Wetting and El. Properties of Biomed. Alloys

7/28/2019 Wetting and El. Properties of Biomed. Alloys

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Wetting and electrochemical properties of biomedical

alloys

Solne Barbotin

Abstract

Wettability of Stainless Steel and Cobalt-Chromium-Molybdenum alloy in two differentsolutions which are Phosphate Buffer Solution (PBS) and PBS plus bovine serum albumin is

examined under application of a potential, using the captive bubble method. It is found thatwettability of both alloys increases with potential decrease, in the two different solutions, thusshowing wettability has a link with potential.

Keywords : wettability, potential, captive bubble method, stainless steel, CoCrMo alloy

I. Introduction

Wettability allows to character-

ize the surface of a material, and

can be linked to the surface en-

ergy, which is an important parameter. In-

deed, it was found that it can be related tofriction and wear phenomena [1]. When

two materials are sliding on each other,

which is the case for hip prosthesis, parti-

cles can detached from one of the material

thus leading to formation of third body

particles. Their presence can lead to in-

flammation of the neighboring tissues, and

thus cause pain to the patient. It is stated

that the better the wettability, the better the

materials sliding properties thus influenc-

ing the wear phenomena.The main goal of this project was then

to study the influence of potential on the

materials wettability. In order to perform

MX/TIC EPFL, semester project. Supervised byAnna Igual and Stefano Mischler.

the tests, the first objective was to design

an electrochemical cell, and then to state

if applying a potential the cathodic way

or anodic way has an effect on the mate-

rials sample wettability. The influence of

the type of alloy on the wettability using

CoCrMo alloy and Stainless Steel (SS) isalso investigated, as well as the influence

of the electrolyte solution on the samples

wettability, using Phosphate Buffer Solu-

tion (PBS) and PBS plus bovine serum al-

bumin.

Then, if it occurs that the potential seems

to have an effect, it will be stated which

effect has potential change on the surface

wettability characteristics.

II. Preliminaries

Wettability characterizes the ease with

which a liquid drop spreads on a surface

solid. It is characterized by the contact

angle theta which depends on the three

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Semester project Autumn 2012 S.Barbotin

surface tensions sg , sl and lg , acting on

the contact line between the solid, liquid

and gazeous phases. At equilibrium, the

sum of the parallele forces at the surface of

the solid is null. Follows the relationship,based on Youngs equation [2]:

cos lg + sl = sg (1)

cos =sg sl

lg(2)

A number of friction and surface phenom-

ena are explicable in terms of the surface

energy of adhesion of the contacting mate-

rial.It is interesting to know the wettability of

a material, because it can be related to the

materials surface energy, through Youngs

equation, as stated before.

In order to increase of an increment dA

a solid or liquids surface, it is necessary

to move a certain quantity of atoms from

the mass to the surface. Since the surface

atoms have less bonds than those inside

the mass, the solid or liquid free energy

increases of a quantity dG. dG = dA.The definition of surface energy comes

from a more general formulation of free

energy change or free enthalpy of a ther-

modynamic system, before surface effects

[2]:

dF = PdV SdT+i

idni + dA (3)

dG = VdP SdT+i

idni + dA (4)

=

F

A

V,T,ni

; =

G

A

P,T,ni

(5)

In a cubic face centered metal, an atom has

12 close neighbors and so, as many bonds.

The energy to provide to break these bonds

is equivalent to the enthalpy of sublimation.

The atoms possess 9 close neighbors if they

belong to a surface which orientation cor-

responds to the Miller indices (111). Thus,according to the simple model exposed be-

fore, the metals surface energy depends on

the crystalline orientation. For a cfc metal,

atoms belonging to the orientation surface

(111), (100) and (110) have respectively 9, 8

and 6 close neighbors. The surface energy

then increase in the order (111) < (100)

< (110). Of maximum compactness, the

(111) orientation surfaces are, thermody-

namically speaking, the most stable.

