surface texture
direction
θ
Direction of view
from Camera
Figure 3: Schematic diagram of experimental
measurements of contact angle in direction parallel to
surface texture.
The wide range of tested materials and surface
morphologies, allow one to compare directly the
roughness and material influences on wetting
phenomenon.
Analysis of Results
Real engineering surfaces are highly irregular
and often anisotropic. Surfaces prepared for this study by
polishing, have unidirectional texturing and they are very
complex. Examples of roughness are presented in Figure
2 and Figure 5. A simple mechanism of wetting on the
rough surfaces can be described as the barriers formed by
the peaks. The advancing contact line can be stopped by
this barrier and the apparent contact angle will be larger.
However once the contact line reaches the maximal point
of the next roughness peak the balance will be lost and
due to surface tension and the gravity force the contact
line will move forward, wetting the valley of the peak up
to the next equilibrium state (Figure 4). However, the real
profile of the surface is much more complex. Therefore,
the measurement of the contact angle can be highly
affected by roughness effects.
vapour
liquid
Wetting of rough surface
R
unequilibrium
solid
equilibrium
0
0.1
0.2
0.3
0.4
0.5 mm
µm
-20
-15
-10
-5
0
5
10
15
Length = 0.503 mm Pt = 21.5 µm Scale = 40 µm
Real surface profile (Ceramic –Process 3)
R
AR
R
2
AR
Kr
Figure 4: Schematic diagram of rough surface wetting,
versus real surface measured profile on ceramic material.
Table 2: Syntheses of the most important parameters
selected by covariance analysis of wetting versus
morphological parameters.
3D morphology parameters Covariance
coefficient
S10z Ten point height 3954.5
S5v Five point pit height 2204.8
Spd Density of peaks 2022.1
S5p Five point peak height 1750.5
Std Texture direction 565.3
SzMaximum height -252.2
StTotal height -251.3
2D Profile parameters Covariance
coefficient
Rmr Relative Material Ratio of
the roughness profile. 148.3
Trc Microgeometric material
ratio 63.5
Pmr Relative Material Ratio of
the raw profile 57.8
KrMean Slope of the
Roughness Motifs 43.6
RONtPeak to valley roundness
deviation 33.0
RONpPeak to reference
roundness deviation 28.8
Psk Skewness of the raw
profile 28.6
Pku Kurtosis of the raw profile -417.5
Prepared surfaces were measured by an
interferometric profilometer and firstly from 3D
morphologies more than 50 different surface parameters
were calculated. Secondly the 2D profile in the direction
parallel to surface texture was extracted and more than
100 different 2D roughness parameters were calculated
for all 24 tested surfaces. This procedure generated huge
Proceeding of 14th International Congress of Metrology in Paris 22-25 June 2009
amounts of data and therefore the statistical method of
covariance has been used to analyse the most important
parameters that can influence the contact angle
measurement. Due to the space limitation only the most
important parameters have been presented in Table 2.
From covariance analysis of 2D profile it can be
noted that three first parameters are linked to the relative
material ratio curve which describes the percentage of
material which is traversed by a cut at a certain level
located with respect to the highest point on the profile.
This curve is known as the Abbott-Firestone curve. For
higher values of these parameters the distance between
the peaks is usually higher and the barrier created by the
next peak needs more energy to be wetted, therefore the
apparent contact angle is higher, which confirms positive
covariance of contact angle and material ratio related
parameters (Rmr, Trc, Pmr). Next parameter Krhas been
found by many researchers as a parameter which well
describes roughness influence [9, 10]. It is defined as
follows:
R
AR
Kr2
where: Kris mean slope of the roughness motifs, R (µm)
is mean depth of the roughness motifs (average of all Ri,
µm), AR (µm) is mean spacing of the roughness motifs.
R and AR are defined in the ISO 12085:1996 standard,
however the Krparameter is only defined in the French
standard E.05.015 and is not defined in the ISO 12085.
In the present study results of roughness influence on
contact angle measurement will be presented in terms of
the Krparameter (Figure 6 and Figure 7).
0
0.1
0.2
0.3
0.4
0.5 mm
µm
-3
-2
-1
0
1
2
3
Length = 0.503 mm Pt = 0.66 µm Scale = 6 µm
Process 1 Kr=13.7 Steel AISI 8630
0 0.1 0.2 0.3 0.4 0.5 mm
µm
-3
-2
-1
0
1
2
3
Length = 0.504 mm Pt = 4.68 µm Scale = 6 µm
Process 3 Kr=81.3 Ceramic
7.13
R
2
AR
Kr
3.81
R
2
AR
Kr
Figure 5: Comparison of mean slope of the roughness
motifs parameter Krfor process 1 and process 3.
