Revealing Negative Pressure: The Combined Capabilities of Photonic and Acoustic Waves

An illustrative representation of a capillary tube filled with liquid. Scientists used optical fibers to encapsulate fluids, enabling the examination and quantification of negative pressure effects, employing sound waves as monitoring instruments. Credit: © Long Huy Dao

Researchers have formulated an avant-garde approach for probing the metastable states of liquids under negative pressure by confining these liquids within optical fibers. This method offers a streamlined avenue for pressure measurement through the interplay of light and acoustic waves, thereby setting the stage for novel advancements in the fields of thermodynamics and chemical processes.

Pressure, defined as force per unit area acting orthogonally to a surface, finds applications in diverse domains, ranging from atmospheric pressure in meteorological studies to blood pressure in medical science, and from the household utilities of pressure cookers to vacuum packaging of food items.

In closed systems, pressure can manifest in distinct ways; extremely high pressures can trigger explosive outcomes, while exceedingly low pressures could induce the system’s collapse or implosion.

Ordinarily, whether it’s high or low pressure, the numerical value assigned to pressure remains positive, with overpressure indicating that the liquid or gas is exerting force against the internal walls of its container—akin to an inflating balloon.

Unique Metastable States in Fluids

Yet, fluids display an intriguing trait: they can occupy a special metastable state characterized by a negative pressure value. In such a metastable state, minute external forces can destabilize the system, triggering its transition into another state—akin to a roller coaster descending rapidly upon a slight nudge.

In recent studies published in Nature Physics, the researchers investigated this peculiar state by employing two groundbreaking techniques. Initially, minuscule quantities—measured in nanoliters—of fluid were sealed within a closed optical fiber system that could accommodate both highly positive and negative pressures. The synergistic action of photonic and acoustic waves within the fluid allowed for meticulous assessments of varying thermodynamic states under different pressure and temperature conditions. Acoustic waves served as sensitive detectors, providing high-precision, spatially-resolved insights into this extraordinary state of matter.

Implications and Techniques for Measuring Negative Pressure

Negative pressure’s influence on a fluid can be conceptualized as a volume contraction, adhering the liquid to the glass fiber capillary through adhesive forces, much like how a droplet of water clings to a fingertip. This results in the fluid undergoing a ‘stretching’ or elongation, resembling a stretched rubber band.

Historically, investigating this exotic state required elaborate setups and rigorous safety measures, especially when working with hazardous substances such as Carbon disulfide. The novel methodology circumvents these issues, enabling precise pressure readings through a simple, miniaturized system utilizing an optical fiber roughly the diameter of a human hair.

Comments from the Scientific Team

Dr. Birgit Stiller, the head of the Quantum Optoacoustics research group at MPL, noted, “Incorporating new measurement technologies with innovative platforms can open doors to exploring challenging phenomena.” The team’s sound wave technology enables highly sensitive detections of temperature, pressure, and strain variations along the fiber, also allowing spatially resolved measurements.

Alexandra Popp and Andreas Geilen, the lead authors, stated that their method brings a more profound understanding of thermodynamic interdependencies in this unique fiber-based system. “The sound wave frequency clearly delineates the observation of negative pressure regimes,” added Geilen.

Future Prospects and Final Remarks

The synergistic use of optoacoustic measurements and hermetically sealed capillary fibers presents opportunities for pioneering research in monitoring chemical reactions in hazardous fluids and investigating challenging thermodynamic states.

Prof. Markus Schmidt from IPHT in Jena, along with Dr. Mario Chemnitz, also from IPHT, emphasized the scientific community’s keen interest in probing and even customizing nonlinear optical phenomena in such fibers.

In conclusion, Birgit Stiller expressed that the collaborative efforts of the research groups in Erlangen and Jena have been instrumental in yielding fresh perspectives on thermodynamic processes using an accessible and manageable optical platform.

Reference: “Extreme thermodynamics in nanolitre volumes through stimulated Brillouin–Mandelstam scattering” by Andreas Geilen, Alexandra Popp, Debayan Das, Saher Junaid, Christopher G. Poulton, Mario Chemnitz, Christoph Marquardt, Markus A. Schmidt, and Birgit Stiller, 25 September 2023, Nature Physics.
DOI: 10.1038/s41567-023-02205-1

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