College of Science and Engineering students with metamaterial object. (Image: University of Minnesota)

University of Minnesota researchers have discovered a new method of moving objects using ultrasound waves. The research — which opens the door for the application of contactless manipulation in industries such as manufacturing and robotics, where devices wouldn’t need a built-in power source to move — was published in Nature Communications.

It’s been demonstrated before that light and sound waves can manipulate objects, but the objects have always been smaller. The team has developed a method that can move larger objects using the principles of metamaterial physics.

By placing a metamaterial pattern on the surface of an object, the researchers were able to use sound to steer it in a certain direction without physically touching it.

“We have known for a while that waves and light and sound can manipulate objects,” said Professor Ognjen Ilic, senior author. “What sets our research apart is that we can manipulate and trap much bigger objects if we make their surface a metamaterial surface, or a ‘metasurface.’”

Using this technique, the researchers can also pull an object toward a source — in addition to moving it forward.

“Contactless manipulation is a hot area of research in optics and electromagnetism, but this research proposes another method for contactless actuation that offers advantages that other methods may not have,” said grad student Matthew Stein, first author.

The team aims to test out higher frequencies of waves and different materials and object sizes in the future.

“I think we’re charting in a new direction here and showing that without physical contact, we can move objects, and that motion can be controlled simply by programming what is on the surface of that object," Ilic said. "This gives us a new mechanism to contactlessly operate things.”

Here is a Tech Briefs interview, edited for length and clarity, with Ilic.

Tech Briefs: What inspired your research?

Ilic: We were interested to study how the energy and momentum of waves can be used to move objects without physical contact. We learned that if we turned the surface of an object into a metamaterial surface (also called a metasurface), we could realize new modes of motion, such as autonomous self-guidance or tractor beaming, that were previously not possible.

Metamaterials are materials that have precisely engineered patterns that give them properties not found in natural materials. These patterns are usually periodic.

Tech Briefs: What were the biggest technical challenges you faced?

Ilic: The process of designing the metasurface pattern is involved and informed by computer simulations. But once we got the designs down, the actual fabrication was done with standard 3D printing.

Tech Briefs: Can you explain in simple terms how the technology works.

Ilic: Waves can exert pressure on objects in their path, but the ability to control this pressure is severely limited by how all “conventional” objects scatter waves. In our approach, we show that if we deliberately pattern the surface of an object, we can precisely control the pressure of the wave at each point on the object (the object can be almost anything, as long as we can etch tiny patterns into it). This new control enables contact-free actuation phenomena (guidance, steering, tractor beaming) that were previously not possible.

Tech Briefs: What’s the next step with regards to testing at higher frequencies?

Ilic: We are interested in higher frequencies as this allows is to make objects smaller and still make it possible to move them without contact. We are also looking at different types of metasurface patterns. This paper has the first results to demonstrate the concept, but there are many (infinite!) ways that a surface can be patterned into a metasurface. We would like to survey and understand which metasurface “unit cell” topologies work best and how this concept can be scaled up or down, depending on the target application.

Tech Briefs: Do you have any advice for engineers aiming to bring their ideas to fruition?

Ilic: In our research, we are always on the lookout for how our skills and knowledge across different areas can come together to give us fresh and new ideas to solve a problem. Here, we combined our knowledge of wave physics with our previous experience in designing metamaterials for very different applications: This combination led us to solve the contactless actuation problem.

Tech Briefs: Is there anything else you’d like to add?

Ilic: We are excited about the future implications of this concept for robotics. The ability to actuate and move objects from a distance and without physical contact could lead to new applications and devices for robotic surgery, guided drug delivery, manufacturing, and potentially even long-range actuation and tractor beams.

One exciting application is to use light (not sound) and optical metamaterials for space propulsion. Sail-like spacecraft could be propelled using light and would not need to carry any fuel on board. Some of our research, and research of others, explores how optical metamaterials can be used for long-range actuation and propulsion.