Transcript
Page 1: Contact angle hysteresis on nano-structured surfaces

Surface Science 540 (2003) 355–362

www.elsevier.com/locate/susc

Contact angle hysteresis on nano-structured surfaces

S.M.M. Ramos a,*, E. Charlaix a, A. Benyagoub b

a Laboratoire de Physique de la Mati�eere Condens�eee et Nanostructures (UMR CNRS 5586) Universit�ee Claude Bernard LYON I,

43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, Franceb CIRIL, B.P. 5133, F-14040 Caen Cedex 5, France

Received 7 March 2003; accepted for publication 3 June 2003

Abstract

We present results from an experimental study on the phenomenon of contact angle hysteresis on solid surfaces

decorated by a random array of nanometric hollows. For weak values of the areal density of defects /d, the hysteresis Hincreases linearly with /d. This evolution is described by a pinning–depinning process of the contact line by individual

defects. At higher values of /d, a collective pinning effect appears and H decreases with increasing /d. In the linear

regime, our experimental results are compared to theoretical predictions for contact angle hysteresis induced by a single

isolated defect on the solid surface. We suggest that the crossover from the individual to the collective pinning effects

could be interpreted in terms of an overlapping of wetting cross sections. Finally, we analyse the influence of both the

size and the morphology (hollows/hillocks) of defects on the anchorage of the contact line.

� 2003 Elsevier B.V. All rights reserved.

Keywords: Contact; Wetting; Surface defects; Ion bombardment

1. Introduction

It is now well established that the wettingproperties of the real solid surfaces are strongly

affected by their roughness and chemical hetero-

geneities. In many practical situations these het-

erogeneities pin the contact line inducing a contact

angle hysteresis. In this case, the static angle be-

tween the solid surface and the fluid interface takes

two different values depending on the contact line

motion; it advances with a contact angle hA and

* Corresponding author. Tel.: +33-0-472-431-218; fax: +33-0-

472-431-592.

E-mail address: [email protected] (S.M.M. Ra-

mos).

0039-6028/$ - see front matter � 2003 Elsevier B.V. All rights reserv

doi:10.1016/S0039-6028(03)00852-5

recedes with hR (hA > hR). In the last 20 years

much theoretical and experimental work has been

devoted to investigation of the motion of a fluidinterface on an heterogeneous surface and of the

contact angle hysteresis [1–10]. From an experi-

mental point of view, it has been shown that

chemical heterogeneities and geometrical defects

(roughness) do not have a similar effects on the

hysteresis. Although the contact angle hysteresis

has been under investigations for many years, the

experimental studies performed up to now dealtessentially with surfaces structured at the micro-

scopic scale, with patterns of a few 10s of mi-

crometers in size. To date, in spite of the many

fundamental and potential applications, the ex-

perimental investigations taking into account the

roughness effects at a nanometer range still remain

ed.

Page 2: Contact angle hysteresis on nano-structured surfaces

356 S.M.M. Ramos et al. / Surface Science 540 (2003) 355–362

limited, probably due to the difficulties usually

encountered for the preparation of well charac-

terized surfaces with controlled defect distribu-

tions.

Recently, we have attacked the problem of

wetting on nano-rough surfaces by investigatingthe phenomenon of contact angle hysteresis on

surfaces structured by a random distribution of

nanometric hillocks [11].

The aim of the present work is to study the

effects of nanometric hollows on the contact angle

hysteresis. In these experiments, we analyse the

influence of both the size and the morphology

(hollows or hillocks) of defects on the contact linemotion. For this purpose, we have processed our

surfaces by using swift heavy ion (SHI) irradia-

tions. As we reported in the recent past, the pro-

jectile impacts at the glass slices surface induce the

formation of nanometric hollows [12]. The hollow

dimensions depend on both the ion mass and the

irradiation energy, whereas the concentration of

such structures and consequently the surfaceroughness is governed by the irradiation fluence.

The role played by topographical defects in the

contact angle hysteresis phenomenon is investi-

gated here via two complementary techniques:

atomic force microscopy AFM observations and

contact angle measurements.

