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A new experimental method to analyse the dewetting properties of polymer surfaces and cationic surfactants
Colloids and Surfaces A: Physicochemical and Engineering Aspects 206 (2002) 125– 133 www.elsevier.com/locate/colsurfa
A new experimental method to analyse the dewetting properties of polymer surfaces and cationic surfactants C. Della Volpe a,*, S. Invernizzi b, D. Maniglio a, S. Siboni a a
Department of Materials Engineering, Uni6ersity of Trento, Via Mesiano 77, Trento 38050, Italy b Synt srl- Zola Predosa(BO), Italy
Abstract In the present paper a new method able to easily evaluate the critical height of rupture of liquid films on various substrate surfaces is described and commented. It is automatic and based on the different IR reflectance of solids and liquids. Its limits and advantages are analysed. The heights of rupture of water have been measured on different kinds of polymeric solid surfaces as well as on surfaces whose water repellency has been increased by different mixtures of substituted ammonium salts, used as autophobic rinse aids. In both cases the effect of experimental conditions and the value of contact angles measured by the Wilhelmy microbalance have been correlated with the height of rupture. The peculiar sawtooth trend of force versus immersion in Wilhelmy experiments, performed by using some of these autophobic substances solutions, is described and interpreted. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Height of rupture; Thin film; Contact angle
1. Introduction The study of hydrodynamic instabilities induced by intermolecular forces in thin films has been motivated by their industrial application in disperse and colloidal systems on one hand and the understanding of diverse biological phenomena on the other. Some typical industrial applications of thin film rupture are in the field of foams and emulsions, thin coatings, oil-recovery, vapour condensation; on the biological front this subject is particularly interesting to understand cell adhesion mechanisms, thin membrane instabilities, tear * Corresponding author. Tel.: + 39-0461-882-409; fax: + 39-0461-881-977. E-mail address: [email protected] (C.Della. Volpe).
film break-up. The phenomenon can be described as the spontaneous dewetting of a surface covered with water (or any other liquid) when the film thickness on the surface drops below a critic value, leaving a dry patch with a wetting meniscus in contact with it. The phenomenon is associated with low energy surfaces and the rupture thickness is critically dependent on the nature of the solid surface, which makes it be a useful quantity to detect differences and modifications in surface characteristics. In literature the phenomenon has been described so far in many papers, in particular much effort have been done by Padday [1] and Sharma and Ruckenstein [2–4] who provided also some experimental data collected on various surfaces, some mathematical models to correlate the rupture height with hole-shape perturbations and
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to describe the stability of thin films. Some works about the correlation between contact angle and dewetting speed [5] are also present. Therefore, it seems that the only rupture height data in literature are those collected by Padday and Doughman et al. [1,3]. To measure the rupture height over a hydrophobic surface is in general not easy; it requires specialised equipment such as cathetometers or fast video cameras, not easily available in an industrial laboratory. Our purpose is to tune a sufficiently accurate and low-expensive method to detect modifications due to aging or to chemical variations induced by the covering with surfactants. We tried to find out a new easy method to estimate the height of rupture by using widely available instruments such as an analytical balance, a micro comparator and a peristaltic pump. In this way it is possible to get a quite precise measurement of the height at a very low cost. We have focused our attention on a peculiar industrial topic, the ability of various ‘rinse-aids’ agents for car washing plants made with mixtures of different cationic surfactants. These agents are, in fact, commonly used to increase the protection against water when the paint reduces its hydrophobicity with the time passing. Together with the development of this new method, we focused our attention on the analysis of the mechanism of thin film rupture, trying to correlate the values of height measured with other surface parameters such as contact angles. To do this we have tested different materials like painted samples (‘fresh’ painted and aged) and polymer plates and compared the data collected with the contact angle measurements made using the Wilhelmy technique. We have also investigated some specific autophobic phenomena (Zisman [6]) that have been observed performing the contact angle measurements in surfactants solutions. As well known, the wetting behaviour of a fluid can be dramatically modified by adding surfactants and their control of wetting processes can lead to strange effects like those ‘unsteady stick-jump motions’ of the contact line described by Garoff [8 – 10]. While performing our measurements on RF-plasma cleaned glass plates in a surfactants solution we have observed a typical autophobic behaviour, the incapacity of the surfactants solu-
tion, once wet an horizontal strip of the surface, to keep wetting it. The typical forced immersion of the Wilhelmy technique leads to a retraction and a pinning of the contact line, a deformation of the liquid – vapour interface and a subsequent jump of the contact line to a close lower energy state. This phenomenon is well pointed out by the Wilhelmy characteristic curves.
