The flow-stick transition has been placed at about 50um for dry mineral particles (Smalley 1964, Jones & Pilpel 1966a,b). Particle systems with particles less than 50um are cohesive; with larger particles flow through an orifice is possible. As particle size increases the flow rate increases, gravitational forces overcome cohesive forces. If flow is observed using a 3mm orifice there is a transition point at about 150um.
Bingham, E.C., Wikoff, R.W. 1931. The flow of dry sand through capillary tubes. Journal of Rheology 2, 395-400.
Jones, T.M., Pilpel, N. 1966a. The flow properties of granular magnesia. Journal of Pharmacy & Pharmacology 18, 81-93.
Jones, T.M., Pilpel, N. 1966b. The flow of granular magnesia. Journal of Pharmacy & Pharmacology 18, 429-442.
Smalley, I.J. 1964. Flow-stick transition in powders. Nature 201, 173-174.
The simple picture is the graph from Smalley (1964); this is crushed quartz passing through a 3mm orifice. A beautifully clear picture of the flow-stick transition ( and probably the first one). At point S flow stops because the gravity forces producing flow are counter-balanced by the cohesive forces in the system. This is at about 50um- an interesting size for all loess people. At point B the orifice is blocked- the particles are simply too large to pass through, simple jamming. Point T is interesting; here is where cohesive forces really begin to be felt; to the left of M cohesive forces are dominant; to the right of M gravity forces are dominant.
Sorry about the sideways nature of this picture. This is from Jones & Pilpel(1966a). Magnesia instead of quartz; a beautiful set of curves; note point S- nicely placed at about 50um, the various aperture sizes are indicated. An interesting observation from the complex picture is that the 3mm aperture was causing a definite 'container' effect. T point is at about 250um, this is clearly observed from the values of flow through the larger apertures. In a general system cohesion effects kick in at sizes below about 250um.
We cite Bingham & Wikoff (1931) because they were possibly the first people to study the flow of granular materials through orifices(?) and because their simple experimental set-up was followed by Smalley (1964) and Jones & Pilpel (1966a). They were more concerned with flow properties- rather than material properties- and did not observe any interesting interactions between gravitational and cohesive forces.
Some loessic relevance? When loess material is falling from the sky to make a loess deposit the size of the particles is important. Say loess has a mode size around 30-50um, just on the cohesive side of that important reference point. The system is cohesive and this causes an open structure to form, limited compaction on deposition- some obviously but not enough to form a compact sediment. Had the particles been somewhat larger a much simpler packing would have been produced. 100um particles can move relative to each other. 50um particles form open structured systems with quite high tensile strengths, and the strength grows as some cementation occurs. Enough tensile strength to form high vertical faces in exposures, and to retain the open structure until loaded and wetted.
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