Cream on
One of the ways of drinking the liqueur Tia Maria is to sip it through a thin layer of cream. If the cream is poured onto the surface of the drink, to a depth of about 2 millimetres, and left to stand for about two minutes, the surface begins to break up into a number of toroidal cells. These cells develop a rapid circulation pattern which continues even if some of the Tia Maria is sipped through the cream. How and why do these cells develop and what is the energy source? (continued)
鈥� We are glad to see that this Last Word question from 1995 inspired a research project by Julyan Cartwright at the Laboratory for Crystallographic Studies in Granada, Spain, Oreste Piro at the Mediterranean Institute of Advanced Studies in Majorca, Spain, and Ana Villacampa at the Lawrence Livermore National Laboratory in California. Their paper 鈥淧attern formation in solutal convection: vermiculated rolls and isolated cells鈥� was published last year in Physica A, (vol 314, p 291). The first author has sent the following account for us 鈥� Ed
鈥� We tried this and were hooked. It is beautiful to watch these patterns form, and how different patterns form in layers of cream of different thickness.
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This is all caused by convection. Convection is the bulk movement of fluid, often associated with temperature differences 鈥� thermal convection. In Tia Maria and cream the convection is a driven by a difference in concentration, and is called solutal convection.
The important component is the alcohol in the Tia Maria. After the cream is poured on top of the liqueur, the alcohol begins to diffuse through the cream layer. When it reaches the surface it alters the surface tension: the more alcohol at the surface, the lower the surface tension. Regions of higher surface tension then pull liquid towards them from the regions of low surface tension. As the surface liquid is pulled away, the liquid beneath these regions of low surface tension takes its place.
But this liquid contains yet more alcohol, because it has come from the part of the cream nearer to the Tia Maria below. It has an even lower surface tension and in turn gets dragged away. This positive feedback mechanism creates convection, which continues as long as there is a concentration difference to sustain it.
This type of convection, driven by surface tension, is called B鈥檈nard-Marangoni convection and it is particularly relevant to thin layers of fluid. It is important in situations like drying paint, and the same capillary or surface-tension forces also cause other types of patterns in alcoholic drinks, like the tears in glasses of wine (The Last Word, 29 April 2000).
The other important mechanism that can cause convection is buoyancy. But buoyancy-driven, or Rayleigh-B鈥檈nard convection, cannot be causing the patterns in Tia Maria because cream is lighter than Tia Maria, so Tia Maria with cream on top is buoyantly stable.
The patterns that form when a fluid starts to convect by either of these mechanisms have been well studied in situations such as rolls of clouds in the sky, or hexagons formed in a frying pan when a layer of oil is heated (The Last Word, 24 February 1996). Tia Maria is an oddity because the patterns are not normal rolls or hexagons.
Similar patterns have also been reported in the scientific literature, in particular in papers written in the early decades of the 20th century. The worm-like patterns in thin layers of cream were called vermiculated rolls, and the toroidal cells in thicker layers, isolated cells. Both appear when there is a surface film on top of the convecting substance that hinders movement between the surface and the bulk of the liquid. In this case, the fatty cream is partially blocking the surface, so these patterns appear.
More recent convection research has tended to ignore these types of patterns, and the old experiments have often been considered as inaccurate because the fluids were impure; the different patterns were thought to be the result of impurity. We have tried to redress this. After seeing the patterns in Tia Maria, we carried out solutal convection experiments with simpler pure fluids that still show the same patterns. You can find a thorough account, both with Tia Maria and more conventional lab chemicals, in Physica A as detailed above.
Julyan Cartwright, Oreste Piro, Ana Villacampa