‘Jamming’, in physics, refers to the way certain types of fluid or fluidlike materials, including powders, become more viscous at higher density. As density increases, the particles get packed more tightly together, until the fluid undergoes a transition. At this point the particles are so tightly packed that the material behaves as a solid rather than a liquid.
To understand what this transition looks like, imagine pouring a stream of sand onto a flat surface. When you pour the sand, it behaves like a liquid. However, when it reaches the surface it forms a pile — instead of spreading out like a liquid. The force of gravity pushing the grains of sand together has caused them to jam and behave like a solid.
Another example is the way sand can sometimes jam in an hourglass. If the sand particles form an archlike structure, the arch itself begins to jam under the weight of the sand above it, which presses the particles together. Increasing the force on the jammed sand by piling more sand on top or trying to push it through won’t unjam it. Only an intervention that disrupts the jammed state (such as shaking the hourglass) will allow the sand to flow again.
Jamming plays a role in many different situations, from transporting powders through pipes in the concrete industry to explaining the way rocks fall in an avalanche. The same physical models can even be used to describe traffic jams, by treating the cars as a granular fluid, flowing along the road.
When we apply force to coffee by tamping, there comes a point at which the particles ‘jam’ and cease to move. This point is the maximum density we can achieve with tamping by hand.