Music Under the Microscope

Droplets on a microfluidic chip can be controlled so precisely that they become a musical instrument

Water droplets are musical. Researchers at the Max Planck Institute for Dynamics and Self-Organization in Göttingen have converted the frequencies of droplets flowing through thin channels into musical notes. This is more than just a gimmick: The fact that droplets can be controlled so precisely that they become musical instruments means they are also of interest with regard to medical diagnostics applications. Scientists are currently developing miniature laboratories on microfluidic chips, for example, which would make it possible to examine minute samples of fluids, such as blood in an extremely small space. The new procedure brings researchers a big step closer to reaching this goal.

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Musical notes and melodies are created when sound waves oscillate at different frequencies. Vibrating guitar strings, air flowing through a flute or a loudspeaker membrane: these are all examples of vibrations that move through the air and reach our ears. The higher the frequency – i.e. the faster the oscillations – the higher the pitch of the resulting note.

Researchers at the Max Planck Institute for Dynamics and Self-Organization in Göttingen have now developed a method of making music using water droplets. The scientists used high AC voltage to control exactly how many droplets flowed through tiny channels on a plastic chip per second. These droplet frequencies were then electronically converted into musical notes. The researchers presented their new process in Scientific Reports, an online research publication of the Nature Publishing Group.

Chips for Medical Applications
The research being conducted with regard to this new musical instrument could also be put to use in practical applications, due to the fact that the researchers have achieved a degree of control over the droplets that could also be promising for medical uses: new diagnostic methods are currently being developed to examine samples of patients’ fluids (such as blood or DNA) in the form of tiny droplets. This approach requires what are known as microfluidic chips – the same technology used by the Göttingen-based researchers. This type of chip is made from transparent plastic traversed by thin channels. These channels transport oil and water. Since these two fluids do not mix, the water forms small droplets in the oil. “Scientists could trap DNA molecules or cells in these droplets to examine them, for example,” explains Jean-Christophe Baret, who heads the team of researchers at the Max Planck Institute in Göttingen.

A Simple yet Tricky Method
Using this method for medical diagnostics requires a high degree of precision when controlling the movement of the droplets. One potential application could be to sort the cells trapped inside the droplets according to specific criteria. This is already possible with the help of electric voltage.

The researchers in Göttingen further developed this method by applying an AC voltage of up to 1000 Volt on the microfluidic chip at flow focusing junctions. This resulted in the formation of tiny water droplets that are produced in the electric field  with a diameter of just a few micrometres, only visible under the microscope. The higher the applied voltage, the quicker the succession of the droplets – the higher their frequency.

The researchers have now translated these frequencies into musical notes. For this purpose, they added a fluorescent agent to the water, so that the droplets emitted light when illuminated with a laser. A photomultiplier tube converted the light into electric signals, which in turn were used to create the respective notes with the help of a sound card.

Putting the Method to the Test: Ode to Joy
The first test was to play a simple note scale. To begin with, the researchers had to “tune” their new musical instrument by correlating different electrical voltages with different pitches. The first melody they played was Ode to Joy. It is easy to recognise the beginning of this Beethoven symphony, even though the intonation of the melody is not free of errors: the frequencies occasionally deviate from the original note by up to five percent, which is roughly equivalent to a half-tone.

These inaccuracies are due to the mechanical properties of the microfluidic chip, as a result of which the intervals between the successive droplets are not always identical. Even the electric voltage needs some time to adopt a new value. The droplet frequency therefore changes with a slight delay, which is audible as a short glissando in the melody, i.e. as a gliding change in pitch, before the final note is reached.

An Entire Laboratory on a Chip
While it is doubtful whether this system will ever make a debut as an electronic musical instrument, the researchers were able to show that the movement of droplets can be controlled with a high degree of precision using electric voltage. This is a significant step in the development of a medical chip laboratory.

What makes this new procedure so important is that it allows many droplets to be controlled at the same time. “In the case of an early cancer diagnosis, for example, it is necessary to examine a large number of the patient’s DNA molecules in order to determine how much of the DNA has undergone mutation,” explains Baret.

The first microfluidic chips suitable for these and other analytic applications are already available on the market. However, if the goal is to select and sort samples according to specific criteria, the method described above is of particular interest: electric fields could be used to sort out infected cells or mutated DNA, for example. In future, one such chip could even replace an entire medical laboratory.