Carla BOSIA Nucleation dynamics in 2d cylindrical Ising models and chemotaxis. The aim of our work is to study the e?ect of geometry variation on nucleation times and to address its role in the context of eukaryotic chemotaxis. We focalized on the process which is at the heart of directional sensing, that is the chemical phase separation process happening on the inner membrane of an eukaryotic chemotacting cell. When a chemoattractant is switched on, thanks to the interplay of the two enzymes P T EN and P I3K in the cytoplasm, it is observed the formation of two complementary domains on the membrane, rich in phospholipids P IP2 and P IP3 . The P IP3 -rich patch is localized on the side of cell exposed to the higher concentration of chemoattrac-tant, while the P IP2 -rich one on the complementary side. Recently, an Ising model with antiferromagnetic interaction has been applied to study this phase separation process. Since the cell geometry plays a pivotal role in chemotaxis (?lopodia protrude from the leading edge to sense the external chemoattractant allowing the whole cell to move toward its maximum), using that model as a starting point we studied the nucleation dynamics on a cylindrical lattice whose radius changes as a function of time. The main assumption is that the phase separation scenario proposed above, holds also for ?lopodia despite their extreme geometry. Our study suggests that the role of these protrusions is to optimize signal detection allowing the cell to identify evenvery small amounts of chemoattractant gradients : the geometry variation has indeed the e?ect of greatly decreasing the timescale of the nucleation process even in presence of very small amounts of chemoattractants.