Bruce Drinkwater is an Engineer and Professor working at the University of Bristol, Bristol, UK. His interests span many applications of ultrasonics including imaging, condition monitoring as well as ultrasonic levitation, tractor beams and haptics. In 2015 he co-invented the world’s first stable ultrasonic tractor beam, in 2017 he demonstrated the levitation of wavelength-scale objects and in 2019 the levitation and manipulation of multiple objects. Potential applications of this technology are diverse, and he is now working on using these ideas for ultrasonically assisted 3D printing and the assembly of living tissue. He is a passionate advocate of the importance of engineering and science to society, be it by improving our living standards or, alongside the arts, worthy of being pursued for its own sake. He has discussed his work on Radio and TV and exhibited at various science festivals, most notably at the Royal Society Summer Science Exhibition in London. In 2016 he spoke at a Star Trek film screening on the subject of tractor beams, thereby bringing together science fact and science fiction.
Describe something that has recently amazed you and how it made you feel.
The tiny scales on the wings of moths are amazing in many ways. The most obvious thing they do is to hold the colours we see when we look at a moth (or butterfly). But moth wings are particularly special as they appear to have been tuned to absorb the ultrasonic waves that bats use to echolocate them – and bats eat moths, so the evolutionary pressure is clear. Under a very high magnification microscope these scales, which are the moth equivalent of hairs, are incredibly finely textured. They look like a delicate lattice structure. I recently helped characterise this structure and investigated how it vibrates in response to the incoming bat ultrasound. But the existence of a structure like this, which is so beautiful and yet so functionally perfect is truly breathtaking. I can’t help but think of the millions of generations of moths going back into the mists of time and how small random changes, alongside the evolutionary pressure, have led to what we see today.
Can you tell me about your research journey?
It’s quite a simple one really. I worked hard at school, studied Mechanical Engineering at Imperial College, stayed on to do a PhD in the same department and then got a Lecturing job at the University of Bristol, where I still work. My choice of PhD topic – the study of ultrasonic transmission across contacting interfaces – turned out to be life changing as I’m still researching ultrasonics. This choice was no great insight on my part, I just chose a topic that seemed interesting and would allow me to work with a clever and supportive supervisor (Professor Peter Cawley). But it turned out to be an incredibly important choice as, not only was my supervisor great, but the team of PhDs I joined was inspirational. This experience made me a life-long collaborator and believer that people are at the heart of research as much as ideas are. Once I got a job on the academic staff at Bristol, I made it my mission to recreate this positive experience for my own students.
Of course my research journey is still ongoing and I’m always thinking about what is next. What actually comes next depends on what I’m currently working on and how well things work. It also depends on down-to-earth things like funding as well as the creative side of things such as new ideas which I may or may not have. So, for me, I feel that this journey has a strong random element to it that only makes sense in hindsight. That’s not to say I blindly plod forward, I’ve got some ideas which I know may take 10 years or more to achieve, so there is a long-term horizon. But what I want to get across is that as a researcher you have to be open to the uncertainty of the journey and I’ve always tried to embrace this.
My understanding of ultrasonics is very limited (apart from medical imaging and scaring off cats from gardens). What are the obstacles and opportunities in using ultrasonics? What are the really interesting developments in the field?
The field of ultrasonics may seem small from the outside, but it’s actually very diverse and offers a huge number of possibilities, often in collaboration with other fields. Ultrasonic imaging is still a very active field, both in medical and engineering applications. Increases in computational power over the last decade have transformed what is possible. I have been working on characterising cracks using an approach that shares a lot in common with a Google search. First, I simulate the ultrasonic reflection from thousands of cracks and store these results. Then, when a measurement is made, I compare this measurement to the stored database, so it’s basically a search routine. This idea could also be used assist and possibly automate diagnosis in medical imaging. As well as these diagnostic uses, there are many physical effects of ultrasound and these have opened up some particularly exciting areas in recent years. On the large scale, ultrasonic haptics can create the sensation of touch in mid-air. This is amazing technology with applications such as in virtual reality environments. On the smaller scale the interaction of ultrasound with cells is still mysterious. Recent very exciting results show that ultrasound can stimulate the brain and reduce tremors in Parkinson’s patents. Whilst this is an incredible result, at present the mechanisms are not understood, so there is still a huge amount of work to be done.
