The dynamics of wetting processes

i!i?“*Ds SURFACESA

Colloids and Surfaces

ELSEVIER

A: Physicochemical

and Engineering

Aspects 93

( 1994)

15-23

The dynamics of wetting processes Robert A. Hayes, John Ralston* Pcwticle mui Surface Technology

Research Group, Ion Work Research Institute,

(inirersity

of South Australia,

The Levels.

S.A. 5095. Austrulicl Received 26 November

1993; accepted 30 April

1994

Abstract The

dynamic

wetting

and dewetting

behaviour

technique.

is briefly reviewed. Slow relaxation

evaporative

process in the case of advancing

velocity dependence more adequately Keywords:

Contact

angle; Dynamic

of polyethylene

wetting; Wetting

Corresponding

author.

0927-7757/9+.SO7.00

SSDI 0927-7757(

(3 1994 Elscvier Science B.V. All rights rcscrvcd 94 )02934-K

studied

by the Wilhelmy is achieved by an

theory describes the contact angle-

theory in the case of forced spreading.

dynamics;

The wetting. or dcwetting, of solid surfaces is central to a seemingly infinite number of everyday processes, both natural and man-made. Examples include droplet spreading on leaves during insccticidc spraying; painting; printing; coating of fibres, particles and monolithic substrates; removal of oil from oil-bearing strata and mineral flotation. The movement and growth of cells on solid surfaces is a significant biological example of wetting. A comprehensive understanding of the fundamentals of such wetting processes is, however far from being renlised. There are essentially two reasons for this deficiency: the complexity of the systems of interest and the nature of the techniques available for their analysis. The large number of solid-liquid systems of interest have to date hampered the development of a global explanation of wetting processes. Systems of practical interest are invariably not three component (solid/liquid/vapour) but arc

(PET),

surfaces. Equilibrium

contact angles. The molecular-kinetic

than the hydrodynamic

1. Introduction

l

terephthalate

effects were observed for PET

Wilhelmy

technique

more complicated in that additional components arc often present for a variety of reasons. Solid surfaces are rarely chemically homogeneous and uncontaminated. A gap still exists between fundamental principles and actual practice although the two are in step in certain instances such as the photographic industry. Most methods ofcharacterisation of wetting processes involve static or equilibrium measurements. By contrast applications typically involve dynamic wetting, or dewetting, where the relevance of static measurements is highly questionable. A wetting balance technique has been used to study the dynamic aspects of wetting processes. Results recently obtained for polyethylene terephthalate (PET) are briefly reviewed [l-3].

2. Esperimental 2. I. Appcmtrrs

Advancing and receding angles were determined by the Wilhclmy method (Fig. 1). A Cahn micro-

16

R A. Huyes und J. RulstonlColloidr

Surfuces A: Physicochem.

Eng. Aspects 93 ( 19941 IS-23

Environmental Enclosure Fig. I. Schematic of experimental

balance (C-2000) was used for force measurements (sensitivity IO-’ mN up to 15 mN). The plate was held at a fixed position during mcasurcmcnts while the liquid was raised and lowered via a motoriscd platform (Time and Precision, UK). This platform assembly, driven by a microstcpping motor allowed scanning of the solid at velocities in the range OOOOS-20mm s- ’ ( f 0.0001) over 150 mm tlYlVd. Both the balance and platform assembly were interfaced to a PC for data acquisition and control purposes. Force and position readings could be acquired continuously at a maximum rate of 20 Hz. The apparatus was housed in a perspcx box in order to control the environment and minimise air currents. The box itself was placed on a vibration isolation table in a controlled environment room (T= 20.0 + 05°C; relative humidity 45 f 5%). The apparatus could be used in two modes, referred to were as either dynamic or static, corresponding to forced or spontaneous three phase contact line (TPCL) movement respectively. In dynamic mode the plate was scanned (in both advancing and receding directions) at constant velocity, from which a force-distance plot was constructed. In static mode. the platform assembly was held stationary and the variation of force with time was measured.

apparatus.

2.2. Motcricils Polyethylene tcrephthalatc was obtained from ICI Plastics (Australia). Grade ‘0’ film of 125 urn thickness was sclcctcd for the present work due to its smooth surface finish. Examination by light microscopy (200 x ) confirmed an essentially smooth surface with low density random blemishes Of iWCB < IO pm’. Peak to valley roughness, assessed by atomic force microscopy, was found to be 20 nm. Plates of PET were cut from larger sheets with their protective coating in place so as to prevent scratching. Plates were thoroughly rinsed in conductivity water prior to wetting experiments. The PET surface yields high quality wetting cycles (Fig. 2) and reproducible contact angle data in conductivity water. The liquid properties are summarised in Table 1.

3. Results and Discussion

One might expect that after forced liquid movement the TPCL would relax quickly for non-

R A. Hayex and J. Ralston/Colloids

Surfaces A: Physicochem.

Eng. Aspects 93 ( 1994) I S-23

17

1 & D A

......

.

D1j adv P

-2

2

0

6 4 distance (mm)

8

10

100

200

300

400

t&w (8)

(iI)

(b)

Fig. 2 s-l.