All remain the most important parameters presented in
Table 3 (RONt- peak to valley roundness deviation;
RONp- peak to reference roundness deviation; Psk -
pkewness of the raw profile; Pku - kurtosis of the raw
profile; S10z - ten point height; S5v - five point pit height;
Spd - density of peaks; S5p - five point peak height; Std -
texture direction; Sz- maximum height; St- total height
[11]) are related to the peaks height, density, roundness
etc. However, assuming that contacts between liquid and
solid are limited to the top part of roughness peaks it can
be noted that from physical point of view all these
parameters defined numbers and size of contacts
liquid/solid and therefore the real contact area.
Results of initial contact angle θ (t=0s) are
presented in Figure 6 and static contact angle θ (t=20s) in
Figure 7. For all measures surfaces the spreading
phenomenon has been observed, after 20s the initial
contact angle decreases by 5 to 15 degrees. The highest
spreading effect was observed for PMMA and ceramic
material and the lowest effect for copper, aluminium and
titanium alloys (Table 3). All measured materials have
hydrophilic properties where θ<90º.
Mean Slope of the Roughness Motifs, K
r
initial contact angle, θ(º)
Initial Contact Angle (t=0s)
0
25
50
75
100
0 50 100 150 200 250
AA7064 Ti-6Al-4V
AISI 8630 Copper
Ceramic PMMA
Figure 6: Experimental results of initial contact angle θ
(t=0s) evolution as a function of roughness parameter Kr.
Mean Slope of the Roughness Motifs, K
r
static contact angle, θ(º)
Static Contact Angle (t=20s)
0
25
50
75
100
0 50 100 150 200 250
AA7064 Ti-6Al-4V
AISI 8630 Copper
Ceramic PMMA
Figure 7: Experimental results of static contact angle θ
(t=20s) evolution as a function of roughness parameter
Kr.
The initial and static contact angle shows similar
trends for the tested range of roughness, however in this
range the minimum value of apparent contact angle can
Proceeding of 14th International Congress of Metrology in Paris 22-25 June 2009
be observed for roughness obtained on surface prepared
by Process 2.
This phenomenon can be explained by the
following hypothesis: when the distance between the two
neighbourhood peaks is small and the height of these
peaks is high relative to the distance, the roughness
effectively creates a small capillary at the surface, which,
due to the capillary phenomenon, can be easy wetted,
making the apparent contact angle smaller (Figure 6 and
Figure 7).
Table 3: Contact angle measurement.
Process
reference Material Krθ º
(t=0s) θ º
(t=20s)
Process 1 Al alloy 29.3 83.2 78.9
Process 2 Al alloy 31.1 87.9 86.7
Process 3 Al alloy 49.1 90.0 81.4
Process 4 Al alloy 224.0 88.2 86.9
Process 1 Steelalloy 13.7 98.0 93.7
Process 2 Steelalloy 23.9 76.5 68.0
Process 3 Steelalloy 32.9 73.4 69.2
Process 4 Steelalloy 69.6 79.8 73.9
Process 1 Ceramic 35.9 41.0 38.1
Process 2 Ceramic 54.3 29.5 19.1
Process 3 Ceramic 81.3 33.9 22.2
Process 4 Ceramic 210.0 57.7 42.1
Process 1 Cu alloy 23.3 84.8 83.5
Process 2 Cu alloy 27.3 56.7 55.3
Process 3 Cu alloy 38.1 63.6 57.9
Process 4 Cu alloy 164.0 82.9 80.8
Process 1 PMMA 36.6 75.8 68.6
Process 2 PMMA 48.9 72.6 57.8
Process 3 PMMA 65.0 78.7 63.6
Process 4 PMMA 217.0 79.0 65.9
Process 1 Ti alloy 32.2 73.9 66.4
Process 2 Ti alloy 26.8 74.7 69.0
Process 3 Ti alloy 39.5 71.5 64.5
Process 4 Ti alloy 96.5 81.5 73.2
This phenomenon is observed for almost all
materials, hence it can be independent of material
properties and more related with surface roughness. For
smooth surfaces the peaks’ heights are too small and for
rough surfaces the distance between the peaks is too large
to create the capillary effect. It should be noted that this
hypothesis has been tested for the unidirectionally
textured surfaces. However, a deeper study of roughness
phenomenon is needed to fully explain and understand
the phenomenon.
Conclusions
From the presented experimental study on the
influence of roughness on contact angle measurements,
the following conclusions can be drawn:
Due to the complexity of real engineering
surfaces the roughness must be considered in
wettability(contact angle) measurements,
The wide range of tested materials confirms that
similar influences of roughness have been
observed for all tested materials,
The following roughness parameters (Rmr, Trc,
Pmr, Kr, RONt, RONp, Psk, Pku) calculated from
2D profile have major influence on apparent
contact angle θ,
Similarly for 3D surface morphology the main
roughness parameters which influence apparent
contact angle θ are: S10z, S5v, Spd, S5p, Std, Sz, St.
Acknowledgments
The Authors are grateful for generous
participation of Mr. Philippe Carval (ALTIMET SAS in
France) in the present study and support in measurement
and analysis of surface morphology.
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