The contractor : abreviated name: Network coordinator

ENSUP.LPS* ENS J. Meunier**

* Director of the ENS: Gabriel Ruget

45 rue d’Ulm, 75 231 Paris Cedex 05, France

* LPS de l’ENS

24 rue Lhomond, 75231 Paris Cedex 05, France

The associated contractors: Scientist in charge:

RHODIA-CRA RHODIA O. Anthony

KUL.DN.VSFM K.U. Leuven J. Indekeu

HULL. SC U. Hull B.P. Binks

UDUS.ITP U. Duesseldorf R. Bausch

UAM.FNS.WL WINS G. Wegdam

ULIS.CFMC U. Lisboa M. Telo da Gama

UOXF.PCL U. Oxford R.K. Thomas

UBOD1.PMOH U. Bordeaux H. Kellay

Participants

The programme of work

The original work plane

Scientific highlights

Soap films and surfactants

Wetting

Publications

Benefits resulting from the network

The post-docs: summary

Interaction with the industry.

Training and mobility. Employment

"The Contractor (ENS, established in France) will act jointly and severally towards the Commission with the following Associated Contractors to perform this Project Programme:

Rhône-Poulenc Industrialisation S.A. (RP) established in FRANCE

In the first stage of the project Rhone Poulenc’s research effort in the Network took place at the research center in Lyon, France (Scientist in charge:V. Bergeron). During the second period Rhone Poulenc changed its status and the company split into two: a pharmaceutical sector called Aventis, and chemical sector named Rhodia. The work performed in the Network was continued within the chemical sector and took place at Rhodia’s research center in Paris, France (Scientist in charge: O. Anthony)

Katholieke Universiteit Leuven (KU Leuven) established in BELGIUM

University of Hull (U. Hull) established in UNITED KINGDOM

Heinrich-Heine-Universitaet Duesseldorf (U. Duesseldorf) established in GERMANY

University of Amsterdam (WINS) established in NETHERLANDS

Fundacao da Universidade de Lisboa (U. Lisboa) established in PORTUGAL

University of Oxford (U. Oxford) established in UNITED KINGDOM

Université Bordeaux I (U. Bordeaux) established in . FRANCE

 

 

 

Research Objectives

Both the stability of long-lived foams and emulsions and the wetting behaviour of fluids on a substrate are determined by molecular interactions. Both are, in fact, manifestations of these interactions at mesoscopic length scales. These molecular interactions are commonly separated into long-range (van der Waals and electrostatic) interactions and short-range so-called 'structural interactions'. Both the wetting transition (from a microscopic to a mesoscopic adsorbed film) and the transition from a common black to a Newton black soap film result from a subtle interplay between these long-and short-range forces. We performed detailed experimental studies of both foam and wetting films as a function of these interactions. Simultaneously, a detailed theoretical description relating the wetting behaviour and film stability to the interactions will be developed. The research objectives were :

1. SOAP FILMS

(1) Determination of the relation between the disjoining pressure and the probability of transition from a common black to a Newton black film.

(2) Determination of the structural interactions that lead to stable Newton black films.

2. WETTING TRANSITIONS

(3) Determination of the generic wetting phase diagram in terms of both the long-and short-range interactions.

(4) Determination of the conditions under which fluctuation-induced critical wetting is observable experimentally.

Methodological Approach and Work Plan

 

 

Experimental 1. Soap and emulsion films.

The three groups at RP, the U. Bordeaux, and the ENS use the same Thin Film Balance (TFB) technique to create and study soap films. This is an experimental technique that can be used to study both thin-film hydrodynamics and equilibrium thin-film forces. Single thin-liquid films are formed in a small hole drilled through a solution-saturated porous glass disk. The porous frit is connected to a capillary tube, which in turn is connected to a syringe pump. The thickness of the film then results from a balance of thin-film and hydrostatic forces, and can thus be changed by applying different external pressures. The RP group and ENS group measures the film thicknesses as a function of pressure using interferometry. This yields a direct link between the interactions and the film thickness: the disjoining pressure isotherm. The U. Bordeaux group, on the other hand, uses ellipsometry. In addition to these standard techniques, this group also used confocal micro-Raman spectroscopy to study the soap films. To our knowledge, this technique had not been used before to study ordering in soap films, the experiments of the group at the U. Bordeaux demonstarte that the technique is well adapted in particular to study ordering in Newton black films.

In addition to this, the U. Hull group has a created a novel instrument to determine the disjoining pressure isotherms of emulsion films. This instrument uses a micropipette technique to directly measure the force exerted on a micron-sized liquid drop as it is pressed up to an interface. The thin film thickness is again determined by optical interferometry. This instrument has yielded the first measurements of forces between 2 interacting surfactant monolayers at oil-water interfaces. The understanding of the stability of these emulsion films is not only of great industrial importance, it is also makes a direct connection with the wetting studies in the project; for these surfactant films at the oil-water interface are studied.

Experimental 2. Wetting.

In order to determine wetting film thicknesses, both the ENS and the WINS group utilise ellipsometry. In addition to this, the WINS group can also perform dynamic light scattering experiments with the same experimental set-up to measure the (capillary wave) surface fluctuations. This is a standard technique for studying interfaces with low interfacial tensions, and is thus well adapted to study the wetting of alkane-water systems in the presence of surfactants, a system studied both at the ENS and at the U. Hull. The latter group has also developed a technique whereby the surface tensions of aqueous surfactant solutions can be measured as a function of the vapour pressure of an adsorbing oil. Using standard thermodynamic relationships, the measured data yields again the disjoining pressure isotherms for wetting films of oil. In a collaboration between the U. Hull and the U. Oxford, the microstructure of the resultant films (under conditions of controlled vapour pressure of the adsorbing oil) have been successfully determined to a resolution of 1 Angstrom using neutron reflectivity with selective deuteration of the components.

