Nucleated dewetting in supported ultra-thin liquid films with hydrodynamic slip

This study reveals the influence of the surface energy and solid/liquid boundary condition on the breakup mechanism of dewetting ultra-thin polymer films. Using silane self-assembled monolayers, SiO$_2$ substrates are rendered hydrophobic and provide a strong slip rather than a no-slip solid/liquid boundary condition. On undergoing these changes, the thin-film breakup morphology changes dramatically -- from a spinodal mechanism to a breakup which is governed by nucleation and growth. The experiments reveal a dependence of the hole density on film thickness and temperature. The combination of lowered surface energy and hydrodynamic slip brings the studied system closer to the conditions encountered in bursting unsupported films. As for unsupported polymer films, a critical nucleus size is inferred from a free energy model. This critical nucleus size is supported by the film breakup observed in the experiments using high speed \emph{in situ} atomic force microscopy.Comment: 8 pages, 9 figures, including supplementary materia

Capillary leveling of stepped films with inhomogeneous molecular mobility

A homogeneous thin polymer film with a stepped height profile levels due to the presence of Laplace pressure gradients. Here we report on studies of polymeric samples with precisely controlled, spatially inhomogeneous molecular weight distributions. The viscosity of a polymer melt strongly depends on the chain length distribution; thus, we learn about thin-film hydrodynamics with viscosity gradients. These gradients are achieved by stacking two films with different molecular weights atop one another. After a sufficient time these samples can be well described as having one dimensional viscosity gradients in the plane of the film, with a uniform viscosity normal to the film. We develop a hydrodynamic model that accurately predicts the shape of the experimentally observed self-similar profiles. The model allows for the extraction of a capillary velocity, the ratio of the surface tension and the viscosity, in the system. The results are in excellent agreement with capillary velocity measurements of uniform mono- and bi-disperse stepped films and are consistent with bulk polymer rheology.Comment: Accepted for publication in Soft Matter, Themed Issue on "The Geometry and Topology of Soft Materials

Fluidics of thin polymer films : boundary conditions and interfacial phenomena

Fließprozesse auf der Mikro- und Nanometerskala sind von enormem grundlagenwissenschaftlichem Interesse und bergen, z.B. in Lab-on-a-Chip-Anwendungen, zudem großes technologisches Potential. Das Verständnis der Stabilität und Dynamik dünner Flüssigkeitsfilme erlaubt Rückschlüsse auf Wechselwirkungen an Grenzflächen und auf das Verhalten von Flüssigkeiten auf molekularer Ebene. Dünne Polymerfilme auf hydrophoben Substraten unterliegen unterschiedlichen Entnetzungsmechanismen wie z.B. der Nukleation von Löchern. Der Fokus dieser Arbeit lag dabei im Speziellen auf der Quantifizierung der hydrodynamischen Randbedingung an der fest/flüssig Grenzfläche, der sog. Sliplänge. Im ersten Teil wurde die Dynamik des Lochwachstums studiert. Hierbei konnte der Einfluss des Substrats auf die Entnetzungsdynamik quantifiziert werden. Zudem hat die Länge der Polymerketten einen enormen Einfluss auf die Sliplänge und somit auf das Fließverhalten. Im zweiten Teil dieser Arbeit wurde die Morphologie des Randwulstes untersucht, der das Loch umgibt. Basierend auf theoretischen Modellen konnte die Sliplänge zur Charakterisierung der Randbedingung bestimmt werden. Es zeigte sich, dass diese mit der dritten Potenz der Kettenlänge des Polymers skaliert, sobald Verhakungen auftreten. An der Grenzfläche selbst ist die Dichte dieser Verhakungen um den Faktor drei bis vier reduziert. Diese Erkenntnisse unterstreichen die Bedeutung von Polymer-Konformationen an der fest/flüssig Grenzfläche für das Fließverhalten.Flow processes on the micro- and nanometer scale are of enormous scientific interest and additionally hold, e.g. in lab-on-a-chip applications, large technological potential. The understanding of the stability and the dynamics of thin liquid films allows for drawing conclusions about the interactions at interfaces and on the behavior of liquids on the molecular level. Thin polymer films on hydrophobic substrates are subject to different dewetting mechanisms such as e.g. the nucleation of holes. The focus of this study lies in particular on the quantification of the hydrodynamic boundary condition at the solid/liquid interface, the slip length. In the first part, the dynamics of hole growth was studied. Thereby, the impact of the substrate on the dewetting dynamics is quantified. Moreover, it is found that the length of the polymer chains exerts a dominating influence on the slip length and, thus, on the flow properties. In the second part of this thesis, the morphology of the rim surrounding the hole is studied. Based on theoretical models, the slip length characterizing the boundary condition is determined. It is shown that the slip length scales with the third power of the chain length of the polymer as soon as chain entanglements occur. At the interface itself, the density of entanglements is reduced by a factor of 3 to 4. These findings emphasize the relevance of polymer conformations at the solid/liquid interface for the flow properties

