1. Counter-intuitive turbulence W. I. Goldburg, University of Pittsburgh, DMR-0201805 Research With only a bit of stirring, the powdered cream you have added to your cup of coffee quickly dissolves (With no stirring, dissolving occurs but it takes much longer.) Now consider the behavior of tiny particles that are so light that they remain on top in spite of the irregular three- dimensional convective motion of the underlying fluid. Here a new effect appears; if the floaters are initially dispersed uniformly on the surface (upper left panel of Fig. 1), the stirring causes them to coagulate rather than disperse. In the tank of water where the experiments were carried out, the coagulation is seen to be almost complete in a fraction of a second. The goal here is to understand and quantify this counter-intuitive effect.

2. Floaters coagulate in roughly 1 sec = 1000 ms

3. Figure 2 is a schematic view of this tank and the sheet of laser light that illuminates the tiny floaters moving about on the surface. The overhead high-speed camera tracks the position and velocity of the particles as a function of time. The figure does not show the pump or the complicated network of pipes which together generate the underlying turbulence, which is quite strong. The coagulation effect being studied here is apparent in the motion of flotsam in a harbor. It is surprising that the phenomenon seems to have escaped quantitative laboratory study. The results obtained here could have important implications for minimizing the effect of pollution spills on the sea and in understanding the behavior microorganisms, such as phytoplankton, which live on the ocean’s surface.

4. Set-up Green laser light illuminates particles floating on the surface

5. Figure 3 helps demystify this coagulation effect. Shown there is a type of organized motion that can be generated by heating a fluid from below. The ordered roll-like structures generated here can sometimes be seen in cloud formations viewed from the air. In our case the fluid is water rather than air, and our interest focuses on the motion of the floaters, I.e. the green circles in that figure. Observe that the floaters collect above the fluid downwellings and flee the “upwellings”. As a result, the particles gather in static straight lines (out of the page), whereas the fluid below is moving. This same coagulation or compression effect is also dramatically present if the subsurface is turbulent rather than highly ordered, as in this figure.

6. Surface Compressibility

7. The effects seen here are also expected in supersonic flows in air, where it is intimately tied to its compressibility. Strong compressibility gives rise to effects that are similar but not identical to the coagulation phenomena studied here. This entire problem is presently the subject of intense theoretical investigation elsewhere. Our quantitative measurements are in striking agreement with computer-generated studies, but an analytical theory is still lacking. Outreach This ongoing study has benefited from the participation of undergraduates, grad students, and colleagues in Germany, who have illuminated a lot of the physics by their computer studies of the coagulation phenomenon under study here. The laboratory experience introduces students at all these levels to the sophisticated equipment required in turbulence studies. Weekly group meetings involve all of us, including the undergraduates, who give their share of the lectures.