Immiscible substances, such as gases, solids or liquids may be included within a reaction site container as a mixer of a liquid sample. Movement of the mixer within the container may help suspend or re-suspend cells or other species. Movement of the mixer also may generate shear forces that can affect cellular activity. In some embodiments, movement of the container brings about movement of the mixer. Containers may be mounted to a rotating apparatus in various orientations to achieve different travel paths of the mixer. Varying the rotation rate and/or the relative densities of the mixer and the liquid sample also may affect the mixer travel path.