Acoustic Manipulation of Tangible Janus Icons on Liquid Droplets

Abstract

Interfaces that couple digital information with physical matter enable computation to be expressed through tangible motion and touch, yet typically rely on embedded actuators, rigid mechanisms, or enclosed environments. Consequently, contactless manipulation and interaction with centimeter-scale tangible elements in open settings remain difficult to achieve. Here, we present PolygonWave, a solid--fluid acoustic interface that enables transport and tangible interaction by coupling airborne ultrasound with liquid-mediated support. The system employs lightweight Janus icons with asymmetric wettability: a superhydrophobic upper surface permits dry touch interaction, while a hydrophilic lower surface couples to a water droplet resting on a superhydrophobic mesh. Focused acoustic fields generated by a 256-element phased array induce lateral forces, enabling programmable motion without mechanical contact. Systematic characterization demonstrates transport of payloads up to 525 mg across variations in icon size, droplet volume, and applied load. Beyond translation, the liquid layer functions as a reconfigurable mechanical element, enabling button-like input with self-recovery and resonance-driven vibro-visual feedback, exhibiting a peak response near 22 Hz for 200 \textmu L droplets. Liquid-mediated acoustic coupling provides a unified mechanism for mechanically expressive, touch-accessible tangible interfaces bridging acoustics, soft matter physics, and physical human--computer interaction.

Figures (4)

• Figure 1: Overview of the solid--fluid acoustic tangible interface. (a) System configuration consisting of a phased-array transducer (PAT) positioned beneath a superhydrophobic mesh, which supports a liquid layer and enables contactless translation of Janus icons through programmable acoustic focusing. (b) Structure of the asymmetric wetting Janus icon, featuring a hydrophobic top surface for dry touch interaction and a hydrophilic bottom surface that couples to the supporting liquid layer. (c) Interaction modalities enabled by the system, including horizontal manipulation of mounted objects, tactile button input, and vibro-visual feedback.
• Figure 2: Transport performance across icon diameter, droplet volume, and applied load. (a) Summary of transport outcomes for combinations of icon size, droplet volume, and load. Each bar indicates the proportion of trials classified as stable trajectory control [V], partial trajectory [IV], lifted without translation [III], grounded without lift [II], or flipped [I]. (b--d) Side-view images of 12 mm icons with increasing droplet volumes (100, 200, and 300 µ L), showing progressive vertical elongation and increased tilt. (e--g) Corresponding images for 15 mm icons, demonstrating more uniform droplet distribution and improved mechanical stability over the same volume range.
• Figure 3: Dynamic stability during transport of an elevated payload. A 24 mm-tall, 10 mm-base 3D-printed tower (137 mg, red PLA) mounted on a Janus icon is translated along a horizontal linear trajectory. The assembly follows a sinusoidal back-and-forth motion at 10 Hz while maintaining capillary support and translational stability. Despite its elevated center of mass, the structure remains stably coupled to the supporting droplet throughout motion (see Supplementary Video 1).
• Figure 4: Tangible interaction enabled by liquid-supported Janus icons. (a) Button-like input through vertical compression of the icon, in which the supporting droplet provides compliant tactile feedback and self-recovery after release (see supplementary video 2). (b) Vibration generated by droplet resonance (see supplementary video 3). Measured vibration amplitude of the icon (15 mm) as a function of modulation frequency for a 200 µ L droplet at 10 V, showing a clear resonance peak around 22 Hz. The solid green lines indicate the classical Rayleigh--Lamb predictions for the $l=2$ and $l=3$ capillary modes of a free spherical droplet of equal volume.