Introduction

Fluid jets1–8 are traditionally generated by forcing liquid through a nozzle—commonly seen in syringes, inkjet printers9–11, or spray systems12. Here, we exploit a nozzle-free jetting approach based on highly focused surface acoustic waves (SAWs), which can be viewed as nanoscale analogs of seismic waves traveling along the Earth’s surface. This method destabilizes the liquid–air interface, overcoming capillary stresses to generate a collimated liquid stream known as a liquid jet13. These jets serve as a platform for free-surface motion, capable of carrying materials within the liquid and transferring them to the intended site. However, a fundamental limitation of SAW-induced jetting is its susceptibility to capillary-driven Rayleigh–Plateau instabilities, which lead to premature jet breakup and constrain jet length13. These instabilities pose a major challenge in high-speed applications, where jet stability is essential for accurate droplet deposition, transdermal drug penetration, and microfabrication processes. Despite its technological promise, most research on SAW-driven jetting has focused on pure liquids14,15, the role of interfacial stabilizers in this context remains poorly understood.

Previous studies in air-blast jetting have shown that surfactants can increase jet breakup length by lowering surface tension and inducing Marangoni stresses that counteract capillary-driven instabilities10,16–21. In free-falling water jets into toluene, surfactant transport across the interface delays jet breakup and increases jet length22. Yet in SAW-driven systems, interfacial deformation occurs so rapidly that surfactants desorb before providing lasting stabilization. Biocompatibility concerns further limit their use in biomedical contexts, motivating the search for an alternative strategy that can withstand high strain rates while remaining non-toxic.

A promising alternative is soft colloidal stabilizers, particularly microgels as cross-linked polymer networks that are swollen by solvent. Unlike rigid colloidal particles, microgels are highly deformable, dynamically adapting at liquid interfaces through adsorption, spreading, and steric interactions23–36. Their distinctive fried-egg morphology features a swollen corona that spreads across the interface while a denser core remains mainly submerged, with a small portion protruding into the air phase37–39. Although microgels are well established for stabilizing emulsions and coatings40,41, their behavior under fast, high-strain interfacial flows such as SAW-driven jetting has not been investigated.

Here, we show that the softness of poly(N-isopropylacrylamide) (PNIPAM) microgels plays a decisive role in stabilizing nozzle-free jets. Microgels with low cross-linker density assemble into entangled, elastic interfacial networks that suppress Rayleigh–Plateau instabilities, producing jets that persist up to 44% longer than those from pure water. In contrast, stiffer microgels lose network connectivity under strain, allowing the air–water surface tension ( ≈ 70 mN m−1) to recover and prompting earlier jet breakup. Due to experimental challenges in capturing dynamic surface tension during jetting, we employ dissipative particle dynamics (DPD) simulations with explicit solvent to gain molecular-level insights. DPD simulations reveal that polymer chains bridging neighboring soft microgels remain entangled during high-speed deformation, thereby preserving a lower surface tension than that of the pure air–water interface throughout jet elongation. Subsequently, we develop a simple scaling argument, balancing SAW-driven kinetic energy with the jet’s surface energy, which quantitatively predicts the observed length enhancement.

By combining experiments, simulations, and scaling analysis, we establish a direct link between the nanometer-scale internal architecture of microgels and centimeter-scale jet stability. This surfactant-free, biocompatible strategy enables controlled jets, opening avenues for precision liquid delivery, high-speed printing, and dynamic control of high-strain interfacial flows.