Langmuir 1993,9, 3255-3258
3266
Multilayered "Pancakes"of Nonvolatile Liquids Close to a Wetting Transition M. P. Valignat, N. Fraysse, and A. M. Cazabat* Physique de la Matibre Condemke, Collbge de France, 11 place Marcelin Berthelot, 75231 Paris Cedex 05,France Received May 28,1993.I n Final Form: August 5,199P We report experimentalstudies of systemswhere a wetting transition occurs as the result of a competition between a long-range attractive interaction promoting spreading and a short-range repulsive interaction opposingit. The wetting behavior is monitored from the shape of tiny nonvolatile microdropleta spreading on smooth surfaces. A step grows at the bottom of the droplet, the thickness of which is a monolayer far from transition, then increases when transition is approached, and ultimately diverges when the liquid becomes nonwetting. At long times, the droplet takes the "pancake*shape first predicted by de Gennes.1 Wetting transitions have been widely studied, both theoretically and experimentally, in the cases where a liquid mixture or a gas phase is put in contact with a solid substrate. The situation where a nonvolatile liquid is in contact with the solid is less usual, because it is not easy to approach the transition in a continuous way. However, interesting features are expected in this case, i.e., thick wetting films controlled by universal laws,' which deserve experimental investigations. Attempts have been made to reach the transition by varying the surface energy of the substrate.2 Here, we rather play with the liquid properties. Note that the transition is not approached via a continuous change in some intensive property like temperature. We consider van der Waals liquids, with purely dispersive interactions with the solid. The bare surfaces are completely wet by the liquid, which means that the longrange part of the disjoining pressure II(zI3 is positive. In the present case, hydrophobic grafted layers are responsible for the repulsive short-range interaction. The wetting behavior on a grafted surface depends on the sign of the spreading tension S, which is the integral of II(z):
In complete wetting, but close to the wetting threshold, i.e., for small, positive values of S, the repulsive shortrange interaction leads to a transition between a molecularly thin film and a thicker film of thickness e, supposed to belong to the long-range part. For nonvolatile liquids, where three-and twdimensional evaporation is negligible, the molecularly thin film is a two-dimensional very dilute gas, the thickness of which is negligible, even a t the molecular scale. The disjoining pressure has the shape drawn in Figure 1,and eq 1 can be rewritten in terms of the long-range part IIdz) of II(z):
For a van der Waals liquid on a homogeneous solid surface, )&I = A/67rz3,where A is the Hamaker constant of the system. The thickness e becomes e = (A/4aS)'f2 (3) The final state of the drop is a 'pancake" of thickness e, Abstract published in Advance ACS Abstracts, October 1,1993. (1) de Gennee, P. G. Reo. Mod. Phys. 1985,57,827. (2) m e r , L.; Silberzan, P. Liquids 1991,3, 421. ( 3 ) Derjaguin, B. J. Colloids USSR 1965,17,191. Churaev, N. V. Rev. Phys. Appl. 1988,23,975. 0
0143-1463/93/2409-3255$04.00/0
"U Schematic shape of the disjoiningpressure for longe
Figure 1.
a
range attractive interaction,obeying II&) = A/6rz9,and a shortrange repulsion. The resulting n(z)curve has an unstable part, and a transition takes place along the horizontal dotted lime, the position of which is determined by a Maxwell construction.The thickness e is calculated by using the general relation 1. If ho is very small, this relation becomes S = eII(e) + J;II(z)dz. Moreover, if e is in the long-range part, S is given by eq 2, from which eq 3 results.
as first predicted by de Gennes. For A = J and S = 10-4 N m-', e = 30 A (although not a SI unit, A is very common for molecular thicknesses and will be used throughout the text). Thickness profiles of microdropleta spreading on smooth surfaces have been extensively studied by various techniques.P6 Spatially resolved ellipsometry is required in the present casew because of ita good lateral resolution. The liquids are light but nonvolatile poly(dimethy1siloxanes), hereafter abbreviated as PDMS, the same as in previous studies,- with a polydispersity index between 1.06 and 1.29. Molecular weights Mp and corresponding surface tensions y are in the ranges (2-52) X lo3g mol-' N m-l, respectively, and can be and (20.6-21.4) X found in Table I. The molecular weights and polydispersity indices are deduced from GPC measurements (courtesy of D. Teyssib). Four-digit values for the surface tensions have been given; they are calculated using interpolation of smoothed y(M,) curves, in order to obtain (4) LBger, L.; Erman, M.; Guinet, A. M.; AussBrB, D.; Strazielle, C.; Benattar,J. J.; Rieutord, F.;Daillant,J.; Boeio, L. Rev. Phys. Appl. 1988, 23, 1047. (5) Beaglehole, D. J. Phys. Chem. 1989, 93, 893. (6) Heslot, F.; Cazabat, A. M.; Levinson, P. Phys. Rev. Lett. 1989,62, 1286. (7) Heslot, F.; Cazabat, A. M.; Levinson, P.; Fraysse, N. Phys. Rev. Lett. 1990, 65, 659. (8) Healot, F.; Fraysse, N.; Cazabat, A. M. Nature 1989,338,640.
