Discussion

Our findings show that microgel softness, tuned via cross-linker density, governs interfacial resilience during rapid deformation. Unlike pure water jets, soft microgels form deformable interfacial networks that suppress Rayleigh–Plateau instabilities, extending jet lengths by up to 44%.

To better understand the rupture of the microgel monolayer upon jet formation, we estimate the areal strain during the jetting process. The interfacial area of a sessile droplet expands significantly, reaching roughly 200%, as the interface evolves into a cylindrical jet. At the same time, our experiments and simulations demonstrate that pre-adsorbed soft microgels undergo substantially larger interfacial deformation than stiff microgels, enabling them to maintain a reduced effective surface tension during rapid stretching. Notably, the estimated areal strain during jet formation ( ∼ 200%) is compatible with the observed ability of individual soft microgels to laterally stretch by approximately a factor of two, as evidenced by our AFM measurements and consistent with previous reports of center-to-center distance at different surface pressures51. Given that jet formation occurs on a rapid timescale, limiting the contribution from diffusive replenishment of particles from the bulk, this agreement supports the conclusion that deformation of pre-adsorbed microgels provides a physically consistent mechanism contributing to jet stabilization under extreme extensional conditions.

To separate the roles of polymer chemistry, interfacial activity, bulk polymer effects, and microgel architecture, we compared soft microgels with linear PNIPAM (M__w = 40 kDa) and PEG (M__w = 35 kDa) (Supplementary Fig. S13). PEG, despite its comparable molecular weight, exhibits a higher surface tension and produces much shorter jets than PNIPAM-based systems, showing that dissolved polymer chains alone are insufficient for stabilization. Instead, PNIPAM-mediated interfacial activity is essential. However, linear PNIPAM and microgels stabilize newly created interfaces differently. Microgels diffuse over their average interparticle spacing on second timescales, far slower than the millisecond jetting process, whereas 40 kDa PNIPAM chains can access the newly created interface within microseconds (Supplementary Fig. S14). Thus, microgel stabilization is governed mainly by the pre-adsorbed interfacial layer, while linear PNIPAM can additionally adsorb during extension. Although the present experiments do not fully isolate network mechanics from other microgel-specific interfacial properties, our results indicate that the deformable microgel architecture contributes beyond PNIPAM-mediated surface activity. This is consistent with microgels forming mechanically responsive layers at fluid interfaces52. More broadly, soft microgels provide a distinct route to jet control, where nanometer-scale crosslink density and softness of a pre-adsorbed layer regulate centimeter-scale jet dynamics through interfacial mechanics, in contrast to high-molecular-weight linear polymers, where bulk extensional stresses may also contribute53–57.

While our system centers on acoustically driven jets, the underlying mechanism likely extends to a wide range of dynamic interfacial processes. Similar mechanically stabilizing behaviors may occur in natural systems, such as bubble bursting at oil-covered water surfaces, where interfacial layers modulate jet formation58. In such scenarios, surface-active species, including lipids, proteins, and microorganisms, can form elastic barriers that mirror the stabilizing behavior of soft microgels. This analogy suggests that our findings may inform a broader understanding of how deformable interfacial structures influence jetting and breakup dynamics in both environmental and technological contexts.

Beyond their fundamental relevance, our results have direct implications for bioprinting and tissue engineering, where controlled jetting of soft, cell-compatible materials is essential. SAWs represent a promising and increasingly adopted approach for non-contact manipulation of biomaterials, facilitating precise cell patterning and assembly in bioprinting applications59. Soft microgels offer a mechanically tunable platform to stabilize biofluid jets without relying on potentially cytotoxic additives or high-viscosity formulations. By enabling long, stable jets, our approach provides a path toward higher precision, reduced clogging, and improved structural fidelity in next-generation biofabrication systems.

Moreover, our results suggest broader applicability in soft matter and interfacial engineering. For example, in foams and emulsions, microgel-based interfacial networks could provide enhanced stability under fluctuating mechanical stresses, potentially delaying film rupture or coalescence. The interfacial scaffolding effect we describe may serve as a unifying design principle for creating far-from-equilibrium systems that resist deformation across multiple length and time scales.

Looking forward, several open questions remain. How do these soft interfacial networks ultimately fail under extreme strain? Can microgel-mediated stabilization be generalized to more complex fluids containing biomolecules or living cells? Could microgels be designed to mimic and reinforce fragile biological structures, such as lipid bilayers60, offering mechanical protection during physiological or processing stresses?

Ultimately, this work introduces a general framework for designing responsive and resilient fluid interfaces. Moving beyond conventional surfactant-based stabilization, we show that mechanically tunable particles can be used to control interfacial behavior under dynamic conditions. These insights not only deepen our understanding of capillary-driven instabilities but also offer practical strategies for advancing technologies in microfluidics, inkjet and bioprinting, and needle-free drug delivery. We anticipate that future efforts to optimize continuous liquid feeding, microgel softness, and driving frequencies will enable the generation of jets several times longer than those currently achievable, expanding the horizons of nozzle-free jetting applications.