Whenever it rains, people generally don’t look too closely at what the drops do exactly when they hit a surface. We generally assume that stuff will get wet and depending on the slope of the surface it’ll run off downhill at some point, probably in a nice, neat flow. Of course, reality doesn’t work that way, as Swiss researchers recently found when they pointed high-speed cameras at simulated raindrops. Their findings were published recently in Applied Physical Sciences, which is sadly paywalled, but the summary article over at phys.org provides some details, including a video.
The researchers set up a 1.2 meter long dry silicate sand surface with a 30° slope on which the drops were released. In the top image you can see two stills of the result, with the full video showing the drops turning into either peanut- or doughnut-shaped forms that gathered significant amounts of sand grains. These grains mix with the water, allowing a single drop to erode significant amounts of material from a slope, more than was previously assumed in existing soil erosion models.
Beyond erosion, these findings also offer insights for similar dynamics in other fields, all thanks to a group of researchers who got curious during a rainy walk and decided to take a closer look.
LOL, isn’t this the definition of the spherical cow joke, HAD referenced today? Love the pictures though. There are many macro studies on this subject just in the US South West alone. Department Of Transportation puts up signs asking people not to disturb their studies. However, this is the first near micro, in lab only, study I have seen.
“LOL, isn’t this the definition of the spherical cow joke, HAD referenced today?”
How so?
That joke is about strange assumptions made to over-simplify. I supposed it doesn’t say here that they tried anything with wind or different slopes.. so I guess this wouldn’t be a perfect picture of EVERY rain and it’s result. But rain falling on a sandy slope… that’s not exactly a strange occurrence like a spherical cow now is it? I’m sure exactly that is happening right now as we speak.
So what were you talking about?
You can get some insight on their lab setup on the phys.org link. The entire controlled environment provided controlled results. They controlled the slope, the thickness of the media on the slope, the size of the media, and got interesting results, you won’t find in nature. No perfect spinning wheels neatly rolling down the hills were present in their comparison video. It was more of a lumpy potato shaped object that rests closer to the source.
But yes in a lab environment where you control all factors you can maximize the media collected and get the perfect shape for rolling on a perfectly flat 30 degree platform and it will roll much further too.
The spherical cow in a vacuum joke is a metaphor an abstraction. I can’t force you to see the parallels.
Erosion through rain (and dust and other stuff) is also a big problem for the leading edge of big windmills.
And airplane props, I imagine.
Wood and composite without leading edge protection, yes. Otherwise not so much
I disagree, metal props on float planes get very messed up. Very sharp jagged edges from water spray.
All props are wear items.
Not just airplane, boats too.
“These shapes only occur at very high spin rates in pure-liquid drops, but occurred at slightly lower rates in the lab experiments due to the water-glycerol mixture used in the lab-based drops.”
Does anyone know why they didn’t use pure water or rain water?
I would guess that they needed to slow the drops down a bit, in order to get clear enough video. The faster shutter speed of the high-speed camera, the lower the resolution. So if you slow down the water droplets a bit, you might be able to go for a higher resolution at a lower shutter speed.
There must also be a reason why they specifically used glycerol. I’m no chemist by far, let me say that. But glycerol is extremely soluble in water, mixing completely with the right mixture. So the glycerol will only act on the water, and not on the surface it touches. Which probably means that it can be taken largely out of the equation. Glycerol does change the viscosity of the water, which does mean that the effect of gravity is slightly altered. But that’s only one extra parameter that needs to be compensated for, which is not so much extra calculation work.
This is my intuition, fed by my general knowledge and my common sense, and by far not fed by any expertise on the matter. :D
I suspected that. The lower viscosity slows it down. But is the effect that dramatic that normal drops can’t be filmed at high speed and slightly slower drops can?
I wonder if the effect on surface tension is not too high.
According to my Google skills, the surface tension of a drop of water decreases when adding glycerol. So it will actually wet the surface better. And that seems to means that I was wrong saying that the glycerol will not act on the surface. If the drop wets the surface better, I think that means that the droplet will experience more drag.
So viscosity and wetting cause variations in the effect of gravity and in drag on the droplet. Two extra parameters to take into account, not one.
Like I said: I’m guessing around a bit, although “theorizing” is a nicer word. ;)
Basically, I have to agree with you. My explanation doesn’t explain why they did it. Maybe all they did it for was to increase their chance of success. Not all drops will form a peanut or a donut, but by mixing glycerol into the water the mixture will have slightly a lower surface tension. That will have some effect on the drop when it impacts the surface after falling from some height. With a small height, it might help forming peanuts or donuts more often.
I don’t know, still just theorizing with my limited knowledge of physics and chemistry.
“Maybe all they did it for was to increase their chance of success”
That seems like a more reasonable explanation. If normal drops are too random they are harder to study. If by adding some glycerol they make it very reproducible that makes it more valuable. Reproducibility is vital to such studies. Other researches can build op top of this and add more variation in fluids to test.
I wonder what the applications are. Perhaps the goal is a model of erosion by rain?
Weeeell, first you need some speed for the jet, and it will help if you got kidney sand to pass. Also a friend has to pass you beer after beer.
After a while you may admire the piece you’ve made.
Woopsie, I made a comment to a naughty comment that went away.
Do this trillions of times and you wear down a mountain.
Another day, another “not a hack” in hackaday…
I remember being a univ student and going out for a meal during a 105 km/h cyclone
Yeah…first time I thought in life “man raindrops can hurt”, I could feel every single drop hitting my body like needles