title: Stability of freely falling granular streams.
abstract: A freely falling stream of weakly cohesive granular particles is modeled and analyzed with the help of event driven simulations and continuum hydrodynamics. The former show a breakup of the stream into droplets, whose size is measured as a function of cohesive energy. Extensional flow is an exact solution of the one-dimensional Navier-Stokes equation, corresponding to a strain rate, decaying like t(-1) from its initial value, γ[over ˙](0). Expanding around this basic state, we show that the flow is stable for short times, γ[over ˙](0)t<<1, whereas for long times, γ[over ˙](0)t>>1, perturbations of all wavelengths grow. The growth rate of a given wavelength depends on the instant of time when the fluctuation occurs, so that the observable patterns can vary considerably.
authors.affiliation: Instituut-Lorentz for Theoretical Physics, 2333 CA Leiden, The Netherlands.
authors.forename: Stephan
authors.initials: S
authors.lastname: Ulrich
authors.forename: Annette
authors.initials: A
authors.lastname: Zippelius