Gravity defying smoke 

My studio now smells like burned post-it notes. If you're interested in the live Q&As consider becoming a supporter on Patreon (I'll see you on the Discord!): stvmld.com/ndg9-y-x You can also discuss this video on REDDIT: stvmld.com/awh34y3s The sponsor is KiwiCo. Get 30% off your first month of any box: kiwico.com/stevemould

You are to things what Tom Scott is to places. You find things, explain what makes them interesting, then explain how they work, Tom goes to places, explains why they are interesting and how they work.

Incense “smoke” is not just suspended ash from combustion. Incense contains a lot of oils and the combustion is incomplete. Note the brown droplets of oil on the glass from the cross section. The smoke is so much heavier out the bottom because it’s mostly uncombusted oil in suspension. If you were to trap and analyze the smoke from above vs below, you’d find a very different oil content ratio.

I’m in the fire service and we do a lot of fire science and learning about fire phenomena. The first thing we learn is all about pyrolysis. It’s a really interesting subject. For example, when any material reaches a certain temperature, it breaks the material down and the chemicals are released into the atmosphere which is, like you said, the flame part. Once this is gone you are essentially left with coal (assuming it’s wood that’s burning) and that will just glow because the gasses are burnt off. We sometimes come out of training runs and our fire kit is pyrolising. Within our initial course the light a fire with lots of timber sheets, they lay out a small section of carpet about 3 meters away and have 2 cups, one empty and one half full of water. Empty cup melts and half full one melts down to the waterlevel (this is to show how much water can absorb the heat) the carpet starts to release nasty looking gasses whilst curling up around the edges, after a while this carpet just bursts into flames even though there is a few meters separation. A good demonstration is to heat up small bits off wood within a glass jar above a Bunsen burner (attach a tube to jar to allow the steam and gasses to exit away from the flame) at first you will get water droplets and then the smoke will come out white where it has a slight moisture content, this will then turn brown/green and this will be the flammable gasses. Once the process is over, the original wooden pieces are now just bits of coal ready for the BBQ. This would be a really cool video and you can then demonstrate flashovers and backdraughts.

"Sorry it's a really bad line" dad jokes on point, Steve, proud of you :)

About the Mexican wave, or as we call it in the US simply the wave, I've seen a situation where there were two waves in the audience moving towards each other. I was genuinely tickled to see that the waves canceled each other out when they met.

I used to do nosework with my dogs, which is a sport where the dog is supposed to pinpoint the source of a specific smell. When you get into that, you learn all kinds of surprising things about how and where air travels within a room (since it affects how the smell travels and how/whether the dog can interpret it). In some of the seminars, we used "smoke pencils" or "smoke pens" to explore what airflow does next to doors, in sunny spots, next to corners, etc. That's what these smoke experiments reminded me of.

The misnomer Mexican Wave amuses me. It was invented in America, and from here reached our neighbor Mexico where it was seen on global television at the World Cup, so the rest of the world associated it with Mexico. In America, we just call it "the wave." I don't know what they call it in Mexico.

My friend was showing me one of these a few weeks ago, and it absolutely blew my mind + had me completely stumped as to how it works. I'm so grateful for this explanation 😬

I have been thinking about this topic recently so thank you for this video - I'm a blacksmith and when I fire up my forge, often times smoke from the coals will cling to the horizontal forge table before very slowly starting to rise (or settling I guess). I figured it was due to the steel table cooling the smoke before it had a chance to rise. It looks quite surreal. This video makes me think that my case, the convection probably doesn't get to pull all the smoke up with it; as the smoke is forced out of the coal mound to all sides, it ends up too far away from the convection stream to get pulled up, and it gets cooled by the table and simply lingers for minutes.

Something i remember as a child is smoke from the chimney falling when there was a snow storm on the way. It was explained to me that the high pressure front of the storm forced the smoke down low.

Once again Steve, always with these interesting science topics.

The post-it note example reminded me of years ago when I was a smoker. Break the butt off of an unburnt cigarette. Pull the cotton out of the paper of the butt. Then take the cellophane that's around the bottom of the cardboard box they come in. Turn the square shaped cellophane upside down and set it on a flat surface. Twist the butt paper into a thin paper tube. Light one end of that paper tube. Use the lit paper to burn a tiny hole in the top of the short side of the cellophane. Then turn the burning paper tube around and stick the unlit end into the hole you just made and let it burn. The smoke will flow from the paper tube to the inside of the cellophane and fall downwards just like the post-it. I have a theory that maybe Steve knows about the cigarette trick and came up with the post-it trick because he didn't want to make cigarettes look cool which is a great thing. Don't smoke, kids.

This channel is freaking amazing. I love that it's never "and that's just how it works" - there's always "but there's something that didn't make sense..." followed by more digging. This is an approach and attitude that we need more of in society - never assuming that we know everything there is to know about a subject, being aware that there is always something more going on that we haven't figured out yet.

