Nucleation dynamics in two-dimensional cylindrical Ising models and chemotaxis

Abstract

The aim of our work is to study the effect of geometry variation on nucleation times and to address its role in the context of eukaryotic chemotaxis (i.e. the process which allows cells to identify and follow a gradient of chemical attractant). As a first step in this direction we study the nucleation dynamics of the 2d Ising model defined on a cylindrical lattice whose radius changes as a function of time. Geometry variation is obtained by changing the relative value of the couplings between spins in the compactified (vertical) direction with respect to the horizontal one. This allows us to keep the lattice size unchanged and study in a single simulation the values of the compactification radius which change in time. We show, both with theoretical arguments and numerical simulations, that squeezing the geometry allows the system to speed up nucleation times even in presence of a very small energy gap between the stable and the metastable states. We then address the implications of our analysis for directional chemotaxis. The initial steps of chemotaxis can be modelled as a nucleation process occurring on the cell membrane as a consequence of the external chemical gradient (which plays the role of energy gap between the stable and metastable phases). In nature most of the cells modify their geometry by extending quasi-onedimensional protrusions (filopodia) so as to enhance their sensitivity to chemoattractant. Our results show that this geometry variation has indeed the effect of greatly decreasing the timescale of the nucleation process even in presence of very small amounts of chemoattractants.2

Magnetisation reversal in Ising ferromagnet by thermal and field gradients

“…The problem of classical nucleation is still challenging. A significant amount of work has already been done [4] , [5] , [6] , [7] in this field. However, all the studied mentioned, so far, have been done for steady (in time) and uniform (over the space) magnetic field.…”