III. Materials and Methods

III.1 Materials

III.1.1 Electrochemical cell

The electrochemical cell built to host the

tests was made of PMMA for the cell itself

Fig. 1(a) and Fig. 1(b), size 74 74 65 mm

(length width height) + lid, a rubber seal

was added in the lid to hold the sample,

size 20 3, 0 mm (rubber ring diameter

ring diameter thickness). The syringes

needle diameter was 0,8mm.

The cell was chosen to be geometrically

squared and not cylindrical, because if

cylindrical, it might would have led to ge-

ometrical deformations of the bubble on

the pictures that were taken with a camera.

The size of the cell was chosen based on

the following concerns : the cell needed tobe big enough to allow good visual appre-

ciation and to allow the reference electrode

placement (height) but also small enough

in order not to waste solution that was at

disposal.

A reference electrode calomel (SCE)

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(a) Closed electrochemical cell (b) Open elctrochemical cell

Figure 1: Electrochemical cell

(a) Reference

electrode

(b) Counter electrode (c) Potentio-stat.

Figure 2: Materials used for the electrochemical part of the experiment

and a counter electrode made of plat-

inum wire were used for the experi-

ment and connected to a potentiostat

Fig. 2(a)Fig. 2(b)Fig. 2(c). The results pre-sented in section IV, for the potential values

are all vs SCE.

III.1.2 Stainless Steel (SS)

The SS sample Fig. 3(a) was of grade 316L,

and having the following dimensions :

- 20 mm diameter

- 5 mm height

The surface of the sample in contact with

the electrolyte during experiment was of

r2 = 3,14cm2.

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(a) Stainless steel (b) CoCrMo

Figure 3: Samples

III.1.3 Cobalt-Chromium-Molybdenum

alloy (CoCrMo)

The CoCrMo sample Fig. 3(b) is of grade

LC ASTM F 1537-00. It was characterized

with X-Ray Fluorescence analysis, giving

the following composition :

- Fe 0.24 %

- Co 64.95 %

- Cr 28.80 %

- Mo 5.68 %

- Ni 0.32 %

It has the following dimensions :

- 19mm diameter

- 6 mm height

The surface of the sample in contact with

the electrolyte during experiment was then

of r2 = 3, 0cm2.

III.1.4 Phosphate Buffer Solution (PBS)

The phosphate buffer solution was pre-

pared with the following quantities :

- 8 g/L NaCl

- 0.2 g/L KCl

- 1.44 g/L Na2HPO4- 0.25 g/L KH2PO4

III.1.5 PBS + bovine serum albumin

The PBS+albumin solution was prepared

with the following quantities :

- 8 g/L NaCl

- 0.2 g/L KCl

- 1.44 g/L Na2HPO4- 0.25 g/L KH2PO4- 0.5 g/L BSA (albumine)

III.2 Methods

III.2.1 Samples polishing

The first step before starting the experi-ment is to mirror polish the samples, using

silicon carbide sandpaper. The polishing

buffers granulometry used were the follow-

ings, with their corresponding grain size,

using water as a lubricant :

- Grade 500 (29-33m)

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- Grade 1200 (13-15m)

- Grade 2400 (6-8m)

- Grade 4000 (4-5m)

Then, the last polishing step was per-

formed on a velvet polishing buffer, us-

ing red lubricant (no water) with diamond

sprayed particles with a size of 3m. Red

lubricant cools down the sample and helps

removing the debris, it is alcohol based.

III.2.2 The captive bubble method

(CBM)

The captive bubble method is used for mak-

ing the experiment in a retrospective way.It is possible to conduct the experiment un-

der condition mimicking the one in situ,

this condition being total immersion in a

fluid : the prosthesis are indeed immersed

in the body synovial fluid. Another rea-

son for using the captive bubble method,

is that because it is immersed in a liquid, it

is possible to apply an electrochemical po-

tential and study the wettability evolution

in function of this potential.

The methods procedure is as follow.

