Tiny, soft polymer particles known as microgels can help stabilize extremely thin liquid jets, according to a study led by researchers at TU Darmstadt, published in Nature Communications. These microgels are less than one micrometer in size—about one-thousandth the width of a human hair—and can be used to reduce the surface tension of water. This effect is similar to that of traditional surfactants, which are commonly used to reduce surface tension in various applications. However, unlike many conventional surfactants, the microgels used in the study are non-toxic and interact with liquids in a different way, offering a potentially safer and more sustainable alternative. To investigate this phenomenon, the researchers used surface acoustic waves to generate extremely thin jets of water. A drop of water containing microgels was used, and a specially designed chip created waves at a frequency of about 64 MHz, forming a jet around 200 micrometers in diameter—nearly the width of a human hair. A high-speed camera recorded the jet's formation and extension, revealing that the microgels, despite being only around 700 nanometers in size, had a significant impact on the jet's stability. The softer the microgels were, the longer the jet remained intact. The key factor in the microgels' effectiveness appears to be their ability to deform. The researchers adjusted the stiffness of the microgels by modifying the cross-linking of the polymer scaffold, which is the three-dimensional, porous structure that forms the microgel. A more cross-linked structure makes the microgels stiffer, while weaker cross-linking makes them softer. Computer simulations helped explain the findings: soft microgels can stretch and deform more at the air-water interface, staying connected and maintaining low surface tension. In contrast, stiffer microgels tend to detach from each other, causing the jet to break up more quickly. The study has potential applications in needle-free drug delivery, where thin, fast-moving liquid jets are used to deliver medication through the skin without traditional needles. Improved control of jet stability could make these systems more reliable, reducing the need for invasive injections and the associated medical waste. The findings could also benefit inkjet printing, where precise control of liquid flow is essential. The research also highlights the novel observation of how microgels behave under extreme and rapid deformation during jet formation, a process that is too fast to be seen with the naked eye and requires high-speed cameras with millisecond-level resolution. The results open new possibilities for both fundamental research and practical applications in various fields.