The experiments Suresh Advani (University of Delaware) and I performed were classified as two-fluid problems. In such problems there are two thermodynamic systems (fluid 1 and fluid 2) that relate to one another across a boundary between the two fluids.
Thermodynamics (thermo) is a well-organized and descriptive language scientists use to examine what happens when two systems (objects in specified spaces) come together. The language of thermo specifies what is exchanged across boundaries when two or more systems interact or relate.
In our two-fluid experiments, an expanding circular droplet of a less-dense, water-based fluid is pushing a more-dense, viscous oil out of the way. This relationship is considered unstable, because the water flows easier than the sluggish oil. Because the oil cannot move fast enough out of the water’s way, the boundary between the two fluids buckles (adopts a wavy pattern like a sine wave).
At the trough of the wave, the watery fluid sees a concave-upward boundary. The oily substance sees a concave-downward one. So, not only are the fluids flowing against one another, exchanging the momentum of movement at their boundaries, but they see this exchange in starkly different ways. Each with its own perspective or point of view (POV).
For this example, then, we see that primitive relationship does not just involve the energy exchanged across a boundary between relating systems, but, also, the unique perspectives of each.