Scientists have developed an innovative method to study the metamorphosis of liquids under negative pressure by encapsulating them in optical fibers. The technology provides a simpler way to measure pressure using light and sound waves, paving the way for new discoveries in thermodynamics and chemical reactions. As a physical quantity, pressure exists in various fields: atmospheric pressure in meteorology, blood pressure in medicine, and even pressure cookers and vacuum-sealed foods in daily life.
Artistic impression of a glass capillary filled with liquid. By encapsulating liquids in optical fibers, scientists observed and measured the negative pressure effect using sound waves as sensors. Image source: ©LongHuyDao
Pressure is defined as the force per unit area normal to the surface of a solid, liquid, or gas. Depending on the direction of force in a closed system, in extreme cases extremely high pressures can lead to explosive reactions, while extremely low pressures in a closed system can cause the system itself to implode.
Overpressure always refers to a gas or liquid squeezing the walls of a container from the inside, like a balloon expanding when more air is added. Regardless of high pressure or low pressure, under normal circumstances, the numerical value of pressure is always positive.
However, liquids have a special property. They can exist in a specific variable state corresponding to the negative pressure value. In this variable state, even small external influences can cause the system to collapse into one state or another. Think of it like sitting on the top of a roller coaster: a slight touch to one side or the other sends you plummeting off the track.
In the current study, the scientists are studying the metamorphosis of liquids under negative pressure. To this end, the research team combined two unique techniques to measure various thermodynamic states in a study published in Nature Physics.
First, tiny (nanoliter) amounts of liquid are encapsulated in a completely enclosed optical fiber, allowing it to have both high positive and negative pressures. Subsequently, through the special interaction of light and sound waves in the liquid, the effects of pressure and temperature in different states of the liquid can be sensitively measured. Sound waves act as sensors that detect negative pressure values, exploring this unique state of matter with high precision and detailed spatial resolution.
(From left to right) Research team leader Birgit Stiller with Andreas Geilen and Alexandra Popp in the laboratory. Image source: ©FlorianRitter,MPL
Effects of Negative Pressure and Measurement Techniques
The effect of negative pressure on the liquid can be imagined as follows: According to the laws of thermodynamics, the volume of the liquid will decrease, but the liquid will be affected by the adhesion force in the glass fiber capillary, just like water droplets sticking to your fingers. This causes the liquid to "stretch". The fluid is pulled apart, like a rubber band being stretched.
Measuring this exotic state often requires sophisticated equipment and heightened safety precautions. High pressure is a dangerous job, especially with toxic liquids. The carbon disulfide used by the researchers in this study is one such liquid. Due to this complexity, previous measuring devices used to generate and determine negative pressure required a large amount of laboratory space and even caused disturbances to systems in steady state.
Using the method described in this article, researchers have developed a tiny, simple device that can make very precise pressure measurements using light and sound waves. The optical fiber used for this purpose is only as thick as a human hair.
Researchers' comments
"When new measurement methods are combined with novel platforms, some phenomena that are difficult to explore with common and established methods become surprisingly accessible. I find this very exciting," said Dr. Birgit Stiller, head of the Quantum Photoacoustics research group at MPL. The research team used sound waves that can be very sensitive to detect changes in temperature, pressure and strain along the optical fiber. In addition, spatially resolved measurements can be made, meaning the sound waves can provide an image of what's going on inside the fiber with centimeter-level resolution along its length.
"Our method allows us to gain a deeper understanding of the thermodynamic dependencies of this unique fiber optic system," said Alexandra Popp, one of the two lead authors of the article.
Another lead author, Andreas Geilen, added: "The measurements revealed some surprising effects. The observations of the negative pressure state become very clear when looking at the frequency of the sound waves."
Potential applications and concluding remarks
Combining photoacoustic measurements with tightly sealed capillary fibers could lead to new discoveries in monitoring otherwise difficult-to-study materials and chemical reactions of toxic liquids in microreactors. It can penetrate into new and inaccessible areas of thermodynamics.
Professor Markus Schmidt from the IPHT Jena and Dr. Mario Chemnitz, also from the IPHT Jena, emphasize: "This new platform of fully sealed liquid-core fibers allows access to high pressures and other thermodynamic environments. It is very meaningful to study or even further customize the nonlinear optical phenomena in this fiber."
These phenomena can unlock previously unexplored potential new properties in the material's unique thermodynamic state.
Birgit Stiller concludes: "Our research groups in Erlangen and Jena are uniquely collaborating on their respective expertise to obtain new insights into thermodynamic processes and states on a tiny and easy-to-operate optical platform."