Each individual grain of powder is unambiguously a solid, but a bulk collection of powder does not behave the way any single solid does. Powders can be poured like water, compressed into rigid blocks, or dispersed into airborne clouds, shifting between behaviors we normally associate with solids, liquids, and gases. This shape-shifting quality is why physicists treat granular materials as something close to a distinct state of matter, one that borrows properties from the classical three but follows its own rules.
Why Powders Do Not Fit Neatly Into One Category
A 1982 paper in the journal Endeavour captured the oddity well: powders “can float like a gas; flow like a liquid; or support a weight in the same way as hard-packed snow.”1Endeavour. Powders—gaseous, liquid and solid That single sentence sums up decades of frustration among scientists trying to classify granular materials. When you dump sugar into a bowl, it streams from the bag the way water pours from a pitcher. Once it lands and settles, it forms a heap that holds its shape like a solid. And if you throw a handful of flour into the air, the fine particles drift and disperse much the way smoke does. The material has not changed chemically between those three moments. What changed is how the particles interact with one another and with the forces acting on them.
This is not just a curiosity. The ambiguity matters in engineering, geology, pharmaceuticals, and food processing, anywhere large quantities of particulate material need to be moved, stored, or shaped. Understanding when and why a powder will act solid, liquid, or gaseous is a practical question worth billions of dollars in industry and, occasionally, a matter of life and death in natural disasters.
When Powder Acts Like a Solid
Leave a pile of sand on a table and it stays put. Press powder into a tablet and the tablet holds together. These are solid-like behaviors, and they arise from friction and interlocking between grains. The most visible sign of this is the angle of repose, the steepest angle a pile of granular material can maintain without collapsing. Pour dry sand slowly onto a flat surface and it forms a cone. The slope of that cone depends on particle shape, size, surface roughness, and the friction between grains.
Research on the angle of repose has shown that particle shape plays a surprisingly large role. Elongated or angular particles interlock more effectively than spheres, and there appears to be an optimal shape that maximizes resistance to flow. Studies have found that for elongated particles, the angle of repose hits a peak at a particular aspect ratio, then can decrease or plateau as particles become more extreme in shape.2Powder Technology. Effect of friction on the angle of repose of elongated particles This is not just about static piles. The same frictional behavior that determines a heap’s slope also governs how much force is needed to make the material start flowing.
Particle size matters too. Simulations and experiments have mapped how the angle of repose changes with grain diameter under Earth’s gravity, and the numerical predictions line up well with experimental data across a range of materials.3PubMed Central. An expression for the angle of repose of dry cohesive granular materials on Earth and in planetary environments Very fine powders tend to have higher angles of repose than coarse grains, partly because cohesive forces between tiny particles (van der Waals attraction, moisture films, electrostatic charge) become more significant relative to gravity as grains get smaller.
Force Chains and How Powder Supports Weight
When you stand on a beach, the sand under your feet does not distribute your weight evenly the way a rubber mat would. Instead, your weight travels through the granular bed along narrow, branching paths called force chains. These are strings of particles in direct contact that carry loads far above the average, while the majority of surrounding grains bear almost nothing. Experiments have shown that most particles in a granular material carry less than the average load, and the number carrying above-average load drops off sharply.4PubMed. Characterization of force chains in granular material The high-load particles form a “strong network” that threads through the bulk, while the low-load majority forms a “weak network” that fills in the gaps.
This is part of why granular materials can feel rigid in some situations and fragile in others. When force chains are well-established and aligned with the applied load, the material resists deformation effectively. But disturb those chains, say by vibrating the container or shifting the load direction, and the structure can collapse abruptly. Numerical modeling of force chains under continuous loading has revealed that their formation, evolution, and sudden reorganization are directly tied to how the grains contact the external load and to the geometric arrangement of particles.5PubMed Central. Deformation and Force Chain of Two-Dimensional Granular Systems under Continuous Loading In sheared systems, the chains become anisotropic, preferentially aligned along the direction of compression, and the distribution of chain lengths shifts compared to an evenly compressed pack.6PubMed. Force-chain distributions in granular systems
Recent experimental work has pushed this further into three dimensions. Using optical techniques to image force chains inside packed spheres and multi-faceted particles under load, researchers found that spheres tend to form strong vertical chains that intensify under increasing pressure, while angular particles develop more interconnected, web-like chain networks.7PubMed Central. Dynamic imaging of force chains in 3D granular media The shape of the grain changes the architecture of the invisible scaffolding holding everything up.
