Doctor Who employs his fictional sonic screwdriver in a vast range of situations that includes opening locks, breaking into computers and cash machines, defusing bombs in addition to rotating screws. However, research suggests that this iconic science fiction device is at least partly based on science fact.
The idea that sound waves carry energy seems intuitively reasonable – think of the physical feeling we humans experience when we get close to powerful loudspeakers with heavy sub-bass found at concerts and clubs. Sound can be felt physically and not just heard. A fantastic demonstration of this phenomenon can be found in acoustic levitation experiments: if the distance between a loud speaker and a reflector is adjusted so that a standing wave is formed, objects can be levitated and held aloft at low-pressure regions known as nodes.
While this looks like spooky action at a distance, it’s purely down to the fact that acoustic waves, like their electromagnetic counterparts, carry momentum. This means they can apply a force, usually called the acoustic radiation force. If the force is stronger than gravity, objects can be levitated. Loudspeakers generally produce linear momentum, so that they can push objects in straight lines.
This may be of great use for repelling Daleks, but useless for turning screws. This is where acoustic vortices come to the rescue: these are acoustic waves with wavefronts shaped in a spiral pattern, called a helix (like one strand of the DNA double helix). This spiral pattern provides acoustic vortices with rotational, rather than linear, momentum. If this momentum can be transferred to an object – such as a screw – the result is a sonic screwdriver.
The first to get close to a real sonic screwdriver was a research team from the University of Dundee who in 2012 created an acoustic vortex with a special medical ultrasound transducer designed for destroying tumours. They used this device to rotate a large disk made from a material which absorbed the rotational momentum of the waves. This was impressive, but it doesn’t replicate many of the sonic screwdriver’s capabilities. We’ve gone a step further by showing that similar devices can be scaled down and used to manipulate microscopic particles.
We created the required swirling sound waves using a number of tiny ultrasonic loudspeakers arranged in a circle. This device, only 10mm in diameter, created acoustic vortices of around 1mm in size. In turn, these tiny acoustic vortices were able to rotate objects measuring between one and 100 microns – about the width of a human hair. If the size of the objects was just right, the acoustic vortex acted like a tiny tornado.
Accoustic vortex, seen in the experiment (top) and in the theoretical predictions (bottom), showing how mm-scale accoustic vortices spin 0.5 micron tracer particles. Colour indicates the rotational energy of the vortex. Bruce Drinkwater/University of Bristol, Author provided
For example, when a mixture of household flour and water was placed in the device, the flour particles were drawn into the vortex core where they were spun around at high speeds. Conversely smaller particles just moved slowly around in circles and were not attracted to the vortex core at all. This millimetre scale means that we now have what could be described as a watchmaker’s sonic screwdriver, potentially capable of undoing the very smallest screws.
So while it’s great to go some way to grounding the imagination of Doctor Who’s scriptwriters in sound science, do these acoustic vortices have any uses in the real world? The answer is yes, but perhaps not in the ways that that Doctor Who might imagine. For example, they could be used to create microscopic centrifuges for sorting biological cells, or for water purification. What makes these possible is that this latest study has shown how different-sized particles behave differently when exposed to tiny acoustic vortices.
More exciting is the knowledge that the particles' motion is also extremely sensitive to their material properties, such as stiffness and density. This could lead to new methods for medical diagnostics. If, for example, healthy cells can be distinguished from unhealthy ones (cancerous cells are thought to be softer then healthy cells), these detections could be possible on a very small scale – perfect for medical diagnostics and for forensics.
It’s likely that acoustic vortices will soon join existing methods as a new tool for the controlled manipulation of tiny and microscopic matter. So sometimes science fact is just as interesting as science fiction – now if someone could just reverse-engineer the TARDIS.