2. Materials and methods We performed our measurements on disc like, metallic samples painted with car paint (25 mm diameter, about 0.5 mm thickness), microscope glass slides (24× 24× 0.15 mm) from Prestige; squared polymer plates (about 20× 20× 0.5 mm) from Goodfellow (uPVC, PTFE, PET, PS). The two surfactants solutions used are a simple solution (concentration 2 g l − 1 in MilliQ-Millipore ultrapure water) of two commercial products, Roquat TO90 (Cromogenia, Barcelona, Spain) N + (CH3)(CH2CH2OH)(CH2COOR)2CH3SO4− and Ro-min HPA (Cromogenia, Barcelona, Spain) N (CH2)2(CRCHNCH2NH2). C6 is a microemulsion obtained by mixing the previous surfactants with oil and isobuthyl alcohol. The exact formulation of all products is covered by industrial secret. The technique to measure the rupture height of a liquid film over a solid surface is based on a weight measurement. The basic experimental setup (Fig. 1) is composed of a single 86.9 mm Ø aluminium made throwed Petri capsule whose cylindrical geometrical proportions are well known and very precise, a microbalance (GIBERTINI E50S), put on a standard antivibration table, and a peristaltic pump (ALTEA AB) with a thin needle at the endpoint of the flexible tube. A regular shape sample is placed on the Petri capsule and totally covered with water, then pumped away very slowly with the thin needle (0.5 ml min − 1). When the water surface breaks over the sample the pump is stopped and the rupture height is then calculated from the weight value (Wwater) read on the balance, by using Eq. (1) There z(T) denotes the value of the water density at a certain temperature T, rPetri is the inner radius of the Petri capsule, Vsample stands for
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the volume of the sample. This setup (sample placed on a container) describes the so called film confined by bulk water’ [7], in which the thin film is located only on the sample surface and not in the surrounding water. In a second time the experimental setup has been modified in order to automating the measurement (Fig. 2). The detection of the water break is made using an infrared diffuse scan opto-switch sensor (OPB704 led, 9926 MEXICO transistor, provided by RS-components) whose output voltage, redirected to a comparison circuitry, drives the remote command of the peristaltic pump (S1-MINI by Altea AB). The circuitry is a simple trigger Schmitt with variable hysteresis built up by means of a LM331NE comparator (see Fig. 3). The value of the hysteresis can be changed by acting on the R6 trimmer in order to adapt the circuit to various kinds of surfaces. The working principle is simple, as the reflectivity of the surface is below a certain threshold (chosen by the operator before the Petri capsule is filled with water) the pump continues to suck; when the water film breaks over the surface, the reflectivity of the surface suddenly changes, the comparator sets its output voltage value to zero and the transistor T1, no more active, switches off
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the pump. For in the most cases we were working with painted metal surfaces we supposed to have an increase in reflectivity passing from the wet to the dry surface. In the case that the variation is negative the measurement can easily be done manually stopping the pump by acting on the switch SW1 or by inserting a two pole switch before the 2nd and 3rd pin of the comparator. The data acquiring has been made via the serial port (RS232) present on the Gibertini E50S and a Keyspan USA28X USB to serial adapter, connected to an Apple iBook. The control program has been written both in REALBASIC and in LABVIEW® (the last one to provide compatibility with MACINTOSH® and WINDOWS® worlds). The test measurements have been made upon disc-like metal surfaces with their upper surfaces rounded because of the mechanical cut starting from a single, wide plate. The point of rupture is supposed to be the upper point of the disc and as shown by Padday [1] the curvature of the surface does not have any effect on the value of the height. Contact angle measurements have been made using a CAHN DCA 322 Wilhelmy microbalance at the speed of 20 mm s − 1 if not specified otherwise.
Fig. 1. The sample placed in a metal cylindrical Petri capsule is immersed in a liquid pumped away from a thin needle. When the liquid film breaks, the pump is stopped and the height is calculated starting from the residual liquid weight.
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tion of the bulk or the declared chemical composition. We have put our data in comparison with those given by Padday for the Teflon –water system and, as if it’s possible to see from the Table 2, the value is in good agreement with our. The literature data refer to PTFE, PE, PMMA and wax, in different solvents, while we have analysed in the same solvent, i.e. water, a wider set of common polymers. We have also tried to include LDPE, PMMA and NY6,6 but if in the case of PMMA and LDPE there have been some setup trouble (floating and low reflectivity) in the case of Nylon the problem is the adsorption of water. Plotting the data of the rupture height for the studied polymers versus the cosine of the contact angles (Fig. 4) it is detectable an opposite behaviour, as the rupture height increases the value of the cosines decreases (which corresponds to an increase of the angle values). hdewetting = Fig. 2. A close view of the Petri aluminium capsule with a disc immersed in water. It is also visible the aspiration needle and the opto-switch, along with and the simple micrometric positioning structure.
3. Results A first set of data comes from the measurement of some polymer-made squared plates (Table 1). In literature the height of rupture is often put in relation with a so-called contact angle or a not well specified ‘equilibrium contact angle’ which probably refers to an advancing angle somehow relaxed. In our set of data we propose a complete characterisation through advancing, receding and ‘stable’ equilibrium contact angle, whose experimental and theoretical definition is specified in a previous work and further described in a second paper presented at this congress [11,12]. The plates have been used as received, only after a standard cleaning phase with solvent and/or detergent; so it is possible (almost certain) that their surface composition, as it commonly happens in commercial polymers, is not the same composi-
(W/z(T))water + Vsample − hsample yr 2Petri
(1)
It is important to note that there is a problem related to the weight of the meniscus along the borders of the Petri capsule at the moment of the rupture, which could lead to significantly overestimated values of the rupture height (Eq. (1) refers to a completely flat liquid surface, as it is drawn in Fig. 1). It is not simply correlated with the advancing or receding contact angles of the liquid on the container material, because the border meniscus is in a non-equilibrium situation. In our case, the choice of the combination of water and the aluminium container appears to reduce the importance of this problem, the facts that the curvature of the border meniscus is not appreciable and that our data for PTFE are in agreement with the literature, let us deduce that the induced error is low at least for the couple. Moreover, only water has been used as a test liquid along the present paper, so that the eventual meniscus at the borders is the same in all the experiments. Other studies will be done changing the material of the Petri and considering different liquids to evaluate the effective importance of this phenomenon and the possibility to compare different liquids in the same containers.
