Contact-Free Surface Tension Measurements of Microgel-Laden Air-Water Interfaces via Acoustic Levitation

Soft, stimuli-responsive microgels are widely used to stabilize foams and emulsions, yet probing their interfacial properties without perturbing the interface remains a challenge. In this work, we show that acoustic levitation provides a simple and fully contact-free approach to quantify the interfacial properties of poly(N-isopropylacrylamide) microgels at the air-water interface. By monitoring the time evolution of the shape of a levitated droplet and using a neural network trained to interpret droplet geometries, we extract surface tension values for microgels with different cross-linker contents (1, 5, and 10 mol %). Under static levitation, dilute microgel dispersions display a characteristic nonmonotonic shape evolution that reflects the competition between evaporation-induced volume loss and the gradual adsorption and spreading of microgels at the interface. The surface tension values obtained from levitated droplets show excellent agreement with conventional pendant drop measurements, confirming the reliability of this shape-based, contact-free method. Furthermore, this approach enables the study of interfacial kinetics at soft interfaces without physical contact, thereby avoiding potential contamination or perturbation of the sample. In addition, acoustic levitation provides a platform for probing dynamic interfacial properties in future studies. Unlike conventional techniques that impose volume oscillations on the droplet─which can generate internal flows, particularly at higher oscillation frequencies─this method offers the possibility to investigate interfacial dynamics while minimizing flow-induced artifacts. Overall, our results establish acoustic levitation as a powerful platform for probing the interfacial behavior of soft colloids and lay the groundwork for future investigations of dynamic interfacial responses using controlled, volume-preserving droplet deformations.