An air bubble is injected from a syringe

into the PMMA cell chamber containing

the solution Fig. 4(b). The bubble is re-

leased from the tip of a needle and floated

3-4 mm to the sample surface held by the

rubber seal in the cell lid, at the top of the

liquid. The bubble size is in the range of 8

to 12 L. Fig. 4(a)

III.2.3 Instruments

For the contact angle measurement a sim-

ple method was used based on the photog-

raphy of finite dimension bubbles Fig. 6(a)

Fig. 6(b). For this, an instrument consisting

of two parts was used :

(a) Image of the bubble obtained with the CBM

(b) Scheme of the bubble introduction in the system

Figure 4: CBM

- a microscope with numerical camera

connected to a computer Fig. 5(a)

- a translation table XYZ allowing sam-

ples positioning in front of the camera

A bubble of 102 L of air is injected

on the downwards surface of the sample,

with a syringe. The bubble image is cap-

tured by the video camera connected to the

computer. Fig. 5(b)

III.2.4 Cathodic and Anodic Steps, Lin-

ear Sweep Voltammetry (LSV)

and Cyclic Voltammetry (CV)

The cathodic and anodic steps consist of

diminishing for the cathodic steps and in-

creasing for the anodic steps the potential,

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(a) Positionning table and camera

(b) System with cell, camera and computer

Figure 5: Operating system

by steps of 100mV, starting from the Open

Circuit Potential (OCP) of the sample im-

mersed in the electrolyte. The OCP is the

potential of the working electrode relative

to the reference electrode when no poten-

tial or current is being applied to the cell.

The bubble is changed in between each po-

tential step, by stirring the electrolyte with

the syringe at the samples surface.

LSV consists of applying a potential

that constantly diminishes or constantly in-creases over time, by steps of 0,002V, with-

out changing the bubble, and taking pic-

tures every 50mV or every 100mV, starting

at the OCP. For CV, it is the same, the only

difference is that the potential starts di-

minishing so going in the cathodic domain

until it reaches the limit potential that was

fixed, and then it starts increasing, going

to the anodic domain. These two methods

for measuring were used to globally see

if there was a change in the contact angle,and in which domain. It was an approach

to apprehend the behavior of the bubble.

III.2.5 Contact angle measurements

This step is also very important for repro-

ducibility. Indeed, the method used for

measuring such a precise angle, is very

important because variations of angles are

sometimes very small.

The drop analysis plugin [3] created by

the Laboratory BIG in EPFL was created

for drops contact angle measurements, but

can also apply to the CBM since it is partic-

ularly accurate for unsharp or noisy bound-

aries, which corresponds to the images ob-

tained with the CBM.

The drop analysis plugin is using a new

approach to measure contact angle and sur-

face tension : the Low Bond Axisymmetric

Drop Shape Analysis (LB-ADSA)In LB-ADSA, the theoretical profile is

not fitted to a discretized drop contour but

is optimized based on an image energy

approach. In this approach, segmentation

and fitting are combined in what can be

seen as a model-based segmentation. The

complete pixel information is used during

the fitting process. This approach is par-

ticularly advantageous when a clear accu-

rate contour detection is difficult because

of unsharp or noisy boundaries. Appli-cation of image energies to segmentation

tasks is an active research domain. Fol-

lowing current proposals, a gradient im-

age energy comprising gradient direction

is used. This energy term has the bene-

fit of being invariant to parametrization.

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(a) Image before fitting (b) Final fitting (c) Fitting with angles

(d) Measurements table

Figure 7: Fitting steps

In addition, it is suggested to account for

pixel value information by a region energy.

Region energies are statistically based and

offer the advantage of having a wide at-

traction range. The image energy approach

has already been applied to the domain of

contact angle measurements (DropSnake

method) and demonstrated its potential for

noisy and/or low contrast data.

Finally, the presented method is applied

to a continuous image of the drop by us-

ing cubic B-spline interpolation. Then, the

evolution procedure takes place in this con-

tinuous domain to avoid inaccuracies in-

troduced by pixelization and discretization.