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We made also some measurements on disc plates to understand the protective properties of some cationic surfactants on old-painted surfaces. To produce an accelerated aging on the paints of our samples we have treated them using an RFplasma reactor at low power (30 W) and short treatment times (30 s). Doing so we have uniformly oxidised the surfaces simulating the natural phenomena induced by the exposure to UV-radiation and to atmospheric agents, obtaining the same results as using an UV-condensation chamber but with a reduced treatment time. We compared the behaviour, by measuring the height of rupture, either on the non-protected surfaces or on the aged disc or on aged and surfactants-covered discs. We found that simple painted discs are much different in their surfaces than expected, due probably to a non-perfect and uniform coverage of the paint, variable heights are measured starting from 150 to 250 mm, changing at the same time the values of the contact angle. The treatment consists in a brief immersion in a cationic surfactants solution or in a formulation called C6, a micro-emulsion of surfactants. A treated surface (30 W power, 30 s time) shows to have an height of rupture equivalent to zero (or not detectable), even if the level of the liquid in the Petri capsule is lower than the sur-
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face of the sample, the latter still remains wet (leaving a thin film on the high energy surfaces due to the disjoining rupture process and whose typical thickness is about some microns or less [1]) (Fig. 5). The data in Table 3 show that the microemulsion seems to increase a little the protection in comparison with the cationic solution. The microemulsion sensibly increases the values of both the contact angles (advancing and receding) making it reach almost the same values than the ‘fresh’ paint. It is realistic that the coverage of the surface is best obtained by the microemulsion. The simple cationic solution in fact is not able to increase significatively the values of the receding contact angle which means that a larger amount of high energy zones are exposed. Both the treated surfaces have values of rupture height very close to the ones of the new paint (about 180 mm). The values in Table 3 represent the first measurement made on the sample. As attended, in fact, the next measurements give rupture height lower because the surfactants tend to be slowly removed by the new water added, increasing the high energy zones exposed to the surface. Plotting all the results (Fig. 6) obtained so far, it is evident that the data of the cosines of the contact angles versus the height of rupture are
Fig. 3. Pump-driving circuit diagram.
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Table 1 Rupture heights and contact angle values measured on PTFE, PS, PVC, PET
PTFE PS PVC PET
H (mm)
qa
qr
qeq
cos qa
cos qr
cos qeq
527 9 42 397 9 22 320 9 23 399933
108.8 93.4 91.9 91.2
74.8 52.3 50.3 59.3
91.7 74.0 72.4 75.8
−0.322 −0.059 −0.033 −0.021
0.262 0.612 0.639 0.511
−0.030 0.276 0.303 0.245
Errors on contact angles are 9 0.5° for advancing and receding and 91° for equilibrium.
quite well linearly correlated; in particular, it seems that the best correlation is obtained with the receding and equilibrium contact angles (Rr = − 0.9266, Req = − 0.9215, respectively). Even the linear correlation of advancing contact angle is not so bad (Ra = −0.8898) and shows that with the increasing of the contact angle one should expect an increase in the values of the height of rupture. This means that rupture height is directly correlate with the hydrophobicity of the surface. We have also tried to apply the mathematical model proposed by Sharma and Ruckenstein [3,4] to predict the critical thickness for thin films near the critical state. The approach of the authors is to consider the small perturbations of the liquid surfaces and to estimate the minimal hole dimensions able to determine the film rupture. The expressions proposed are function of the critical film thickness (at which the film is thermodynamically unstable) and is assumed to be close to the rupture height in a low vibration environment. Á Ãhc =r tan r[ 1+cos r−1] Í (1−cos r) Ã hc =r 2 Ä
for for
r5
y 2
y 5r 5110° 2
(2) We have also calculated the value of the rupture height using Eq. (2) (which is valid for pure liquids) with our experimental advancing contact angle data (assuming r =720 mm for the critical hole radius as suggested by the authors). The values obtained (Table 4, col. 2) are in quite good agreement with the ones measured on the untreated samples, but differ consistently with those on surfactants treated samples. This could be due
to the fact that some surfactants molecules can detach from the surface and solve in water causing a lowering of the surface tension of the liquid and an increase of the free energy of the solid: as a consequence the measured heights expected to be lower than those calculated. Moreover, this can, in principle, change the height of the borders meniscus, as previously discussed. 2hc (1−cos r)2 2hc = 1− e r sin r r sin r sin2 r
(3)
Otherwise it must be noted that the contact angles used by Sharma and Ruckenstein are not always well defined, some of the values called ‘equilibrium’ seem in fact too large and are probably advancing contact angle. Moreover, using our receding contact angle to calculate the rupture heights values is not a solution, while the results obtained are close to the measured values on the treated samples, they loose their agreement with the polymers values (Table 4, col. 3). The values from true equilibrium are equally far from both the treated and the untreated surfaces (Table 4, col. 4) Using the Eq. (3) we have also calculated the critical hole radius r starting from our data of contact angle and rupture height (see Table 4, col. 5). The results are in the range 260–770 mm radius, which can be considered quite realistic values considering that they come from a theoretTable 2 Comparison between our data and literature data of contact angle and rupture height on PTFE
PTFE a
Our data (mm)
qa
Literature (mm)
qa
527 942
108.8
510a
110
Padday [1].
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Table 3 Values of rupture heights on the painted discs with or without aging and surfactants immersion
‘Fresh’ painted disc RF-treated disc Cationic surfactants
Corrupted microemulsiona ‘Fresh’ microemulsion
Fig. 4. Values of rupture heights and contact angles on the analysed polymeric samples.
ical approach. The contact angle measurements have been performed also after a surfactants treatment, the aim was to observe whether some autophobic effect on the surfaces previously treated with cationic surfactants was detectable. To do this we have made some Wilhelmy immersions, from which we have observed some bizarre behaviour in the wetting and dewetting stage. The Wilhelmy immersion curve of a clean RF-plasma treated (in oxygen flux, 50 W, 10 min) slide in microemulsion solution shows, as visible in the Fig. 7, the plot force versus immersion assumes an irregular saw-tooth shape. This phenomenon can be related to an autophobic behaviour due to the surfactants solution incapacity to wet the surfactants covered surface, at first; in fact; a clean and high energy-surface strip is wet by a surfactants layer, but then this coverage becomes a wetting barrier and the meniscus retracts because of the autophobic interaction. While the plate is immersed in the liquid, the Liquid– Vapour line is deformed in order to avoid the wetting, but since the sample is forced into the liquid, the wetting is
Fig. 5. On some high energy surface the thin film does not break, even if the liquid level is below the upper side of the sample.
