3 droplets with circulating chemical fronts can store information. Image Credit: IPC PAS, Grzegorz Krzyzewski

The ‘Chit’ – a One Bit Chemical Memory Unit

In traditional computer science, information is stored in bits, while in quantum computer science the unit is quantum bits, i.e. qubits. Research done at the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw has shown that chemistry can also be used to store information. When three droplets are placed in contact with each other and oscillatory reactions occur, it becomes a chemical bit, called the ‘chit’.

Consumer electronics such as computers, smartphones and digital cameras all work with memory chips. In all electronic memory, zeros and ones are detected, stored and read by physical occurrences such as the change in electrical or magnetic properties of the medium, or the flow of electricity.

Prof. Jerzy Gorecki and Dr. Konrad Gizynski from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in Warsaw have shown a different type of working memory based on chemical phenomena. A single bit is can be stored in three adjoining droplets. Chemical reaction fronts in the droplets propagate cyclically, steadily and in a manner that is strictly defined.

The so-called Belousov-Zhabotinsky (BZ) reaction forms the chemical foundations of the memory created by the IPC PAS researchers. The sequence of the reaction is oscillatory, which means that when one cycle is complete, the reagents necessary to start the next cycle are reconstituted in the solution. There are normally anything from tens to hundreds of oscillations before the reaction stops. The oscillations go together with a regular change in the color of the solution. This is caused by the reaction catalyst, ferroin. The Warsaw researchers used ruthenium as the second catalyst. The introduction of ruthenium was critically important because it results in the BZ reaction becoming photosensitive. When the solution is lit by blue light, the oscillations stop. This reaction makes it possible to control the course of the reaction.

Prof. Gorecki explained that their idea for the chemical storage of information was simple. In previous experiments, they had discovered that when BZ droplets come into contact with each other, chemical fronts can be propagated from droplet to droplet. The team decided to look for the tiniest droplet systems in which excitations could take place in various ways, with at least two droplets remaining stable. The researchers then assigned a logic value of 0 to one sequence of excitations, while the other was designated as a 1. In order to switch between excitations states, i.e. to force a specific change of memory state, light could be used.

Experiments were conducted in a vessel filled with a thin layer of lipid solution in oil (decane). Tiny amounts of oscillating solution were added to the system when a pipette was used to form droplets. The droplets were located above the ends of optical fibers that had been positioned at the base of the vessel. Several rods extending from the base of the vessel prevented the droplets from sliding off the optical fibers by immobilizing each one.

Pairs of coupled droplets were studied and four types (modes) of oscillation identified:

  1. Droplet 1 excites droplet 2.
  2. Droplet 2 excites droplet 1.
  3. Both droplets excite each other simultaneously.
  4. Both droplets excite each other alternately, i.e. when one is excited, the other one is in the refractory phase.

Dr. Gizynski explained that in paired droplet systems, it most often happens that one droplet excited the other. Only one mode of this type is however always stable and two were required. Although all droplets are made up of the same solution, they never have exactly the same size. This results in the chemical oscillations in each droplet occurring at a slightly different speed. The droplet in a pair oscillating the slowest begins to adjust its rhythm to that of the faster one. Even if it were possible to use light to force the slower oscillating droplet to excite the faster one, the system would always return to the mode in which the faster droplet stimulated the slower one.

The team then looked into triplets of adjoining droplets arranged in a triangle with each droplet touching its two neighbors. Chemical fronts in this configuration can propagate in many ways. Droplets could oscillate simultaneously, or in anti-phase, or two droplets could oscillate at the same time and force oscillations in the third, etc. The researchers were mostly interested in rotational modes where the chemical fronts passed from droplet to droplet in a 1-2-3 sequence, or in the opposite direction (3-2-1).

A droplet in which the BZ reaction proceeds excites rapidly, but it returns to its initial state much slower. It can become excited again only when it has reached its initial state. If the excitation were to reach droplet 3 too quickly in the 1-2-3 mode, it would not be able to get through to droplet 1 to start a new cycle, as droplet 1 would not have reached its initial state yet. This would result in the rotational mode disappearing. The researchers only focused on rotational modes that could achieve multiple repetitions of the excitation cycle. This mode has an added advantage in that the chemical fronts that circulates between the droplets resembles a spiral wave. Spiral waves are characterized by increased stability.

Further experimentation revealed that both of the rotational modes that were studied are stable. If a system enters either of them, it remains in that mode until the BZ reaction stops. By correctly selecting the length and time of illumination of relevant droplets, the team proved that they could change the direction of rotation of the excitations. The triplet droplet system with its multiple chemical fronts was therefore capable of storing one of two logic states permanently.

Dr. Gizynski commented that the chemical bit has in fact a slightly bigger potential than the traditional bit. The rotational modes that were used to record states 1 and 0 had the shortest oscillation times of 19.5 and 18.7 seconds respectively. If the system were to oscillate slower, there might be an additional third logic state. Gizynski noted that this third state could be used to verify the correctness of the record, instead of using it to store information.

The full study was published in the journal Physical Chemistry Chemical Physics.