In daily life, maybe everyone has the experience of accidentally dripping coffee on the table, but I don’t know if you have noticed such a magical phenomenon: if you don’t wipe off the coffee dripping on the table in time, wait After it dries, the stain that forms on the tabletop is a ring that is dark on the outside and light on the inside.
This phenomenon is called the coffee ring effect.
Unlike pure water, in a sense, brewed drinks such as coffee and tea can be classified as a solution (strictly speaking, a mixture of a solution and a suspension).
The dark brown color of coffee comes from the charcoal black substances after roasting and extraction of coffee beans. After the coffee is brewed, these substances become several small particles suspended in the liquid. Small particles are responsible for the color of the coffee stains we accidentally drip onto our clothes or tabletops.
For most ordinary people, the coffee ring effect is just an accidental phenomenon in daily life, but in the eyes of physicists, there is a big mystery behind it.
Different evaporation rates lead to coffee rings
Although the coffee ring effect is named after coffee, in a broad sense, it should be a scientific phenomenon that occurs when a solution evaporates on a solid surface. The formation process of the observable colored ring represented by the coffee ring is also relatively intuitive: when the solution drops on the desktop or paper, the droplet evaporates and does not form a circle that gradually shrinks in proportion but A ring shape with more solute particles on the outside and fewer solute particles on the inside is formed.
This is because when the water in the droplet evaporates, the interaction between the solid-liquid interface between the table and the liquid and the gas-liquid interface between the liquid and the air surface leads to inconsistent evaporation rates of the water at different positions of the droplet, which in turn promotes the formation of coffee rings.
At the interface between the desktop or paper surface and the droplet, the evaporation rate of water is faster than at the interface between the droplet and the air.
In this way, the water molecules in the middle of the droplet will carry solutes and particles to the edge of the droplet and the desktop or paper to replenish. Eventually, after the water evaporates, the small particles of solute continue to accumulate at the edge, slowly becoming the ring we see.
In 1997, physicists Sidney Nagol and Thomas Witten of the University of Chicago and other researchers published an academic paper on the coffee ring effect in the internationally authoritative journal “Nature”. In this article, the coffee ring effect is formally described for the first time. Since then, scientific research on how to break the coffee ring effect has also emerged.
There are big applications behind small discoveries
The coffee ring effect is not only an interesting little phenomenon in life, but its appearance actually brings some troubles and troubles to people’s daily production and life. For example, the uneven inkjet of printers is caused by the coffee ring effect.
To solve these problems, the researchers set out to find out how to form a uniform solid layer after the liquid droplets evaporate.
Later, scientists discovered that to solve this problem, they needed to change the nature of the suspended particles. The researchers found that different particle shapes can change the properties of thin films at the interface between air and liquid, thereby affecting the evaporation process of these substances.
Under the same conditions, oval particles will change the interface between air and liquid. This discovery directly reveals the effect of particle shape on evaporation, and then effectively guides people to improve printing and painting methods.
In addition, the coffee ring effect is also “sparkling” in the biological field. In diagnostics, scientists combine the coffee ring effect with biosensing technology to detect biomarkers in body fluids such as saliva and blood. Researchers at Vanderbilt University in the United States built on this research to create a biological method for the rapid detection of malaria.
Not only that, but the coffee ring effect also draws attention to the fact that various solutes in the same solution can be separated according to the particle size.
For example, the particles concentrated on the outermost side of the coffee ring have the smallest diameter, and the particles concentrated on the inner side of the coffee ring have a larger diameter. Scientists have used this principle to separate related substances.
The famous British physicist and mathematician Newton began to pay attention to gravitation after being hit on the head by an apple, which then influenced the development of physics in the following years. Perhaps the mysteries of science are hidden in some subtle phenomena in daily life, waiting for people to explore.
Next time when you are drinking coffee, if you accidentally drip the coffee on the table, you might as well wipe it off slowly, and at the same time imagine a new application for the coffee ring effect.