Q: What are the convection patterns or structures that I see in the popcorn oil while heating and before adding the popcorn?
A: Natural convection can arise from the effect of temperature differences on fluid density and on fluid surface tension (if there is a free surface). In the case of no free surface, heating from below (also cooling from above), and fluid expansion upon heating, Rayleigh (or Rayleigh-Benard) convection “rolls” occur, in which there are tubular patterns of convection caused by cool (less dense) fluid sinking and displacing warm, less dense fluid.
In the case of a shallow layer of oil in a clean, flat skillet heated from below, one will often observe more complex patterns. These are the so-called Benard-Marangoni convection cells and they are due to the interaction of temperature-dependent density and surface tension. The cells often assume a remarkably uniform, hexagonal order when viewed from above. Tubular convection rolls may also occur when there is a free surface.