0

Force traces for PET/water:

C-D

Table

and F-G

(a) force-distance;

represent spontaneous

relaxation

(b) force-time.

A-C,

D-F

and G-H

represent forced movement

at 0.1 mm

for 100 s.

I

Properties of liquids used Liquid

Water Glycerol-water l

Literature

(70.5%

v/v)

Density, I,

Viscosity, q

Surface tension’, ylv

Conductivity.

pcm-’

(PI

(mN m-‘)

(S

O.YYHZ’

o.OlOOt’

72.8

I.21

0.82X6b

67.9


h’

m-‘1

values at 20 ‘C.

b Mcasurcd

with Ilaake

F Wilhclmy

method, clean glass phtc.

viscometcr.

viscous liquids such as water. However, this is clearly not the case (Fig. 2) and one may readily calculate that the TPCL velocity is of the order of 0.3 urn s-t 100 s after cessation of forced movement. One may ask the following questions. When does the TPCL come to equilibrium? How do we define equilibrium in the context of a Wilhelmy type experiment? What is the mechanism by which the TPCL relaxes to equilibrium after forced advancing and receding movement? In order to answer these questions, further relaxation studies of extended duration (up to 24 h) were performed (Fig. 3). The effect of liquid vapour pressure on the relaxation process was investigated. It is clear that the rate of equilibration is diff’erent in the advancing case from that for receding move-

ment and further that relaxation is dependent upon liquid vapour pressure. At extended times extraneous effects such as evaporation and condensation will impact on the measured force (F) i.e. capillary forces (F,,) will not be the only component of F. This situation is summarised in Fig. 4. In control experiments we have been able to estimate the contributions of evaporation and condensation to measured force. By comparing the estimated rate of force changes, due to extraneous effects, to the measured force, dF/dr, we are able to determine the approximate magnitude of dF,,/dt. When dF,,,/dt is zero the TPCL is at equilibrium. While there is good agreement between the measured force-time traces (Fig. 3) and the predictcd ones (Fig. 4(b)) for advancing relaxation there are clear differences in the receding case. The

R A. Hayes and 1. RalstonjColloidv

Surfaces A: Physicochem.

Eng. Aspects 93 ( 1994) 15-23

time (s)

(4

I I

0 (bJ

4,ooo

12,000

8,m

18,000

20,000

time (8)

Fig. 3. Spontaneous relaxation traces for PET/water recorded after forced TPCL pressure:(b) low water vapour pressure. The receding trace is noisy at low pH,O.

following model of TPCL relaxation is proposed to explain our results. The force on the TPCL during relaxation after forced movement is in the direction of that movement. In the advancing case pinning of the TPCL due to roughness/heterogeneity prevents the rapid

movement at

1 mm s-‘. (a) high water vapour

movement necessary to satisfy the force/energy imbalance. Instead the three phase zone comes to equilibrium via an evaporative mechanism. As a result equilibration is more rapid at lower water vapour pressures. In the receding case the TPCL is not pinned after forced movement. The direction

RA.

Huws and 1. Rulston,‘Colloia!v Surfaces A: Phyicochem.

Eng. Aspects 93 ( 1994) 15-23

3.2. Forced liquid movement -

19

dynamic contact

angles

nmo

la)

F

T/me -

(h1 Fig. 4. Prcdictcd rate dcpcndcncc of (a) I:_, fore

and

(h) measured

F.

of

SpolltilnCoUS movement and evaporation are identical. Evaporation cannot, therefore satisfy the force/energy imbalance at the TPCL. This slow relaxation behaviour was not confined to the PET surface. It was an observation characteristic of all low energy solids studied (PMMA, PTFE and hydrophobised silica) and. as we have discussed in detail elsewhere [Z], cannot be explained by solvent absorption.

The dynamic advancing contact angle data for water on PET (Fig. 5) obtained in this study agree well with those obtained by optical methods [4]. In contrast the receding contact angle was found to be velocity independent in the range studied (0.001-0.2 cm s-l). The dynamic contact angle data for glycerolwater on PET are displayed in Fig. 6. At higher viscosity we see that both advancing and receding liquid movement is velocity dependent in the range studied (0.001-0.5 cm s-l). The measured 0-V relationship was examined in the context of the molecular-kinetic [l] and hydrodynamic [ 5-71 theories. The molecularkinetic theory was found to describe the experimental data better over the velocity range studied. In addition the molecular-kinetic theory produced parameter values (Tables 2 and 3) that were physically reasonable. While the hydrodynamic theory gave reasonable values of the static contact angle (by extrapolation to V=O) the SIOPC OT the (IJ-V plot gave slip lengths that were smaller than atomic dimensions and thercforc physically unreasonable.