Table 1

Irradiation fluence /d (hollow concentration), average distance betw

(advancing and receding) contact angles hA and hR collected for the s

/d (·109ions cm�2)

k (nm) aPb (%) aKr (%) P

h

Reference

surface

– 0 0 1

0.5 447 1.8 – 1

1.0 316 3.5 – 1

3.0 182 10.0 – 1

5.0 141 – 1.6 –

6.0 129 19.0 – 1

7.0 119 – 2.3 –

10.0 100 29.7 3.3 1

15.0 82 41.0 – 1

20.0 71 50.5 6.4 1

40.0 50 75.5 –

60.0 41 87.9 17.8

80.0 35 – 23.2 –

The contact angle measurements were done optically with an accurac

2. Experimental section

2.1. Surface processing

The starting material is constituted of glass sli-ces with a low roughness value purchased from

Prolabo. The surfaces of these slices were pro-

cessed twice: first, hollows of nanometric size were

created by using swift heavy ions irradiations at

Grand Acc�eel�eerateur National d�Ions Lourds

(GANIL). In order to modify the hollow dimen-

sions, two different kinds of ions were used: 208Pb

ions of 250 MeV energy and 78Kr of 240 MeVenergy. All the irradiations were performed at

room temperature with fluences extending from

5 · 108 to 8 · 1010 cm�2. For the different irradia-

tion conditions we have listed in Table 1, the

corresponding average distance (k) between two

neighbouring defects and the fraction of the sur-

face covered by the defects (a). The a values were

determined by using the following relation, whichis based on a direct impact model [13]

a ¼ 1� expð�/dAÞ ð1Þ

where A is the defect cross-section and /d is the

irradiation fluence.

Second, in order to obtain contact angles large

enough to be accurately measured and to have a

een hollows k, coverage rate of the surface by defects a, andamples irradiated with Kr or Pb ions

b Kr

A (�) hR (�) hA (�) hR (�)

00.0 89.0 100.0 89.0

00.0 86.0 – –

01.0 84.0 – –

02.0 83.0 – –

– 101.0 89.0

05.0 83.0 – –

– 104.0 91.0

03.0 83.0 103.0 86.0

02.0 83.0 – –

01.0 84.0 106.0 87.0

99.0 86.0 – –

96.0 86.0 101.0 88.0

– 99.0 90.0

y of 3�.

Page 3: Contact angle hysteresis on nano-structured surfaces

Fig. 1. 3D view of a virgin (a) and an irradiated (/d ¼ 1010

Pb cm�2) sample (b).

S.M.M. Ramos et al. / Surface Science 540 (2003) 355–362 357

wide range of variation of the contact angle hys-

teresis, octadecyltrichlorosilane (OTS) molecules

were grafted on the tailored surfaces. The prepa-

ration method of these surfaces is explained below

briefly. Prior to the OTS deposition, the samples

were cleaned according to the following procedure:rinsing thoroughly in hot water with Micro90 de-

tergent in an ultrasonic bath for �30 min, and

rinsing again in deionised water. The cleaned

substrates were immersed in a reaction bath con-

sisting of a freshly prepared millimolar solution of

OTS in toluene (Prolabo, 99%) for 1 h. To remove

all excess reactants, the samples were rinsed in two

successive chloroform ultrasound baths. Finallythe samples were rinsed in deionised water, and

excess water droplets were blown away with dry

N2 gas. In order to provide a reference surface the

OTS molecules were also grafted on a virgin

sample.

2.2. Surface characterization

An Explorer Nanoscope atomic force micro-

scope (AFM) operating in a tapping mode was

used. The surfaces were probed under ambient

conditions with silicon tips having a nominal ra-

dius curvature of �15 nm. Each sample was pro-

bed at least three different locations, the images

were flattened (to eliminate the experimentally

obtained bowing effects) and no other filtering wasdone. Prior to characterize the topographic mod-

ifications induced at the irradiated surfaces, both

the pristine sample and the as-grafted monolayer

onto glass slices were examined by AFM. The rms

roughness values, measured on a scanned zone of 1

lm2, are of 0.07 and 0.13 nm, respectively. These

low roughness values evidence the high quality of

both the starting sample and the monolayergrafting, which constitute our reference surfaces.

