Separating composite liquids

A method of separating components of whole blood, the method comprising:

• transferring a liquid comprising whole blood into a blood processing vessel adapted to be mounted on a rotor;
• separating the liquid comprising whole blood into at least a first component comprising cells and a second component by rotating the blood processing vessel on the rotor;
• transferring the first component into a membrane volume defined by a flexible membrane movable from a contracted configuration to an expanded configuration, an inlet into the membrane volume is in fluid communication with an outlet of the blood processing vessel such that the first component flows from the blood processing vessel into the membrane volume for further separation, the flexible membrane is seated in a chamber volume defined by a stepped side wall of a cell separation chamber such that upon transferring the first component into the flexible membrane the flexible membrane expands from the contracted configuration to the expanded configuration, in the expanded configuration the flexible membrane contacts the stepped side wall to give the flexible membrane an expanded, stepped configuration;
• separating cells from the first component in the membrane volume by rotating the cell separation chamber;
• and
• removing the cells from the membrane volume, thereby allowing the flexible membrane to contract from the expanded, stepped configuration to the contracted configuration.