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.

Provided is a stirring apparatus that can drive a stirring body with a simple structure and without applying excessive force. The stirring apparatus is provided with: the stirring body, which is a two-circle roller having an axis of rotation and two stirring fins; first and second drive shafts with axial centers not matching the axis rotation; a first shaft coupling, which connects the first drive shaft on a first stirring fin side of the axis rotation; a second shaft coupling, which connects the second drive shaft on a second stirring fin side of the axis of rotation; and a driving unit that exclusively carries out rotary drive of only either of the first and second drive shafts. The driving unit is provided with a regulating member that makes a teeth angular range of a first gear face a power-driven gear without play at a rotary position where the boundary between the teeth angular range and a missing teeth angular range of the first gear faces the power-driven gear and makes a teeth angular range of a second gear face the power-driven gear without backlash at a rotary position where the boundary between the teeth angular range and a missing teeth angular range of the second gear faces the power-driven gear.

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.