Lockdown Challenge: Rice and bean ski jump

Lockdown Challenge #45: Sort your sliders from your rollers

Sorting stuff out takes energy. You might think that only applies to clearing up after a party, but it’s the same all over the place in engineering too. Stuff just wants to be more chaotic: disorder – entropy – always increases unless energy is fed in. Mixtures stay mixed without energy to sort them out. If you want to get pure oxygen out of the air, metal out of ore, or whisky out of beer, you have to put in energy. And the same principle works with beans and rice.

‘But for beans and rice’, you say, ‘you can use a sieve!’. The little ones wriggle through leaving the big ones behind. But wait a minute, you can’t use a sieve if the beans and rice grains are the same size. But there is another way of unmixing some same-size mixtures. Where some grains slide and others roll, a chute can do it. Kinetic energy and parabolic trajectories do the unmixing. Here’s how to try it yourself:

You’ll need a roller-slider mixture: round grains like mung beans (beansprout beans) and long or flat grains like rice. For the separating machine, you’ll need a chute, which could be a cheap-as-chips cardboard tube or a PVC gutter. A ‘ski jump’, a short horizontal section at the end of the chute is helpful, as it allows the rollers to fly further without changing the sliders’ trajectory much. This can be as short as 1 or 2cm. In dry weather, sliders can get a static electric charge and stick to a plastic channel like PVC. Line the channel with aluminium foil or maybe just paper to stop this. Finally, you need small oblong buckets such as the bottoms of milk containers to catch the flying grains.

Prop up your chute up at an angle and try the sliders. Too horizontal and they won’t move at all, while too steep and they’ll go too fast. Now try positioning the buckets. Dribble the rollers and sliders separately onto the top of the chute. Adjust the chute angle and height, and the position of the catcher buckets after each test, until you can pot most of the sliders into the first bucket and most of the rollers into the second bucket. There are many designs that will work, but to give you an idea, the chute in the photograph below was 7cm across by 60cm long, sloping at 33°.

Now mix the sliders and rollers, and trickle them from your fingers onto the top of the chute. Look at the proportion of each grain in each bucket. How effective is your grain sorter? Can you get 90 per cent or more pure grains out of the mixture?

Why does this work? Rolling grains accelerate, their speeds going up as the square root of distance travelled. Now sliding grains also accelerate, but at a slower rate because of the absorption of much of their gravitational energy by friction. The graph shows theoretical trajectories of sliders (in red) and rollers (in black). They leave the chute horizontally and fall into the buckets (blue). The grains should follow a parabolic curve, where their height decreases with horizontal distance x as x-squared. Take some pics or videos to check out the real grain trajectories.

And finally: what about a grain feed system to replace your fingers? In industry there are vibratory feeder chutes, ‘shibblers’, which can deliver a steady stream of grains.

If you liked this, you will find lots more fun science stuff in Neil Downie’s books, like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org. In line with this experiment, Neil’s current work includes developing a new ventilator system to support people with breathing difficulties – get more information on this great project here (Exovent.org). 

There is the back catalogue of Lockdown Challenges from the past year to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM.