Blurred Boundaries in Flow – Science Spectrum

This phenomenon can even be observed in stagnant water. This is the case, for example, when a constant wind sweeps a material that floats from the top towards the edge of a pond. The remaining surface waters that are cleaned in this way also move in the direction and find a similar situation as in a clogged river. It hits the film of dirt and dives down in front of it – in this case not under it, but again as a subtle stream close to the ground. This creates a closed cycle that continues perfectly as long as the wind blows.

Amazing physics hides behind many everyday objects. I felt for many years Hans Joachim Schlichting He discusses these phenomena and explains them in his column for readers of the “Scientific Spectrum”. Schlichting is Professor of Educational Physics and worked at the University of Münster until his retirement.

The velocity of the flow plays a role in how prominent the Thoreau-Reynolds wave is. Once a velocity of 23 cm per second is exceeded, “capillary waves” can appear in an upward direction in front of the line, and their behavior is determined primarily by surface tension. The resulting ripples indirectly draw attention to the line, especially since the solid film remains undisturbed.

© Courtesy of Harald Berner (Details)

capillary waves | If the speed is sufficient, minute ripples can form upstream of the boundary line. On the fender side, the surface composition does not allow this.

The most recent research interest in Thoreau-Reynolds waves on the one hand refers to ocean currents under the influence of various substances occurring everywhere. On the other hand, this phenomenon has long found its way into the laboratory investigation of the effect of surface-active substances in a more general sense.

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