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article in newscientist.com Biology provides many examples of self-propelling microorganisms in the quasi-static low-Reynolds-number limit. Spiroplasma are tiny helical-shaped eubacteria lacking a rigid cell wall. They propel through viscous fluids by sending kinks or domain walls between regions of opposite handedness down their helical body. A simple elastic model for the domain-wall propagation is formulated and studied using hydrodynamic simulations and scaling arguments, giving good agreement with recent video-microscopy observations [J. Shaevitz, et al. Cell 122, 941 (2005)]. It is shown that the observed helical bacterial pitch angle Ψ ∼ 35° is optimized for maximal speed and efficiency. Conference: Modeling and Computer Simulations of Microswimming and Bacterial Motility TU Munich, Garching (Munich) - 17-19.09.2007 Dr. V. Lobaskin, Prof. L. Bocquet and Prof. R.R. Netz
The behavior of a single collapsed polymer under shear flow is examined using hydrodynamic simulations
and scaling arguments. Below a threshold shear rate γ* the chain remains collapsed and only deforms slightly,
while above γ* the globule exhibits unfolding/refolding cycles. Hydrodynamics are crucial: in the free draining
case γ scales with the globule radius R as γ ~ R^-1 , while in the presence of hydrodynamic interactions
γ ~ R. Experiments on the globular von-Willebrand protein confirm the presence of an unfolding transition at
a well-defined critical shear rate. These findings explain the function of the von-Willebrand factor, a
protein present in our circulatory system, to stop bleeding under high shear-stress conditions as found
in small blood vessels. Shear-flow induced unfolding of polymeric globules |
Model for Self-Propulsive Helical Filaments: Kink-Pair Propagation
Shear-induced unfolding triggers adhesion of von Willebrand factor fibers