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# Schulmann, Nava

PhD (2008-2011)

Monday 18 June 2012, by

Nava Schulmann has curently Postdoc position in Weizmann Institute of Science.

My Phd project aims at understanding macroscopic fiber systems with statistical physics concepts. In practice I perform numerical simulations on fiber systems under compression - see a cool movie here and theoretical calculations. For a deeper description of my project see here..

### **Understanding Macroscopic Fiber Systems with Statistical Physics Concepts**

Many natural systems such as cellulose fibers, hair, DNA, tobacco mosaic virus, or manufactured materials like ropes, wires or … spaghetti are made from fibers and assembled for instance as fiber bunches or fiber networks. The mechanical and statistical mechanic behavior of individual fibers is now well understood; one hair fiber, one rope or one rigid or semi-flexible macromolecule can be described by the physical concepts of torsion, extension and curvature rigidities, and by the topological concepts of twist and writhe. The properties of matter made by many interacting microscopic fibers have also been investigated to some extent, particularly in the limit where fiber rigidity is small enough for thermal forces to play a predominant role. Macroscopic or more rigid systems, where temperature plays a negligible role are much less understood. We have recently studied fiber stacks [Europhys. Let., 2003, 64, 647], and shown that statistical physics concepts from the thermal systems can be used to understand many properties of the macroscopic systems. In particular, we have shown that the frozen curvature heterogeneities give rise to an effective temperature that controls material properties as stack compressibility or assembly shape : the method allows for instance to predict the cone shapes often observed at the tips of ropes. The aim of this project is threefold. As a first step we want to confront the mean-field theoretical predictions for nearly-aligned, two and three dimensional fiber stacks with numerical simulations. In particular, we want to understand the accuracy of the effective temperature analogy and the limits of the mean-field approximation. Next we plan to explore the effects of slight to strong misalignment of the fibers. The limit where the stacks are made from isotropically distributed fibers is particularly relevant to understand materials as paper or glasswool. As a third and longer-term objective, we plan to investigate the dynamical behavior of entangled fibers. Known disentanglement processes as chain reptation play a crucial role in understanding the viscoelastic behavior of high molecular weight materials such as molten polymers, we expect that the extension of such kinetic concepts to macroscopic fiber systems dominated by solid friction will be useful to understand disentanglements in fibers bunches, as those that control hair combing or wool and cotton carding.