Theoretical

Two different methods are available for a theoretical description of fluid interfaces. The first one is the phenomenological Landau theory. Although this is a mean-field theory, it has proved extremely useful in uncovering new physical phenomena in the context of wetting transitions. Moreover, recent progress allows for a quantitative comparison with experiment, as was shown explicitely in the ENS/KU Leuven collaboration. Both the KU Leuven and U. Duesseldorf groups have a strong expertise in this field. The second theoretical method is the use of density functional theory, which has the advantage of incorporating a more realistic description of the molecular interactions, but has been proven tedious to implement for real systems. Recently, however, quantitative predictions have come out of such theories, which have led to a large experimental and theoretical activity in low-temperature physics. The question whether it can also be used to describe quantitatively more complicated molecules at higher temperatures is still open, has been pursued within the contract by the U. Lisboa group.

Task (1) The relation between the disjoining pressure and the probability of transition from a common black to a Newton black film.

Subtask (1a) Measurement of the disjoining pressure for the systems described above (RP-U.Hull) and emulsion films (U.Hull)

 

Subtask (1b) Measurement of the transition probabilities from a common black to a Newton black film on the soap film systems (RP-U.Bordeaux)

 

Subtask (1c) Adaptation of the nucleation model for wetting films to soap films (KU Leuven-U.Duesseldorf-ENS)

Task (2) The structural interactions that lead to stable Newton black films

Subtask (2a) Measurement of the degree of local ordering in the Newton black films studied under task(1) using Raman microscopy (U.Bordeaux)

 

Subtask (2b) Measurement of the microscopic structure of single films using neutron reflectivity (U.Oxford)

 

Subtask (2c) Development of a theoretical description of the local ordering using primarily Density Functional Theory (U.Lisboa) and also Landau theory (KU.Leuven-U.Duesseldorf)

 

Subtask (2d) Contrasting studies using deuterated solvents and surfactants (U.Oxford-U.Bordeaux)

Task (3) The generic wetting phase diagram in terms of both the long- and short-range interactions.

Subtask (3a) Direct measurement of the generic wetting phase diagram as a function of long-and short-ranged interactions for the oil-water systems (ENS-WINS)

 

Subtask (3b) Theoretical description of the phase transition sequence: Landau theory with inclusion of long-range forces (KU Leuven-U.Lisboa-ENS)

Task (4) Observation of a fluctuation-induced critical wetting transition

Subtask (4a) Measurement of surface and interfacial tensions as a function of temperature and salinity for the oil-water surfactant systems (U.Hull-WINS)

 

Subtask (4b) Equilibrium film thickness measurements for the same systems (ENS-WINS)

 

Subtask (4c) Direct measurement of the capillary-wave fluctuations in the vicinity of the transition (WINS)

 

Subtask(4d) Neutron reflectivity experiments on the same systems to determine equilibrium thicknesses and fluctuation amplitudes of the wetting films (U.Oxford).

 

Subtask (4e) Theoretical description of the fluctuation effects on the wetting transition (U.Duesseldorf-KU Leuven)

 

 

 

 

 

 

At ENS (Paris), the transition from a common to a Newton black film was studied using a dilute nonionic surfactant solution as a function of the disjoining pressure in the film. A hysteresis of the transition was observed: for a given range of disjoining pressures either of the two possible states of the film may be observed (the common black film or the Newton black film). This important new result confirms the first order nature of the transition between the thin and the thick soap film. In addition, the ENS studied the probability of nucleation of a Newton black film from a common black film. For a given fixed disjoining pressure the probability of transition was measured and found to be exponential (i.e., it follows a Poisson law) as predicted for a first order transition. The activation energy deduced from these measurements is found to be a linear function of the disjoining pressure. The theory of nucleation in wetting films developed in the Dusseldorf group was adapted to soap films, and agrees quantitatively with these last experimental results. A joint publication involving the different groups has been submitted. [1] [Subtasks 1a, 1b, 1c, [collaboration ENS/Dusseldorf, involving three young researchers financed by the TMR network: V. Casteletto, I. Cantat, D. Sarker]

The Hull group has investigated a problem closely related to the common-to-Newton black film transition: flocculation transitions in emulsions. These in fact correspond to wetting transitions occurring with emulsion thin films. The effects of surface charge and electrolyte concentration were successfully modelled in terms of the disjoining pressures across emulsion films. This work "spun-out" of the TMR network and was co-funded by an industrial sponsor [2] [Subtask 1a].

Soap films are constituted by two monalayers separated by water. Consequently, the behaviour of a single monolayer is important for the understanding of the properties of the soap film as a whole. The group in Hull has used ellipsometry to determine the dynamic and equilibrium adsorption of non-ionic surfactants at the air-water surface plus the subsequent adsorption of oil vapours onto the surfactant monolayers. The ellipsometric signals were successfully modelled and shown to be consistent with results from tensiometry plus neutron reflection experiments, the latter having been done in collaboration with the U. Oxford group. Importantly, this group has also made the first observations of the effects of adsorbed oil vapours on foam formation and stability. [3] [Subtask 2b].

At RP, soap films where studied in the Thin-Film Balance for there ability to make the transition from a common to a Newton black film. In particular Cationic surfactants with the addition of various salt additives where investigated. Although it has been long known that the addition of salt to ionic surfactants can induce Newton black film formation via screening of the electrostatic double layer, what is less clear is how salts which have large hydrophobic moeties effect the Newton black film transition. A systematic study using a series of hydroxy-2-naphthoic acids was performed, (e.g. 3-hydroxy-2-naphthoic acid is pictured below).

