Diary: Calculating with droplets

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Diary: Calculating with droplets
Diary: Calculating with droplets

Calculations with droplets


Science journalists in Germany read, it's not much talked about in the industry, a lot, but above all weekly "Science" and "Nature". This duopoly plays an amazing role in the reputation of the researchers who write in it, but also in global science communication. It is best to read both sheets in their online pre-release, so that you can celebrate the hottest topics in your own daily sheet as soon as they appear on the Internet, even days before the printed edition is in front of you.

Of course I do the same thing, and it takes time. But if you don't read, you remain stupid and lose. In addition to the specialist articles, the comments and almost journalistic features in both magazines are quite useful, written for almost everyone who has ever held a stylus in research.

I was recently impressed by the reporting in "Nature" on the 4th climate report of the CPCC - including the still open problems in climate research (in the April issue of "Spektrum" we also publish an article on this aspect). And in Science, a curious headline just caught my eye: "Can Droplets and Bubbles Think?" (Science, Vol. 315, pp. 775, 828, and 832). So: Can droplets and bubbles think? Or, in order not to rave about watery artificial intelligence, a little more modestly: can you calculate with droplets? That will have to be proven. What seems odd today doesn't always have to stay that way.

I have to think of another computing device that once went exactly the opposite way: from the popular universal device to today's curiosity: the slide rule (in the April issue of Spektrum we are putting a small monument to the forgotten instrument). My grandfather, a seasoned electrical engineer in Regensburg, I can't imagine without this mute helper. During debates, he would pull out his little Faber-Castell pocket model every moment to explain a particularly tricky subject to me, who was practicing fractions. With multiplication, division and three-digit precision he had an explanation ready for almost all (his) world problems. And in his world, the Bavarian Forest, he was always successful. As he once explained to his astonished grandson, in the 1930s he had helped to “electrify” a number of communities, i.e. to supply them with electricity. Yes, even in these backwoods areas, which were still isolated at the time, there was life before electricity. And on the wall in my grandfather's living room, between all sorts of self-killed deer antlers, a certificate flaunted: honorary citizen of the municipality of so-and-so, in recognition of the electrification.

But back to the calculating droplets. How does that work? This leads to the delicate field of microfluidics, where researchers have been handling the tiniest droplets since the 1990s. In the nanoliter range, water or similar chemicals behave very differently from raindrops. Adhesive forces become so strong that nanodroplets can be guided freely along conductor tracks on surfaces, for example, driven by electric fields.

Active ingredients for pharmaceuticals can thus be efficiently mixed on the smallest scale. In the narrowest channels, no wider than a tenth of a millimeter, droplets (alternatively also bubbles) can now be pushed through. At the junctions, the nanostructure “decides” whether to continue swimming to the left or to the right, depending on where the current encounters the least resistance. And since a droplet itself increases the resistance in the chosen channel branch, a subsequent droplet will move to the other channel.

In tiny ring channels, whole droplets can be sent through in series (and thus store information) and fed back into other channels (and thus change other information). The basic Boolean operations AND, OR, and NOT can be represented with some refinement – the droplet calculator is born. Response times in milliseconds seem feasible. The chemist Irving R. Epstein asks at this point in "Science": "Can in fluido compete with in silico or in cerebro?"

They seem suitable at least for simpler control and memory functions, says the expert, and labyrinth puzzles have already been solved with them. Higher complexities promise ingredients from nonlinear chemical dynamics. Then it would be over with the oddity. Perhaps in a few decades we will also be using such liquid calculators, probably without even noticing it. I suspect it won't quite be enough for "thinking".

Reinhard Breuer

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