H (mm)
qa
qr
qeq
181 922 0 183 180 182 156
94.0 68.4 81.8 81.7 81.7 80.0
47.0 25.3 29.9 31.0 31.0 30.0
72.2 50.5 59.7 59.9 59.9 58.7
202 185 233
84.0 83.0 86.9
42.5 35.5 46.2
65.1 62.1 68.1
The error values of H are estimated to be around 10%. Errors on contact angles are 90.5° for advancing and receding and 91° for equilibrium. a In this case, because of the accidental evaporation of the isobuthyl alcohol, the microemulsion resulted not well homogeneous.
inevitable and when the line tension reaches a critical value to keep the dry surface is no more convenient and so the LV line jumps ahead. In this way the wetting is a discontinuous mechanism and takes place in subsequent jumps of the meniscus, which is reflected by an analogous behaviour in the immersion versus force diagrams (Fig. 7(A)), the force values in some cases overcome those measured in recession mode. The autopho-
Fig. 6. Rupture heights vs. contact angles (advancing, receding and equilibrium) of the whole measurements (polymers and treated or untreated samples).
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Table 4 Experimental and calculated (r= 720 mm) rupture height from advancing, receding and equilibrium contact angles
PTFE PS PVC PET ‘Fresh’ painted disc Cationic surfactants
Corrupted microemulsion Fresh microemulsion
H (mm)
Hcalc. ADV (mm)
Hcalc. REC (mm)
Hcalc. EQ (mm)
rcalc. ADV (mm)
5279 42 3979 22 3209 23 3999 33 1819 22 183a 180a 182a 156a 202a 185a 233a
496 381 372 368 366 344 344 345 340 349 347 355
341 261 253 289 239 158 163 163 158 219 186 235
378 338 334 344 291 292 292 287 310 300 320 333
765 609 496 621 227 302 298 300 261 328 303 355
The calculated critical hole size for the tested samples are also reported. a Estimated error 9 10%.
bic behaviour is well visible also by considering the low values of the receding force (about 100 mg) which describes the wetting difficulty of the surfactant solution over a surfactant covered surface. The maximum height of the peaks reduces along the immersion. This reduction is stronger to that due to buoyancy and can be interpreted as a partial spreading of the surfactants beyond the meniscus three-phase line. After a subsequent immersion in water, the glass surface being covered with a surfactants layer, the values of contact angle turn from 0 advancing and receding (RFtreated glass in water) to 91.7 advancing and 41.3 receding. The low value of the receding contact angle lets suppose that the surfactants were not able to homogeneously cover the glass surface, leaving some high energy spots exposed. It is well known how a compact packing of the first layer is required to provide a good surface covering. The fact that on a such high energy sample (plasmatreated glass) the surfactants are not able to full cover the surface is probably reflected in lower energy samples like the aged painted discs, in which the values of the receding contact angles are still low after the immersion in the surfactant solution. It seems to be somehow better the behaviour of the microemulsions which rise the receding contact angles to values quite close those of the ‘fresh-painted’ non treated disc.
4. Conclusions A new experimental method to determine the height of rupture of a thin liquid film has been successfully tested on different materials as polymers or painted disc plates (untreated or artificially aged and then treated with cationic surfactants solutions used as ‘rinse aids’ agents). All the measurements have been performed in water. The data are quite precise and the values are in substantial agreement with literature data eventually obtained through other, more complex, methods. The relevance of the weight of the meniscus along the border of the Petri capsule at the moment of the film break has been considered negligible in the case of the couple aluminium–water but it should be evaluated in the case of other couples. The data collected on various surfaces seem to indicate a qualitatively good correlation between the values of the cosines of the contact angles (particularly the receding and equilibrium angles) and the height of rupture, both for the polymeric samples and for the painted ones. It has been shown a marked differentiation between the normal and the aged samples and the effect of the surfactants covering has been well detected. The experimental data have also been compared with the Sharma and Ruckenstein model
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of the hydrophobicity of the surfaces or of the rinse-aids agents. Focusing the attention on the analysis by the Wilhelmy method of the dynamics of the spreading of the surfactants solution, we have provided a description of the autophobic effect during immersion in terms of force measurements. With the Wilhelmy technique, in fact, it has been possible to detect the ‘stick-jump motion’ of the meniscus by the sawtooth shape of the curves and so to test the goodness of the surfactant covering. The new technique appears to be a cheap and sufficiently reliable method to measure the rupture height. Nevertheless many other tests are in progress to get a wide set of data to improve the statistical analysis and to test new materials, liquids and to evaluate the effect of the border meniscus weight for different couples of test liquids and container materials.
References
Fig. 7. Immersion of a RF-Plasma-treated microscope glass, (50 W, 10 min, oxygen) (A) in the microemulsion solution and then (B) in ultrapure water. It is evident the sawtooth shape typical of an interface restructuring in (A) and the surfactants coverage in (B) showed by the-increase of the contact angle. The surfactant solution is no more able to wet well the surface, showing an autophobic behaviour. The contact angle calculated in (B) is 91.7° adv. and 41.3° rec.
but an incomplete agreement was found, probably because of a loss of surfactants from the treated surface into the liquid. Also the critical hole radius has been calculated and the results are significantly constant. Starting from these considerations the rupture heights measured through the presented method can be considered a valid index
[1] J.F. Padday, Spec. Discuss. Faraday Soc. 1 (1970) 64. [2] A. Sharma, E. Ruckenstein, J. Colloid Interf. Sci. 113 (1986) 456. [3] A. Sharma, E. Ruckenstein, J. Colloid Interf. Sci. 133 (1989) 359. [4] A. Sharma, E. Ruckenstein, J. Colloid Interf. Sci. 137 (1990) 443. [5] K.R. Shull, T.E. Karis, Langmuir 10 (1994) 334. [6] Zisman, Contact Angle, Wettability and Adhesion, Adv. Chem. Ser. n. 43, American Chemical Society, Washington, 1964 Chapter 1. [7] F.J. Holly, in: E.M.alcom Schrader, I.G.eorge Loeb (Eds.), Modern Approaches to Wettability, Plenum Press, New York, 1992 Chapter 8. [8] B. Frank, S. Garoff, Langmuir 11 (1995) 4333. [9] B. Frank, S. Garoff, Langmuir 11 (1995) 87. [10] B. Frank, S. Garoff, Colloids Surf. 116 (1996) 31. [11] C. Della Volpe, D. Maniglio, M. Morra, S. Siboni, Oil Gas Sci. Technol. 56 (2001) 9. [12] C. Della Volpe, D. Maniglio, M. Morra, S. Siboni, The determination of a ‘stable-equilibrium’ contact angle on heterogeneous and rough surfaces, this congress.
