I'm a sort of person who's easily distracted by a sidequest.
Like, if we're cooking dinner then after a while my wife will say
"What are you doing?" because for the last 10 minutes I've been
turning the tap on and off or something like that
because I've noticed that I can generate laminar flow
but only if I turn the tap in a very specific sequence of turns.
(It's actually quite interesting, I might make a video about that.)
But the point is, it happens to me quite a lot and, like, for example
Recently we were packing away some groceries, and we bought this huge bottle of shampoo
But the the top of the bottle had broken and the way I'd pulled it out the bag
The shampoo was pouring out onto the ground, and I noticed that the way it hit the ground
It was interesting, the behavior was interesting, so I just stood there watching the shampoo
dribble onto the ground, because I feel like if you notice something that's interesting
and you think further investigation is required, there's no time like the present.
So two hours later, with some tweaking of variables
I'd noticed some very unusual behavior indeed
and I've reproduced it here under what I generously call laboratory conditions.
It's interesting, isn't it? You see these chaotic
jets of shampoo firing in all different directions
appearing and disappearing.
Watching it in slow motion by the way.
This is about a fifth of normal speed.
I now know this is called the Kaye effect. K-a-y-e.
And it's a result of the fact that shampoo is shear-thinning
which means that it's
viscous, it's thick
but its viscosity goes down under shear force.
So shear forces like this, if you've got two things sliding against each other
shear force in the case of a liquid is like, if you've got a body of liquid and
at the top it's traveling this way, at the bottom it's traveling this way
then you've got a gradient of velocity
through the liquid as it's changing direction, so you've got a shearing, a shear force like that
and when you apply a shear force to this shampoo the viscosity goes down.
So why does that lead to these jets appearing?
Well, at the base you've got this bed of shampoo that's viscous
and incoming you've got a jet of shampoo that's also viscous,
but because the shampoo piles up on the tray
occasionally you'll have the incoming jet hit the bed of shampoo at an angle
and when it does that, it slides along
and that sliding is a shear force, you have a shear force then.
So in the boundary between the jet that's incoming and the bed
you have a thinning of the shampoo, the viscosity goes down
and it creates a layer of lubrication, so the jet is able to continue sliding
further than it otherwise would.
But because you've got this jet coming in with a decent amount of force,
it actually pushes down and creates a dip in the bed of shampoo.
So now you've got a jet coming in like this, and then it loops out and it has a ramp that fires off.
And actually that loop gets more and more extreme until eventually
it's firing back in on itself and it's killing the the incoming jet,
which is why it's chaotic, which is why the jets come and go.
So why is your shampoo shear thinning in the first place? Well, part of the answer is
by design. Like, it's not the soap content that makes shampoo
viscous and shear thinning, it's an additive. It's a molecule called a viscosity modifier.
The flow properties of your shampoo are fine-tuned by the manufacturer.
Viscosity and shear thinning are useful properties in their own right, like
you don't want the shampoo or the shower gel to run through your fingers
or slip off your hands, so viscosity is good.
But at the same time when you rub it in your hair or rub it on your body
you don't want that to be a laborious process,
you want it to become less viscous when you rub it in, so shear thinning is also useful.
But it's also about perception: it turns out that consumers equate
thickness, viscosity with
luxuriousness, and actually manufacturers tweak the viscosity for different markets.
It turns out the average shampoo that is marketed to a male demographic is more viscous
than the shampoo marketed to a female demographic.
For some reason men prefer their shampoo thick, who knew.
But also, like, for kids
they've found out, literally people research this stuff
the shampoo and shower gel for kids
shear thins at a much higher shear force
so it takes more force for the viscosity to go down.
And that makes sense. Like, I've got kids.
If, you know, I want my shower gel to stay in their hands
because kids can't control their limbs, it's really annoying. And you want it to be
to be very stable as they kind of flail around in the bath or the shower.
So it makes sense.
So how do these viscosity modifying molecules work?
How do they add viscosity and how are they shear thinning?
Well, they're polymers, which means they're really long-chain molecules,
a bit like these chains of beads here.
And normally they're all tangled up with each other, they can't move very easily
and so they add viscosity, they add thickness.
But if you apply a shear force like this
the molecules spread out, they smear out and they line up.
When they're lined up like this they can more easily flow over each other
so the viscosity goes down. They're shear thinning.
There's still a part of the explanation missing though because
shampoo and shower gel aren't the only shear thinning liquids.
Ketchup, mayonnaise,
custard, paint, hand sanitizer, they're all shear thinning.
But I haven't been able to reproduce the Kaye effect with any of those.
And I think I know why:
There's a time component to this shear thinning effect.
So you apply a shear force to a liquid, it becomes less viscous,
but when you remove the shear force it doesn't suddenly become fully viscous again,
it takes a bit of time, and in the case of shampoo it's pretty quick.
So to get the shampoo out of the bottle in that nice stream
you apply a shear force, it has a shear force as it comes out the bottle.
But by the time the stream has reached the bed of shampoo that it's falling on to, the viscosity has returned.
But in the case of hand sanitizer or ketchup or all those other liquids,
it's still "un-viscous" by the time it reaches the floor
so you don't see the effect.
As a side note, if you want to reproduce this effect at home
you need to have good control over the stream of shampoo,
and I recommend pouring it out of a beaker, it makes the whole process a lot easier.
I mean in general I think life can be improved by pouring things out of a beaker.
So next time you notice something strange, dig a little deeper.
You might find something amazing.
And share it with me, because I'd like to make a video about it.
Right, it's puzzle time again. This time the scenario is:
you are on an island, and on this island there are only three types of people:
knights, knaves and jokers.
Knights always tell the truth, knaves always lie, and jokers can do either.
And you meet three people: Erin, Faro and Gobi.
And Erin says that Faro is a joker.
And Faro says that Gobi is a joker.
And Gobi says that Erin is a joker, so it's like a little loop.
E says F is a joker, F says G is a joker and G says E is a joker.
The puzzle is this: if exactly one of them is a joker, how many of them are knights?
It's interesting, isn't it? As always, I'm not gonna give you the answer
because it's very satisfying to get it for yourself and I wouldn't want to rob you of that.
Having said that, you can check your solution using the link in the description
but not until you've given it a go yourself.
I mean, I can't police it so there you go, solution in the description there.
I found this puzzle on a website called brilliant.org who sponsor this video
and working through puzzles and problems is absolutely
the best way to learn. It's also just really fun.
I like the way Brilliant have laid out the puzzles.
And I like the way they've curated the sequence, it really helps you on your learning journey.
Like, the steps are really well thought out.
So if you want to think like a scientist and think like a mathematician, then why not try it for free.
Go to brilliant.org/SteveMould
The link is also in the description.
As a bonus, the first 200 people to click on that link
will get 20% off annual premium membership should they choose to upgrade.
I hope you enjoyed this video. If you did, don't forget to hip subspibe
and I will see you next time.
Also click on the notification bell. That's just a new thing that I have to say at the end of videos now
so there you go. But it's true, you know, you should do it, you should click on the notification bell.
Bye!