4. Conclusions Spontur~eous liquid ~rrorenre~rt.Results for water on PET indicate that after forced movement of the TPCL the attainment of thermodynamic equilibrium does not coincide with macroscopic mechanical equilibrium. After advancing movement, equilibration at the TPCL occurs over several hours via an evaporative mechanism, allowing the

Tdbls 2 Molecular-kinetic

parameters

from non-linear

least-squares amdysis of data’ for PET/water

n

I

7” h

AG

(cme2)

(cm)

ts-‘)

(kJ mole’)

This investigation”

I.0 x IO”

9.9 x Io-S

I.3 x IO”

37.5

Blake [A]

9.5 x IO”

1.0 x IO“

2.7 x IOJ

41.3

Source of data

l

Advancing

data only.

b Static angles mcasurcd

100 s after ccssntion of forced movement

to allow comparison

with results of other workers.

20

R A. Hayes and 1. RalstonKolloiak

Sur/ctces A: Physicochem.

Eng. Aspects 93 ( 1994) 15-U

receding 0

40

0

0

00

0

1

I

-3

-2

oB

ooouml I -1

log (V,

I 0

cm/s)

3 0.0

0.2

(h) Fig. 5. Wetting kinetics for waler on PET (0, with hydrodynamic

0.6

04

1.0

1.2

this study; A. Blake [4]: (a) comparison

with molecular-kin&

theory:(b)

comparison

theory.

angle to be extracted. The corresponding receding angle was not found to bc experimentally accessible. As a result contact angle hysteresis could not be evaluated although it is clear that its magnitude is significantly less than that measured soon after cessation of forced movement. static

0.6

Velocity (cm/s)

Contact an&s measured soon after (i.e. within minutes of) forced movcmcnt of a TPCL have questionable thermodynamic significance due to strcsscs that arc not macroscopically obvious by, for cxamplc. a goniomctric tcchniqus. In contrast the tcnsiomctric technique can be used to indirectly

R A. Huyes and 1. Rulston/Coiloia!s Surfaces A: Physicochem.

lzygtz~, 5-000

0.10

0.05

Fig. 6. Wetting

probe zone.

kinetics for glycerol-water with hydrodynamic

the stressed

I

I

0.15

0.20

21

-

Velocity (cm/s)

thl

theory; (b) comparison

Eng. Aspects 93 ( 1994) IS-23

nature

on PET

0,

this study: 0,

Pctrov and Pctrov [7]:

(;I) comparison

with molecular-kinetic

theory.

of the three phase

Forcrd liquid I~OI’CIW~I~. The molecular-kinetic [4] theory describes the experimental 0-V depen-

dence for polar liquids on PET better than the hydrodynamic [S-7] model. This conclusion may be demonstrated by comparing the 0-V dependences predicted by the two models (Fig. 7). Over

22

R A. Huyes and J. RalstoniColloi&

Surfaces A: Ph_vsicochem E’ng. Aspects 93 ( 1994) 15-23

Table 3 Molecular-kinetic

parameters

Source of data

from non-linear

least-squares analysis of data for PET glyceroLwater

n

i

K

AG

(cm-‘)

(cm)

(s-l)

(kJ mol-‘)

Advancing This investigation

5.5 x 10”

1.3 x lo-’

9.2 x lo’

43.9

Petrov and Petrov [ 73

1.6 x IO”

7.9 x lo-*

2.3 x lo5

41.7

Receding This investigation

5.9 x IO”

1.3 x IO’

1.8 x IO’

47.9

Petrov and Petrov [7]

3.9 x IO”

I.6 x IO-’

5.7 x IO’

50.7

1

log (Velocity, Fig. 7. Comparison experimental

data;

of molecular-kinetic

n.

(-)

and

hydrodynamic

(---)

cm/s) theories

for glycerol-water

advancing on PET: 0,

static contact angle.

velocity r:mge the hydrodynamic theory fits the experimental data well, with deviations at both low. and particularly, high velocity. In con-

advantage of producing physically meaningful.

trast the experimental

Acknowledgemenls

a limited

tensiometric

data are well

fitted by the molecular kinetic model over the entire velocity range studied (O.OOl-0.5 cm s-l). The molecular-kinetic model has the additional

parameter

This research work was Australian Research Council.

values that are

supported by the We are grateful to

R A. Huyes

and

1. Ralsron/Colloiuk Surfuces A: Phyicochem

Mr. A. Robinson for assistance in the areas of computer programming and device control.

Eng. Aspects 93 ( 1994) IS-23

[ 21

R.A. Hayes and J. Ralston. Colloids Surfaces. 80

[3]

R.A. Hayes and J. Ralston.

-TSl_ T.D.

Blake. AIChE

[I]

R.A. Hayes and I. Ralston. I.

( 1993)

429.

Colloid Interface Sci., 159

Langmuir,

10

Int. Svmtx Mechn.

Paper la. New Orleans.

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Lk.

( 1993 J137. ( 1994) 340.

Thin

Film Coat..

i988.

[S]

R.G. Cox. J. Fluid Mech..

[6]

O.V.

Voinov.

Fluid

Dyn.. 1I ( 1976) 714 (English translation). Peirov and PG. Petr&, Colloids Surfaces.

[7]

J.G. (1992)

143.

Mekh.

168 (1986)

Zhid. Gaza, 5

169.

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