A new experimental method to analyse the dewetting properties of polymer surfaces and cationic surfactants C. Della Volpe a,*, S. Invernizzi b, D. Maniglio a, S. Siboni a a
Department of Materials Engineering, Uni6ersity of Trento, Via Mesiano 77, Trento 38050, Italy b Synt srl- Zola Predosa(BO), Italy
Abstract In the present paper a new method able to easily evaluate the critical height of rupture of liquid films on various substrate surfaces is described and commented. It is automatic and based on the different IR reflectance of solids and liquids. Its limits and advantages are analysed. The heights of rupture of water have been measured on different kinds of polymeric solid surfaces as well as on surfaces whose water repellency has been increased by different mixtures of substituted ammonium salts, used as autophobic rinse aids. In both cases the effect of experimental conditions and the value of contact angles measured by the Wilhelmy microbalance have been correlated with the height of rupture. The peculiar sawtooth trend of force versus immersion in Wilhelmy experiments, performed by using some of these autophobic substances solutions, is described and interpreted. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Height of rupture; Thin film; Contact angle
1. Introduction The study of hydrodynamic instabilities induced by intermolecular forces in thin films has been motivated by their industrial application in disperse and colloidal systems on one hand and the understanding of diverse biological phenomena on the other. Some typical industrial applications of thin film rupture are in the field of foams and emulsions, thin coatings, oil-recovery, vapour condensation; on the biological front this subject is particularly interesting to understand cell adhesion mechanisms, thin membrane instabilities, tear * Corresponding author. Tel.: + 39-0461-882-409; fax: + 39-0461-881-977. E-mail address: [email protected] (C.Della. Volpe).
film break-up. The phenomenon can be described as the spontaneous dewetting of a surface covered with water (or any other liquid) when the film thickness on the surface drops below a critic value, leaving a dry patch with a wetting meniscus in contact with it. The phenomenon is associated with low energy surfaces and the rupture thickness is critically dependent on the nature of the solid surface, which makes it be a useful quantity to detect differences and modifications in surface characteristics. In literature the phenomenon has been described so far in many papers, in particular much effort have been done by Padday [1] and Sharma and Ruckenstein [2–4] who provided also some experimental data collected on various surfaces, some mathematical models to correlate the rupture height with hole-shape perturbations and
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to describe the stability of thin films. Some works about the correlation between contact angle and dewetting speed [5] are also present. Therefore, it seems that the only rupture height data in literature are those collected by Padday and Doughman et al. [1,3]. To measure the rupture height over a hydrophobic surface is in general not easy; it requires specialised equipment such as cathetometers or fast video cameras, not easily available in an industrial laboratory. Our purpose is to tune a sufficiently accurate and low-expensive method to detect modifications due to aging or to chemical variations induced by the covering with surfactants. We tried to find out a new easy method to estimate the height of rupture by using widely available instruments such as an analytical balance, a micro comparator and a peristaltic pump. In this way it is possible to get a quite precise measurement of the height at a very low cost. We have focused our attention on a peculiar industrial topic, the ability of various ‘rinse-aids’ agents for car washing plants made with mixtures of different cationic surfactants. These agents are, in fact, commonly used to increase the protection against water when the paint reduces its hydrophobicity with the time passing. Together with the development of this new method, we focused our attention on the analysis of the mechanism of thin film rupture, trying to correlate the values of height measured with other surface parameters such as contact angles. To do this we have tested different materials like painted samples (‘fresh’ painted and aged) and polymer plates and compared the data collected with the contact angle measurements made using the Wilhelmy technique. We have also investigated some specific autophobic phenomena (Zisman [6]) that have been observed performing the contact angle measurements in surfactants solutions. As well known, the wetting behaviour of a fluid can be dramatically modified by adding surfactants and their control of wetting processes can lead to strange effects like those ‘unsteady stick-jump motions’ of the contact line described by Garoff [8 – 10]. While performing our measurements on RF-plasma cleaned glass plates in a surfactants solution we have observed a typical autophobic behaviour, the incapacity of the surfactants solu-
tion, once wet an horizontal strip of the surface, to keep wetting it. The typical forced immersion of the Wilhelmy technique leads to a retraction and a pinning of the contact line, a deformation of the liquid – vapour interface and a subsequent jump of the contact line to a close lower energy state. This phenomenon is well pointed out by the Wilhelmy characteristic curves.
