Editor’s Note: This post on the Brazil Nut Effect was written by Steve Grimmer, Little River’s Artist Mechanic.
In our planning meeting last week, I had to be reminded by my colleagues that the Brazil Nut Effect is a phenomenon wherein larger particles will tend to rise to the top of a mixture when it is shaken or otherwise disturbed.
The classic example is of course, a can of mixed nuts: open one up and you’re almost guaranteed to see several Brazil Nuts at the top. I happen to like them, so it’s a positive effect for me. My mom told me as a child that she and her siblings called them Troll Toes when she was a girl in the 1950’s so I suppose there’s room for disagreement on that point. Keeping with the Snack Food Physics theme, this also explains why when you get to the bottom of your trail mix, you’re left with sunflower seeds and salt. Here’s a fun and informative video from Nicole Sharp at her great blog, FY! Fluid Dynamics, aka FYFD:
Here at Little River, when we send our Memphis and Carbondale color-coded-by-size media out to clients, we typically put the very fine black and red fractions in a plastic bag separate from the coarser yellow and white media. The black and red particles are a small proportion of the mix, so it’s hard to see them when everything is evenly distributed; it appears at first glance as though we forgot to add them. If we simply pour the red and black on top of the coarse particles in the shipping buckets, the vibration of the truck during transport is enough to bring coarse particles up to the top and cover the fines!
Out in the field, the same effect can be seen in almost any rocky or gravelly stream. The bed of the stream is invariably covered with large stones or cobble, while fine sediments sink down below out of the flow. This is one of the mechanisms that stabilizes a channel and slows erosion and is known as Riverbed Armoring. It has been recognized for many years and generally been understood as a product of fluid mechanics rather than the behavior of a granular system: smaller particles tumble along until they land in a gap between larger particles and find shelter from the water flow. Periods of higher flow roll the big particles over on top of the smaller sediment, and the bed becomes sorted. We don’t tend to see this in our Emriver stream tables because our media is specifically sized to model erosion and sediment transport, rather than inherently stable systems. A very stable stream model wouldn’t be very interesting, would it?
There are other forces at work, though, as University of Pennsylvania Professor of Geosciences Douglas J. Jerolmack and colleagues have discovered. In a paper published in the journal Nature in 2015 they show that in addition to suspended and bedload transport of sediment, there is very slow creep of particles below the zone of hydrological interaction. This is a result of shear force from moving particles on the stream bed interacting with fine particles several layers below. The finding has implications in predicting the onset and volume of particle erosion in different hydrological events because the substrate holding the steam bed has structurally reorganized and moved downstream since the last flood. That doesn’t quite get to the armoring question, though.
In research that builds on this, Jerolmack and his team use the same instrument, a circular flume, to show vertical movement of larger particles in the stream bed substrate. This flume is a clear plastic toroid about 36cm across, with a channel width of 2.5cm. The granular media is made up of clear plastic beads between 1.5mm and 3mm diameter, and the fluid used is a light oil. The lid of the flume rotates and is fitted with paddles to induce flow in the system, and a camera takes pictures of the particles inside the channel illuminated with a sheet laser.
Jerolmack and his team published a paper in the journal Nature in 2017 that demonstrates not only the afore-mentioned horizontal sub-bed transport, but also vertical sorting of particles, with the larger grains rising to the top of the bed over time. The team documented the effect both at vigorous levels of bedload and suspended transport and at very low levels. This finding shows that our understanding of granular physics applies to riverbeds in motion along with the hydrologic forces acting on the surface particles. This research has wide-ranging applications outside fluvial geomorphology, from explaining why soil on hillsides tends to have coarse particles on top to helping predict landslides and avalanches.
So, we have a couple of experiments to try here at Little River Research and Design. In our Emflume1, we’ll add a coarser fraction to our color-coded media and do a time-lapse video to see if we can track horizontal or vertical movement in the subsurface layers. We can also add some plastic riprap to the media and watch for the same effect.
In our Em3 stream table, I’d like to insert a polycarbonate wall running the length of the model and use that as a window to watch for the same particle movement as the stream flows along the opposite side of the wall. In both cases, friction between the particles and the polycarbonate wall may have an effect, and I don’t have a sheet laser to photograph mid-channel.
Lastly, I’m going to pay more attention to stream bed armoring out in the field, and will think about the processes that make streams and rivers so fascinating!