Images of Hexagonal Patterns near Onset
The images on this page are from E. Bodenschatz, J.R. de Bruyn, G. Ahlers, and D.S. Cannell, Phys. Rev. Lett. 67, 3078 (1991). They are also discussed by E. Bodenschatz, W. Pesch, and G. Ahlers, Annu. Rev. Fluid Mech., to be published.
Just below the bifurcation to hexagons, fluctuations become visible in the right-center of the image. The circles in the upper left are caused by a dust particle. The blemish in the lower right is in the optics, not in the pattern.
Just above onset, a transient pattern of hexagonal convection forms and spreads in time. Note the six fronts of rolls which emanate from the hexagons. This illustrates that hexagons really are a superposition of three sets of rolls.
After some time, the patch of hexagons in the previous image has spread to cover a larger area. The boundaries are now determined by the very slight inhomogeneity of the sample thickness which causees a spatial variation of the control parameter.
As the control parameter is reduced, there is hysteresis and the pattern persists even below the original onset. This pattern is a steady state. A very slight further reduction of the temperature difference causes it to disappear.
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