When Powder Flows Like a Liquid
Tilt a container of sand past a critical angle and it avalanches. Feed powder through a hopper and it streams out of the opening in a way that looks remarkably fluid-like. This liquid-like behavior is one of the most practically important properties of granular materials, and it does not follow the same rules as true liquids.
In a real liquid, flow rate through an opening depends on the depth of liquid above it, because the pressure at the bottom increases with height. Granular flow is different. In a tall silo, the pressure at the bottom saturates beyond a certain fill height because of friction between grains and the silo walls, an effect that redirects some of the weight sideways. Research using a specially designed silo that removed this wall-friction effect found that when friction was eliminated, the flow rate did change with pressure in a straightforward linear way, but only up to a point. Beyond a certain ratio of silo width to opening size, the effect of pressure on flow became negligible, and the classic empirical flow law took over.8Physics of Fluids. External pressure dependence of granular orifice flow: Transition to Beverloo flow This is one of the ways granular “liquids” diverge from actual liquids: the pressure-flow relationship is fundamentally different.
You can also force powder into a fluid-like state artificially by blowing gas through it from below, a process called fluidization. A landmark classification from the 1970s divided powders into four groups based on how they respond to upward airflow. Some powders expand smoothly and uniformly before bubbles form. Others start bubbling immediately. Very fine, cohesive powders resist fluidization altogether and tend to channel, letting gas escape through cracks rather than lifting the particles evenly. Coarse, dense particles can form spouted beds where a central jet punches through the material.9Powder Technology. Types of gas fluidization These categories remain a standard reference in chemical engineering because they dictate reactor design, drying processes, and coating operations.
When Powder Behaves Like a Gas
At the far end of the spectrum, when grains are widely dispersed and moving fast with lots of space between them, the collection starts behaving like a gas. Physicists have formalized this as “granular gas” theory: a system of particles bouncing around and colliding, losing energy with each collision because the impacts are not perfectly elastic. Unlike molecules in a real gas, which maintain their kinetic energy indefinitely in an insulated container, granular particles slow down and eventually stop unless energy keeps being pumped in.10PubMed. Kinetic approach to granular gases
This dissipation leads to some strange effects. In a cooling granular gas where no external energy is added, kinetic energy drops continuously. But researchers have shown that as particles lose energy and begin clumping together, the number of free-flying particles decreases. Under certain conditions, the “temperature” of the gas (measured as average kinetic energy per particle) can actually increase even as total energy drops, because fewer particles share the remaining energy. This counterintuitive heating-while-cooling effect echoes behavior seen in certain astrophysical systems with long-range gravitational interactions.11PubMed Central. Increasing temperature of cooling granular gases
Kinetic theory has been extended to handle these granular gases at moderate densities, treating particles as inelastic rough spheres that can spin as well as translate. The math gets complicated because you need to track not just velocity but also rotation for each particle, but the underlying picture is straightforward: it is gas physics with an energy leak at every collision.12Journal of Physics A: Mathematical and Theoretical. Moderately dense granular gas of inelastic rough spheres
The Jamming Transition
One of the most active areas in granular physics is understanding the transition between the fluid-like and solid-like states. When you shake a jar of powder vigorously, the particles flow freely. Stop shaking and they jam, locking into a rigid arrangement almost instantly. This jamming transition is not the same as freezing. There is no temperature change involved, and the particle arrangement is disordered, not crystalline. Instead, particles simply crowd together until they cannot rearrange anymore, and the whole collection suddenly resists deformation.
