Spinodal dewetting

The unstable situation caused when a spread him begins to dewet the surface has been studied [32, 33]. IDewetting generally proceeds from hole formation or retraction of the him edge [32] and hole formation can be a nucleation process or spinodal decomposition [34]. Brochart-Wyart and de Gennes provide a nice... 

Fig. 33(a,b) shows a series of snapshot pietures as a result of a eomputer experiment probing the kineties of dewetting. The loeal darkness of eaeh snapshot indieates the loeal eoverage of the substrate surfaee. Coverage fluetuations (white spots) appear rather early and get rapidly amplified. The substrate regions, eovered with polymer, have very irregular surfaee initially and are eonneeted with many weak links later, these hnks disappear, and the droplets of adsorbed polymer eompaetify, a pattern similar to spinodal deeomposition. 

The bilayer morphology of thin asymmetric films of may be unstable. A regularly corrugated surface structure of the films was ascribed to spinodal transition into a laterally phase separated structure, where the surface morphology depended on the polymer incompatibility and the interfacial interactions [347, 348]. Recently, the phase separation and dewetting of thin films of a weakly incompatible blend of deuterated PS and poly(p-methylstyrene) have been monitored by SFM [349, 350]. Starting from a bilayer structure, after 454 h at T= 154 °C the film came to the final dewetting state where mesoscopic drops of... 

For iJ (do) < 0, Tq > 0 for every q and fluctuations decrease exponentially after they are excited. On the other hand, if II do) > 0 a wavevector qc can be found so that Tg g < 0 and the given fluctuation mode is amplified resulting in a decomposition of the film. Typical size of drops which result from the spinodal dewetting of a fihn is given by the most amplified capillary wave dTq jdq = 0) with... 

When a polymer solution is allowed to evaporate from a surface, dewetting instabilities can arise, producing modulated patterns like spinodal structures (68). Section IV addresses the applicability of these interesting phenomena to making opto-electronical devices. 

The dewetting rim is deformed by the morphological instability. The wave number of the deformation and the critical diameter of the dry patch depend upon the initial film thickness. The rim instability and the origin of the initial holes (e.g. spinodal surface instability) determine the final arrangement of the droplets. From the calculation of the surface free energy of deformed rims, we showed that the rim instability is due to the three-dimensional structure of the rims. However, estimation of the node density is complicated by the two kinds of kinetics, the expansion mte of the dry patches and Ae growth rate of the rim fluctuations. 

Higgins, A.M., Jones, R.A.L. Anisotropic spinodal dewetting as a route to self-assembly of patterned surfaces. Nature 404(6777), 476-478 (2000)... 

Herminghaus, S., Jacobs, K., Mecke, K., Bischof, J.,Fery, A., Ibn-Elhaj, M., Schlagowski, S. Spinodal dewetting in liquid crystal and liquid metal films. Science 282(5390), 916-919... 

Below ec, there are two dewetting mechanisms (a) a macroscopic film is metastable and dewets by nucleation and growth of dry zones (b) a microscopic film is unstable and spontaneously breaks into a multitude of droplets. Capillary waves are amplified and this mechanism is called spinodal decomposition, by analogy with what happens in phase transitions. 

Fig. 1.28. Dewetting of microscopic films through the spinodal decomposition mechanism for a polystyrene film deposited on a silicon wafer (Reiter, G.) (a) spontaneous nucleation of the hole (b) final stage, in which the microdroplets arrange themselves into polygons. The length of the black bar is 10 lm. Film A has thickness 25 nm and film B has thickness 45 nm...

Fig. 1.30. Liquid (PDMS)/liquid (fluorinated PDMS) dewetting (Buguin, A., Martin, P.) (a) nucleation and growth (note the circular shape of the holes) (b) spinodal decomposition...

On a fundamental level, the mechanisms controlling the way liquid films recede are somewhat similar to those involved in phase transitions. The film can be metastable and dewet by nucleation and expansion of dry zones. It can also be unstable and dewet spontaneously through amplification of capillary waves. The latter case is referred to as spinodal dewetting. 

We will deal primarily with the dewetting of metastable films. We will also discuss the case of spinodal dewetting (of more restricted practical interest), which is limited to very thin films (e < 10 nm) and to ultra-pure conditions to avoid the nucleation of dry zones at specks of dust. 

We now focus our attention on the case A < 0, which implies an unstable film. Equation (7.34) tells us that, in this instance, the energy of the film decreases when the thickness is modulated with a spatial periodicity q greater than g. If g g < 1, the thickness modulation gets amplified in time, which leads to spinodal dewetting. 

FIGURE 7.12. Images of the spinodal dewetting of a nanoscopic film of PDMS deposited on a silicon wafer (a) for short times, showing the amplification of the thickness fluctuations (courtesy O. Rossier) (b) for long times (courtesy G. Reiter) (c) also for long times, but on a liquid substrate (courtesy A. Buguin). 

Since then, spinodal dewetting at short times has been studied in some detail. It has been possible to monitor by AFM the thickness modulation and its amplification as a function of time. The literature contains discussions of the behavior of other materials. Nevertheless, results are typically poorly reproducible and the appearance of holes at short times (observed in ultra-thin polystyrene films less than 10 nm thick) is often unexplained. These anomalies arc usually blamed on flawed preparations... 

In the case of liquid crystal films, for which the orientation of molecules is fixed at the walls, the distortion energy of the molecular axis can dominate over the van der Waals energy, and spinodal dewetting can take place in thicker films, which makes it easier to observe (see refercncc ). 

Spinodal dewetting has been studied mostly for the case of the negligible slip. But the dewetting behavior with a weak slip (with... 

M e) (i.e, M[l —0) —> 0) poses numerical problems. By using algorithms, which preserve the nonnegativity of f[x, y, it has been possible to demonstrate, that spinodal dewetting patterns can be described quantitatively by thin-hlm equations. 

M. Rauscher, R. Blossey, A. Miinch, and B. Wagner, Spinodal dewetting of thin films with large interfacial slip implications from the dispersion relation, Langmuir, 24,12290-12294 (2008). 

R. Fetzer, M. Rauscher, R. Seemann, K. Jacobs, and K. Mecke, Thermal noise influences fluid flow in thin films during spinodal dewetting, Phys. Rev. Lett., 99,114503 [2007). 

---