1,376 research outputs found
Formation of Kinneyia via shear-induced instabilities in microbial mats
Kinneyia are a class of microbially mediated sedimentary fossils. Characterized by clearly defined ripple structures, Kinneyia are generally found in areas that were formally littoral habitats and covered by microbial mats. To date, there has been no conclusive explanation of the processes involved in the formation of these fossils. Microbial mats behave like viscoelastic fluids. We propose that the key mechanism involved in the formation of Kinneyia is a Kelvin–Helmholtz-type instability induced in a viscoelastic film under flowing water. A ripple corrugation is spontaneously induced in the film and grows in amplitude over time. Theoretical predictions show that the ripple instability has a wavelength proportional to the thickness of the film. Experiments carried out using viscoelastic films confirm this prediction. The ripple pattern that forms has a wavelength roughly three times the thickness of the film. This behaviour is independent of the viscosity of the film and the flow conditions. Laboratory-analogue Kinneyia were formed via the sedimentation of glass beads, which preferentially deposit in the troughs of the ripples. Well-ordered patterns form, with both honeycomb-like and parallel ridges being observed, depending on the flow speed. These patterns correspond well with those found in Kinneyia, with similar morphologies, wavelengths and amplitudes being observed
From modular invariants to graphs: the modular splitting method
We start with a given modular invariant M of a two dimensional su(n)_k
conformal field theory (CFT) and present a general method for solving the
Ocneanu modular splitting equation and then determine, in a step-by-step
explicit construction, 1) the generalized partition functions corresponding to
the introduction of boundary conditions and defect lines; 2) the quantum
symmetries of the higher ADE graph G associated to the initial modular
invariant M. Notice that one does not suppose here that the graph G is already
known, since it appears as a by-product of the calculations. We analyze several
su(3)_k exceptional cases at levels 5 and 9.Comment: 28 pages, 7 figures. Version 2: updated references. Typos corrected.
su(2) example has been removed to shorten the paper. Dual annular matrices
for the rejected exceptional su(3) diagram are determine
- …