14,249 research outputs found
Smectic-Nematic Phase Transition as Wrinkling Transition in a Stack of Membranes
We point out that the smectic-nematic phase transition may considered as a
transition of a stack of membranes in dimensions, in which the
layers become so wrinkled that they interpenetrate each other are no longer
distinguishable.Comment: Author Information under
http://www.physik.fu-berlin.de/~kleinert/institution.html . Latest update of
paper also at http://www.physik.fu-berlin.de/~kleinert/137
Solvent evaporation of spin cast films: crust effects
When a glassy polymer film is formed by evaporation, the region near the free
surface is polymer rich and becomes glassy first, as noticed long ago by
Scriven et al. We discuss the thickness of this "crust" and the time interval
where it is present -before freezing of the whole film. We argue that the crust
is under mechanical tension, nd should form some cracks. This may be the source
of the roughness observed on the final, dry films, when the solvent vapor
pressure is high (and leads to thin crusts).Comment: 8 pages, 2 EPS figures. submitted to Europ. Phys. J. (E
Instabilities during the evaporation of a film: non glassy polymer + volatile solvent
We consider solutions where the surface tension of the solvent is smaller
than the surface tension of the polymer. In an evaporating film, a plume of
solvent rich fluid, then induces a local depression in surface tension, and the
surface forces tend to strengthen the plume. We give an estimate (at the level
of scaling laws) for the minimum thickness required to obtain this instability.
We predict that the thickness a) is a decreasing function of the solvent vapor
pressure b) should be very small (<1 micron) provided that the initial solution
is rather dilute. The overall evaporation time for the film should be much
longer than the growth time of the instability. The instability should lead to
distortions of the free surface and may be optically observable. It should
dominate over the classical Bernard-Marangoni instability induced by cooling.Comment: 8 pages, 2 EPS figures. submitted to Eur. Phys. J. (E
Phase transitions of nematic rubbers
Single crystal nematic elastomers undergo a transition from a strongly
ordered phase N to an "isotropic" phase I. We show that: (a) samples produced
under tension by the Finkelmann procedure are intrinsically anisotropic and
should show a small (temperature dependent) birefringence in the high
temperature I phase. (b) for the I->Ntransition via cooling there is a spinodal
limit but for the N->I transition via heating there is no soft mode at the
standard spinodal temperature. (c) the N->I transition is reminiscent of a
martensitic transformation: nucleation of the I phase should occur in the form
of platelets, making a well defined angle with the director.Comment: 7 pages, 3 figures (To appear in Europhys. Lett.
Dewetting dynamics of stressed viscoelastic thin polymer films
Ultrathin polymer films that are produced e.g. by spin-coating are believed
to be stressed since polymers are 'frozen in' into out-of-equilibrium
configurations during this process. In the framework of a viscoelastic thin
film model, we study the effects of lateral residual stresses on the dewetting
dynamics of the film. The temporal evolution of the height profiles and the
velocity profiles inside the film as well as the dissipation mechanisms are
investigated in detail. Both the shape of the profiles and the importance of
frictional dissipation vs. viscous dissipation inside the film are found to
change in the course of dewetting. The interplay of the non-stationary
profiles, the relaxing initial stress and changes in the dominance of the two
dissipation mechanisms caused by nonlinear friction with the substrate is
responsible for the rich behavior of the system. In particular, our analysis
sheds new light on the occurrence of the unexpected maximum in the rim width
obtained recently in experiments on PS-PDMS systems.Comment: 11 pages, 10 figure
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