3-hydroxy-2-naphthoic acid

Three different salts were used to vary the position of the hydroxy group along the phenol alpha, meta and para positions. In this way the chemical formula is the same but the structure of the salt is varied. These structural variations effect the ability of the system to generate the tight packing needed to for a Newton Black film. Therefore, although it is still necessary to add enough salt to screen the electrostatic double layer, this is not a sufficient condition for Newton black film formation with these systems. Results from this study revealed that Newton black film transitions can be controlled using this concept and Rhone Poulenc has benefited from these studies to design new salts for controlled Newton black film formation. [subtasks 1a,1b, involving one TMR postdoc: Ian Harrison]

At RHODIA, Katerina Karagianni worked with new Rhodia diblock copolymers to investigate the structural features of the molecule that control the transition from a common to a Newton black film. These polymeric molecules are termed Rhodiblocks and a wide variety of structures exist. Most interesting is the work performed with copolymers that contain one charged block and one hydrophobic block. Foam film properties were measured by Dr. Karagianni at the ENS laboratory in Paris, while at the same time she built two foam testing devices in Rhodia’s research laboratory to compare macroscopic foam stability to microscopic film stability. Dr. Karagianni found that this class of polymers is very similar to classical surfactant molecules but their film properties are quite different. In fact, presumably due to counterion binding along the charged portion of the chain, osmotic forces within the charged portion of the film monolayer significantly reduced the salt effects for common to Newton black film transitions. Moreover, it was found that the strongest effects were seen after the charged block exceeded ten monomer units. [subtasks 1a,1b, collaboration RHODIA / ENS involving TMR postdocs: Ian Harrison and Dr. Karagianni]

Surprisingly, very little effect on Newton black film stability was seen throughout the series of different di-blocks tested. This result was also found for macroscopic foams made from these co-polymers. Subsequent studies are now underway with triblock polymers with a flexible non-ionic polymer as a spacer between the hydrophobic and charged portion of the original di-blocks. [subtasks 2, collaboration RHODIA / ENS involving one TMR postdoc: Ian Harrison]

Likewise, the systems studied, hydroxy-2-naphthoic acids, in Task 1) above, were also used to help understand the stability of Newton black films formed from cationic surfactants. In this case the structural criteria of the monolayer and the strength of the lateral forces in the monolayer where used to control Newton black film stability. It was found that extremely stable Newton black films of Cetylpyridinium chloride and 3-hydroxy-2-naphthoic acid could be formed. Combining NMR, surface and film elasticity measurements it was shown that this stability is due to the increase in the surface elasicity due to the combined interaction of the p electrons in the phenol groups of the salt and those within the pyridinum group of the surfactant, coupled with the direct electrostatic interactions of the cationic and anionic groups present. The finding that so-called p bonds can be used to enhance lateral binding of the monolayer and thus increase film stability has important industrial implications. [subtasks 2, involving one TMR postdoc: K. Kariagianni]

Another parameter that is of key importance for the stability of soap films is their molecular structure. Their simple structure, two monolayers of surfactant separated by interstitial water, allows for accurate measurements of their Raman and infrared spectra. The water layer can reach a very small thickness at the end of the draining process, for relatively large amounts of added salt in the case of ionic surfactants, or for low humidity rates. Investigating the properties of these ultra thin films via Raman and Infrared spectroscopy brings forth some novel features about their molecular structure. The main techniques used to study such films, which can reach molecular dimensions, are Raman spectroscopy, infrared spectroscopy and ellipsometry. While the latter technique allows for a precise measurement of the thickness of the film, the first two techniques, which are complementary, probe the structure of the interstitial water as well as the order of the aliphatic chains. In this respect, study of the Bordeaux group's covers an area where there have been few available experimental results.

This group started measurements of Raman spectra of black soap films obtained using an ionic surfactant (SDS) with variable amounts of electrolyte (NaCl). The use of salt allows to obtain films with different thickness. On a typical spectrum (figure 1) one can clearly discern the water band which is broad and represents the O-H stretching modes of the water molecules and the finer peaks associated with the C-H stretching modes of the aliphatic tails of the surfactant molecules. These measurements gave the possibility to look for differences in the organisation of the two types of black films: the common black film which is usually of about 100Å in thickness and the Newton black film which is much thinner and can be of 40Å or so in thickness. The main finding from the Raman spectroscopy was that the Newton black films present more conformational order in the hydrophobic part of the monolayers constituting the films. In addition, the lateral order of the chains in the thinner films is better than in their thick counterparts. Whether the thicker films present more surface roughness leading to more disorder in the aliphatic part of the chains or whether the thinner films are just more compact (area per molecule smaller than in the thick films) was left unanswered. From the Raman measurements one could also deduce the water core thickness which was found to be between 10 and 20 Å in the Newton black films [4, 5] [Subtask 2a] and [6] [Subtask 2a. Collaboration Bordeaux/RP involving two TMR young researchers: C. Berger and I. Harrison] .

In addition to the Raman spectroscopy this group also started using ellipsometry in a systematic way to measure the total thickness of the films used. Using this combination of experimental techniques the group was able to determine the water core thickness and the total thickness of the film separately and with good accuracy. Figure 2 illustrates the results obtained for the water core thickness using both Raman and ellipsometry.

One can note the clear transition between the common black films and the Newton black films in this figure obtained for an anionic surfactant (Sodium Dodecyl Sulfate or SDS) upon addition of an electrolyte (NaCl) to screen the electrostatic interactions between the surfactant headgroups. The existence of two branches for the thickness as a function of salt concentration clearly shows the first order nature of the transition between common and Newton black films. Other surfactants were used in similar studies: two cationic surfactants (C14Tab and C16Tab) and a nonionic surfactant (C12E5). For the two cationic surfactants, this group has observed a variation of the thickness of the film versus electrolyte concentration. However, the films obtained with the cationic surfactants only showed one branch with a limiting value of the water core thickness at a little more than 45Angstroms. This is at least twice as much as the thickness observed for the SDS system. In a sense one can say that true Newton black films for these two surfactants were not observed. Whether the transition and its nature changed completely or whether the barrier of energy separating the two states increased is unanswered at this point. The nonionic surfactant however did allow to study very thin films. The group in Bordeaux noticed that the films obtained with the nonionic surfactant (C12E5) were very sensitive to the humidity rate in the experimental cell used to shield them. This group therefore proceeded to do controlled humidity measurements by using saturated salt solutions at the bottom of the cell to obtain different humidity rates. For a saturated atmosphere films of about 200 Å for the water core was obtained. This thickness decreased to 140 Å for 85% relative humidity and then decreased to about 3 Å for a relative humidity of 75%. This effect is shown in figure 3 where the reduction in the thickness of the water core is seen through the reduction of the water signal. Clearly the relative humidity in the cell plays a major role in setting the water core thickness of soap films. Similar observations were also made on the ionic surfactant system but the change was more gradual.