Fig. 7(a) show the approximation curve. It

(the green curve) can be fitted to the bub-ble contour by modifying width, heigth

and other parameters, and once a close

fitting is reached manually, an automatic

fitting was performed using the gradient

energy approximation, leading to the fit-

ting in Fig. 7(b).

The angle measured by the program is

the one called on Fig. 7(c) and presented

in Fig. 7(d) as CA[], which is the angle

formed by the air bubble. The angle of

interest for us, is the one formed by the

water, that is to say the angle on the

figure, which is the one calculated as 1802and presented in the results.

For the program to be able to analyze

the bubble, the images must be black and

white and the bubble must stand upwards

like in Fig. 8(b). Since most of the images

taken were in color and with the bubble

downwards, a plugin flipping the image

and changing color to black and white was

applied. For the other images, because

there was some problems concerning thepixels colorization as seen on Fig. 8(a), a

moving average of 2 was vertically and

horizontally applied, which means a color

mix is done between two adjacent pixels,

in order to get rid of this problem. It was

primordial to get rid of it because it would

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(a) Color picture of the bubble

(b) Black and white picture of the bubble

Figure 6: Exemple of the two image types taken

have interfered with the energy gradient

fitting.

IV. Results

IV.1 Electrochemical cell conception

A functional cell was obtained, allowing

good visibility of the samples surface, andso of the bubble evolution. The pictures

taken were of good quality considering the

electrolyte in which the pictures were taken

was a liquid, rendering good quality more

difficult to obtain than with the sessile drop

method, where the electrolyte is air.

(a) Problem encountered with black and white pic-

tures

(b) Final image

Figure 8: Black and white image steps

IV.2 SS in PBS solution

Fig. 9(a) with CV shows a drop in contact

angle when decreasing the potential. The

contact angle starts diminishing at -0,6 V,

going from 30 up to 10 when reaching

-0,95 V, at a constant rate. Then, the poten-

tial evolution was reversed and increased

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0

5

10

15

20

25

30

35

-1,

2

-1

-0,

8

-0,

6

-0,

4

-0,

2

0

0,

2

0,

4

0,

6

0,

8

Watercontactang

le[]

Poten0al[V]

CVSSPBS

CVSSPBS

(a) Contact angle in function of potential

-8

-7

-6

-5

-4

-3

-2

-1

0

-1,3 -0,8 -0,3 0,2 0,7

log(|i|)[A]

Poten0al[V]

CV

(b) Current curve

Figure 9: SS PBS CV

anodically, reaching 0,6 V with no more

evolution of the contact angle, thus being

stationnaire at 101.

For the cathodic steps, Fig. 10(a), the con-

tact angle starts decreasing at 0,35 V until

0,5 V for the first test and until 0,6 V for

the 2nd test. At 0,5 V, for the first test, the

contact angle stops decreasing and is sta-

tionary, whereas for the 2nd test the contactangle keeps decreasing at a constant rate.

Test 2 had to be stop at 0,6 V. Indeed, final

values of contact angles are so low when

decreasing potential, that for cathodic steps

test 2, the contact angle of bubbles at poten-

tials lower than -0,6 V were not measurable.

0

2

4

6

8

10

12

14

-0,8 -0,6 -0,4 -0,2 0

Watercontactangle[]

Poten0al[V]

SSPBSCathodic

SSPBS

cathodic2

SSPBS

cathodic1


-0,9

-0,8

-0,7

-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0

-2,E-04

-2,E-04

-1,E-04

-1,E-04

-9,E-05

-7,E-05

-5,E-05

-3,E-05

-1,E-05

0 500 1000 1500 2000 2500 3000 3500

Poten&al[V]

Currenti[A]

Time[s]

SSPBScathodicsteps1current

SSPBScathodicsteps2current

SSPBScathodicsteps1poten?al

SSPBScathodicsteps2poten?al

(b) Current and potential curves

Figure 10: SS PBS cathodic steps

The sample surface was so hydrophilic thatthe air bubble didnt stick to it anymore,

thus rendering impossible further contact

angle measurements.

