Not all materials behave like fluids or rigid solids; there are some which are neither fluids nor solids and so fall somewhere in between. It is exactly there that Discrete Element Modelling (DEM) shines bright!

DEM is a particle-based simulation method. In simple terms this means that each particle in the simulation is treated as a distinct object (rather than treating the material as a continuous mass like we do in Fluid Dynamics, for instance). So, in DEM, each particle has its own position, velocity, shape and behaviour and what DEM does is that it tracks these particles when they move, collide, stick, slide or roll over time. In short, DEM calculates contact forces, such as friction, cohesion and restitution between particles and between particles and walls. 

Think sugar granules falling into a sack. Each granule in this case would be represented individually, and these individual granules' paths, pile shape, their bouncing, sticking or crushing can all be visualised and/or measured. Furthermore, you can change particle size, shape or stickiness and see how the flow changes. So, this particle-level detail makes DEM especially useful when you are studying materials which do not behave neither like fluids nor like solids, when particle interactions dominate the system, such as with powders, grains, tablets, as well as when you want to understand segregation, breakage or jamming which emerge from how particles behave individually and collectively. 

DEM is used when you want to avoid bridging, arching and flow inconsistencies in hopper and silo design, in mixing and blending when you want to assess segregation risks to improve homogeneity, in tablet coating or compaction, to simulate mechanical stresses and surface contact. It is also used in conveying and transport to optimise equipment and reduce breakage and dusting as well as in additive manufacturing to model powder spreading and deposition. 

On the flip side DEM can be computationally intensive and calibration is not an easy thing, however it provides insights into problems that are otherwise not so clear and so trial-and-error driven, which wastes time and resources and lacks precision. 

Let's take milk powder as a practical example.

In milk powder production spray drying is followed by bulk powder handling operations, conveying storage, mixing and packaging. Milk powder is cohesive, hygroscopic (absorbs moisture) and often fragile and during post-drying handling, manufacturers might face issues such as segregation due to particle size variation leading to uneven composition, caking and clumping in hoppers or silos, inconsistent flow rates during packaging, excessive dust generation during pneumatic conveying and product degradation from mechanical stress. 

All of these issues can in fact be addressed using DEM. In dairy, where food safety, consistency, and hygiene are non-negotiable, DEM offers a way to proactively address flow issues and product damage before they reach the production floor or the customer. Not only can dairy producers optimise on hopper design and wall angles to prevent blockages, but they can also reduce downtime due to flow stoppages or cleaning, improve product uniformity and reduce waste as well as inform decisions on process parameters, such as airflow, velocity and drop height in conveying systems.