2. Materials and methods We performed our measurements on disc like, metallic samples painted with car paint (25 mm diameter, about 0.5 mm thickness), microscope glass slides (24× 24× 0.15 mm) from Prestige; squared polymer plates (about 20× 20× 0.5 mm) from Goodfellow (uPVC, PTFE, PET, PS). The two surfactants solutions used are a simple solution (concentration 2 g l − 1 in MilliQ-Millipore ultrapure water) of two commercial products, Roquat TO90 (Cromogenia, Barcelona, Spain) N + (CH3)(CH2CH2OH)(CH2COOR)2CH3SO4− and Ro-min HPA (Cromogenia, Barcelona, Spain) N (CH2)2(CRCHNCH2NH2). C6 is a microemulsion obtained by mixing the previous surfactants with oil and isobuthyl alcohol. The exact formulation of all products is covered by industrial secret. The technique to measure the rupture height of a liquid film over a solid surface is based on a weight measurement. The basic experimental setup (Fig. 1) is composed of a single 86.9 mm Ø aluminium made throwed Petri capsule whose cylindrical geometrical proportions are well known and very precise, a microbalance (GIBERTINI E50S), put on a standard antivibration table, and a peristaltic pump (ALTEA AB) with a thin needle at the endpoint of the flexible tube. A regular shape sample is placed on the Petri capsule and totally covered with water, then pumped away very slowly with the thin needle (0.5 ml min − 1). When the water surface breaks over the sample the pump is stopped and the rupture height is then calculated from the weight value (Wwater) read on the balance, by using Eq. (1) There z(T) denotes the value of the water density at a certain temperature T, rPetri is the inner radius of the Petri capsule, Vsample stands for
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the volume of the sample. This setup (sample placed on a container) describes the so called film confined by bulk water’ [7], in which the thin film is located only on the sample surface and not in the surrounding water. In a second time the experimental setup has been modified in order to automating the measurement (Fig. 2). The detection of the water break is made using an infrared diffuse scan opto-switch sensor (OPB704 led, 9926 MEXICO transistor, provided by RS-components) whose output voltage, redirected to a comparison circuitry, drives the remote command of the peristaltic pump (S1-MINI by Altea AB). The circuitry is a simple trigger Schmitt with variable hysteresis built up by means of a LM331NE comparator (see Fig. 3). The value of the hysteresis can be changed by acting on the R6 trimmer in order to adapt the circuit to various kinds of surfaces. The working principle is simple, as the reflectivity of the surface is below a certain threshold (chosen by the operator before the Petri capsule is filled with water) the pump continues to suck; when the water film breaks over the surface, the reflectivity of the surface suddenly changes, the comparator sets its output voltage value to zero and the transistor T1, no more active, switches off
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the pump. For in the most cases we were working with painted metal surfaces we supposed to have an increase in reflectivity passing from the wet to the dry surface. In the case that the variation is negative the measurement can easily be done manually stopping the pump by acting on the switch SW1 or by inserting a two pole switch before the 2nd and 3rd pin of the comparator. The data acquiring has been made via the serial port (RS232) present on the Gibertini E50S and a Keyspan USA28X USB to serial adapter, connected to an Apple iBook. The control program has been written both in REALBASIC and in LABVIEW® (the last one to provide compatibility with MACINTOSH® and WINDOWS® worlds). The test measurements have been made upon disc-like metal surfaces with their upper surfaces rounded because of the mechanical cut starting from a single, wide plate. The point of rupture is supposed to be the upper point of the disc and as shown by Padday [1] the curvature of the surface does not have any effect on the value of the height. Contact angle measurements have been made using a CAHN DCA 322 Wilhelmy microbalance at the speed of 20 mm s − 1 if not specified otherwise.
Fig. 1. The sample placed in a metal cylindrical Petri capsule is immersed in a liquid pumped away from a thin needle. When the liquid film breaks, the pump is stopped and the height is calculated starting from the residual liquid weight.
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tion of the bulk or the declared chemical composition. We have put our data in comparison with those given by Padday for the Teflon –water system and, as if it’s possible to see from the Table 2, the value is in good agreement with our. The literature data refer to PTFE, PE, PMMA and wax, in different solvents, while we have analysed in the same solvent, i.e. water, a wider set of common polymers. We have also tried to include LDPE, PMMA and NY6,6 but if in the case of PMMA and LDPE there have been some setup trouble (floating and low reflectivity) in the case of Nylon the problem is the adsorption of water. Plotting the data of the rupture height for the studied polymers versus the cosine of the contact angles (Fig. 4) it is detectable an opposite behaviour, as the rupture height increases the value of the cosines decreases (which corresponds to an increase of the angle values). hdewetting = Fig. 2. A close view of the Petri aluminium capsule with a disc immersed in water. It is also visible the aspiration needle and the opto-switch, along with and the simple micrometric positioning structure.
3. Results A first set of data comes from the measurement of some polymer-made squared plates (Table 1). In literature the height of rupture is often put in relation with a so-called contact angle or a not well specified ‘equilibrium contact angle’ which probably refers to an advancing angle somehow relaxed. In our set of data we propose a complete characterisation through advancing, receding and ‘stable’ equilibrium contact angle, whose experimental and theoretical definition is specified in a previous work and further described in a second paper presented at this congress [11,12]. The plates have been used as received, only after a standard cleaning phase with solvent and/or detergent; so it is possible (almost certain) that their surface composition, as it commonly happens in commercial polymers, is not the same composi-
(W/z(T))water + Vsample − hsample yr 2Petri
(1)
It is important to note that there is a problem related to the weight of the meniscus along the borders of the Petri capsule at the moment of the rupture, which could lead to significantly overestimated values of the rupture height (Eq. (1) refers to a completely flat liquid surface, as it is drawn in Fig. 1). It is not simply correlated with the advancing or receding contact angles of the liquid on the container material, because the border meniscus is in a non-equilibrium situation. In our case, the choice of the combination of water and the aluminium container appears to reduce the importance of this problem, the facts that the curvature of the border meniscus is not appreciable and that our data for PTFE are in agreement with the literature, let us deduce that the induced error is low at least for the couple. Moreover, only water has been used as a test liquid along the present paper, so that the eventual meniscus at the borders is the same in all the experiments. Other studies will be done changing the material of the Petri and considering different liquids to evaluate the effective importance of this phenomenon and the possibility to compare different liquids in the same containers.
