In Figure 2 the resulting liquid distribution in a well due to the orbital movement of a microplate is shown. If the effect of friction and surface forces is neglected the free surface of the fluid, which is a face of an equal pressure niveau, forms a rotational paraboloid. The acting forces to a volume element dm within the free surface are gravity and centrifugal force. In a co-moving reference frame the fluid appears to rotate along a stationary wall. Interesting values are the minimal and the maximal height of fluid in depence of amplitude r0 and mixing frequency n. Further Information about calculating liquid distribution can be found in .
The choice of suitable operating parameters for orbital mixing, especially the mixing frequency n and the amplitude r0, is depending on:
- microplate fill volume VF
- well geometry (diameter DW, height h)
- surface tension of fluid and construction material σ
- fluid density ρ
- and kinematic viscosity of the fluid ν.
The most important requirement for an effective mixing process is the formation of a macroscopic flow. As microplate well volumes decrease the impact of surface tension increases because of the low volume/surface ratio of the usually thin and tall well geometry. For this reason it is necessary to generate a high centrifugal acceleration to achieve an intensive macroscopic flow. A large number of commercially available instruments have been developed for use with larger laboratory vessels and they are not designed to generate a centrifugal acceleration which is required for processing small volumes. The labour required for surface enlargement must be delivered by the centrifugal force. The increased centrifugal force exceeds the surface tension at a critical shaking frequency