Some features of the water band from the Raman spectra suggested that the water in the core of these films had a different structure than bulk water as will be discussed below. Since a ionic surfactants and an electrolyte were used to change the thickness of the soap films, part of the observed spectral changes could be due to the higher ionic strength of the aqueous solution in the core of the film. Since then, the group in Bordeaux extended its study to nonionic surfactants. Here, for the nonionic surfactant, no ionic effects due to the presence of electrolyte are present. The spectral changes observed can solely be attributed to the confinement of the water molecules. In order to observe this effect more clearly, experiments using Infrared spectroscopy were carried out. Infrared spectroscopy is much more sensitive to water than Raman spectroscopy and brings new complementary information about the structure and organization of these films. The spectra can be obtained in a couple of minutes instead of two hours, which was the case for the Raman spectroscopy. The changes observed using Raman spectroscopy are shown in figure 4a,b. The spectral changes are present for both ionic and nonionic surfactants. The first type shows a shift of the peak of the water band while the second type shows a reduction of the low wave number part of the spectrum. Infrared spectroscopy confirmed these changes in the spectrum of the water as the thickness of the film reached 20 or 30 Å. This can be seen in figure 5a for the ionic surfactant (SDS) where spectra of films of different thickness are compared to each other. The film with a thickness of 830 Å presents a water band, which is very similar to that of bulk water and similar to results of simulations of the water band (using the properties of bulk water). Clearly films of less than 30Å or so in water core thickness show a water band which is different than their thicker counterparts and different from the simulations assuming the water to be similar to bulk water. The low wavenumber part is clearly suppressed. The effect is somewhat different for ionic surfactants and nonionic surfactant (as seen in figure 5a,b). The ionic surfactant (5a) makes for a shift in the position of the maximum of the water band to higher wave-numbers with an increase of the intensity at high wavenumbers and a corresponding decrease at the low wave number part. The effect seen with the nonionic surfactant, which shows no shift in the peak position but a decrease of the low wave number part , figure 5a) is usually attributed to confinement effects that were previously observed for water confined in reverse micelles for example. The spectrum of water consists of many components, which reflect the different kinds of organization of the water molecules. One expects that confinement can change the organization of these molecules and therefore change the spectrum. The second effect seen for the ionic surfactant (a shift of the maximum to higher wave numbers) is attributed to the higher ionic strength of the aqueous core. The group in Bordeaux has evidenced both effects by studying very thin films.

In addition this group also studied the draining of vertical soap films. While for rigid films the draining can be understood using simple arguments, the draining of mobile films appears more delicate. Using Infrared spectroscopy allows to follow this draining and the reduction in thickness of the mobile films as a function of elapsed time. For all the studied surfactants, a simple draining law was obtained. The origin of such an exponential drainage law is still not clear at the present time but may reflect the fact that these films drain mainly by the process of marginal regeneration postulated by C. Mysels a long time ago. The theory predicts the rate of descent of a constant thickness patch but not how the thickness of the film at one point evolves versus time so the comparison is somewhat difficult to make quantitatively as one needs some additional assumptions.

While doing these draining experiments, a stratification phenomenon was observed for the C12E5 surfactant. By examining the CH stretching part of the spectrum while the film drains, an abrupt change occurred at the end of the draining process. The CH stretching bands intensity suddenly dropped by a factor of two. Basically, the film enclosed a bilayer of surfactant at the beginning of the draining; at the later stages this bilayer gets ejected from the film and the CH band intensity decreased by a factor 2. This part of the spectrum is shown in figure 6. Note that the intensity drops abruptly at a specific time. By subtracting the final CH stretching band intensity from the initial spectrum, one recover the CH stretching part of the bilayer directly. As can be seen in this figure, the bilayer shows higher band intensity with a smaller width. The chains within the bilayer seem to be better ordered than the aliphatic part of the soap film (which may be considered as an inverted bilayer). The study of the draining of these films using infrared spectroscopy which is relatively fast allowed us to observe the stratification in the film and extract the CH stretching spectrum of the structure (bilayer) inside the film. [7]. [Collaboration Bordeaux/RP, involving two TMR young researchers: C. Berger and I. Harrison]

Part of this work namely the work on C14Tab and C16Tab was motivated by discussions with the RP group of Vance Bergeron and Ian Harrison.

In close collaboration with V.Bergeron (Rhodia) a foam film drainage apparatus suitable for using a neutron beam to study a single foam film was constructed in the Oxford group. The objective was to investigate by more direct means the effects of stratification in the drainage of the film that had been observed in the Rhodia group. The stratification was assumed to be caused by lamellar structure of some kind within the film. Neutron reflection could in principle observe this structure directly.

Although Bergeron had already made several such pieces of apparatus and used them extensively to study film drainage, the neutron experiment required a much larger area of film and this posed a number of severe technical problems, only some of which were overcome. The apparatus worked extremely well in the laboratory but, in the harsher environment of the neutron beam (ambient vibration, etc.) was less easy to control. Nevertheless, in the special case of the surfactant AOT the Oxford group were able to follow the drainage and to identify internal structure of the kind expected from the earlier Rhidia results. This work is being written up for publication [8] . [subtask 1a/2b/2d. Collaboration Rhodia/Oxford, involving one TMR young researcher: T. Ederth]

As well as studying the drainage of a single foam film, the group in Oxford though it would be interesting to explore the internal structure of the foam film in a real foam. This group created foams of AOT in 1 m high 10 cm diameter quartz tubes and studied the internal structure at various stages of drainage using small angle neutron scattering. Again, this group was able to identify stratification within the foam film. The results are being written up for publication [9] .[ subtask 2b/2d, involving one TMR young researcher: T. Ederth]

Stratification in foam films has been observed in mixtures of polyelectrolytes and oppositely charged surfactants (also at Rhodia). It was thought that if stratification occurs in a foam film, which has two air/solution interfaces, it might also occur at the single air/water surface of a solution. The group in Oxford has observed that this has indeed turned out to be the case. In the system sodium polystyrene sulphonate/dodecyl trimethylammonium bromide a trilayer of surfactant and polymer of a very well defined thickness forms at the surface under a number of circumstances. However, although the same system has shown stratification in its foam drainage, the drainage experiments seem to be associated with quite different structural features and it has not been possible yet to reconcile the two observations.[subtask 2b and 2d.]