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We made also some measurements on disc plates to understand the protective properties of some cationic surfactants on old-painted surfaces. To produce an accelerated aging on the paints of our samples we have treated them using an RFplasma reactor at low power (30 W) and short treatment times (30 s). Doing so we have uniformly oxidised the surfaces simulating the natural phenomena induced by the exposure to UV-radiation and to atmospheric agents, obtaining the same results as using an UV-condensation chamber but with a reduced treatment time. We compared the behaviour, by measuring the height of rupture, either on the non-protected surfaces or on the aged disc or on aged and surfactants-covered discs. We found that simple painted discs are much different in their surfaces than expected, due probably to a non-perfect and uniform coverage of the paint, variable heights are measured starting from 150 to 250 mm, changing at the same time the values of the contact angle. The treatment consists in a brief immersion in a cationic surfactants solution or in a formulation called C6, a micro-emulsion of surfactants. A treated surface (30 W power, 30 s time) shows to have an height of rupture equivalent to zero (or not detectable), even if the level of the liquid in the Petri capsule is lower than the sur-
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face of the sample, the latter still remains wet (leaving a thin film on the high energy surfaces due to the disjoining rupture process and whose typical thickness is about some microns or less [1]) (Fig. 5). The data in Table 3 show that the microemulsion seems to increase a little the protection in comparison with the cationic solution. The microemulsion sensibly increases the values of both the contact angles (advancing and receding) making it reach almost the same values than the ‘fresh’ paint. It is realistic that the coverage of the surface is best obtained by the microemulsion. The simple cationic solution in fact is not able to increase significatively the values of the receding contact angle which means that a larger amount of high energy zones are exposed. Both the treated surfaces have values of rupture height very close to the ones of the new paint (about 180 mm). The values in Table 3 represent the first measurement made on the sample. As attended, in fact, the next measurements give rupture height lower because the surfactants tend to be slowly removed by the new water added, increasing the high energy zones exposed to the surface. Plotting all the results (Fig. 6) obtained so far, it is evident that the data of the cosines of the contact angles versus the height of rupture are
Fig. 3. Pump-driving circuit diagram.
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Table 1 Rupture heights and contact angle values measured on PTFE, PS, PVC, PET
PTFE PS PVC PET
H (mm)
qa
qr
qeq
cos qa
cos qr
cos qeq
527 9 42 397 9 22 320 9 23 399933
108.8 93.4 91.9 91.2
74.8 52.3 50.3 59.3
91.7 74.0 72.4 75.8
−0.322 −0.059 −0.033 −0.021
0.262 0.612 0.639 0.511
−0.030 0.276 0.303 0.245
Errors on contact angles are 9 0.5° for advancing and receding and 91° for equilibrium.
quite well linearly correlated; in particular, it seems that the best correlation is obtained with the receding and equilibrium contact angles (Rr = − 0.9266, Req = − 0.9215, respectively). Even the linear correlation of advancing contact angle is not so bad (Ra = −0.8898) and shows that with the increasing of the contact angle one should expect an increase in the values of the height of rupture. This means that rupture height is directly correlate with the hydrophobicity of the surface. We have also tried to apply the mathematical model proposed by Sharma and Ruckenstein [3,4] to predict the critical thickness for thin films near the critical state. The approach of the authors is to consider the small perturbations of the liquid surfaces and to estimate the minimal hole dimensions able to determine the film rupture. The expressions proposed are function of the critical film thickness (at which the film is thermodynamically unstable) and is assumed to be close to the rupture height in a low vibration environment. Á Ãhc =r tan r[ 1+cos r−1] Í (1−cos r) Ã hc =r 2 Ä
for for
r5
y 2
y 5r 5110° 2
(2) We have also calculated the value of the rupture height using Eq. (2) (which is valid for pure liquids) with our experimental advancing contact angle data (assuming r =720 mm for the critical hole radius as suggested by the authors). The values obtained (Table 4, col. 2) are in quite good agreement with the ones measured on the untreated samples, but differ consistently with those on surfactants treated samples. This could be due
to the fact that some surfactants molecules can detach from the surface and solve in water causing a lowering of the surface tension of the liquid and an increase of the free energy of the solid: as a consequence the measured heights expected to be lower than those calculated. Moreover, this can, in principle, change the height of the borders meniscus, as previously discussed. 2hc (1−cos r)2 2hc = 1− e r sin r r sin r sin2 r
(3)
Otherwise it must be noted that the contact angles used by Sharma and Ruckenstein are not always well defined, some of the values called ‘equilibrium’ seem in fact too large and are probably advancing contact angle. Moreover, using our receding contact angle to calculate the rupture heights values is not a solution, while the results obtained are close to the measured values on the treated samples, they loose their agreement with the polymers values (Table 4, col. 3). The values from true equilibrium are equally far from both the treated and the untreated surfaces (Table 4, col. 4) Using the Eq. (3) we have also calculated the critical hole radius r starting from our data of contact angle and rupture height (see Table 4, col. 5). The results are in the range 260–770 mm radius, which can be considered quite realistic values considering that they come from a theoretTable 2 Comparison between our data and literature data of contact angle and rupture height on PTFE
PTFE a
Our data (mm)
qa
Literature (mm)
qa
527 942
108.8
510a
110
Padday [1].
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Table 3 Values of rupture heights on the painted discs with or without aging and surfactants immersion
‘Fresh’ painted disc RF-treated disc Cationic surfactants
Corrupted microemulsiona ‘Fresh’ microemulsion
Fig. 4. Values of rupture heights and contact angles on the analysed polymeric samples.
ical approach. The contact angle measurements have been performed also after a surfactants treatment, the aim was to observe whether some autophobic effect on the surfaces previously treated with cationic surfactants was detectable. To do this we have made some Wilhelmy immersions, from which we have observed some bizarre behaviour in the wetting and dewetting stage. The Wilhelmy immersion curve of a clean RF-plasma treated (in oxygen flux, 50 W, 10 min) slide in microemulsion solution shows, as visible in the Fig. 7, the plot force versus immersion assumes an irregular saw-tooth shape. This phenomenon can be related to an autophobic behaviour due to the surfactants solution incapacity to wet the surfactants covered surface, at first; in fact; a clean and high energy-surface strip is wet by a surfactants layer, but then this coverage becomes a wetting barrier and the meniscus retracts because of the autophobic interaction. While the plate is immersed in the liquid, the Liquid– Vapour line is deformed in order to avoid the wetting, but since the sample is forced into the liquid, the wetting is
Fig. 5. On some high energy surface the thin film does not break, even if the liquid level is below the upper side of the sample.
