The mitotic spindle is an essential cytoskeletal structure that enables chromosome separation during cell division. This project seeks to identify the physical principles that control spindle assembly by using a simplified biomimetic system composed solely of microtubules and molecular motors. We will use motors of opposite polarities combined with dynamic microtubules to understand how these components organize through active phase separation. Indeed, preliminary experiments have demonstrated that such reconstituted systems can spontaneously form bipolar structures resembling mitotic spindles. We now propose to encapsulate these molecular components in compartments of controlled geometry to reconstruct a minimal bipolar structure capable of elongating, retracting, and separating its organizing poles. This multidisciplinary approach will combine biochemical and physicochemical techniques, advanced microscopy, and quantitative analysis of the spatial and temporal evolution of the system. The experimental work will be closely coupled with theoretical modeling in collaboration with Prof. Jean-François Joanny (Collège de France) to develop a physical model of active phase separation that will provide better understanding of self-organization mechanisms at the subcellular scale in living organisms.