The group at the ENS have studied the superspreading which is the property of some trisiloxy ethoxylates surfactants to increase the spreading velocity of a drop of water on an hydrophobe support. The main conclusion is that the acceleration of the spreading results from a large Marangoni effect. [collaboration RP/ENS involving a TMR young researcher: D. Sarker] [10].

Following a hypothesis of Bonn (ENS) and Bergeron (RP) that superwetting by certain surfactant solutions could be associated with structure formation near the interface, the group in Oxford and the group in Hull investigated three systems, AOT, didodecyl dimethylammonium bromide (DDAB), and some trisiloxy ethoxylates. The first two do show quite complex long range structure at the interface for almost any kind of surface, whereas these groups were unable to observe any such structuring for the siloxy surfactants. However, the structure exhibited by DDAB has proved so interesting that it has become a major project in itself. Dilute solutions of DDAB are known to consist of unilamellar vesicles. However, what is surprising is that these vesicles adsorb at the surface (all surfaces so far investigated) intact and actually form a three or four layer structure, in which the vesicles are monodisperse. No such surface structure has previously been observed.[11] [collaboration Oxford/Hull]

  WETTING

 

 

The original theory of wetting transitions by Cahn considers only short-range forces and predicts a first-order transition between a partial wetting state (characterised by a microscopically thin film, the presence of discrete droplets, and nonzero contact angle) and a complete wetting state (macroscopically thick film, zero contact angle). Subsequently developed theories predicted the possibility of (short-range) critical wetting and those taking long-range forces into account as well, also of long-range critical wetting.

Just before the network was initiated, the first experimental observation of (long-range) critical wetting had been made by the ENS group in collaboration with the K.U. Leuven group in the industrially relevant (model) system of pentane on water. At the very begining of the network, the experiments at the ENS show that this second order transition was preceeded by a first order one (figure 7) [12] [Alkane/water mixtures are of importance for the petroleum industry and their wetting behaviour is important for secondary oil recovery [13]. [Subtasks 3a, 3b. Collaboration ENS/K.U. Leuven].

These systems also provide an interesting test case for the theory of the line tension [14-16] [Subtasks 3a, 3b, involving a young TMR researcher: H. Dobbs].

The first step towards a theoretical description of the observed phenomenon of long-range critical wetting was the introduction of long-range forces (in a perturbative way) into a Cahn-type theory [17] [Subtasks 3a, 3b. Collaboration K.U. Leuven/ENS]. It was found that, if the temperature at which the Hamaker constant changes sign (marking the critical wetting transition) is above the (first-order) wetting temperature predicted on the basis of short-range forces only (Cahn theory), then there is a sequence of changes in the wetting state from partial wetting to frustrated complete wetting (mesoscopically thick film, but still nonzero contact angle of droplets that sit on top of this film) and, finally, to a complete wetting state [18-21], [Subtasks 3a, 3b. Collaboration K.U. Leuven/ENS involving a young TMR researcher: H. Dobbs]. The term `sequential wetting' was coined for this scenario.

In the meantime, a critical examination of experimental and model-based determinations of adsorption enthalpies of n-alkanes adsorbed at the vapour/water interface had been performed [22]. [Subtasks 3b involving a young TMR researcher implied: H. Dobbs].

While it was found in the course of the above work that Cahn theory, augmented by long-range forces in conjunction with Dzyaloshinskii--Lifshitz--Pitaevskii theory, was sufficient to compute the critical wetting temperature, this standard, continuum Cahn theory had to be modified in a way as to treat the first layer of adsorbed alkane molecules in a discrete fashion in order to describe states of low adsorption properly [23]. [Subtasks 3b. A young TMR researcher involved: H. Dobbs].

Whereas experimentally, the two transition temperatures in sequential wetting were found to be shifted in parallel when the salt concentration in the hexane/brine system or, adopting the equivalent carbon number concept, the chain length of the alkane was varied[19,20] [Subtasks 3a, 3b. Collaboration K.U. Leuven/ENS involving a young TMR researcher: H. Dobbs].the case of a particular model system was considered in which the amplitudes of the two leading terms of the long-range forces can be tuned in such a way as to produce a critical endpoint, at which the two transition temperatures coincide. Among other things, an asymptotic relation between the contact angle and the film thickness in the frustrated complete wetting state at the first-order transition near the critical endpoint was deduced in this system [24] . An experimental check of this prediction is underway on a real system which displays such a critical endpoint needs in the ENS group.

As regards the lifetime of metastable states and the hysteresis properties of these systems, the dynamics of wetting layer formation was studied experimentally and theoretically. In addition, the diffusivional growth of super-critical wetting droplets has been investigated at the wetting transition, and in the partial- and pre-wetting regimes. In all cases an asymptotic scaling behavior has been found with regime-dependent growth exponents. A question for future investigations is the influence of hydrodynamic flow in such growth processes which e. g. dominate in spreading phenomena of wetting profiles. For an ordered array of critical volatile wetting droplets the formation of a superlattice has been predicted. The underlying mechanism for this is an Ostwald-ripening-like competition process between the droplets. [25], [Subtasks 3a, 3b. Collaboration K.U. Leuven/ENS/U. Dusseldorf, involving a young TMR researcher: R. Blossey].

Based on the experience gained with extended versions of Cahn's theory, simplified approaches to estimating the wetting behaviour of alkanes on water, from the non-equilibrium initial spreading coefficient, for example, were developed and applied successfully [26], [Subtasks 3b. Collaboration K.U. Leuven/ENS involving a young TMR researcher : H. Dobbs].

A second major achievement, by the group at ENS, was the experimental observation and theoretical description of short-range critical wetting in van der Waals fluids, for the very first time [27,28] [Subtasks 3a]. In binary mixtures of methanol and n-alkanes, the nature of the wetting transition was shown to change from first-order to critical - via tricritical wetting - the closer the wetting temperature of the mixture was to its consolute temperature at which the region of two-phase liquid--liquid coexistence ends and methanol and alkane become completely miscible). For nonane (and methanol, of course), the transition was continuous, i.e. critical while for undecane, it was clearly first-order [27, 29] [Subtasks 3a]. In the framework of a theory based on a spin-1 Ising model and mean-field approximation, the change of the characteristic exponent, which is indicative of the order of the wetting transition, was successfully calculated -- in good agreement with the experiment [30] While it had been conjectured that the presence of long-range forces that favour wetting would necessarily lead to first-order wetting -- even if short-range forces favoured critical wetting -- recent work within the above-mentioned model, extended for long-range forces, showed that the slight perturbation introduced by weak long-range forces will still allow the transition to look like a critical one due to the fact that the wetting layer is slightly off of two-phase coexistence [30, 31]. [Subtasks 3a, 3b. Collaboration K.U. Leuven/ENS]. This phenomenon is caused by gravity since the wetting phase at the alkane/vapor interface is methanol (or, to be precise, the methanol-rich liquid phase), which is the phase of higher mass density; therefore, the droplet of methanol at the alkane/vapor interface has a slightly higher chemical potential than the bulk methanol.

The Lisboa group has studied the wetting properties of a fluid adsorbed on a support by means of the Density Functional Theory, which is more rigorous than mean field theory for these problems. Explicit calculations of the substrate-liquid and substrate-gas density profiles where carried out and used to evaluate the asymptotic expansion for interfacial potential of a system with long-range interactions. The range of validity of the asymptotic expansion is checked by comparing it with the interface potential obtained numerically through the constrained minimisation of the density functional free energy.

Depending on the parameters of the fluid-fluid and substrate-fluid interactions, the Lisboa group found a first order or a critical wetting transition. In a limited range of parameters, the critical wetting transition is preceded by a first prder transition between a microscopic and a mesoscopic film, thus corroboring the previous calculations and experimentsfor alkane on brine described just above. This group find that the behaviour of the alkanes on brine is not universal, since it requires fine-tuning of the fluid-fluid and fluid-substrate interactions.

Finally this group has investigated the influence of the sort- and long-range force on the location of the first-order transition. They found that for the models studied, the long-range forces cannot be treated pertubatively. Thus, for this type of model, it is not possible to separate the effects of short- and long-range forces as done in Landau theories, where the long-range forces are treated pertubatively [32] [Lisboa, subtask 3b involving a TMR young researcher: A. Gonzalez].

The wetting properties of a model colloid-polymer mixture adsorbed at a solid substrate was studied by means of the same theory. Explicit calculations of the substrate-liquid (colloid-rich) and substrate-gas (colloid-poor) density profile were carried out. The effective interface potential of the system was calculated for a range of parameters by constrained minimisation of the DFT free energy. Currently experiments on such a system are being performed at ENS.

Depending on the parameters of the substrate-colloid and substrate-polymer interactions, the Lisboa group find first order wetting or drying transitions. The wetting transition is always preceded by a first order layering transition between a microscopic and a mesoscopic film, thus corroborating previous calculations. The dryng transition has a simpler standard structure. [33] [Lisboa, subtask 3b involving a TMR young researcher: A. Gonzalez]

The observation of both short-range and long-range critical wetting, as well as the discovery of sequential wetting and the realisation of tricritical wetting, along with the theoretical understanding and description of these phenomena, represent milestones in the research on wetting phase diagrams.

The wetting behaviour of octane on aqueous solutions of glucose was described in the literature as very complex. This behaviour was examined and elucidated in a collaboration between the ENS group and the group in Hull using measurements of spreading coefficients (Hull) and oil film thickness (Paris). The conclusion is very simple: in contrast to a previous study, no wetting transition was found for this system. [34] [Subtasks 3a. Collaboration ENS/U. Hull. One young TMR researcher implied: D. Segal]

In a collaboration, the ENS group and the Hull group provided a quantitative experimental determination of the fluctuation repulsion that stabilises wetting film in systems for which the interfacial tension of one of the two interfaces bounding the wetting film is low. This is achieved by studying the wetting of different n-alkanes on water, in the presence of a surfactant (AOT). The interfacial tensions as a function of temperature and salinity for the oil-water surfactant systems where measured in Hull [subtask 4a] and the film thickness was determined by ellipsometry at the ENS [subtask 4b]. The results agree well with the theoretical calculation of the fluctuation repulsion of a membrane under vanishing surface tension. It was shown that these experiments gives a new method to determine the bending rigidity of a surfactant monolayer. [Subtask 4e. Collaboration ENS/Hull involvingne TMR young researcher: D. Segal] [35]

The effect of the fluctuations was also examined in a system without surfactant film: diphasic mixtures of cyclohexane and methanol in which the methanol phase wet the cyclohexane phase above the wetting temperature. In this system the low interfacial tension is due to the proximity of a consolute critical point. However, the origin of the fluctuation repulsion force is similar in this system and in a system with surfactant forces : it is the entropy. The wetting film thickness was measured above the wetting transition as a function of the temperature and for two of theses systems, the first one was made with deuterated methanol and the second one with hydrogenated methanol. Due to the density difference between the hydrogenated and the deutered methanol, the film thickness are different and consequently the fluctuation repulsion forces are larger in the deuterated film which is the thinnest. The gravitational energy and the Hamaker constant being proportional to the density difference between the two phases of the mixture, in the absence of fluctuations one expects a wetting film thickness independent of the temperature. In practice, the film thickness varies as the difference between the critical temperature and the temperature of the system to the power -0.2±0.1 in agreement with the renormalisation group which takes into account the fluctuations (exponent -0.1) [subtasks 4b, 4e. Collaboration ENS/Lisboa/WINS involving a TMR young researcher: D. Finistein] [36].

