1. Why Gel Ink Stays Put in the Pen but Flows When You Write
When you stand a gel ink pen tip-down on a desk, the ink does not drip. However, when you press the ball against paper and move it, the ink flows out smoothly to create a dark line. Pilot, a pen manufacturer, explains that gel ink ballpoint pens are water-based inks with a gelling agent added. [1]
Normally, the ink has high viscosity, which is the measure of a liquid's stickiness or resistance to flow. But during writing, the viscosity drops to levels similar to standard water-based ballpoint pens. This allows for writing with light pressure and minimal bleeding. [1]
So, the same ink changes its thickness depending on whether it is sitting still or being written with. Is this property unique to pen ink, or does it appear elsewhere?
2. Why Ketchup Won't Pour Until You Shake or Tap the Bottle
When you turn a ketchup bottle upside down, the ketchup often refuses to come out. But if you shake the bottle or tap the bottom, it suddenly pours out in a rush. News explanations describe ketchup as showing solid-like properties until a certain limit of force is exceeded. [5]
Once that limit is crossed, it flows like a much thinner, smoother liquid than before. It is thought that shaking or tapping provides the force needed to cross this limit. [5]
It seems that pen ink and ketchup share a similar nature. But what about paint?
3. Why Paint Spreads Easily When Applied but Stays in Place Afterward
Imagine painting a wall. When applying paint with a brush or roller, force is applied, so you want it to spread easily. But if it kept flowing after application, it would drip down the wall. A patent publication explains this problem by noting that highly thixotropic paints have their apparent viscosity lowered when mixed or applied. [6]
This makes them easy to apply. After application, the paint becomes stationary, so its viscosity rises again. This prevents it from flowing and stops drips. [6]
Pen ink, ketchup, and paint are all connected by the idea that they flow when force is applied and settle when at rest. What is the name and definition of this property?
4. Thixotropy: Gels That Turn into Flowing Sols When Stirred
A gel is a state that is firm and wobbly, like jelly. When stirred, it becomes a flowing state called a sol. Dictionaries define thixotropy (also known as *yohen-sei*) as the property where a gel turns into a sol when stirred and returns to a gel when left alone. Yogurt and paint are often cited as examples. [2]
The IUPAC (International Union of Pure and Applied Chemistry) glossary defines a thixotropic gel almost identically. It describes a gel whose viscosity decreases when shear (a force that slides surfaces past each other) is applied, and returns to its original viscosity when the shear stops. [3]
Research reviews offer a deeper explanation. They note that when a stationary substance is stirred to create flow, its viscosity decreases over time. When stirring stops, the viscosity recovers. The key feature is that this takes time. It is distinguished from simple changes where viscosity drops immediately as force increases. [4]
Does this property appear in places far removed from pens?
5. Why Space Pen Ink Is a Gel When Still and a Liquid When Moving
The Fisher Space Pen, known for writing in space, had its patent filed in 1965. Its ink is thixotropic: it acts like a solid gel when sitting still, but becomes a fluid when the ball moves. The inside of the pen is pressurized with nitrogen gas to about 240 kilopascals (about 35 psi). [7]
A famous story claims that NASA spent millions developing a pen for space while the Soviet Union used pencils. Scientific American notes this is a myth. The development cost of about $1 million was paid by Fisher himself, not NASA. When NASA bought 400 pens in 1968, the price was $2.39 each, which was cheaper than the usual $3.98. [7]
The ink property used in regular pens is also used in space pens. Does this property exist in the ground, not just in pens?
6. Why Sand Can Suddenly Act Like a Liquid When Force Is Applied
Quicksand is ordinary sand saturated with water, where friction between grains is reduced, according to science news. A 2005 paper in the journal Nature, by researchers including Khaldoun, showed that sand can suddenly behave like a liquid with just a tiny increase in force. Reports state that a force increase of about 1% caused grain flow to increase by a factor of one million. [8]
However, this article does not use the word "thixotropy." Instead, it uses the term "liquefaction." Dictionaries also describe the temporary liquefaction of solid sand layers during earthquake vibrations as an example of thixotropy. [2]
Here, we list it as an example of a similar property. From pens to sand, a similar property appears in both.
7. Experiments You Can Try and Topics You Can Look Up
If you want to investigate this yourself, you can try it at home with an adult. Place ketchup on a plate. Compare how it comes out when you gently tilt the bottle versus when you close the lid, shake it lightly, and then pour it. For pens, hold it tip-down above paper without touching it to see if it drips. [1]
You can also visit a stationery store. Try writing with a gel ink pen and other types of pens to compare how they feel. Pilot’s support page lists the types and features of ballpoint pens. [1]