Experiments on fine powders with controlled attraction between particles found that at the jamming threshold, clusters of particles pack together in a metastable state at a specific density that depends on how strongly the grains attract each other. Near the transition, the relationship between stress and packing fraction follows a pattern similar to what is seen in foams, emulsions, and colloidal suspensions, suggesting that jamming is a surprisingly universal phenomenon across very different soft materials.13PubMed. Jamming threshold of dry fine powders A foam going from squishy to rigid and a powder going from flowing to stuck may be expressions of the same underlying physics.
Dilatancy and the Wet-Sand Paradox
If you have ever walked on wet sand at the beach, you may have noticed something odd: the sand around your foot appears to dry out momentarily as you step down. This is Reynolds dilatancy, named after the 19th-century physicist Osborne Reynolds. When a tightly packed granular material is forced to deform, say by your foot pressing into it, the grains need to rearrange. To slide past one another, they have to temporarily push apart, expanding the pore space between them. In wet sand, this expansion sucks water into the newly created voids, making the surface look dry around the pressure point.14PubMed. Dilatancy in slow granular flows
Dilatancy also matters in engineering. Compacted soil or powder that must expand before it can shear is initially stronger than loosely packed material, because extra force is needed to push the grains apart. But once dilation begins, the material can weaken rapidly. This is one reason why densely packed slopes or embankments can fail suddenly rather than gradually: the initial resistance masks the fact that once movement starts, the material loosens and the resistance drops.
The Brazil Nut Effect and Size Segregation
Open a can of mixed nuts after a bumpy car ride, and the largest nuts are sitting on top. This “Brazil nut effect” is one of the most studied phenomena in granular physics, and it demonstrates another way powders defy expectations. In a normal fluid, heavy things sink. In a vibrated granular bed, large particles rise.
Three-dimensional imaging of mixed-size particles during vibration has revealed why. As large particles get jostled, their elongated axes rotate toward vertical. Once oriented upright, they present less horizontal cross-section, which allows smaller particles to percolate downward through the gaps. The downward migration of small grains pushes large grains upward by a simple mass-balance argument, and the vertical orientation gives large grains the most room to move upward.15Scientific Reports. Size segregation of irregular granular materials captured by time-resolved 3D imaging Other work has identified three complementary mechanisms behind size segregation: void filling (small particles dropping into gaps beneath large ones), convection rolls driven by wall friction, and a kinetic-theory effect called thermal diffusion.16PubMed. Mechanisms in the size segregation of a binary granular mixture The mix of mechanisms means that segregation can be suppressed, reversed, or amplified depending on container shape, vibration frequency, and the size ratio of particles.
When the Ground Turns to Liquid
Soil liquefaction during earthquakes is one of the most dramatic real-world demonstrations that granular materials can switch states. Saturated sandy soil that is perfectly solid under normal conditions can suddenly lose all its strength during seismic shaking, causing buildings to tilt or sink into what was, moments earlier, firm ground. The mechanism involves water trapped between grains. When shaking compresses the grain structure, water pressure in the pore spaces spikes. If it rises high enough, the water effectively pushes the grains apart, eliminating the grain-to-grain contact that gives the soil its strength. The material transitions from solid-like to fluid-like in seconds.
Experiments and simulations have shown that this transition is essentially a rigid-to-fluid switch: structures resting on the surface sink to their buoyant depth within the granular layer, just as they would in a true liquid. The presence of water has a dramatic influence on the stability of anything sitting on the surface, because the effective weight of each grain is reduced by buoyancy.17PubMed. Sinking during earthquakes: Critical acceleration criteria control drained soil liquefaction Recent work has also shown that liquefaction can happen under conditions previously thought safe, even far from the earthquake’s epicenter, when water drainage through the soil during shaking creates pressure gradients that strip away grain contacts from the top down via a propagating compaction front.18PubMed Central. Drainage explains soil liquefaction beyond the earthquake near-field
Industrial Headaches With Powder Flow
If you have ever watched a saltshaker clog and then suddenly dump a clump, you have experienced on a tiny scale the same problem that plagues factories handling bulk powder. In hoppers and silos, powder can form stable arches across the outlet, blocking flow entirely. It can also form “ratholes,” hollow channels through the middle of the material that leave most of the powder stuck to the walls.