Concerning the anodic steps curves in

Fig. 11(a), the contact angles are fluctuating

with the potential between 25 and 12, the

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0

5

10

15

20

25

30

-0,3 -0,2 -0,1 0 0,1 0,2 0,3

Watercontactangle[]

Poten0al[V]

SSPBSAnodicsteps

SSPBS

anodic1

SSPBS

anodic2

SSPBS

anodic3


-0,3

-0,2

-0,1

0

0,1

0,2

0,3

0,4

0,0E+00

2,0E-06

4,0E-06

6,0E-06

8,0E-06

1,0E-05

1,2E-05

1,4E-05

1,6E-05

1,8E-05

0 1000 2000 3000

Poten&al[V]

Currenti[A]

Time[s]

SSPBSanodicsteps1current



SSPBSanodicsteps1poten=al




Figure 11: SS PBS anodic steps

contact angle globally diminishing with

increasing potential.

0

2

4

6

8

10

12

-1,5 -1 -0,5 0

Watercontactan

gle[]

Poten0al[V]

SSPBS+albuminCathodic

LSVSSalbumin

cathodic1

SSalbumincathodic2

SSalbumin

cathodic3


-1,8E-03

-1,6E-03

-1,4E-03

-1,2E-03

-1,0E-03

-8,0E-04

-6,0E-04

-4,0E-04

-2,0E-04

0,0E+00

-1,4 -0,9 -0,4 0,1

Currenti[A]

Poten.al[V]

LSVSS

PBS

+albumin

cathodic1

(b) LSV

-1,2

-1

-0,8

-0,6

-0,4

-0,2

0

-2,5E-03

-2,0E-03

-1,5E-03

-1,0E-03

-5,0E-04

0,0E+00

0 500 1000 1500 2000 2500

Poten&al[V]

Currenti[A]

Time[s]

SSPBS+albumincathodic2current

SSPBS+albumincathodic3current

SSPBS+albumincathodic2poten@al

SSPBS+albumincathodic3poten@al

(c) Cathodic steps

Figure 12: SS PBS+albumin cathodic

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0

5

10

15

20

25

-1,7 -1,2 -0,7 -0,2 0,3

Watercontactangle[]

Poten0al[V]

LSVCoCrMoCathodic

LSVCoCrMo

cathodic1

LSVCoCrMo

cathodic2

LSVCoCrMo

cathodic3


-5,0E-02

-4,0E-02

-3,0E-02

-2,0E-02

-1,0E-02

0,0E+00

1,0E-02

-2 -1,5 -1 -0,5 0

Currenti[A]

Poten.al[V]

LSVCoCrMoPBScathodic1



(b) Current in function of potential

Figure 13: LSV CoCrMo PBS cathodic

IV.3 SS in PBS+albumin solution

For SS in PBS+albumin solution only the

cathodic behavior presented in Fig. 12(a)

was studied, because it appears to be

the most interesting one. The tests werestopped when contact angle measurements

were not possible anymore because of hy-

drogen evolution.

With albumin, the initial wettability of

the sample is already very good. At OCP,

the contact angle is of only 11, 8 and 6

0

5

10

15

20

25

30

35

-1,3 -0,8 -0,3 0,2

Watercontactang

le[]

Poten0al[V]

CoCrMoCathodicSteps

CoCrMo

cathodic1

CoCrMo

cathodic2


-1,3

-1,1

-0,9

-0,7

-0,5

-0,3

-0,1

0,1

-4,0E-03

-3,5E-03

-3,0E-03

-2,5E-03

-2,0E-03

-1,5E-03

-1,0E-03

-5,0E-04

0,0E+00

0 1000 2000

Poten&al[V]

Currenti[A]

Time[s]

CoCrMoPBScathodic1current

CoCrMoPBScathodic2current

CoCrMoPBScathodic1poten?al

CoCrMoPBScathodic2poten?al


Figure 14: CoCrMo PBS cathodic steps

respectively for the first cathodic test, 2nd

and then LSV test. Eventhough there weresome small contact angle fluctuations, the

contact angle decreases with decreasing

potential, until it reaches a very low value

of 1-3. The LSV test shows a later decrease

in contact angle, and also a later hydrogen

evolution, thus reaching lower potential

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0

5

10

15

20

25

-1 -0,5 0 0,5 1

Watercontactan

gle[]

Poten0al[V]

LSVCoCrMoAnodic

LSVCoCrMo

anodic1

LSVCoCrMo

anodic2


-1,0E-04

0,0E+00

1,0E-04

2,0E-04

3,0E-04

4,0E-04

5,0E-04

-0,6 -0,4 -0,2 0 0,2 0,4 0,6

Currenti[A]

Poten.al[V]

LSVCoCrMo

PBSanodic2


Figure 15: CoCrMo PBS anodic

than the other two tests.

As for SS in PBS, values reached contact

angles as low as 11, thus presenting a

very good wettability.

IV.4 CoCrMo in PBS solution

For the CoCrMo in PBS solution, concern-

ing the anodic behavior, one can see in

Fig. 15(a) the contact angle remains con-

0

2

4

6

8

10

12

-1,5 -1 -0,5 0

Watercontactang

le[]

Poten0al[V]

CoCrMoPBS+albuminCathodic

LSVCoCrMo

albumin

cathodic1

CoCrMo

albumin

cathodic1

CoCrMo

albumin

cathodic2


-1,4

-1,2

-1

-0,8

-0,6

-0,4

-0,2

0

-4,5E-03

-4,0E-03

-3,5E-03

-3,0E-03

-2,5E-03

-2,0E-03

-1,5E-03

-1,0E-03

-5,0E-04

0,0E+00

0 500 1000 1500 2000

Poten&al[V]

Currenti[A]

Time[s]

CoCrMoPBS+albumincathodicsteps1current

CoCrMoPBS+albumincathodic2current

CoCrMoPBS+albumincathodic1potenal

CoCrMoPBS+albumincathodic2potenal


-7,E-04

-6,E-04

-5,E-04

-4,E-04

-3,E-04

-2,E-04

-1,E-04

0,E+00

-1,4 -1,2 -1 -0,8 -0,6 -0,4

Currenti[A]

Poten.al[V]

LSVCoCrMoPBS+albumincathodic1

(c) Current in function of potential

Figure 16: CoCrMo PBS+albumin cathodic

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0

2

4

6

8

10

12

-1 -0,5 0 0,5

Watercontactangle[

]

Poten0al[V]

CoCrMoPBS+albuminAnodic

LSVCoCrMo

albumin


0,0E+00

5,0E-06

1,0E-05

1,5E-05

2,0E-05

2,5E-05

3,0E-05

-0,7 -0,2 0,3

Currenti[A]

Poten.al[V]

LSVCoCrMoPBS+albuminanodic1


Figure 17: CoCrMo PBS+albumin anodic

stant while increasing the potential. For

the first test, the contact angle is of 210,5,

and for the 2nd is of 10,5. Some prob-

lems occured with the current file of the

first anodic test, thus it was not possible to

0

5

10

15

20

25

-1 -0,5 0 0,5

Watercontactangle[]

Poten0al[V]

SSPBSaverage

SSPBS

Cathodicaverage

SSPBS

anodic

average

(a) SS contact angle in function of potential

0

5

10

15

20

25

30

-1,80 -0,80 0,20

Watercontactangle[]

Poten0al[V]

CoCrMoPBSaverage

LSVCoCrMo

PBScathodic

average

CoCrMoPBS

cathodic

stepsaverage

LSVCoCrMo

PBSanodic

average

(b) CoCrMo contact angle in function of potential

Figure 18: SS and CoCrMo PBS cathodic and

anodic average

present it in Fig. 15(b)

When looking at the cathodic curves,

both LSV and cathodic steps curves in

Fig. 13(a) and Fig. 14(a) respectively, it is

clear the global behavior is a diminutionof contact angle when there is diminution

of potential. For the LSV curves, the 2nd

test shows a different behavior than the

two other ones, because this test was per-

formed after an anodic test, without any

polishing in between the tests. The first

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and 3rd tests show a decrease in contact an-

gle when reaching -1,0 V. If compared with

the cathodic steps curve, it can be stated

that the decrease in contact angle starts

later when performing the LSV measure-ments than when performing the cathodic

steps, for which it starts at around -0,7 V.

