Did you eat cereal this morning? Or have you walked on a gravel path? Maybe you had a headache and had to take a pill? If you answered any of these questions with a yes, you interacted with a granular system today.
Scientists classify any collection of small, hard particles – such as puffed rice, sand grains or pills – as a granular system.
Even though everyone has interacted with these kinds of systems, describing the physics of how the particles collectively act when they are close together is surprisingly hard.
Granular systems sometimes move like a fluid. Think of an hourglass where sand, a very typical granular material, flows from one half of the glass to the other. But if you’ve run on a beach, you know that sand can also act like a solid. You can move over it without sinking through the sand.
As a geologist, I’m interested in understanding when a granular system flows and when it has strength and behaves like a solid. This line of research is very important for many agricultural and industrial applications, such as moving corn kernels or pills in a pipeline or shoot.
Understanding when a granular system might flow is also essential for geologic hazard assessments. For example, geologists would like to know whether the various boulders making up the slope of a mountain are stable or whether they will move as a rockslide.
Transferring forces between grains
To understand the behavior of a granular system, scientists can zoom in and look at the interactions between individual grains. When two particles are in contact with each other, they can transfer forces between each other.
Imagine this scenario: You have three tennis balls – the grains in this experiment. You place the tennis balls in a row and squeeze the three balls between your hand and a wall, so that your hand presses against the first ball. The last ball is in contact with a wall, but the middle ball is free floating and touches only the other two balls.
Tennis balls can act as grains in this simple granular system experiment. When you push against the tennis ball on the end, you exert a force, which acts upon the other two balls and eventually the wall.
Jeremy Randolph-Flagg
By pushing against the first ball, you have successfully transferred the force from your hand through the row of three tennis balls onto the wall, even though you’ve touched only the first ball.
Now imagine you have many grains, like in a pile of sand, and all the sand grains are in contact with some neighboring grains. Grains that touch transfer forces between each other. How the forces are distributed in this granular system dictates whether the system is stable and unmoving or if it will move – such as a rockslide or the sand in an hourglass.
