Langmuir 1993,9,601403
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Molecular Networks in the Spreading of Microdroplets M. P. Valignat, N. Fraysse, A. M. Cazabat,* and F. Heslott Collage de France, Physique de la matiare Conden.de, 11, place Marcelin Berthelot, 75231 Paris Cedex 05, France Received September 17,1992. In Find Form: November 9,1992 We report here an experimentalinvestigationof the spreadingof OH-terminatedpoly(dimethyleiloxane) droplets on bare and on modified silica. On bare silica,droplets take a remarkable shape,with a shoulder at a thickneee corresponding to the gyration radius of the molecules. The effect disappeare if the surface is made hydrophobic. We interpret these features 88 being due to the formation of a network of molecules anchored onto the solid. For weak anchoring sites, the spreading goes on through the network. If strong anchoring sites are available, the spreading of microscopic dropleta eventually stops. Silicone derivatives, such as poly(dimethylsiloxanes) (PDMS), are polymer molecules widely used as a base for adhesive compounds. AB a matter of fact, they spread spontaneouslyover most solid surfaces and are therefore able to achieve homogeneous coatings with thin liquid films. However, wetting properties are only a part of the requirements for adhesion, and modified PDMS, having enhanced interactions with the substrate, are needed. Examples are OH-terminated PDMS on hydroxylated silica surfaces. However, the exact role of the hydroxyl groups is not yet fully understood. We report here an experimental investigation of the spreading of these oils on oxidized silicon wafers, with bare or modified silica surface. The shape of spontaneously spreading microdroplets results from the balance between liquidaolid interactions and friction processes. For nonvolatile liquids, well below their two-dimensional critical temperature, one usually observessteplikethicknessprofiles,with distinct successive molecular layers.’ The spreading of such droplets is satisfactorily accounted for with a model of an incompressible, stratified liquid.2 However, a very different behavior is observed with modified PDMS oils, on which the trimethyl ending groups are replaced by hydroxyl groups. These oils are referred to as PDMS-OH. Several light oils with narrow molecular weight distribution have been investigated (molecular weights Mp = 8,11.8,15,29,35,45 kg/mol; polydispersity index ranging from 1.13 to 1.50; gel chromatography courtesy of D. Teyssi6). Three substrates were used (a) bare silicon wafers (11113 crystal face, 2 in. diameter, phosphorus doping, resistivity 4-6 Q-cm,prime quality, manufacturer Pensilco), covered with natural oxide (a 20-30 A thick silica layer), cleaned by UV irradiation under ozone, therefore with available silanol sites; (b) silicon wafers incubated with hexamethyldisilazane (“HMDZ-wafers”), i.e. with a layer of trimethyl groups, the thickness of which is around 4 A, gr&d on the silica, silanol groups are no longer available,but interactionswith the underlying silica are not tota€ly screened; (c) silicon wafers on which a Langmuir-Blodgett layer of behenic acid (35 A thick) has been deposited (‘LB-wafers”, courtesy of J. P.Bourgoin, M. Vandevyver, and A. Barraud). Here, oil does not feel the silica any more. Thickness profiles of microdroplets spreading on these Surfaces have been studied using high-resolution ellip-
* To whom all correspondence should be mailed. + Now
at the &ole Normale SupBrieure.
(1) Heslot, F.; Frayme, N.; Cazabat, A. M. Nature 1989,338,640. (2) de Gennes, P. G.; Cazabat, A. M., C. R.Acad. Sci., Paria 1990,310,
1601.