H (mm)
qa
qr
qeq
181 922 0 183 180 182 156
94.0 68.4 81.8 81.7 81.7 80.0
47.0 25.3 29.9 31.0 31.0 30.0
72.2 50.5 59.7 59.9 59.9 58.7
202 185 233
84.0 83.0 86.9
42.5 35.5 46.2
65.1 62.1 68.1
The error values of H are estimated to be around 10%. Errors on contact angles are 90.5° for advancing and receding and 91° for equilibrium. a In this case, because of the accidental evaporation of the isobuthyl alcohol, the microemulsion resulted not well homogeneous.
inevitable and when the line tension reaches a critical value to keep the dry surface is no more convenient and so the LV line jumps ahead. In this way the wetting is a discontinuous mechanism and takes place in subsequent jumps of the meniscus, which is reflected by an analogous behaviour in the immersion versus force diagrams (Fig. 7(A)), the force values in some cases overcome those measured in recession mode. The autopho-
Fig. 6. Rupture heights vs. contact angles (advancing, receding and equilibrium) of the whole measurements (polymers and treated or untreated samples).
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Table 4 Experimental and calculated (r= 720 mm) rupture height from advancing, receding and equilibrium contact angles
PTFE PS PVC PET ‘Fresh’ painted disc Cationic surfactants
Corrupted microemulsion Fresh microemulsion
H (mm)
Hcalc. ADV (mm)
Hcalc. REC (mm)
Hcalc. EQ (mm)
rcalc. ADV (mm)
5279 42 3979 22 3209 23 3999 33 1819 22 183a 180a 182a 156a 202a 185a 233a
496 381 372 368 366 344 344 345 340 349 347 355
341 261 253 289 239 158 163 163 158 219 186 235
378 338 334 344 291 292 292 287 310 300 320 333
765 609 496 621 227 302 298 300 261 328 303 355
The calculated critical hole size for the tested samples are also reported. a Estimated error 9 10%.
bic behaviour is well visible also by considering the low values of the receding force (about 100 mg) which describes the wetting difficulty of the surfactant solution over a surfactant covered surface. The maximum height of the peaks reduces along the immersion. This reduction is stronger to that due to buoyancy and can be interpreted as a partial spreading of the surfactants beyond the meniscus three-phase line. After a subsequent immersion in water, the glass surface being covered with a surfactants layer, the values of contact angle turn from 0 advancing and receding (RFtreated glass in water) to 91.7 advancing and 41.3 receding. The low value of the receding contact angle lets suppose that the surfactants were not able to homogeneously cover the glass surface, leaving some high energy spots exposed. It is well known how a compact packing of the first layer is required to provide a good surface covering. The fact that on a such high energy sample (plasmatreated glass) the surfactants are not able to full cover the surface is probably reflected in lower energy samples like the aged painted discs, in which the values of the receding contact angles are still low after the immersion in the surfactant solution. It seems to be somehow better the behaviour of the microemulsions which rise the receding contact angles to values quite close those of the ‘fresh-painted’ non treated disc.
4. Conclusions A new experimental method to determine the height of rupture of a thin liquid film has been successfully tested on different materials as polymers or painted disc plates (untreated or artificially aged and then treated with cationic surfactants solutions used as ‘rinse aids’ agents). All the measurements have been performed in water. The data are quite precise and the values are in substantial agreement with literature data eventually obtained through other, more complex, methods. The relevance of the weight of the meniscus along the border of the Petri capsule at the moment of the film break has been considered negligible in the case of the couple aluminium–water but it should be evaluated in the case of other couples. The data collected on various surfaces seem to indicate a qualitatively good correlation between the values of the cosines of the contact angles (particularly the receding and equilibrium angles) and the height of rupture, both for the polymeric samples and for the painted ones. It has been shown a marked differentiation between the normal and the aged samples and the effect of the surfactants covering has been well detected. The experimental data have also been compared with the Sharma and Ruckenstein model
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of the hydrophobicity of the surfaces or of the rinse-aids agents. Focusing the attention on the analysis by the Wilhelmy method of the dynamics of the spreading of the surfactants solution, we have provided a description of the autophobic effect during immersion in terms of force measurements. With the Wilhelmy technique, in fact, it has been possible to detect the ‘stick-jump motion’ of the meniscus by the sawtooth shape of the curves and so to test the goodness of the surfactant covering. The new technique appears to be a cheap and sufficiently reliable method to measure the rupture height. Nevertheless many other tests are in progress to get a wide set of data to improve the statistical analysis and to test new materials, liquids and to evaluate the effect of the border meniscus weight for different couples of test liquids and container materials.
References
Fig. 7. Immersion of a RF-Plasma-treated microscope glass, (50 W, 10 min, oxygen) (A) in the microemulsion solution and then (B) in ultrapure water. It is evident the sawtooth shape typical of an interface restructuring in (A) and the surfactants coverage in (B) showed by the-increase of the contact angle. The surfactant solution is no more able to wet well the surface, showing an autophobic behaviour. The contact angle calculated in (B) is 91.7° adv. and 41.3° rec.
but an incomplete agreement was found, probably because of a loss of surfactants from the treated surface into the liquid. Also the critical hole radius has been calculated and the results are significantly constant. Starting from these considerations the rupture heights measured through the presented method can be considered a valid index
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