You and your team are famous for the work in levitation and tractor beams. There’s a magical quality, or science fiction-esque element, to this work. Seeing particles defying gravity and moving with no visible force has an unreal element to it. Is this a common reaction? Does it still amaze you?
Amazement at acoustic levitation experiments is normal. Although I’ve seen levitation work many thousands of times now, I still think it’s cool and still find myself staring at it and thinking what else could I levitate. It’s not magic, but like any good magic, it defies our expectations – in everyday life levitation just does not happen! I also have vivid recollection of the first time we got the tractor beam to work. Levitation experiments that don’t work are quite disappointing – it’s a binary phenomenon, in that it either works or it does not. We experienced months of such disappointment. My colleague Dr Asier Marzo cracked the experiment through incredible patience, and when he had got it working, he called me and a group of other researchers in to see it. And, in that moment I knew we had created the first ever ultrasonic tractor beam. It was quite a special moment – hard to describe – certainly a moment of wonder.
You can find out more about these holographic acoustic tweezers by reading this recently published research paper.
What do you need to be able to levitate a particle? And how easy is it for non-specialists to make their own levitator?
You need a signal generator, a source of high amplitude sound and a means of controlling the sound field. Ultrasound is preferable to audible sound due to the high intensities needed – these intensities would make an audible levitator deafening. Ultrasound, and particularly high frequency ultrasound is preferable and safer. Although care should always be taken as the effect of ultrasound on hearing is not an area that has been well studied. The simplest way to control the field is to point the speaker at a reflector and this creates an interference pattern known as a standing wave. The same effect can be achieved by pointing two speakers directly at one another. For the source a tweeter loudspeaker works, as does an array of parking sensors. The loudspeakers have to be arranged carefully, but it’s quite a reliable experiment. We have published various instructions on how to make levitators and I’ve had lots of emails about levitators that people have made. I know that a 10-year-old made one, with a bit of help from his dad (but not much).
What are the future applications for levitators and tractor beams? How large/heavy a particle can you levitate?
The physical principles of acoustic levitation are scalable in size, both up and down. The trouble with larger objects is that the power consumption of the devices becomes an issue, so my research is focused on levitating the small and the very small. At these small scales there are not many competing technologies and so ultrasound has clear advantages. And by very small I’m usually thinking about cells. One idea I’m working on is a device that could be integrated with a microscope and enable the viewer to manipulate anything they see, making microscopy much more interactive. The ultrasonic forces can also be used to squeeze the cells and, by measuring how much they deform, we can potentially diagnose diseases and track biological processes. Of course, I’d also like to be able to levitate large objects such as human beings, sadly, for the moment the funding for such a large-scale project is just not there.
How does wonder and awe fit into the life and work of a research scientist?
The majority of research is relatively slow and detailed work, and I love this aspect. But occasionally you discovery something new and this is a truly wonderful experience. I wish there were more of these moments, but perhaps their scarcity makes them all the more valuable.
We often think about scientists being curious but often universities and businesses crush the curious spirit (bureaucracy, deadlines, fear of failure tec.). What are the best environments for fostering curiosity and how as a team leader can you keep that spirit alive?
For me, collaborating with other researchers either by just talking about research ideas or working together on longer term projects is hugely important for my motivation. It’s also the main source of the creative element of my job – coming up with new research directions. Some time for quiet thought is also important and this time can be hard to find in the modern world. When I get overwhelmed with bureaucracy and deadlines, as does happen, I try to remind myself about the parts of my job I enjoy the most and this helps me stay focused. I’m also fortunate in that the University of Bristol have done a good job of insulating me from the worst of the bureaucracy that is out there in the university system. I hope that by being personally motivated and enthusiastic about my research, I can pass some of this excitement onto others.
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