Acoustic Levitation for Particle-Armored Liquid Robots

A particle-armored liquid robot is a field-actuated soft machine rather than a miniature rigid robot. Its liquid body can change shape, flow around obstacles, engulf a compatible object and merge with a neighbor, while a particulate skin prevents the liquid from wetting the environment. The 2025 Science Advances study that introduced the Particle-armored liquid roBot, or PB, used a Hielscher UP400S ultrasonic transducer (400 W, 24 kHz) with a 22 mm sonotrode to exert acoustic radiation force from a distance. The force pushed and rolled the PB across ordinary substrates; the transducer was inclined 10 degrees for navigation tasks and oriented vertically for merging.

From Acoustic Propulsion to Acoustic Suspension

True acoustic levitation changes the boundary condition. Instead of using a progressive wave to overcome rolling resistance, an opposed sonotrode-reflector pair produces a standing pressure field that supports the robot against gravity. Removing the floor eliminates solid contact, but it also removes the friction that helped convert acoustic force into rolling motion. A practical machine therefore needs two functions: a vertical trap for lift and an independently controllable gradient or progressive wave for translation. The result is best viewed as a hybrid acoustic workcell with levitate, steer, merge and dock modes.

The Working Principle of Acoustic Levitation

Acoustic levitation uses acoustic radiation forces to suspend an object without mechanical contact. In a typical ultrasonic levitator, a sonotrode emits high-frequency sound toward an opposing reflector. The incident and reflected waves interfere to form a standing wave containing alternating pressure nodes and antinodes. A particle-armored liquid robot placed near a stable node experiences a time-averaged acoustic force that counteracts gravity, while pressure gradients restore small displacements and keep the robot trapped. The reflector spacing, acoustic frequency and vibration amplitude must be tuned to the robot’s size, mass and shape. Additional horns or controlled movement of the acoustic field can provide lateral steering, rotation and docking. Unlike acoustic cavitation, which involves the growth and collapse of bubbles inside a liquid, acoustic levitation primarily relies on radiation pressure transmitted through the surrounding air. Excessive sound pressure should nevertheless be avoided because it can deform the liquid core, dislodge armor particles or destabilize the robot.

Levitation as Lifting Mechanism

In an air-coupled levitator, the desired force is the time-averaged acoustic radiation force created by scattering and absorption of the sound field. A reflector sends the incident wave back toward the horn, and the two waves interfere to produce nodes and antinodes. A dense millimetric object in air can be trapped near a stable pressure node when the upward radiation force balances its weight and the force gradient restores small displacements. Cavitation, by contrast, occurs inside a liquid when negative pressure cycles drive bubbles. The two phenomena can coexist only if enough acoustic energy enters the droplet, but they should not be conflated.

What are Particle-Armored Liquid Robots?

Conventional liquid marbles are droplets coated with hydrophobic powder. Capillary forces hold the particles at the liquid-gas interface, and particle rearrangement gives the surface a shell-like response. The PB concept strengthens that architecture by loading substantially more particles per unit liquid volume. In the reported fabrication route, a cuboid ice precursor was coated with flake-like PTFE particles with an average basal-plane diameter of 34 micrometers. When the ice melted and contracted, the particles jammed into a dense, uniformly distributed layer around a prolate water core.
For the geometries studied, PBs carried about 59% more particle mass than conventional liquid marbles, with a reported packing ratio of 0.983 rather than 0.956. At a liquid volume of 75 cubic millimeters, an aspect ratio near 1.55 gave the highest measured compression resistance; its maximum compression force was 252% greater than that of the comparison liquid marble. Too slender a shape weakened the sides, so more armor and more elongation are not automatically better.
That reserve of mobile surface particles lets a PB survive transient increases in interfacial area. The reported robots squeezed through hydrophilic pillar arrays, engulfed hydrophilic glass beads, merged under vertical acoustic loading, skimmed across water, transported reagents and discharged their contents when a surfactant fractured the armor. The word robot here describes externally controlled, task-capable matter; the PB has no onboard actuator, sensor or power source.