sometry. The experimental setup has been described el~ewhere.~ Thicknesses are measured by single-wavelength ellipsometry (A = 6328 A), with a focused measurement spot. Lateral resolution is 30 pm by 120 pm; the measurement stability over one day is 0.2 A rms and the time constant of the detection electronics 100 ms. The experiments are performed at room temperature and pressure, under a constant flow of clean nitrogen gas. Typical records can be found in Figures 1-3 for the three surfaces. While the profile on the LB wafer is similar to the ones of normal (methyl-terminated) PDMS oils (with a 7 A thick monolayer of molecules flat on the surface growing from a steep thicker edge3*4),the others are strikingly different. A shoulder develops, the height H of which scales as the square root of the molecular mass (Figure41, and is close to the gyration radius R, of the polymer molecule. This radius is approximatelyR (A)= 5.5 IW2, N being the number of monomers (Mp 7&). On HMDZ wafers, the spreading goes on ‘through” this structure which finally fades out when the maximum thickneea becomes smaller than R,. This is not so on bare wafers, where the spreading stops, although there are still molecules available aboveR,. The final shape is striking, with a flat part at the top, connected to the shoulder by a steep edge (“steep” with the scales relevant for ellipsometric profiles, i.e., thickness scale in A and spatial scale in mm). For the three surfaces at short times, where the central part of the drop acts as a reservoir, the drop’s radius R, measured at 3.5 A height (i.e., the radius of the first monolayer) increases like the square root of the time t. Then, a ‘diffusion-like” coefficient D1 can be defined: R = (Dlt)1/2,which is found to scale as the inverse of the bulk viscosity q (Figure 5). D1 is larger on the LB wafer than on the HMDZ wafer. The lowest value is on the bare wafer. Note that no assumption is made about the real diffusive nature of the process: As the measured radius scaleslike t1/2, it is convenientto characterizethe dynamics of growth by a parameter which has the dimensions of a diffusion coefficient. The behavior at long times is much more substratedependent. For LB and HMDZwafers, a slow 2Ddiffusivelike growth of the monolayer is the late p r o c e ~ .The ~ associated “diffusion” coefficient DO is approximately
-
(3) Heelot, F.; Cazabat, A. M.; Levineon, P.; Fraywe, N., Phye. Rev. Lett. IQW,65,559. (4) Heelot, F.; Cazabat, A. M.; Levinnon, P. Phye. Rev. Lett. 1989,62, 1286. (5) The relation &(A) 5.51Vl/* ie obtained from a molecular model of PDMS. In ref 6, a slightly different value, RJA) B 6 W Pb given. In fact, only the order of magnitude b relevant here.
0743-7463193/2409-0601$04.0010 Q 1993 American Chemical Society
Valignut et al.
602 Langmuir, Vol. 9, No. 2, 1993
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o I 2 mm Figure 1. Ellipaometric thickness profiles of a microdroplet of PDMS-OH on a wafer covered with a Langmuir-Blodgett layer of behenic acid. The baaeline at approximately 62A corresponds to the total thickness silica plus deposited layer. T w o molecular steps, 7 A thick, are clearly visible. First profie after 23 h (light line), second profile after 52 h (heavy line). Oil molecular weight: M = 8OOO. Polydispersity index 1.13. At longer times, all the morecules are in the f i t monolayer, which ultimately breaks up by two-dimensional diffusion. -3
140! 120
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Figure 4. Characteristic thickneee H versus oil molecular weight: logarithmic scales on both axes; crosses, HMDZ wafers; circles, bare wafers. A line with slope 0.5 has been drawn as a guide for the eyes.
A
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-2 -1 0 1 2 mm Figure 2. Same oil on a wafer incubated with HMDZ. First profile after 98.6 h (light line), second profie after 140 h (heavy line). The baseline at 22 A corresponds to the total thickness of silica plus grafted trimethyl groups. The molecular steps are still visible, but another characteristic thicknew H,around 45 A, seem to be present. At longer times, the same behavior as for the previous drop is observed. Obviously, the time scales are longer in this case. The ellipeometricmeasurement is not relevant around the center of the drop, where the elope of the interface is too large.
3
4
5 6 7 8
(CP) Figure 5. Diffusion coefficients D1 at short times versus oil viscosity: D1unit, 10-l2 m2 8-l; logarithmic scales on both axes; squares, LB wafers; crosses, HMDZ wafers; circles, bare wafers. Lines with slope -1 have been drawn as a guide for the eyes. 100
1000
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-2 -1 0 1 2 mm Figure 3. Same oil on a bare wafer. First profile after 507 h (lightline),second profile after 792h (heavyline). Silicathickness was 30 A. No molecular step is still visible. The characteristic thickness H at 63 A is weli defined, constant during the whole spreading process. No further changes have been observed after two more months.
0.0101 in both cases. For the bare wafer, no further spreading was observed during two more months (DO= 0) (Figure 6). Let us propose some interpretation for these findings, Le., the occurrenceof the gyration radius as a characteristic length, and the dependence of the spreading rate in oil viscosity and substrate nature.
