Curious fringe around a beet slice*

If a slice of beet is placed on a plate with a thin layer of beet juice, one can observe a clear fringe around the beet, where the color is weaker than the rest of the juice. This phenomenon, often seen in kitchens, has been noted as a curious observation. The hypotheses in literature were inconsistent and limited, which motivated us to revisit this phenomenon. Using a motorized confocal displacement sensor, we measured the liquid surface profile across the fringe in a time-resolved manner. Our measurements suggest that a suction flow, induced by the porousness of the beet slice, causes a “dimple” – a small concave depression – to form on the liquid surface. This dimple is responsible for the visual fringe. While surface tension tends to smooth out the dimple, viscous stresses act against this tendency if the liquid film is sufficiently thin. Our scaling analysis correctly estimates the order of magnitude of the critical film thickness​, below which the dimple remains stable for a long time. We also captured the dimple formation dynamics by numerically solving the lubrication equation with the Young-Laplace equation. This work provides a new interpretation for a commonly observed phenomenon and demonstrates a powerful technique for characterizing liquid surfaces.