# Jetting with gels: Soft microgel networks stabilize and extend nozzle-free water jets

## TL;DR

This paper demonstrates that tuning the nanoscale softness of microgels can stabilize and extend nozzle-free water jets driven by surface acoustic waves, offering a biocompatible alternative to surfactants for high-speed jet applications.

## Contribution

It introduces a novel, biocompatible microgel network strategy to stabilize high-speed jets by suppressing surface tension recovery and delaying instabilities, validated through experiments, simulations, and scaling analysis.

## Key findings

- Microgels extend jet length by up to 44%.
- Soft microgels maintain network cohesion under strain.
- Simulation reveals entangled polymer bridges sustain reduced surface tension.

## Abstract

The stability of high-speed liquid jets is crucial for applications ranging from precision printing to needle-free drug delivery, yet it is fundamentally limited by capillary-driven breakup. A common strategy to stabilize jets is to use surfactants to lower surface tension. However, in nozzle-free jetting driven by surface acoustic waves (SAWs), extreme deformation rates cause conventional surfactants to desorb, calling for alternative strategies to stabilize jets. In particular, we find that tuning the nanoscale softness of PNIPAM microgels provides a robust, biocompatible strategy to overcome this limitation. Soft, low-cross-linker-density microgels form elastic interfacial networks at the air-water interface that suppress surface tension recovery, delay Rayleigh-Plateau instabilities, and extend SAW-driven jet lengths by up to 44%. In contrast, stiffer microgels lose network cohesion under strain, leading to rapid jet breakup. To gain molecular-level insights, we perform dissipative particle dynamics simulations, which reveal that polymer bridges in soft microgels remain entangled during elongation, maintaining a reduced effective surface tension. Finally, a simple scaling analysis, balancing the SAW-driven kinetic energy imparted to the droplet against the surface energy required to form a jet, quantitatively predicts the observed length enhancement. This surfactant-free, biocompatible approach lays the foundation for long-lived jets, enabling precision needle-free drug delivery, high-speed printing, and other high-strain interfacial flow applications.