For polymer melta at equilibrium,the only characteristic length in the bulk is the monomer size. The gyration radius play a role for dynamical procesees if entanglementa are present. As a matter of fact, a transient shoulder hae been observed previously with high molecular weight no111181 (Le., trimethyl-terminated) PDMEL6 However,the oils we use here have short chains, and no entanglementa are expected to occur. Actually, we did not observe the shoulder with normal PDMS in this range of molecular weight.a.31~ Our idea ie that the gyration radius appears as a characteristiclength in the spreadingprocesebecause some ~~~
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Silbenan, P.;Uger, L. Macromolecules 1992,25,1267,
Langmuir, Vol. 9, No.2, 1993 603
Microdroplet Spreading 100
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Figure 7. A two-month-olddroplet of PDMS-OH(heavy line), similar to the one on Figure 3, has been placedin dichloromethane for 1 h, then rinsedwith dichloromethane. The process has been repeatedtwice. The remaining oil (light line) correspondsto the anchored network.
monomers can adsorb and form a layer with reduced mobility. The present situation is reminiscent of the interactions between plates in a polymer melt: The range of the repuleive force is then comparable to the gyration radius of the coils.’ Let us discuss our results along this line: On bare wafers, the OH-ends of the polymer can firmly anchor on available silanol groups, possibly by hydrogen bonding. After some time, many chains will have both ends anchored and form a network, with typical extension R,. The remaining chains flow through the mesches of the network till they find one empty surface site. Such a behavior was first discussed by Bruinemas in the case of entangled polymericchainsof which all monomers could adsorb. Here, we have assumed that only the chain ends can anchor firmly enough not to be displaced by the flow and that no entanglements occur. This assumption is suggested by the fact that normal PDMS of low molecular weight does not show any shoulder on bare wafers.S4 Therefore, the unusual behavior of the OH-terminated oils is actually due to the presence of the hydroxyls. The molecules are strongly anchored on the surface (they are (7) de Gemes, P. G., C. R. Acad. Sci. Paris 1987,305, 1181. (8) Bruinema, R.~acromokcu~eo 1990,23, 276. (9) Thin remark is due to one reviewer. As a matter of fact, no specific treatment hae been performed to increase the number of silanol groups: the plasma in used for cleaning.
not removed by dichloromethane, see Figure 7) and the layer does not spread. Let us note that the observation of a network with thickness R,instead of a compact ‘brueh” of extended molecules anchored by only one end suggests that the number of available silanol groups on the silica is not very high.B For HMDZ wafers, interactions are weaker because ailanolsare no longer available. However,REstill appears as a characteristic length in the profiles. This means that OH ends interact strongly enough to be adsorbed for a significant time. This residence time is finite, because the monolayer continues to spread. Here, adsorption is apparently reversible. For LB wafers, the interaction is too weak to produce significant anchoring: No structure at R, is observed in this case. The dependence of D1 on oil viscosity and substrate characteristicshas to be discussed now. For simpleliquids or light polymers without slippage at the surface one can expect D1 to scale as 7-l) Le., the bulk viscosity 7 to be a relevant parameter even at the molecular scale. Slippage effects are associated with entanglementaprocesses;hence these do not occur here. Therefore, the inverse dependence of D1 in 7 is plausible. D1 also depends on the surface and is higher for the LB wafer, where the friction is less. Such an influence of the surface energy has already been reported for normal PDMS.4
Conclusion We presented here an experimental observation of an anchored network of polymer molecules occurring during a spreading process. A simple model relevant for light polymers without surface slippage accounta satisfactorily for the molecular mass and viscosity dependence of the measured parameters. It is worth noting how sensitive the droplet shape and dynamics are with respectto surface interactions. Such an ellipsometriccharacterization might well be a powerful tool for adhesion studies.
Acknowledgment. Helpful discussions with P.G.de Gennes and S.Granick are gratefully acknowledged. We also thank D. Teyssi6 for the characterization of the PDMS-OH by gel chromatographyand J. P.Bourgoin for the deposition of the Langmuir-Blodgett layers. Remarks suggested by one reviewer have been included in the text.
