Many years ago I was having a quiet drink with a university friend. I had Baileys, and I'm sure that what she had was Tia Maria and orange juice - or at least, one of those two and something else; but anyway, it tasted like chocolate. A few years after that, I tried that combination and it was absolutely revolting, so plainly I'd remembered something wrong. In fact, I was unlucky enough to acquire a nice big bottle of Tia Maria only to discover I don't actually like it very much - and today it suddenly occurred to me that it might actually serve an excellent scientific purpose!
My first mistake was to just go ahead and pour a layer of cream onto a glass full of Tia Maria without reading the book first. I put far too much cream on, and also poured it in in my usual lumberjack style:
This, on the other hand, is not enough:
It should be about 2mm thick, and poured down the inside of the glass. (In fact they recommend poured off a spoon, but my second method worked just as well.)
Well, the first result - with the much-too-thick cream layer - was interesting; there was a doughnut-shaped area in which the liquid was clearly circulating:
Wondering why it wasn't working, I then made another attempt to find the right book - and finally did, then wished I'd done that first. I tried a lot of repairs on that first glass. I tried to get most of the cream out with a spoon. Nothing really seemed to work, except the formation of a few cool holes like the one in the video above. When I couldn't think of anything else to do, I stuck the spoon in and stirred it all up to see what would happen! Nothing much did, except the formation of five or six darker spots on the surface, arranged in a circle.
I then re-read the instructions again, and filled another wine glass, this time with only a little cream poured as described above. With the first few sprinkles, I noticed an interesting "beach-like" effect. It happened too slowly and subtly for the video to work, but you'll see the dark lines appearing around the edge of the cream, creating two quite different cream-scapes:
Actually I took an awful lot of videos, including one of a tiny white speck which was slowly tumbling over and over itself, a millimetre or two below the surface - but I've restricted myself to two. Below is what happened when I added the correct amount of cream . . .
Curious cells formed, the shape of squishy defrosting onion rings. Tia Maria seemed to be rising up from the middle, and the cream seemed to be sinking at the edges!
Anyway, because I botched the first experiment and because I wanted to fill the glasses adequately, to provide sufficient alcohol, I now have plenty of spare creamy Tia Maria. If nobody turns up fast enough to help me with it, my plan is to make a chocolate cake. The less creamy Tia Maria will be added to the mixture; the creamier samples and spooned-off cream will go into whipped cream, as will a little coffee and some grated chocolate!
Now your mouth's watering too much to think, what's actually going on? It's convection. Not the well-known convection caused by molecules of liquid moving around to transport and diffuse heat, a hot place to a cold place - but convection of concentration. The alcohol from the Tia Maria can diffuse through the cream; when it reaches the surface the surface tension plummets. Surrounding areas with high surface tension draw this liquid towards themselves. As the liquid is displaced, Tia Maria moves upwards to fill it, again reducing the surface tension . . . It's a positive feedback mechanism. So it'll go on as long as the Tia Maria feels like it.
This is a particularly lovely story because the reader asked the question back in 1995, and three scientists from my old foreign exchange university, la Universidad de Granada in Spain, actually investigated this and wrote a scientific paper. What a trial those experiments must have been!
You can read about the science here in the Times, and watch a much better experiment - with single cream and Tia Maria on a plate - on this video. And by the way, one day I really hope to acquire a distinctly better camera. I lost my old one about 18 months ago, in London, I think, and have been cursing myself ever since.
(P.S. Did you notice the coaster? All right, here you go. "Rooster" to you folks across the pond . . .)