arXiv subjects

Karin Jacobs

Publications and source records attributed to Karin Jacobs.

23 records · Page 2Linked to original sources

arXiv2001

Shape of a liquid front upon dewetting

We examine the profile of a liquid front of a film that is dewetting a solid substrate. Since volume is conserved, the material that once covered the substrate is accumulated in a rim close to the three phase contact line. Theoretically, such a profile of a Newtonian liquid resembles an exponentially decaying harmonic oscillation that relaxes into the prepared film thickness. For the first time, we were able to observe this behavior experimentally. A non-Newtonian liquid - a polymer melt - however, behaves differently. Here, viscoelastic properties come into play. We will demonstrate that by analyzing the shape of the rim profile. On a nm scale, we gain access to the rheology of a non-Newtonian liquid.

cond-mat.soft↗

arXiv2001

A Universal Nucleation Mechanism for Solvent Cast Polymer Film Rupture

It is shown that the intrinsic stress in solvent cast polymer coatings plays a key role in the nucleation of holes in the film. Nucleation is important because it is meanwhile clear that heterogeneous nucleation is the only relevant rupture mechanism for the technologically relevant thickness regime well above 100 nm. The most striking feature is that in contrast to what has been widely believed, the number density of holes scales not algebraically, but exponentially with the film thickness.

cond-mat.soft↗

arXiv2001

A Transient Instability in Thin Films of nCB Liquid Crystals

A transient surface instability is reported in thin nematic films of 5CB and 8CB, occurring near the nematic-isotropic phase transition. Although this instability leads to patterns reminiscent of spinodal dewetting, we show that it is actually based on a nucleation mechanism. Its characteristic wavelength does not depend markedly on film thickness, but strongly on the heating rate.

cond-mat.soft↗

arXiv2001

The Glass Transition of Thin Polymer Films: Some Questions, and a Possible Answer

A simple and predictive model is put forward explaining the experimentally observed substantial shift of the glass transition temperature, Tg, of sufficiently thin polymer films. It focuses on the limit of small molecular weight, where geometrical `finite size' effects on the chain conformation can be ruled out. The model is based on the idea that the polymer freezes due to memory effects in the viscoelastic eigenmodes of the film, which are affected by the proximity of the boundaries. The elastic modulus of the polymer at the glass transition turns out to be the only fitting parameter. Quantitative agreement is obtained with our experimental results on short chain polystyrene (Mw = 2 kg/mol), as well as with earlier results obtained with larger molecules. Furthermore, the model naturally accounts for the weak dependence of the shift of Tg upon the molecular weight. It furthermore explains why supported films must be thinner than free standing ones to yield the same shift, and why the latter depends upon the chemical properties of the substrate. Generalizations for arbitrary experimental geometries are straightforward.

cond-mat.soft↗

arXiv2001

Why do Thin Polymer Films Melt at Lower Temperature? A Continuum Approach

We have investigated the reduction of the glass transition temperature, $T_g$, for thin supported films of particularly small molecular weight ($M_W$ = 2 kg/mol) polystyrene, and found good agreement with earlier studies on larger molecules. By combining the eigenmodes of the films with mode coupling theory, we arrive at a simple and predictive model which is in quantitative agreement with the data. Its only fitting parameter is the shear modulus governing the relevant fluctuations. The weak dependence of the latter upon the molecular weight of the polymer is in accordance with the general observation that the reduction of $T_g$ is largely independent of chain length at sufficiently low molecular weight.

cond-mat.soft↗