What needs to be said concerning the

first and 2nd curves of the cathodic steps,

is that the first test was performed chang-

ing the bubble between each steps, as writ-

ten in section 3.2.3, whereas the second test

was performed letting the same bubble in

place between each potential steps. The

two curves have however the same shape,contact angle decreasing at the same poten-

tial.

For both LSV and cathodic steps, the

range of contact angle evolution is of

102, except for test 2 in LSV for which

the range is of 40,5.

Thus, CoCrMo presents a good wettabil-

ity, final values ranging between 20-10.

Graphs showing together cathodic and

anodic contact angle behavior, using an

average of the different tests values forCoCrMo and SS in PBS are shown in

Fig. 18(a) and Fig. 18(b), so as to have a

global view.

IV.5 CoCrMo in PBS+albumin solu-tion

As well as for SS in PBS+albumin, the ini-

tial wettability of CoCrMo in PBS+albumin

is much higher than without albumin.

When going to the anodic domain inFig. 17(a), as for CoCrMo in PBS and as

expected, the contact angle doesnt change

and stays at 10,50,5. The test had to be

stopped at 0,4 V due to corrosion of the

sample.

Concerning the behavior when going

to the cathodic domain in Fig. 16(a), the

contact angle is again globally decreasing,

eventhough the three presented curves are

quite different. The first test shows a very

slight contact angle decrease, and had tobe stopped at -1,2 V because of hydrogen

evolution. A bigger decrease might have

been observed otherwise. For the 2nd test,

the contact angle starts increasing (1) then

decreases continuously when reaching -0,8

V up until -1,2 V. The 3rd curve presents

some fluctuations, but a final contact angle,

when reaching -1,2 V, much lower than at

OCP.

The range of contact angle variation hereis of 4,50,5 for cathodic steps, and of

only 10,5 for LSV. The contact angle

which is finally reached is very low : 3-

5, thus indicating a very good wettability

of the sample.

V. Discussion

CoCrMo has a contact angle range of vari-

ation smaller than SS, and this might affect

future possible applications in link with

potential (amplitude between 7 to 20 and

4 to 11 for SS in PBS and PBS+albumin

respectively, then 5 to 11 and 1 to 5 for

CoCrMo in PBS and PBS+albumin respec-

tively).

Concerning the addition of albumin, the

results in Fig.12, 16 and 17 state that the ini-

tial wettability is much higher than when

only immersed in PBS. It is because the

albumin molecules are adsorbed at the sur-face, thus modifying the surface chemistry

and so increasing the wettability.

About the anodic steps, for SS, the con-

tact angle seems to be fluctuating while

increasing the potential. This can be due to

two different reasons. The first one would

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be that while increasing the potential in the

anodic domain, there is oxidation of the al-

loy, which change of oxidation state with

the increasing potential, thus leading to dif-

ferent wettability properties. The secondone would be that, because the bubble is

changed between each measurements, its

volume might varies of few L due to the

pipettes uncertainty, and so leading to con-

tact angle variations. Also, when injecting

the bubble, it happens the syringe nearly

touches the sample, thus maybe leading to

small imprecisions. Knowing that for the

CoCrMo sample the contact angle is very

stable along with the potential increase Fig.

