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Photo credit: Pixabay Michelle Francl "confesses":"If the tea is cold, I will heat it in the microwave before drinking it. Franzl, a chemist at Bryn Mawr College in the United States, wrote in an article titled "A chemist's cup of tea" that a film floats on the surface of herbal tea, which makes it difficult for her to drink it-she has to reheat the tea or squeeze a little lemon juice before brewing it.

This film did not initially attract the particular attention of Caroline Giacomin. Until a classmate from China Taiwan complained to Jia Keming that the film on tea was too much for him to bear, so he did not plan to drink tea anymore.

Jia Keming is a doctoral student in the Department of Health Science and Technology at the Federal Institute of Technology in Zurich, Switzerland. She is part of a research group that focuses on scientific problems related to interfaces. When she discussed her doctoral research project with her tutor, the tutor provided her with some topics worth studying. To Jia Keming's surprise,"the interface of tea" was listed among them. In addition to her previous conversation with her classmates, she decided to study this membrane herself. Eventually, her findings were published in the journal Fluid Physics.

In Jacoming's teacup, sometimes she could see the shiny film of tea, sometimes she couldn't. Moreover, if you wait for a while to drink the brewed tea, you will find that the tea film cracks like ice. But if you can't see it with the naked eye, does this membrane really not exist? What factors are involved in membrane fragmentation? Is it possible to keep the membrane from rupturing?

Chemists paved the way for tea In the 1990s, there were two chemists with a passion for tea: Michael Spiro and Deogratius Jaganyi. They wrote fourteen papers on tea, seven of which explained the chemistry of tea films, including chemical composition and various factors affecting tea film formation, and more importantly, they provided chemical kinetic explanations for "sometimes seeing tea films in cups, sometimes not seeing them."

For scientists, the laboratory is the perfect place to make tea-a glass beaker of a few hundred milliliters makes an excellent teacup, and a constant temperature water bath allows them to control the temperature of "tea" relatively precisely. Spiro and Jaganyi put the black tea bag into a beaker and brewed it with 80℃ water for 5 minutes. Then they took out the tea bag and let the tea sit for a while.

Image source: Pixabay Previously, some scientists thought that tea film was when tea was brewed with boiling water, and the waxy layer on the tea "floated" to the surface of the water. But when Spiro and Jaganyi brewed tea with distilled water from the lab (with minimal inorganic, organic and other impurities), no tea film appeared. This shows that relying solely on tea leaves and higher water temperatures does not produce tea films, and certain components in the water must play a key role. They confirmed through further experiments that calcium ions and bicarbonate ions are the key to inducing tea film formation, but calcium ions or bicarbonate ions alone cannot make tea film "appear," but must be a combination of the two.

In addition, acidity and oxygen concentration will also affect the formation of tea film. For example, the greater the alkalinity, that is, the higher the hardness of water, the easier it is to form a tea film. Moreover, if the air is replaced by nitrogen, it is difficult to see the tea film, so the formation of the tea film must involve oxygen and oxidation reactions, which is also one of the differences between tea scale and scale.

At the same time, using scanning electron microscopy, mass spectrometry, microanalysis and other testing methods, Spiro and Jaganyi further analyzed the composition of the tea film: the tea film is actually composed of organic substances (mainly carbon, hydrogen, oxygen) and inorganic substances (including carbonate and hydroxide). Among them, almost all calcium ions and sodium ions come from water, while potassium ions, manganese ions and aluminum ions almost all come from tea. In particular, they showed that the carbonates and hydroxides in the film existed independently as insoluble compounds, and that the organic matter provided physical support for these insoluble inorganics-calcium carbonate particles "waiting" on the surface of the organic matter as seen by scanning electron microscopy (SEM).

Spiro and Jaganyi also attempted to write the molecular formula for tea membranes. They speculated that after standing for an hour, a "tea film molecule" would consist of about 45 carbon atoms, 50 hydrogen atoms, 40 oxygen atoms and 2.7 divalent metal ions, and its molar mass could even reach about 1400 grams.

How much energy barrier a chemical process needs to cross, that is, how much energy is needed to make the reaction process proceed smoothly, is a problem of great concern to chemists. By precisely adjusting the temperature of tea at rest, and according to the Arrhenius equation, Spiro and Jaganyi calculated the activation energy for tea film formation: 34 kJ / mol. This is a relatively high energy barrier, greater than the activation energy required for diffusion (15.8 kJ / mol, calculated from Stokes-Einstein relations). Diffusion includes diffusion of ions in solution, and diffusion of gases from air into solution. Only when the reactants "meet" and collide after diffusion, that is, chemical reaction occurs, it is possible to form tea film.

Photo credit: Michelle Francl But when you make tea in a teacup, sometimes the temperature drops so quickly that a lot of heat is lost before the tea film can form. On the contrary, if you use a ceramic teapot with good heat preservation, the tea dissipates slowly, so you can usually see the tea film and leave more tea dirt in the teapot, and these tea dirt rich in minerals (such as calcium ions and magnesium ions) can also induce the formation of the next tea film. This coincides with Jia Keming. 'If you want to see the film in your cup all the time, you'd better not wash it,' Mr. Jia quipped.

Based on the research of the two chemists above, Jia Keming wanted to observe the film from a rheological point of view and analyze the mechanical properties of the film, not the chemical properties.

