Pressure, as a physical quantity, is encountered in numerous fields, from meteorology to medicine. It is defined as a force per unit area acting perpendicular to a surface of solid, liquid, or gas. Typically, pressure is always positive in nature, pushing against the walls of its container. However, liquids possess a peculiar characteristic – they can exist in a metastable state corresponding to a negative pressure value. In this unique state, even the slightest external influence can cause the system to collapse into one state or another. In their ground-breaking research, scientists from the Max Planck Institute for the Science of Light in Erlangen have been exploring the metastable state of liquids with negative pressure.
To gain insights into thermodynamic states and measure negative pressure, the scientists combined two unique techniques – liquid-filled optical fibers and sound waves. In a study published in Nature Physics, the researchers encapsulated tiny amounts of liquid in a fully closed optical fiber, enabling the measurement of both highly positive and negative pressures. The interaction of optical and acoustic waves in the liquid allowed for sensitive measurements of pressure and temperature in different states. Sound waves acted as sensors, providing detailed spatial resolution and exploring the unique state of matter with high precision.
The influence of negative pressure on a liquid can be understood through the analogy of a water droplet sticking to a finger. According to the laws of thermodynamics, the volume of the liquid will decrease, but adhesive forces keep the liquid in the glass fiber capillary. As a result, the liquid is “stretched,” similar to a rubber band being pulled apart. Measuring this exotic state traditionally required complex equipment and safety precautions, especially when dealing with toxic liquids. However, the researchers developed a simplified setup using thin optical fibers, making precise pressure measurements possible with light and sound waves.
By combining optoacoustic measurements with tightly sealed capillary fibers, the scientists discovered surprising effects and gained a deeper understanding of the thermodynamic dependencies in the unique fiber-based system. The frequency of the sound waves revealed the clear observation of the negative pressure regime. This new method not only enables the monitoring of chemical reactions in toxic liquids but also opens up new possibilities for investigating previously difficult-to-access materials and microreactors. The fully sealed liquid core fibers provide access to high pressures and other thermodynamic regimes, unlocking unexplored properties in materials.
Dr. Birgit Stiller, the head of the Quantum Optoacoustics research group at MPL, expressed her excitement about the potential of combining new measurement methods with novel platforms. The sound waves used in the research group’s experiments are highly sensitive to temperature, pressure, and strain changes along an optical fiber. This spatially resolved measurement technique provides a detailed image of the situation inside the fiber at centimeter-scale resolution. Alexandra Popp, one of the lead authors of the study, highlights that the method allows for a deeper understanding of thermodynamic dependencies. Andreas Geilen, the other lead author, emphasizes the surprising effects revealed through their measurements.
The discovery of this new method for measuring negative pressure has significant implications for the field of physics. Researchers can now explore previously uncharted areas of thermodynamics and potentially unlock new phenomena and properties within materials. Furthermore, the simplicity and precision of the setup make it a valuable tool for investigating various scientific applications involving pressure and thermodynamic states. As technology continues to advance, it is crucial to embrace innovative approaches that enable a better understanding of the natural world and push the boundaries of scientific knowledge.
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