Fire being the special case is a really interesting insight. Thank you for the great video

This was very interesting! I especially liked the comparison with crowds exhibiting fluid like behaviour. This made me think of the terrifying scenario of crowd surges that often cause hell incarnate (eg crush injuries in festivals or a nightclub) but can be modelled fairly accurately with fluid dynamics. I didn’t expect to think about that so thank you for another great video!

I've needed this video since Christmas when my dad got one of those! Thank you so much for explaining!

This was described to me by a science teacher as creating a small vacuum or low pressure zone, where air is moving up but the smoke gets thick enough in the zone and so heavy enough to fall.

That's awesome! We always had neat incense burners at the local flea market, but I've never seen one that had a smoke waterfall! It's usually Grim Reapers and Dragons with smoking mouths and eye holes.

2:34 I have never rolled my eyes harder than hearing "for a fixed smoke flux..."

The questions we didn't know we needed answered!

This is exactly what I was looking for. I want to make Backflow cones out of my regular cones and didn't know how far to drill the hole thank you for this video!

Is there an upper limit on the size of smoke fall devices? It'll work on incense cones or rolled post-it notes, but what about something like a log with a hollowed core?

So many forces at work to make such a simple looking piece of art. Love it!!

I absolutely love how along the way of learning about this cool new trinket I now want, I also learned two very cool things about smoke and burning, how the way we conceptualize them is only due to circumstance. Amazing insights! I love thinking about things in completely new ways and you do a fantastic job at bringing that out!

Fluid Dynamics always freaks me out. It is easily one of the most complicated fields of mechanics; even the _simplified models_ are hard to work with a lot of the time, and involve a lot of Navier-Stokes equations and things like that.

I remember seeing this in a NightHawkInLight video almost a decade ago! Very cool effect.

Perfect timing. One of my friends just gave me one of these as a gift and I've been wondering myself. I love it!

Thanks for yet another great video! Maybe an idea for a future video: I have been wondering how blades cut on the molecular level. I.e. a very sharp blade does not need much force at all to separate a material, is there any particular molecular/atomic level interaction between the blade and the material being cut? Does the blade just wedge itself in between the target material atoms mechanically pushing them apart, or is there more to it than that?

I had wondered how these work for a while. I had assumed they had to be cooled down in a refridgerator or they had an ice cube inside. Thanks for the explanation.

I remember learning to make a smoke waterfall with..... Smokable products.... just by pooling the smoke in my mouth and pushing it out slowly. it always blew my mind. Thanks for showing me how it works!

Few days ago I asked myself similar questions, thank you for deepening topic.

Steve you killed me with "it's a really bad line" your humor is spot on XD

I've never seen the excitability of bubbles quite so clearly, but I DO love watching the activation of those sodium acetate heat packs, and now I know what to call them!

We have one of these and, today, I discovered something else weird about these incense cones. Try placing one on a flat-based candle holder (as used for ball candles, for example). When I did this today, I discovered that the smoke simply pools at the base of the cone. I've also noticed something about the smoke generated by these cones that may play into the physics - it seems to be much more oily than you'd expect (judging by the residues on our 'waterfall' burner) - which may play into the speed at which it cools down, as well as providing a greater density when suspended. Fascinating stuff - good work.

Great video as always, btw I think the convection still happens inside the tube but its forced to compress and go out, cooling along the way and getting dense .

Never seen one of these before but they do look really cool. Now I want one myself.

Here's another question for you: what causes the regular "puff-puff-puff" pattern of smoke coming out at 5:20? Cool video! I've been wanting one of these things for a while, haha.

If you want to get into exploring convection currents, you might try setting up something for Schlieren photography/videography. Tons of fun and an excuse to purchase an expensive mirror!

thanks for this video. i bought one of these for my wife and ive found it fascinating that they managed to get what i thought was hot smoke to fall

Weird, I just randomly came across these incense burners on Amazon the other day. I think it was the first time I’ve ever seen them, that I can remember anyways. I just assumed the ceramic burner had to be placed in the freezer prior to burning in order to cool the smoke to the point it would descend down instead of rise

Great video! As always (in my extremely biased opinion), the more fluid dynamics videos the better. This whole exercise feels exactly like the kind of thing we'd get as a homework question for my Convection class during grad school fluid dynamics... so much so that I am really tempted to dig out my notes and textbook from that class, which is an expert example of nerd-sniping, so well done I guess. Anyways, I definitely agree that entrainment is the core reason why it "sometimes falls and sometimes doesn't fall". The combustion is heating the air around it, setting up a free convection flow due to the density differences, which then entrains air around it to flow too due to shear setup by viscosity. The area of effect around the plume of this entrainment acts in some ways similar to a boundary layer, at least insofar as the effect obviously diminishes with increased distance, though I can't remember what shape that "boundary layer like" entrainment takes, or if it follows the same sort of parabolic or log-law behavior as a normal boundary layer. (To get the plume shape I think you need to know both the momentum and thermal boundary layers and I think I remember that if you use a boussinesq approximation to simplify N-S you can analytically derive the thermal boundary layer profile, and that it is steeper than the momentum boundary layer). Anyways, get the tracer (i.e. smoke particles) far enough away and they won't be entrained. Or you can just block the flow communication entirely with a giant acrylic enclosure, which works too. I am sure there are dozens of fascinating CFD and flow visualization papers out there on this topic which can show in beautiful detail exactly what is happening (partly because buoyancy driven flows and free convection are fundamentally important in tons of applications, but mostly because buoyancy driven flows almost universally make stunningly gorgeous CFD or flow-vis videos).

