Coffee dripped onto table, so scientists found a way to detect malaria

In life, everyone has the experience of accidentally spilling coffee, tea, or drinks. While you are busy and flustered, I wonder if you have noticed such a magical phenomenon - if you accidentally spill it, or drip coffee on the table, after it dries, the stain formed will be a ring with a dark outer layer and a light inner layer.

Source: Baidu Images

Unlike transparent pure water, brewed drinks such as coffee and tea can be classified as solutions in a sense (strictly speaking, a mixture of solutions and suspensions), and the substances dissolved in water intuitively show their color.

Take coffee, which we often drink, for example. Its dark brown color comes from the carbon black substances after coffee beans are roasted and extracted. After coffee is brewed, these substances become several small particles suspended in the liquid. This is the case with coffee and tea stains that we accidentally drop on clothes or desks. Although we know that the color of these stains is caused by small particles, what makes these small particles eventually become a ring?

Source: Xinhuanet

1

How do people know about the coffee ring effect?

For most ordinary people, the coffee ring effect is just an accidental phenomenon in life. But in the eyes of physicists, there is a lot of mystery behind it. In 1997, physicists Sidney Nagel and Thomas Witten from the University of Chicago published a paper on the "coffee ring effect" in the journal Nature.

In this article, the coffee ring effect was formally described for the first time, but the focus of this paper was still on suspended spherical particles. Later, with the continuous deepening of similar research, people gradually understood the role of the shape of suspended particles. Subsequently, research results on how to break the "coffee ring effect" also continued to emerge. For example, a team of physicists at the University of Pennsylvania hoped to change the shape of particles in the solution to destroy the coffee ring effect and find a new way to achieve a uniform deposition of solid particle layers.

Source | Baidu Encyclopedia

2

What causes the coffee ring effect?

Although the coffee ring effect is named after coffee, in a broad sense, it should be a 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 a table or paper, the evaporation of the droplet does not form a circle that gradually shrinks in proportion, but forms a ring shape with more solute particles on the outside and fewer solute particles on the inside.

Source | Chemical Instrument Network

This is because when the water in the droplet evaporates, it is affected by the different interactions between the solid-liquid interface between the desktop and the liquid and the gas-liquid interface between the liquid and the air surface, resulting in different evaporation rates of water at different positions of the droplet, which leads to the formation of coffee rings. At the interface between the desktop or paper and the droplet, the evaporation rate of water is faster than that at the interface between the droplet and the air. In this way, the water molecules in the middle of the droplet will carry the solute and particles to the edge of the droplet and the desktop or paper for replenishment. After the water finally evaporates, the small solute particles continue to accumulate at the edge and form a "ring".

Source | Baidu Encyclopedia

3

Small discoveries with big applications

The coffee ring effect is not only an interesting little phenomenon in life, its appearance actually brings some "annoying" troubles to people's production and life. For example, uneven inkjet of a printer is also a manifestation of the coffee ring effect.

To solve this problem, scientists began to look for ways to form a uniform solid layer after the droplets evaporated. Later, they found that in order to solve this phenomenon, they needed to change the properties of the suspended particles. Scientists found that different particle shapes can change the properties of the film at the interface between air and liquid, thereby affecting the evaporation process.

Under the same conditions, elliptical particles will change the interface between air and liquid. Although this research has not yet fully matured, scientists are still committed to removing the "coffee ring effect" by changing the shape of suspended particles. However, this discovery directly reveals the effect of particle shape on evaporation and effectively guides people to improve printing and painting methods.

Scientists study the effect of particles of different shapes on evaporation

Source | "Control of Droplet Evaporation on Oil-Coated Surfaces for the Synthesis of"

In addition, the coffee ring effect is also "shining" in the biological and medical fields. In medical diagnosis, scientists combine the "coffee ring effect" with biosensor technology to detect biomarkers in body fluids such as saliva and blood. Researchers at Vanderbilt University in the United States have developed a method for rapid detection of malaria based on this.

As a highly contagious disease, how to quickly detect malaria is an important means of preventing and controlling the spread of infection. In this detection method, researchers add three types of particles to the liquid to be tested (one that can emit red fluorescence, one that can emit green fluorescence, and one that is magnetic but cannot emit fluorescence). When a water droplet containing these three particles dries under the action of a magnetic field, the two types of particles that can emit fluorescence will gather at the edge of the droplet, presenting a yellow ring composed of green and red light. The magnetic particles will react chemically with a special protein secreted by the pathogen Plasmodium and connect to the green fluorescent particles. Both colored particles will remain, presenting a red ring + green circular spot. The determination of color and shape can most intuitively show whether there is Plasmodium infection.

Source | Science World

Not only that, the coffee ring effect also makes people pay attention to the fact that multiple solutes in the same solution can be separated according to the different particle sizes. For example, the particles concentrated on the outermost side of the coffee ring have the smallest diameter, and the diameter on the inner side of the coffee ring is larger. This principle can be used to achieve the separation of substances.

Newton was hit on the head by an apple, and then he began to pay attention to gravity; perhaps the mysteries of science are also hidden in these subtle phenomena in life. The next time you drink coffee, if coffee accidentally drips on the table, you might as well wipe it up slowly and imagine new applications of the coffee ring effect.