A method of mixing a fluid includes at least partially unfolding a collapsible bag bounding a compartment, the collapsible bag containing in the compartment at least a portion of an elongated drive line or drive shaft and an impeller secured to the drive line or drive shaft, the impeller including a plurality of impeller blades that are pivotable relative to the drive line or drive shaft, at least one of the plurality of impeller blades being in a collapsed position. A fluid is delivered into the compartment of the collapsible bag. The drive line or drive shaft is then rotated so as to rotate the impeller within the compartment and mix the fluid therein, the at least one of the plurality of impeller blades pivoting from the collapsed position to an expanded position as the impeller is rotated within the compartment.