0 1993 American Chemical Society
3256 Langmuir, Vol. 9, No. 11,1993
Valignat et al. Table I
Mp (kg mol-’)
y (109 N m-’1
2 3.8 2 3.8 5.8 13 28.4 52 5.8 6.7 7.3 9.7 13 13 16
20.6 20.8 20.6 20.8 20.90 21.07 21.24 21.37 20.90 20.93 20.96 21.01 21.07 21.07 21.10
e (A) 23 m
30 m
16.8 33 38 72 21 42 m
30 m
38 81
S (109 N m-l) 0.06
yc (109 N m-l)
substrate nature C16l C16l C162 C162 trimethyla trimethyla trimethyls trimethyla
20.66
21.37 X lP3N m-l, with our smoothed values. This is not the case for the yeobtained with the two light oils (molecular weights 5.8 X lo3 and 13 X lo3 g mol-'), which are far from transition. Relation 3 is not well obeyed for such small film thicknesses (e = 16.8 and 33 A) which do not really belong to the long-range part, and are modified by layering. For the two heavier oils, yc is farily constant, and takes a plausible value. In cases i and iii, the procedure is more intricate, because five layers (vacuum, PDMS, grafted layer, silica, silcon) are present. Only estimates can be given, and we did not try more. We assume the grafted layer to behave like PDMS, which is a good approximation in case iii, but less in case i, and we treat silica and silicon as a single medium M. We now have a four-layersystem: 1,vacuum; 2, PDMS (thickness z ) ; 3, grafted layer (thickness d); 4, medium M. If the grafted layer resembles PDMS, A123 = 0. The Hamaker constant A l 2 ~is calculated for the system vacuum-PDMS-silica-silicon for a "PDMS" thickness of (13) Hirasaki,G.J. In Interfacial phenomena in petroleum recouery; Morrow, N. R., Ed.; New York, 1991 (preprint 1988); pp 23-99. (14) Valignat, M. P.;Fraysse,N.; Cazabat,A. M.;Heslot, F.; Levinson, P. Thin Solid Films, to be published.
3258 Langmuir, Vol. 9,No. 11, 1993
+
d and a silica thickness of 20 A, i.e., A ~ Z M = A(e + d , 20 A) in eq 4. With these values, nl,(z),S, and yc are calculated. The results are reported in Table I. On the two C16 layers, the values obtained for the critical surface tension are the same, yc = 20.65 X N m-l, a rather typical value on these substrates, which is satisfactory. On the polymer layers, thick films were observed only very close to the transition. The lighter oils did not even show stepped-pyramidal profiles: the main feature was the diffusion of the liquid into the underlying polymer layer (Figure 3). This diffusion, monitored by the change in the drop volume with elapsed time, becomes very slow close to the transition. However, the largest values of S are probably underestimated, because the local composition of the layer just below the drop is changing. This might be the case for Mp= 5.8 X lo3on Mp = 8 X lo3,and Mp= 9.7 X lo3on Mp = 11.8 X lo3. The lowest values are satisfactory, the ycvalues being close to the surface tension of the polymer layer, as expected. e
Valignat et al.
h a conclusion, we have obtained experimental evidence of the possibly thick pancakes predicted by de Gennes' close to a wetting transition, where an attractive longrange interaction is almost exactly balanced by an antagonist short-range one. We approached the transition by taking advantage of the variation of the surface tension of light polymers with molecular weight, which is easier than adjusting the critical surface tension of the substrate. Despite the assumptions in the numerical analysis, fairly good quantitative agreement is obtained with available theories. Acknowledgment. Fruitful discussions with P. G. de Gennes are gratefully acknowledged. Experimentaldetails available in the thesis of J. B. Brzoska have been very helpful. We thank F. Tiberg for helping with the surface treatments. The ellipsometric setup was assembled by F. Heslot and P. Levinson.
