This is a very simple operation. A highly viscous fluid is getting squirted, toothpaste-like, into another fluid. Without any outside prompting, the fluid forms first loops, then bonded figure-eights, and then surprisingly chic-looking fused triangles.
A small number of forces can create a surprisingly large number of patterns. In this case, all that’s needed is a space with inflow and outflow areas, a supporting fluid, and a viscous fluid getting squirted into that supporting fluid. First the fluid forms loops that, over time, squish together closer and closer and grow taller and taller. Eventually the looped ends merge together.
Then, as the fluid approaches the “outflow” area and accelerates, it pulls on the merged loops. Because each loop is joined with the rest of the fluid at one point on either side and one point at the center, it can resist the flow at these points. The rest of the loop get stretched out into triangles before it zips out of sight.
Just for fun, here’s a video of the same principles working on a grouping of viscous droplets. Notice how they, as a group, grow “taller” in the middle and stretch out to either end.
[Source: Microfluid Lab]