As early as 1678, Robert Hooke proposed Hooke's law-for solids, the stress in a material at a given pressure is linear with strain (the degree of deformation). Such materials are called Hooke elastic solids. Nine years after Hooke published his paper, Isaac Newton solved the problem of shear fluid flow and formulated Newton's law of viscosity (also known as Newton's law of internal friction). Fluid refers to liquid or gas, when the fluid flows under the action of external forces, internal stress will be generated to resist external forces. Newton pointed out that there is a linear relationship between the shear stress of a fluid and its flow rate, and fluids that conform to this law are called Newtonian fluids, such as water and alcohol.

But not all material motions can be explained by Hooke's law or Newton's law of viscosity. There is a class of materials that under certain conditions exhibit the characteristics of Hooke solids, such as elastic deformation (transient, recoverable deformation), but under other conditions behave like fluids, i.e. viscous flow (persistent, irrecoverable flow). Rheology is the study of such exotic materials. Rheology, according to Eugene Bingham, an American chemist, is a new branch of science that studies the deformation and flow of materials.

For Jacoming, she wanted to know both the elasticity and the viscosity of the membrane. In addition, the film is located between water and air, so she chose a double-cone interface rheometer.

It is worth mentioning that to describe the mechanical properties of tea films, Jia Keming has to use the "modulus" index. Similar to Hooke and Newton's laws, modulus is a measure of the relationship between stress and strain. In addition, for complex materials such as tea films, the corresponding moduli are elastic modulus (G′) and viscous modulus (G″), which can also be called storage modulus and loss modulus.

To determine the role of calcium ions, Giacomin prepared six different concentrations of calcium carbonate solutions (0, 10, 25, 50, 100, and 200 mg / L), which contained almost no other metal ions, and used these solutions instead of water to make tea. To Jia Keming's surprise, she did not see the tea film, but the invisible film was "seen" by the rheometer.

Jia Keming found that when the shear stress amplitude was fixed (0.3%) and the dynamic time scan was performed, the elastic modulus of the tea film was greater than the viscous modulus (G′ > G″) for solutions with calcium carbonate concentrations of 50, 100 and 200 mg / L, i.e., it was solid; when the calcium carbonate concentration was lower than 50 mg / L, the tea film was fluid (G″ > G′). In other words, the lower the concentration of calcium carbonate, the more it can make the tea film flow. In addition, similar to the results obtained by Spiro and Jaganyi, when tea is brewed with ultrapure water (almost no metal ions are present), not only is the tea film not visible, but it is also not detected by rheometers.

As mentioned above, we often see cracked tea films, so Giacomo wanted to see what stress amplitude would make this film crack. If "modulus" is used to indicate when a film will fracture (film strength), it is when the loss modulus is greater than the storage modulus. Here, instead of elasticity and viscosity, the words energy storage and loss give us a more intuitive sense of why the membrane breaks.

When the CaCO3 concentration is higher (100 and 200 mg / L), the tea film can be broken at a lower shear stress amplitude (0.5%), i.e. G″ > G′. However, when the concentration of CaCO3 is reduced to 50 mg / L, a higher stress amplitude (0.8%) is required to crack the tea film, so the tea film dominated by energy storage (G′ > G″) is more ductile and less fragile. However, for 10 and 25 mg / L, no matter how the stress changes, the loss modulus is always greater than the elastic modulus, and the tea film is difficult to form like a fluid.

In Jia Keming's eyes, tea film is a shiny and beautiful thing. So to see the film, she concludes her essay with the advice: "Don't wash the teacup."

How about lemon tea? One of the most common teas on the market is lemon tea. This is not just because of the lemon flavor, but there are certain scientific principles behind it. Spiro and Jaganyi had already discovered that citric acid inhibited the formation and growth of tea membranes. This is because citric acid can react with metal ions such as calcium ions to reduce the concentration of free metal ions, and calcium ions are the key to forming tea films. Jia Keming found that after adding additional citric acid, although the tea film could not be seen, the rheometer "indicated" that the tea film was still present, but the modulus of the tea film was reduced at this time, that is, citric acid softened the tea film, making the film easier to stretch, while also increasing its mechanical strength.

These stronger membranes play an important role in bottled drinks, says mr jarkomin.

In fact, we are unlikely to see a floating film on bottled tea drinks with the naked eye, mostly because it contains citric acid or other complexes that inhibit tea film formation. Moreover, when tea film is unavoidable, such as in milk tea drinks, a little citric acid can also stabilize the tea film by increasing the mechanical strength of the tea film.

Franz writes that every time she squeezes lemon juice into her black tea, it brings her back to her elementary chemistry class in college and reminds her of mid-term exams that make her shudder. At the time, her chemistry professor Sherry Rowland asked them,"Why does lemon make tea lighter? Please write down the corresponding chemical equation. "

Reference link:

https://aip.scitation.org/doi/10.1063/5.0059760

https://doi.org/10.1038/364581a0

https://doi.org/10.1016/0308-8146(94)90005-1

https://doi.org/10.1016/0308-8146(94)90004-3

https://tea-biz.com/2021/09/19/the-physics-of-black-tea-film/

https://www.nature.com/articles/s41557-020-0445-0

Rheology: An Historical Perspective. By R.I.Tanner and K. Walters

This article comes from Weixin Official Accounts: Global Science (ID: huanqiukexue), written by Wang Yibo, revised by: Erqi

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