Capillary-driven flow induced by a stepped perturbation atop a viscous film

Thin viscous liquid films driven by capillarity are well described in the lubrication theory through the thin film equation. In this article, we present an analytical solution of this equation for a particular initial profile: a stepped perturbation. This initial condition allows a linearization of the problem making it amenable to Fourier analysis. The solution is obtained and characterized. As for a temperature step in the heat equation, self-similarity of the first kind of the full evolution is demonstrated and a long-term expression for the excess free energy is derived. In addition, hydrodynamical fields are described. The solution is then compared to experimental profiles from a model system: a polystyrene nanostep above the glass transition temperature which flows due to capillarity. The excellent agreement enables a precise measurement of the capillary velocity for this polymeric liquid, without involving any numerical simulation. More generally, as these results hold for any viscous system driven by capillarity, the present solution may provide a useful tool in hydrodynamics of thin viscous films.Comment: Accepted for publication in Physics of Fluid

Self-Similarity and Energy Dissipation in Stepped Polymer Films

The surface of a thin liquid film with nonconstant curvature is unstable, as the Laplace pressure drives a flow mediated by viscosity. We present the results of experiments on one of the simplest variable curvature surfaces: a stepped polymer film. Height profiles are measured as a function of time for a variety of molecular weights. The evolution of the profiles is shown to be self-similar. This self-similarity offers a precise measurement of the capillary velocity by comparison with numerical solutions of the thin film equation. We also derive a master expression for the time dependence of the excess free energy as a function of the material properties and film geometry. The experiment and theory are in excellent agreement and indicate the effectiveness of stepped polymer films to elucidate nanoscale rheological properties.Comment: 5 pages, 4 figures, article accepted for publication in Physical Review Letter

Solid Surface Structure Affects Liquid Order at the Polystyrene/SAM Interface

We present a combined x-ray and neutron reflectivity study characterizing the interface between polystyrene (PS) and silanized surfaces. Motivated by the large difference in slip velocity of PS on top of dodecyl-trichlorosilane (DTS) and octadecyl-trichlorosilane (OTS) found in previous studies, these two systems were chosen for the present investigation. The results reveal the molecular conformation of PS on silanized silicon. Differences in the molecular tilt of OTS and DTS are replicated by the adjacent phenyl rings of the PS. We discuss our findings in terms of a potential link between the microscopic interfacial structure and dynamic properties of polymeric liquids at interfaces

Influence of slip on the Rayleigh-Plateau rim instability in dewetting viscous films

A dewetting viscous film develops a characteristic fluid rim at its receding edge due to mass conservation. In the course of the dewetting process the rim becomes unstable via an instability of Rayleigh-Plateau type. An important difference exists between this classic instability of a liquid column and the rim instability in the thin film as the growth of the rim is continuously fueled by the receding film. We explain how the development and macroscopic morphology of the rim instability are controlled by the slip of the film on the substrate. A single thin-film model captures quantitatively the characteristics of the evolution of the rim observed in our experiments