Negative pressure is a rare and challenging-to-detect phenomenon in physics. Using liquid-filled optical fibers and sound waves, researchers at the Max Planck Institute for the Science of Light (MPL) in Erlangen have now discovered a new method to measure it. In collaboration with the Leibniz Institute of Photonic Techno­logies in Jena (IPHT), the scientists in the Quantum Opto­acoustics research group, led by Birgit Stiller, can gain important insights into thermo­dynamic states.

Liquids exhibit a peculiar charac­teristic. They can exist in a specific metastable state corresponding to a negative pressure value. In this metastable state, even a tiny external influence can cause the system to collapse into one state or another. One can imagine it as sitting at the top of a roller coaster: the slightest touch on one side or the other sends you hurtling down the tracks. In their current research, the scientists are examining the metastable state of liquids with negative pressure. To achieve this, the research team combined two unique techniques to measure various thermodynamic states.

Initially, nanoliters of a liquid were encapsulated in a fully closed optical fiber, allowing both highly positive and negative pressures. Subsequently, the specific interaction of optical and acoustic waves in the liquid enabled the sensitive measure­ment of the influence of pressure and temperature in different states of the liquid. Sound waves act as sensors for examining negative pressure values, exploring this unique state of matter with high precision and detailed spatial resolution.

The influence of negative pressure on a liquid can be envisioned as follows: According to the laws of thermodynamics, the volume of the liquid will decrease, but the liquid is retained in the glass fiber capillary by adhesive forces, much like a water droplet sticking to a finger. This results in a stretching of the liquid. It is pulled apart and behaves like a rubber band being stretched. Measuring this exotic state typically requires complex equipment with heightened safety precautions. High pressures can be hazardous endeavors, parti­cularly with toxic liquids. Carbon disulfide, used by the researchers in this study, falls into this category. Due to this complication, previous measurement setups for generating and deter­mining negative pressures have required signi­ficant laboratory space and even posed a disturbance to the system in the metastable state. With the method presented here, the researchers have instead developed a tiny, simple setup in which they can make very precise pressure measurements using light and sound waves.

“Some phenomena which are difficult to explore with ordinary and established methods can become unexpectedly accessible when new measurement methods are combined with novel platforms. I find that exciting,” says Birgit Stiller, head of the Quantum Optoacoustics research group at MPL. The sound waves used by the group can detect temperature, pressure, and strain changes very sensi­tively along an optical fiber. Further­more, spatially resolved measure­ments are possible, meaning that the sound waves can provide an image of the situation inside the optical fiber at centi­meter-scale resolution along its length. “Our method allows us to gain a deeper understanding of the thermo­dynamic dependencies in this unique fiber-based system,” says Alexandra Popp. Her colleague Andreas Geilen adds: “The measure­ments revealed some surprising effects. The observation of the negative pressure regime becomes abundantly clear when looking at the frequency of the sound waves.”

The combination of optoacoustic measure­ments with tightly sealed capillary fibers enables new discoveries regarding the monitoring of chemical reactions in toxic liquids within otherwise difficult-to-inves­tigate materials and micro­reactors. It can penetrate new, hard-to-access areas of thermo­dynamics. “This new platform of fully sealed liquid core fibers provides access to high pressures and other thermodynamic regimes,” says Markus Schmidt from IPHT in Jena, and Mario Chemnitz, also from IPHT in Jena, emphasizes: “It is of great interest to investigate and even tailor further nonlinear optical phenomena in this type of fiber.” These phenomena can unlock previously unexplored and potentially new properties in the unique thermodynamic state of materials. Birgit Stiller concludes: “The colla­boration between our research groups in Erlangen and Jena, with their respective expertise, is unique in gaining new insights into thermo­dynamic processes and regimes on a tiny and easy-to-handle optical platform.” (Source: MPL)

Reference: A. Geilen et al.: Extreme thermodynamics in nanolitre volumes through stimulated Brillouin–Mandelstam scattering, Nat. Phys., online 25 September 2023; DOI: 10.1038/s41567-023-02205-1

Link: Quantum Optoacoustics, Max Planck Institute for the Science of Light, Erlangen, Germany