Predicting when these blockages will happen is harder than it sounds. Traditional methods based on measuring a powder’s flow properties under standard conditions often underestimate the risk. Research in pharmaceutical manufacturing has shown that accurately predicting flow obstruction requires understanding the stress state the powder actually experiences at the outlet, which depends on the geometry of the hopper and the fill conditions.19PubMed. Risks of Powder Flow Obstruction in Hopper and Bin Discharge in Solid Dosage Form Manufacture should be Predicted Under The Active Stress State In some cases, vibration is used to break up arches and keep material flowing, but the relationship between vibration and arch formation is not straightforward. Studies on cohesive fine powders in vibrated silos have found that arches can actually form as a consequence of vibration at certain accelerations, rather than being broken up by it.20Particuology. Arch formation mechanism and discharge process of cohesive fine powder in a vibrated silo
Static charge adds another layer of complication. As powder particles tumble against one another and against container walls, they build up triboelectric charge, the same effect that makes your hair stand up after rubbing a balloon on it. In industrial powder flows, this charge buildup can increase friction, reduce flowability, and cause clogging in fluidized bed reactors and pneumatic conveying systems.21PubMed Central. Triboelectric charge saturation on single and multiple insulating particles in air and vacuum The finer the powder, the worse the problem tends to be, because smaller particles have a higher surface-area-to-mass ratio, so electrostatic forces become large relative to gravity.
Powder in Manufacturing and 3D Printing
The dual solid-and-fluid nature of powder is not just a physics curiosity; it is something engineers exploit deliberately. In metal additive manufacturing, thin layers of metal powder are spread across a build platform and selectively melted by a laser or electron beam, building a solid part one layer at a time. The powder needs to flow smoothly enough to form even layers (liquid-like behavior) but then hold still while the laser works (solid-like behavior). Research on recycled titanium alloy powders for 3D-printed biomedical implants has found that reconditioned powder can actually flow better than mixed or single-batch recycled powder, and the printed parts can match or exceed the mechanical strength of parts made from brand-new powder.22Metallurgical Research & Technology. CT-scan, SEM, EDX, flowability, rheology, permeability and tensile test analysis of recycled Ti6Al4V powders for 3D printing
In pharmaceutical 3D printing, powders are extruded directly through a nozzle to create drug delivery systems with customized dosages and release profiles. Here the challenge is different: the powder must flow consistently into the extruder, melt at the right temperature, and hold its printed shape as it cools. Flowability of the starting powder blend has been identified as one of the critical parameters for successful printing, alongside the melting behavior of the polymer carriers and the printing temperature.23PubMed Central. 3D Printing Direct Powder Extrusion in the Production of Drug Delivery Systems: State of the Art and Future Perspectives A powder that jams in the feed mechanism or flows unevenly produces tablets with inconsistent drug content, which is not a cosmetic issue in medicine.
Landslides and the Rheology of Flowing Earth
At geological scales, the solid-liquid duality of granular materials becomes a question of life and safety. A hillside of soil and rock is a solid until it is not. Once a landslide initiates, the flowing mass of debris can behave in ways that range from solid-like (slow creeping) to fully fluid-like (fast-moving debris flows), depending on how fast the material is shearing and how much water is mixed in.
Geophysicists classify these flows using a “flow index” that captures where on the solid-to-liquid spectrum a moving mass sits. At one extreme, the material resists flow regardless of how fast you try to push it, behaving like a rate-independent solid. At the other extreme, it flows like a classic fluid with constant viscosity. Between those poles, the material can thin under shear (becoming more fluid-like as it speeds up) or thicken under shear (becoming more resistant as it accelerates), with the latter case sometimes leading to sudden jamming of a flow in progress.24Elsevier / Earth-Science Reviews. Classic, modern, and physics-based rheological laws for geophysical granular flows in a landslide hazard chain Predicting which behavior a given slope will exhibit after failure is one of the central challenges in landslide hazard assessment, and it depends on grain size distribution, water content, slope angle, and the volume of material involved.
The same physics that makes a jar of cornstarch mixed with water harden when you punch it and flow when you stir gently operates, at vastly larger scales, in the rocky debris racing down a mountainside. The difference is that in the kitchen you can wash your hands afterward.