The sessile drop method was used to charac-

terize the wettability properties of processed sur-

faces. The samples were introduced in a glass

chamber and the water drop was put onto the

substrate through a microsyringe. Steady-state

advancing contact angles, hA, and receding contact

angles hR, were measured using �1 ll drops ofdeionized water (with a surface tension c ¼ 72:0mNm�1). All measurements were made at room

temperature in a saturated atmosphere in order to

minimize the evaporation problems. At least five

different measurements were performed on differ-

ent areas of each sample. This procedure allows

one to take into account a possible non-uniformity

of the surface probed by the contact angle.

3. Results and discussion

3.1. Surface topography

Fig. 1 shows two 3D views of AFM micro-

graphs recorded, respectively, on a pristine and anirradiated surface. The circular hollows visible in

Fig. 1b result from the individual interaction of

each energetic ion at the solid surface. As we re-

ported in the recent past [12], this specific effect

(i.e., crater formation in the amorphous silica)

supports the previous observation of the isotropic

compaction measured by a decrease of the width

of the irradiated sample [14]. For all samples

Page 4: Contact angle hysteresis on nano-structured surfaces

Fig. 2. Top view and surface profiles recorded on glass slices: (a) irradiated with 5· 109 Kr cm�2 of 450 MeV energy; (b) irradiated with

1010 Pb cm�2 of 250 MeV energy and (c) irradiated sample (/d ¼ 6� 109 Pb cm�2) after OTS grafting.

358 S.M.M. Ramos et al. / Surface Science 540 (2003) 355–362

investigated a good agreement between the hol-

lows density and the irradiation fluence was found.

Profile line scans performed over exposed areasare given in Fig. 2, where the marks indicate the

border between the damaged zone and the sur-

rounding region. For the study of the hollow di-

mensions more than 40 impacts were analyzed and

both a mean basal diameter D and a mean depth hwere extracted from these measurements. The

following values were obtained: DPb ¼ 72� 6 nm;

hPb ¼ 3� 1 nm; DKr ¼ 22� 3 nm; hKr ¼ 0:6� 0:2nm for Pb ions and Kr ions, respectively. It is

worth noticing that the confidence in the depthmeasurements decreases at low hollow diameters.

For small diameters, close to the tip probe di-

mensions, the measured values are probably un-

derestimated.

Finally, the surface characterization was com-

pleted by AFM observations after OTS grafting on

the irradiated samples. A typical result is displayed

Page 5: Contact angle hysteresis on nano-structured surfaces

(a)

(b)

0

5

10

15

20

25

30

0 5 10 15 20 25 30

φd (defects.cm-2)

H(m

N.m

-1)

0

5

10

15

20

25

30

0

5

10

15

20

25

30

0 20 40 60 80 100

φd (defects.cm-2)

H(m

N.m

-1)

∆S

(mN

.m-1)

PbKr

Fig. 3. (a) Contact angle hysteresis H and wettability contrast

DS versus the areal defect concentration /d and (b) zoom of the

H curve in the low /d regime.

S.M.M. Ramos et al. / Surface Science 540 (2003) 355–362 359

Fig. 2c. No significant patterns formation at the

surface is observed and the film seems to follow

very well the surface topography. Meanwhile, the

OTS grafting induces a small (�7%) decrease in

the hollow dimensions. The following values for

the average diameter d� and for the mean depth h�

were found: d�Pb ¼ 68� 7 nm; h�Pb ¼ 2:7� 0:8 nm;

d�Kr ¼ 20:5� 2:5 nm; h�Kr ¼ 0:4� 0:2 nm, for Pb

ions and Kr ions, respectively. From the ensemble

of our observations we have no evidence about the

OTS deposition inside the hollows.