The groups in Oxford and Hull have investigated a number of aspects relating to the spreading of oil films on water in the presence of surfactant. In particular they were looking for conditions for partial wetting of the surface with the objective of determining the distribution of surfactant in the partial wetting layer. This would have followed up in more structural detail earlier work done by Meunier and Binks. They examined the structure of partially wetting oil layers on solutions of AOT and explored the issues of the applicability of the Gibbs isotherm to tension data using neutron reflection as a means of studying composition at the mixed surface. Finally, we synthesized surfactants terminated at the hydrophobic end by aromatic rings, since there were predictions that this would favour partial wetting. Unfortunately, there were difficulties in obtaining sufficiently pure products for the latter work to progress satisfactorily. [Subtask 4d. Collaboration Oxford/Hull]. [37]

The dynamics of the spreading of a drop which impacts on an hydrophobic surface was studied in a collaboration between ENS and RHODIA (RP). It was shown that the suppression of the rebound of the drop at the impact which is observed when a small amount of a polymer is added to the water (patent RHODIA) is due to the large increase of the elongationnal viscosity. [38] [collaboration RHODIA (RP)/ENS]. This work was commented in the News and views section of Nature and commented in more than 20 popular journals and radio or television emissions. (figure 8)

The Lisboa group has studied a Langevin equation describing non-equilibrium depinning and wetting transitions. Attention was focused on short-range attractive substrate-interface potentials. The group has confirmed the existence of first order depinning transitions, in temperature-chemical potential diagram, and a multicritican point beyond which the transition becomes a non-equilibrium complete wetting transition. The coexistence of pinned and depinned interfaces occurs over a finite area, in line withother non-equilibrium systems that exhibit first order transitions. In addition, the group found two types of phase coexistence, one of which is characterised by spatio-temporal intermittency (STI). A finite size analysis of the depinning time was used to characterised the different coexisting regimes. Finally, astationary distribution of characteristic triangles or facets was shown to be responsible for the structure for the structure of the STI phase. [subtasks 4e. Collaboration ENS/Lisboa/WINS, involving a TMR young researcher: F. de los Santos] [39]

The same group has analysed within mean-field theory as well as numerically a KPZ equation that describes non-equilibrium wetting. Both complete and critical wetting transitions were found and characterised in detail. For one-dimensional substrates the critical wetting temperature is depressed by fluctuations. In addition, this group has characterised a region in the space of parameters (temperature and chemical potential) were the wet and non-wet phases coexist. Finite-size analysis of the interfacial detaching times indicates that the finite coexistence region survives in the thermodynamic limit. Within this region the Lisboa group has observed (stable or very long-lived) structures related to spacio-temporal intermittency in other systems. In the interfacial representation these structures exhibit perfect triangular patterns that have been characterised by their slope and size distribution. [subtasks 4e. Collaboration ENS/Lisboa/WINS, involving a TMR young researcher: F. de los Santos] [40]

*1 V. Casteletto, I. Cantat, D. Sarker, R. Bausch, D. Bonn, J. Meunier, "stability of soap films: hysteresis and nucleation of a black film. to appear

2 A paper is currently in press: R. Aveyard, B.P. Binks, J. Equena, P.D.I. Fletcher, P. Bault and P. Villa, Langmuir, 2002, in press.

3 B.P. Binks, P.D.I. Fletcher, V.N. Paunov and D. Segal, Langmuir, 2000, 16, 8926-31.

4 B. Lecourt, A. Malaplate, D. Blaudez, H. Kellay, J. M. Turlet,, "Raman Spectroscopy of Black Soap Films", J. Chem. Phys. 108, (3), 1998, p:1284-1289.

5 F. Capelle, F. Lhert, D. Blaudez, H. Kellay, and J. M. Turlet, "Thickness and organization of black films using confocal micro-Raman spectroscopy"

Colloids and Surfaces A. 171, (2000), 199-205.

6 Charles Berger, D. Blaudez, H. Kellay, B. Desbat, and J. M. Turlet "Observation of a single bilayer in a soap film", Submitted to Langmuir

*7 Charles Berger, Florent Capelle, Ian Harrison, D. Blaudez, H. Kellay, B. Desbat, and J.M Turlet "A systematic study of black soap films using ellipsometry, Raman and Infrared spectroscopy", submitted to J. Chem. Physics

*8 Ederth, T, Follows, D, Thomas, RK, Bergeron, V, Penfold, J, "Direct observation of layering in a draining single foam film".submitted

9 Ederth, T, Thomas, RK, Heenan, R, Direct observation of layering in a the film in a real foam" submitted

*10 Li, ZX, Lu, JR, Fragneto, G, Thomas, RK, Penfold, J, Binks, BP, Fletcher, PDI, "Neutron reflectivity studies of AOT monolayers adsorbed at the oil/water, air/water and hydrophobic solid/water interfaces", Coll. Surf. A 135 (1998) 277

*11 S. Rafaï, D. Sarker, V. Bergeron, J. Meunier, D. Bonn, "Superspreading on hydrophobic surfaces", submitted to Langmuir.

12 N. Shahidzadeh, D. Bonn, K. Ragil, D. Broseta, J. Meunier, Phys Rev. Lett 80 (1998) 3992

*13 K. Ragil, D.Bonn, D. Broseta, J. Indekeu, F. Kalaydjian, J. Meunier, ``The wetting behavior of alkanes on water'', Journal of Petroleum Science and Engineering 20 (1998) 177-183

14 H. Dobbs, ``Line tension of n-alkanes on water from a Cahn-type theory'', Langmuir 15 (1999) 2586-2591

15 H. Dobbs, ``The elasticity of a contact line'', Physica A 271 (1999) 36-47

16 H. Dobbs, ``The modified Young's equation for the contact angle of a small sessile drop from an interface displacement model'', International Journal of Modern Physics 13 (1999) 3255-3259

*17 J.O. Indekeu, K. Ragil, D. Bonn, D. Broseta, and J. Meunier, ``Wetting of alkanes on water from a Cahn-type theory: effects of long-range forces'', Journal of Statistical Physics 95 (1999) 1009-1043

*18 J.O. Indekeu, K. Ragil, D. Broseta, D. Bonn, and J. Meunier, ``Wetting phase transition and petroleum reservoirs'', in: C. Caccamo et al. (eds.), New Approaches to Problems in Liquid State Theory, NATO ASI Ser. C, Vol. 529, Kluwer, Dordrecht, 1999, p. 337--344

*19 E. Bertrand, H. Dobbs, D. Broseta, J. Indekeu, D. Bonn, and J. Meunier, ``First-order and critical wetting of alkanes on water'', Physical Review Letters 85 (2000) 1282-1285

20 J.O. Indekeu, ``Must thin--thick transitions precede long-range critical wetting?'', Physical Review Letters 85 (2000) 4188-4188;

