Feb 2, 2017
Researchers at the University of Bristol have discovered a way to create a simplified tractor beam – using readily available parts and spending less than £70 in the process. Here’s how.
What is a tractor beam?
The fundamental principle of tractor beams isn’t new. Essentially, it involves the use of sound waves to move macroscopic objects or to create patterns in flowing water or resting sand (there are plenty of YouTube videos, illustrating what it looks like in practice). Sound is a mechanical wave, carrying enough momentum to generate acoustic radiation forces. When strong enough, these waves can be used to levitate objects against gravity.
However, tractor beams are about more than sonic levitation (pushing objects around using sound). Tractor beams are based on similar fundamental principles as those used to form optical traps. As the name suggests, the beams literally pull objects, trapping small beads (and sometimes even insects) at their foci.
Asier Marzo, Research Assistant at the University of Bristol, explains further: “When you move the tractor beam, the particle moves, but otherwise, the trap is static. It can levitate small plastics; it can also levitate a fly and small biological samples. It’s quite handy.”
Where can it be used?
Acoustic levitation is already becoming a fundamental tool in many different fields, including pharmaceuticals, microscopy and lab-on-a-chip scenarios. Remarkably, it can not only be used to levitate biological samples, but even heavier objects like small animals.
Acoustic tractor beams have the potential to revolutionize contactless manipulation. Marzo’s 3D printed version can be used to achieve the same results, at a greatly reduced cost.
He explains further: “We generated acoustic holograms that surround the trapped particles from all directions. Three types of holograms were proven to be the most effective; those that were shaped as tweezers, tornadoes, or bottles. To generate the holograms, we used dozens of tiny speakers all emitting with the same amplitude and frequency but with different phases, creating three-dimensional interference patterns.”
Creating a tractor beam with 3D printing
Traditionally, creating a tractor beam is complicated, not to mention expensive. However, Marzo and his team have successfully developed a working tractor beam, which is fully operational in both air and water – all using just one electrical signal and a passive wave modulator.
“The technique,” Marzo explains, “can generate an acoustic tractor beam using only a single electrical signal. This will reduce the cost and complexity of tractor beams, making them a more affordable technology for manipulating and analysing levitated samples. With our new research, now everyone can have an acoustic tractor beam.”
After the initial design phase, Marzo developed a 3D printed version. In addition to publishing details in a paper in APL (Applied Physics Letters), he’s also released a step-by-step guide, demonstrating how to make it yourself.
The passive wave modulator is a type of acoustic lens designed to alter the transmitted or reflected waves. The research team’s passive wave modulator can be created in two different ways – either with a collection of different sized tubes, or using a carefully contoured surface. Both can be created using a desktop 3D printer.
The team is passionate about making their research open access and making science accessible to as many people as possible. As a result, they’ve released step-by-step instructions, detailing how to build your own acoustic tractor beam, using readily available components for under £70. Some basic skills are required (such as soldering), but otherwise, it’s a relatively easy introduction to electronics.
Marzo and his team encountered a few challenges along the way. Creating a suitable design for 3D print was difficult, as some of the holes are less than two millimeters in diameter. The printer can create narrower conduits, but clogging can become an issue.
Additionally, obtaining cheap, readily available electronics was challenging. The team had to test several possibilities before finding the best ones for the job. Eventually, the device was developed with three different designs, with trapping profiles suitable for objects of different sizes. Despite this, it was still tricky to trap objects larger than half the wavelength of sound – in the case of practical frequencies (just above audible levels) this restricts trappable object size by a few millimeters.
The researchers wanted to prove that acoustic beams can be used to generate pulling forces. However, their findings opened up a range of possibilities for the future. For example, samples of blood could be levitated for optical inspection, and chemical compounds could be merged without containers, eradicating the risk of contaminations. It would even be possible to move kidney stones within the body, without the need for an incision.
In the past, generating an acoustic tractor beam required a phased array of over 50 channels, with each channel made up of a signal generator and amplifier. These complex electronics were a real obstacle to its application.
DIY tractor beams resolve this issue and have several potential uses. According to Marzo, ‘micro-gravity research’ has already intrigued and motivated biologists enough to develop their own applications for the device.
Currently, the 3D printed version can only move the particle up and down. However, it’s significantly more cost-effective and far less complex than traditional models. In the future, the team plans to create a comprehensive kit, using it to show school students how to create a tractor beam and demonstrating what can be achieved with one.
Their hope is that, through sharing their design and spreading the word, this will open the way for limitless uses and improvisations within the maker community. It remains to be seen how users will implement their own tractor beam!