15 and 17, the first hypothesis can seem

doubtfull. Since when increasing poten-

tial in the anodic domain for CoCrMo for-

mation of different chromium oxides take

place, it appears unusual that iron oxides

formation would influence the wettability

and not the one of different chromium ox-

ides. Otherwise, the different chromium

oxides have the same wettability. The rea-

son that for increases of potential to the

anodic domain the contact angle doesntchange, is that there is already formation

of a passive film at the sample surface be-

fore starting the measurements. Indeed,

the sample in contact with air already oxi-

dizes, and then when immersed in the elec-

trolyte, there is even more oxidation, thus

no change in the contact angle is observed.

The CV curve in Fig. 9(a) shows an in-

teresting behavior. When the potential de-

creases the contact angle decreases as well,but once the potential change direction and

starts increasing, the contact angle doesnt

increase up until it reaches its initial value.

It says constant, and this is because the

surface chemistry changed with the poten-

tial decrease, and thus wont be the same

anymore.

It is difficult to predict quantitatively the

contact angle behavior. On one hand, it

seems like the decrease in contact angle is

quantified (see V 1rst paragraph) but on

the other hand, the starting contact angle

is never the same, eventhough the OCP is

the same.

Also, it is interesting to remark that for

some tests, the wettability was so good

that the bubble wouldnt stick to the sam-

ple anymore. This case happened mostly

when albumin was added to the elec-

trolyte.

Finally, the obtained results show a qual-itative repeatability for a decrease in po-

tential for both SS and CoCrMo, with or

without albumin. For the increase of po-

tential, it would be necessary to perform

more tests, since some differences were ob-

served.

VI. Conclusion

An electrochemical cell was successfullydesigned, and allowed to perform good

contact angle measurements via the captive

bubble method. This experimental method

used to perform the tests, revealed itself to

be very good. Indeed, the obtained photos

are of nice quality, the contrast between the

bubble and the fluid being very clear.

Concerning the contact angle results, it

appears the same behaviour is observed

for SS and CoCrMo alloy. Increasing the

potential in the cathodic domain increaseswettability and so decreases contact angle.

The contact angle decrease can vary from

2 up to 20 in the PBS solution, and from

1 to 8 in the PBS+albumin solution, show-

ing that the quantitative behaviour cannot

be clearly defined. Since it is clear decreas-

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ing the potential increases wettability, it

could lead to improvement of alloys sur-

face wettability in metallic hip prosthesis.

Future work should try to investigate

more precisely the influence of surface en-ergy on wear and friction behaviour and if

small increase in wettability (e.g. 51 con-

tact angle decrease in PBS+albumin) has or

not a significant effect on surface energy, as

well as investigating more the influence of

an increase in potential on wettability and

the quantitative behaviour between poten-

tial and wettability.

Acknowledgements

I would like to thank my supervisors Anna

Igual and Stefano Mischler for their contin-

uous help and patience. I also thank the

Tribology and Interfacial Chemistry Group

(TIC) and lATELIER IMX of EPFL for their

collaboration. I address a special thank to

Yann Barbotin and Pierre Mettraux who

took on their time to help me resolve all

the encountered problems.

References

[1] E. Rabinowicz, Influence of surface en-

ergy on friction and wear phenomena,

Journal of Applied Physics, vol. 32, no. 8,

pp. 14401444, 1961.

[2] D. Landolt, Corrosion et chimie de sur-

faces des mtaux. PPUR, 1997, vol. 12.

[3] A. Stalder, T. Melchior, M. Mller,

D. Sage, T. Blu, and M. Unser, Low-bond axisymmetric drop shape anal-

ysis for surface tension and contact

angle measurements of sessile drops,

Colloids and Surfaces A: Physicochemical

and Engineering Aspects, vol. 364, no. 1,

pp. 7281, 2010.

[4] A. Igual-Munoz and S. Mischler, Inter-

laboratory study on electrochemical

methods for the characterization of

cocrmo biomedical alloys in simulated

body fluids (efc 61), European Federa-tion of Corrosion, 2010.

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Appendix

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(a) Cell

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(b) Lid

Figure 19: Technical drawings19

Documents

Wetting and El. Properties of Biomed. Alloys