E. Bertrand, D. Bonn and J. Meunier, ``Bertrand, Bonn and Meunier Reply",

Physical Review Letters 85 (2000) 4189-4189

*21 D. Bonn, E. Bertrand, N. Shahidzadeh, K. Ragil, H.T. Dobbs, A.I. Posazhennikova, D. Broseta, J. Meunier, and J.O. Indekeu, ``Complex wetting phenomena in liquid mixtures: frustrated-complete wetting and

intermolecular forces'', Journal of Physics: Condensed Matter 13 (2001) 4903-4914

22 H. Dobbs, ``Enthalpies of adsorption of n-alkanes adsorbed at the vapor/water interface'', Langmuir 16 (2000) 4749-4751

23 H. Dobbs, ``Predicting wetting behavior from surface adsorption data using Cahn theory'', Journal of Chemical Physics 114 (2001) 468-473

24 V.C. Weiss and J.O. Indekeu, ``Contact angle at the first-order transition in sequential wetting'', Physica A 301 (2001) 37-51

*25 D. Bonn, E. Bertrand, J. Meunier, and R. Blossey, ``Dynamics of wetting layer formation'', Physical Review Letters 84 (2000) 4661-4664

*26 H. Dobbs and D. Bonn, ``Predicting wetting behavior from initial spreading coefficients'', Langmuir 17 (2001) 4674-4676

27 D. Ross, D. Bonn, and J. Meunier, ``Observation of short-range critical wetting'', Nature (London) 400 (1999) 737-739

28 D. Ross, D. Bonn, J. Meunier, "Wetting of methanol on the n-alkanes: observation of short-range critical wetting", J. Chem. Phys. 114 (2001) 2784

*29 D. Ross, D. Bonn, A.I. Posazhennikova, J.O. Indekeu, and J. Meunier, ``Crossover from first-order to critical wetting: short-range tricritical weeting'', Physical Review Letters 87 (2001) 176103

*30 A.I. Posazhennikova, J.O. Indekeu, D. Ross, D. Bonn, and J. Meunier, ``Crossover between first-order and critical wetting at the liquid--vapour interface of n-alkane/methanol mixtures: tricritical wetting and critical prewetting'', Journal of Statistical Physics 95 (1999) 1009

*31 J.O. Indekeu, A.I. Posazhennikova, D. Ross, D. Bonn, and J. Meunier, ``Crossover from first-order to critical wetting in methanol on alkanes'', Journal of Physics: Condensed Matter 14 (2002) 4777

32 A. Gonzales, M.M. Telo da Gama, "Density functional theory of long-range critical wetting", Phys. Rev. E 62 (2000) 6571

33 A. Gonzales, M.M. Telo da Gama, "Wetting and structure of colloidal dispersions at solid substrates", Discussion meeting of the Royal Society on interactions, structure and phase behaviour of colloidal dispersions, October 2000, Londres, U.K.

*34 E. Bertrand, D. Bonn, J. Meunier and D. Segal, Phys. Rev. Lett., 86 (2001) 3208.

*35 E. Bertrand, D. Bonn, H. Kellay, B.P. Binks and J. Meunier, Europhys. Lett. 55 (2001) 827.

*36 D. Fenistein, D. Bonn, S. Rafaï, G.H. Wegdam, J. Meunier, O. Parry, M. Telo da Gama, "what controls the thickness of wetting layers near bulk criticality?", Phys. Rev. Lett. 89 (2002) 96101

*37 Binks, BP, Crichton, D, Fletcher, PDI, MacNab, D, Li, ZX, Thomas, RK, Penfold, J, "Adsorption of oil into surfactant monolayers and structure of mixed surfactant-oil films", Coll. Surf. A, 146, 299-313 (1999)

*38 V. Bergeron, D. Bonn, J.-Y. Martin, L. Vovelle "Contolling droplet deposition with polymer additives" Nature, 405, 772 (2000)

This letter was commented in the News and Views section of Nature and commented in more than 18 popular journals and radio or TV emissions

39 F. de los Santos, M.M. Telo da Gama, M.A. Munoz, "Stochastic theory of non-equilibrium wetting", Europhysics Lett. 57 (2002) 803

40 F. de los Santos, M.M. Telo da Gama, M.A. Munoz, "Nonequilibrium wetting transition with sort range forces", submitted to Phys. Rev. E

In addition to the presence of an industrial team in our network, collaborations with the industry have been established. For instance:

- Members of the Institut Français du Pétrôle participated to task 3 (wetting)

- The group in Hull investigated foam film formation and stability and the mechanisms of anti-foam action in collaboration with Unilever, Vlaardingen, allowing this group to secure their research on the subject

- The ENS group collaborated with Pierre Fabre recherche on emulsions

- One member of the staff in RHODIA (V. Bergeron) joined the ENS group.

- The two TMR postdocs in RHODIA (RP) are engaged by RHODIA:

 

Ian Harrison was hired by RP immediately following in his TMR network

 

Katerina Karagianni had the opportunity to be recruited and finally hired by Rhodia (CRA,Paris) as a research engineer (March 2002)

These appointments are permanent positions and were due to the training they received during the work as post-docs within the contract.

- Harvey T. DOBBS, TMR postdoc in K.U. Leuven then in Paris found an emploment in UK: Patent Officer in Wales, UK.

- Jonathan P. WALLINGTON TMR postdoc in K.U. Leuven found an employment at the end of his contract in Motorola Company, Swindon, UK

- Volker C. WEISS, TMR postdoc in K.U. Leuven is postdoc with Flemish Fund for Scientific Research

- Antonio Gonzalez postdoc at U. Lisboa has found a position back at the University of Salamanca

- Francisco de los Santos is a post-doc of H. E. Stanley at Boston University

- Mauro Sellito had a M. Curie scholarship at the ENS at Lyon when he left

Lisbon.

- Florence Biguenet, TMR postdoc at U. Dusseldorf has obtained a permanent position at the Universite de Haute Alsace, Mulhouse-Colmar

- Isabelle Cantat, TMR postdoc at U. Dusseldorf has obtained a permanent position at the Universite de Rennes