Moisture content, temperature, particle size, and time of storage at rest can have a tremendous impact on your material's flow properties. How your material flows is a function of it's cohesive properties. As such, conditions that effect the cohesive strength of a solid will have an impact on its handling capabilities.

Moisture Content

The rule of thumb is "as the moisture content of a solid increases, so does it's cohesive strength".

Even though there may be only a slight increase in moisture, say from 1% to 1.5%, the flowability of your material can be significantly impacted. Hygroscopic materials can experience significant moisture increases simply by being exposed to humid air.  Your material flow properties should be measured using a representative value for moisture content.

Temperature

Cohesiveness is also affected by the solid's temperature.

Some materials are sensitive to increases in temperature (e.g. room temperature to 150 deg F). Others are sensitive to constant temperature. These conditions can be simulated in the laboratory through flow properties tests.

Soybean meal, for example, when stored in a bin or hopper at 90 deg F, is relatively easy to handle. At 100 deg F and greater, soybean meal becomes extremely difficult to handle and capable of bridging over very large openings.

Particle Size

Another rule of thumb is, "as a bulk solid becomes finer, it also becomes more cohesive and subsequently more difficult to handle".

Fibrous and angular particles are usually more cohesive than particles that are rounded. Typically, particles that are consistently ¼" and larger are not cohesive arching problems. They can however, form interlocking arches.

Time of Storage at Rest

As a solid remains at rest in a bin or hopper, it can become more cohesive and difficult flowing.

At rest, the compaction loads due to head pressure can produce a strong cohesive bond. A chemical reaction, crystallization, or adhesive bonding can also cause this. Sometimes, after a cohesive arch is broken up, say by somehow initiating flow, the material can revert back to its original flow condition and not exhibit a similar cohesion if left at rest again. On the other hand, some materials will time and time again, bridge and rathole even after flow is re-initiated.