Researchers have managed to guide floating objects through an aquatic obstacle course using only soundwaves. This innovative technique, inspired by optics, shows great potential for applications in medicine, such as precise drug delivery.
In 2018, Arthur Ashkin was awarded the Nobel Prize in Physics for creating optical tweezers, which are laser beams that can manipulate tiny particles. Although optical tweezers are beneficial for various biological tasks, they require highly controlled and stable conditions to function effectively.
“Optical tweezers work by forming a light ‘hotspot’ to trap particles, similar to a ball dropping into a hole. However, in the presence of other objects, creating and moving this ‘hole’ becomes challenging,” explains Romain Fleury, the head of the Laboratory of Wave Engineering at EPFL’s School of Engineering.
Fleury, along with postdoctoral researchers Bakhtiyar Orazbayev and Matthieu Malléjac, have been working for the past four years on moving objects in unpredictable, dynamic settings using soundwaves. Their approach, known as wave momentum shaping, is not influenced by an object’s surroundings or physical characteristics. All that is required is the object’s position, and the soundwaves take care of the rest.
“Instead of trapping objects in our experiments, we gently guided them around, akin to maneuvering a puck with a hockey stick,” Fleury elaborates.
The unorthodox method, supported by the Swiss National Science Foundation (SNSF) Spark program, has been detailed in Nature Physics in collaboration with researchers from the University of Bordeaux in France, Nazarbayev University in Kazakhstan, and the Vienna University of Technology in Austria.
Simple yet promising
If we consider soundwaves as the hockey stick in Fleury’s analogy, then a floating object like a ping-pong ball would be the puck. In the lab’s trials, the ball floated on the water surface in a large tank, and its position was monitored by an overhead camera. Audible soundwaves produced by a speaker array at both ends of the tank steered the ball along a predetermined path, while another array of microphones “listened” to the feedback, known as a scattering matrix, as it interacted with the moving ball. By combining this scattering matrix with the camera’s location data, the researchers could calculate the optimal momentum of the soundwaves in real-time as they nudged the ball along its path.
“The method is based on the conservation of momentum, making it incredibly simple and versatile, and that’s why it is so promising,” Fleury remarks.
He mentions that wave momentum shaping draws inspiration from the optical technique of wavefront shaping, commonly used to concentrate scattered light. However, this is the first instance of applying this concept to moving an object. Furthermore, the team’s technique is not restricted to guiding spherical objects on a path; they have also used it to manage rotations and manipulate more intricate floaters, like an origami lotus.
Simulating internal conditions
After successfully guiding a ping-pong ball, the researchers conducted additional experiments involving stationary and moving obstacles intended to introduce complexity to the system. By safely navigating the ball around these scattering objects, they demonstrated that wave momentum shaping could perform effectively even in dynamic, uncontrolled settings, such as the human body. Fleury highlights sound as a promising tool for medical applications due to its harmlessness and non-invasiveness.
“Certain drug delivery methods employ soundwaves to release encapsulated drugs, making this technique particularly appealing for directing a drug precisely towards tumor cells, for instance.”
The technique could revolutionize biological analysis or tissue engineering tasks where touching cells for manipulation could lead to damage or contamination. Fleury also envisions 3D printing applications for wave momentum shaping, such as organizing microscopic particles before solidifying them into an object.
In the long run, the researchers believe their method could also be effective with light, but their immediate objective is to transition their sound-based experiments from a macro to micro scale. They have already secured SNSF funding to conduct experiments under a microscope, using ultrasonic waves to move cells around.