3.2. Wetting properties

3.2.1. Data processing

From the experimental measurements of both

advancing (hA) and receding (hR) contact angles

(given in Table 1) we have determined, for each

processed surface, two fundamental parameters

for the wetting investigations. The first one is the

wettability contrast, i.e., the difference of spread-

ing coefficient on a solid surface with chemicaldefects Sd and a pristine one, S0. This parameter is

defined as DS ¼ Sd � S0;¼ cðcos hd � cos h0Þ wherecos hi (i ¼ d or 0) is the average contact angle de-

fined as cos hi ¼ ðcos hA þ cos hRÞ=2. The second

parameter is the contact angle hysteresis, defined

as H ¼ cðcos hR � cos hAÞ. The values obtained for

these two parameters in our reference (unirradi-

ated) surface are DS ¼ 0 mNm�1, H ¼ 13:2mNm�1, respectively.

The evolution of both the wettability contrast

DS and the contact angle hysteresis H versus the

defect concentration is displayed in Fig. 3. The

figure shows that DS varies very slightly with the

defect density. Although not shown here, a similar

evolution is observed on surfaces irradiated with

Kr ions. In both cases, one can estimate from dataof Table 1 and Fig. 3 that the chemical heteroge-

neity does not exceed DðcSV � cSlÞ ¼ 4 mNm�1,

indicating that a chemical heterogeneity of the

defects, due to an eventual small variation in the

coverage of the surface by OTS molecules, can be

reasonably neglected.

From Fig. 3 it is clearly evidenced that the

contact angle hysteresis is not a monotonic func-tion of the defect concentration. Two different re-

gimes are identified:

• At low defect concentration, /d < /c (where /c �6� 109 defects cm�2 for Pb and /c � 20� 109

defects cm�2 for Kr), H grows linearly with /d.

This variation suggests that each defect pins

the contact line individually. The slope of the

straight line fitting the experimental points cor-responds to the total energy W dissipated by

one defect on the hysteresis cycle. From our ex-

periments we found the following energy values:

WPb ¼ 2:2� 10�16 Nm and WKr ¼ 5:7� 10�17

Nm.

• At higher defect concentration (/d > /c), H de-

creases with increasing /d. This behaviour indi-

cates that, despite the rather large mean distancewhich remains between two individual impacts

(e.g., for /d ¼ 2� 1010 Kr cm�2 the average

Page 6: Contact angle hysteresis on nano-structured surfaces

360 S.M.M. Ramos et al. / Surface Science 540 (2003) 355–362

distance between two defects is still of 71 nm

and the defect surface coverage is 6%) an over-

lap in the deformation of the contact line occurs.

It appears from these curves a converse corre-lation between the value of the critical defect

concentration /c and the hollow diameters.

However, additional data are required to confirm

this trend.

Note that we do not discuss here the evolution

of the contact angles versus the defect concentra-

tion. This evolutions is similar than observed for

surfaces decorated by a random distribution of thenanometric hillocks. We have largely discussed

such an evolution in a previous study devoted to

the wetting properties of nanorough surfaces [11].

Fig. 4. Schematic representation of both the distortion of the

contact line induced by one isolated defect (a) and the inter-

action of two wetting cross-sections. This interaction occurs

when k < 2gmax (b).

3.2.2. Data analysis

To quantify the defect force exerted on the

contact line in the linear regime of the hysteresis,

we compare our experimental results with theavailable predictions [3] concerning the behaviour

of a contact line in the presence of a single, lo-

calized defect. In this approach, the equilibrium

position of the triple line on a solid surface results

from the action of two opposed forces: the force

due to the defect which pins the contact line and

the elastic restoring force which tends to bring

the line back to the undisturbed original posi-tion. The latter force is proportional to the

amplitude of the line deformation. The deforma-

tion of the contact line induced by the presence of

a defect is schematised in Fig. 4a. This deforma-

tion is assumed to result from a topographical

defect on a planar surface. Considering the case

where the balance between these forces leads to an

hysteresis and taking into account the fact thatthese defects should anchor the contact line in

both advancing and receding motions, the total

energy dissipated by one defect around an hys-

teresis cycle is given by dimensional analysis as:

W ¼ f 2

Kð2Þ

where f is the defect force and K is the spring

constant characteristic of the restoring force,

written as K ¼ ð2pcð1� cos h0ÞÞ= lnðL=d�Þ, whered� is the size of the defect while L is the range of

deformation of the contact line. In this approach,the maximum amplitude of the contact line de-

formation gmax can be determined by the following

relation: gmax ¼ f =K.From our experimental results we determine the

value of three parameters: the spring constant K,the interacting force between the surface and the

triple line f and the maximum amplitude of the

contact line distortion gmax. In our calculations weassume that the range of deformation of the con-

tact line L corresponds to the capillary length j�1

of the liquid and we use the following experimental

data: j�1 ¼ 2:7 mm, c ¼ 72 mNm�1 and h0 ¼ 95.

The values obtained for f and gmax are presented

in Table 2. These results lead to the following re-

marks: (i) the defect force f is directly correlated to

the defect size, in agreement with what is expectedfrom the wetting of a heterogeneous surface with

purely geometrical defects; (ii) the maximum de-

formation amplitude gmax of the contact line by a

Page 7: Contact angle hysteresis on nano-structured surfaces

Table 2

Defect force f , maximum deformation amplitude of the contact

line gmax, wetting cross-section r and fraction of surface ac in-fluenced by gmax values determined from our experimental re-

sults

Ion f (nN) gmax : (nm) r (nm2) ac (%)

Kr 1.5 37.0 4300 57.6

Pb 3.2 69.0 14957 59.2

S.M.M. Ramos et al. / Surface Science 540 (2003) 355–362 361

single isolated defect is correlated to the average

distance between two defects k. As can be seen

from Table 1, k > 2gmax at low defect concentra-

tions (/d < /c), whereas k < 2gmax at /d P/c re-

gime (e.g., for /c ¼ 6� 109 Pb cm�2, k ¼ 129 nm

< 2gmax ¼ 138 nm). These results suggest the fol-

lowing criterion for the transition between the two

anchorage (individual or collective) regimes at thecontact line k � 2gmax. As a consequence the crit-

ical defect concentration /c should be given by

/c � 1=4g2max.

On the other hand, notice that the defect force

values given in Table 2, are in good accordance

with those about the influence of the nanometric

hillock structures on the wetting properties [11]. In

that study the contact line is pinned by hillocks of19 nm diameter. In the linear regime of the hys-

teresis a defect force f ¼ 3:5� 10�9 N was found.

This value is very close to the values presented in

Table 2 indicating that, at a nanometric scale, the

anchorage force exerted by an isolated defect on

the contact line does not seem to depend on the

defect morphology (hollows/hillocks).

The hysteresis behaviour observed in the regimeof higher defect concentration is more intricate.

The slope inversion in the H curve occurs at quite

low fluence values. In the greater part of the flu-

ence range the distance between two defects is

large enough so that their individual characteris-

tics remain unchanged because the probability of

ion impact overlapping remains low. Therefore the

occurrence of this different regime of variation ofthe hysteresis has necessary relationship with the

onset of collective effects in pinning. In order to

clarify this particular hysteresis regime, we suggest

that two different mechanisms could be responsible

of this behaviour. The first one is related to the

range of action of one individual defect on the

contact line. By identifying gmax with the range of

action of the defect on the contact line we define a

wetting cross-section, r ¼ pg2max, whose values are

reported in Table 2. In addition, inserting both the

r and /c values in relation (1), we also determined

the fraction of surface ac influenced by gmax. As

typified in Fig. 4b, and in agreement with ac valuesgiven in Table 2, the individual wetting cross-sec-

tions interact at a fluence much lower than re-

quired for overlapping two neighbouring defects.

The presence of such a destructive interaction at

the contact line could correspond to the crossover

between the individual to the collective pinning

effects, which occurs at a low critical value /c.

