Sample tube with integrated mixing plunger head

A method comprising:

• inserting an insertion portion of a shaft through an opening in a cap of a container and into a shaft-receiving feature in a plunger head that is positioned within the cap by a retaining feature, wherein the shaft-receiving feature is a hole extending through the plunger head;
• applying a force greater than a first threshold amount to the shaft after the insertion portion of the shaft is fully inserted into the shaft-receiving feature of the plunger head, thereby causing the retaining feature to release the plunger head;
• and
• reciprocating the shaft, after the plunger head has been released from the retaining feature, such that the plunger head is reciprocated within an interior volume of the container.

An apparatus comprising:

• a cap with a capping surface and one or more sidewalls extending away from the capping surface along a direction having a major component that is parallel to a normal of the capping surface;
• a plunger head, the plunger head having an outer diameter and sized to fit within an interior of a container with which the cap is to interface;
• a retaining feature;
• and
• an opening in the cap, wherein:
• the plunger head includes a shaft-receiving feature to receive a shaft that is insertable through the opening,
• the shaft-receiving feature is a hole that extends through the plunger head,
• the plunger head is positioned within the cap by the retaining feature such that the opening is aligned with the shaft-receiving feature,
• the retaining feature releases the plunger head when a force higher than a first threshold amount is applied to the plunger head in a direction facing away from, and normal to, the capping surface, and
• a largest dimension of the plunger head is larger than a smallest dimension of the opening in the cap.