Multiple droplets of a magnetic fluid have been used to create all of the fundamental logic circuits within a computer. Created by researchers in the US, the circuits are made by having the interacting droplets move through a matrix of interconnected tracks while under the influence of an applied magnetic field. While still at an early stage, the research could provide a new platform for creating lab-on-a-chip technologies, as well as provide insights into the fundamental physics of collective behaviour.
While electronic computers process abstract information, it is often the case that their ultimate function is to control a real physical system such as a manufacturing or sensing process. Now, Manu Prakash and colleagues at Stanford University in California have created a system that combines the control of both information and matter at the same time.
Their circuits are based on droplets of water that contain magnetic nanoparticles, which are sandwiched between a thin layer of oil and a piece of glass embedded with iron tracks. When a rotating magnetic field is applied, it creates a set of rotating energy minima.
Ratcheting and repulsion
If the base was just a sheet of iron with no tracks, the droplets would travel around in circles, following the energy minima created by the field. However, by carefully designing the iron tracks and incorporating breaks at the right places, the researchers can create a “ratchet” effect whereby every complete rotation causes a droplet to move into an adjacent energy minimum. Therefore, instead of travelling in circles, a droplet moves in a specific direction through the circuit. Furthermore, by creating two tracks that are mirror images of each other, two droplets will rotate in opposite directions in response to the same field.
The droplets also repel each other because of a combination of hydrodynamic and magnetic forces. Prakash says that this mutual repulsion between droplets allows the team to create “the droplet equivalent of a transistor” in which the presence (or absence) of one droplet will dictate the path taken by another droplet. Using this, and denoting the binary numbers 1 and 0 by the presence and absence of a droplet, respectively, the researchers created droplet logic gates. These gates can perform the complete set of Boolean-logic operations, which forms the basis of computer programs.
By combining these gates appropriately, says Prakash, the researchers could, in principle, execute any computer program. Crucially, Prakash explains, all of the droplet logic operations run at exactly the same clock frequency – that of the applied field. This enables the parallel processing crucial to electronic logic, in which, for example, the output of one logical operation can depend on two inputs, each of which is, in turn, the output of separate previous logical operations. The universal clock frequency ensures that both inputs arrive at exactly the same time and are read correctly by the logic gate.
Manipulating matter
Prakash stresses that the purpose of the research is not to supersede the electronic computer, but instead to enable what he describes as “algorithmic manipulation of matter”. A droplet moving through such a circuit can encase molecules or cells, thereby delivering them to particular places or ordering them in specific ways. This could be useful for the chemical or biological analysis of samples in lab-on-a-chip systems or in the directed or self-assembly of larger structures from components carried in droplets.
Manoj Chaudhary of Lehigh University in Pennsylvania says that he found the research “quite fascinating”. “People have been working on what we call digital fluidics for quite some time,” he says, “but they have been controlling the motion of droplets one after another. The researchers have tried to build a new philosophy here by controlling multiple droplets simultaneously.” Chaudhary suspects that the research may have applications to fundamental physics by building in noise or nonlinearity, and looking for signs of collective behaviour emerging from large numbers of interacting droplets.
In the video below, Manu Prakash and colleagues explain how their droplet logic circuits work.
The research is described in Nature Physics.