The second mechanism that should explain theslope inversion in the experimental hysteresis curve

is linked to a possible formation of nano-bubbles

at the water-OTS interface as shown in AFM in-

vestigation by Tyrell and Attard [15]. As these

nano-bubbles appear at the non-wetting OTS

surface we can suppose that their formation

should be favored by the presence of roughness

defects at sufficiently high concentration, and thatin return they screen those defects, which would

result in a decrease of the contact angle hysteresis.

4. Conclusion

In this work we have investigated the phenom-

enon of the contact angle hysteresis on surfacesdecorated by a random distribution of nanometric

hollows. Our experimental results confirm the ex-

istence of two different regimes on the hysteresis

evolution that depend on the defect concentration.

At low /d values, each defect individually pins the

contact line inducing a linear increase of H with

/d. The decrease of hysteresis with increasing /d,

observed at higher /d values indicates that a col-lective pinning effect on the contact line occurs.

Such a behaviour seems to be the signature of

topographical nanometric defects on the hysteresis

phenomenon.

In addition, our results also exhibit two other

features: first, the trap efficiency of the contact line

by the topographical defects seems to be insensitive

to the defect morphology (hollow/hillock). In fact,the pinning effect of the line by hollow or by hillock

structures is quantitatively quite similar. Second,

Page 8: Contact angle hysteresis on nano-structured surfaces

362 S.M.M. Ramos et al. / Surface Science 540 (2003) 355–362

the break up in the linearity of H curve occurs at a

critical defect concentration /c, which is directly

correlated to the defect size. The low values of /c

seem reasonably described by a destructive inter-

action of wetting cross-sections at the contact line.

Finally, this work is pursued at present by per-forming complementary experiments using wider

defect concentrations and liquids of different sur-

face tensions and also by carrying out numerical

simulations in order to better understand the in-

fluence of the defect characteristics (e.g., size and

density) on the motion of the contact line on a

topographically nano-structured surface.

References

[1] P.G. de Gennes, Rev. Mod. Phys. 57 (1985) 827.

[2] R.E. Johnson, R.H. Dettre, in: E. Matijevic (Ed.), Surface

and Colloid Science, vol. 2, Interscience, New York, 1969.

[3] J.F. Joanny, P.G. de Gennes, J. Chem. Phys. 81 (1984)

552.

[4] M.O. Robbins, J.F. Joanny, Europhys. Lett. 3 (1987) 729.

[5] J.-M. di Meglio, Europhys. Lett. 17 (1992) 607.

[6] J.-M. di Meglio, D. Qu�eer�ee, Europhys. Lett. 11 (1990) 163.

[7] A. Paterson, M. Fermigier, Phys. Fluids 9 (1997) 2210;

A. Paterson, M. Fermigier, P. Jenffer, L. Limat, Phys. Rev.

E 51 (1995) 1291.

[8] V. De Jonghe, D. Chatain, I. Rivollet, N. Eustathopoulos,

J. Chem. Phys. 87 (1990) 1623.

[9] J. Bico, C. Tordeux, D. Qu�eer�ee, Europhys. Lett. 55 (2001)

214.

[10] A. Prevost, E. Rolley, C. Guthmann, Phys. Rev. Lett. 83

(1999) 348.

[11] S.M.M. Ramos, E. Charlaix, A. Benyagoub, M. Toule-

monde, Phys. Rev. E 67 (2003) 031604.

[12] S.M.M. Ramos, B. Canut, A. Benyagoub, M. Toule-

monde, Nucl. Instrum. Meth. B 191 (2002) 456.

[13] J.F. Gibbsons, Proc. IEEE 60 (1972) 1062.

[14] A. Benyagoub, S. L€ooffer, M. Rammensee, S. Klaum€uunzer,

G. Saemann-Ischenko, Nucl. Instrum. Meth. B 65 (1992)

228.

[15] J.W. Tyrell, P. Attard, Phys. Rev. Lett. 87 (2001) 176104.


Top Related