I have one of these incense burners and it's really soothing to just watch with some soft music on, when I've had a stressful day. I've always wondered what causes it to work.

5:51 this I used to do in front of the fireplace as a child. I discovered when I rolled up pages of newspaper, the smoke would flow out the other end when I put the tip into the fire in the fireplace. The smoke would then flow around until enough would accumulate and reach the flames where the gas would ignite causing the flame to travel to the opposite end of the newspaper tube.

I once saw this effect with a lollipop stick, or possibly a q-tip that had had the cotton pulled off, when I was a kid. I have no idea anymore why it was set on fire, but because the paper was rolled so tightly into a solid tube, the inner hole was very narrow and the smoke came out the other end in a jet rather than falling out like a liquid. It was cool enough for me to remember it even now. Unfortunately, they don't seem to make those out of paper anymore, it's all plastic, so even though I've thought about recreating it several times, I've never been able to. Although it's not like I am constantly buying lollipops to see what their sticks are made out of, so maybe they are still around somewhere.

Expanding on your notes around 7:15: Smoke is flammable, for the reasons which you gave here. If one were to collect a bunch of smoke and introduce a flame to it (in a mixture of oxygen), then the smoke would ignite. That is why smoldering fires produce a lot of smoke but firey fires do not, I think.

This is great. The smolder/fire is an interesting set of stages. Thinking of how it builds with lump charcoal and grilling. Starts with a smolder, and then builds to a flame

I literally just saw one of these for the first time a week ago and had no idea what it was. Thanks for the video!

Wait this was actually really cool. It’s crazy how we don’t question common but incredible phenomena like this 😅

I’d love to know where you can get such beautiful „smokefall ceramics“ as shown in the video 🤔

Interesting fire moment I had recently: I had an overgrown ditch (steep grade) connected to a lawn that gets mowed. There was a mat of dried grass across the face of the ditch, and when I lit the righthand edge, the fire slowly crept across sideways, like a giant incense stick, until it reached the left hand edge, but the fire never moved into the mowed lawn, because there was no fuel, no mat of dried grass. Reminded me of the Forest Service’s clearcut lines that act as firebreaks, helping prevent widespread forest fires.

Great video. Thanks. Have seen those waterfall incense burners around but, couldn't imagine they actually worked. They actually look pretty cool too.

Kudos on your incense cutting skills, them blighters are so fragile 😆

Thanks for the video - but Michael Faraday covered much of this - brilliantly - in his Ri Xmas Lectures on the Chemical History of the candle in 1848 (yes - nearly 175 years ago). His book on the lectures is in the public domain, and various people have tried to recreate his demonstrations on video (some are quite tricky to set up).

I have been wondering about this for a while! I love my backflow incense burner. It's such a pretty and serene thing

Very cool video. These things kinda confused me so I was quite happy to see you tackled the explanation o f how this effect happens. Thank you for the video.

Smoke gases are mainly H2O ( lighter than air), and CO2 (heavier), The visible parts are droplets and solid matter (heavier). Smoke cones evaporate a lot of oil, so the mix is potentially quite heavy.

Pretty sure my videos prompted the invention of these things. I did a few videos on early RU-vid about making smoke waterfalls with rolled sticky notes and these popped up a short time later. People used to do a similar thing as a cigarette trick but I think it was forgotten for a time between the 90's and early youtube.

I didn't even know these were a thing and I'm now learning more about them than I ever thought I would

I liked this video so much I ordered 2 burners. Thanks Steve.

Today I learned: In the UK, "the wave" is called "a Mexican wave."

I used to do this as a little boy with printer paper. I still think smoke and fire are the most captivating thing.

There is two parts to this. Fire/heat and the particle/fluid dynamics. For the second part you can use an utrasonic fogger or mister (cheap on amazon) to play with the "smoke". Much safer and reusable. Getting a nice thick flow of "steam" down a path was actually more complex than I thought it would be. I always loved the slow fall waterfall look.

Thats the first time I'm truly tempted to do patreon for those live Q&A

When it comes to investment, diversification is key. That is why I have my interests set on key sectors based on performance and projected growth. They range from the EV sector, renewable energy, Tech and Health.

Can you fill a bottle with smoke and seal it? Will it stay in there forever if properly sealed? Could you create a "smoke globe" this way?

We used to demonstrate Brownian motion using smoke from a burning paper straw. You hold the straw at an angle to the horizontal and light the high end. As it burns the (waxed paper) smoke falls down the straw because it's actually heavier than air and being at the bottom of the flame inside the tube, no convection currents exist top push it upward. (The smoke was collected in a dish under a microscope and viewed to see the motion). A candle shows the same behaviour if you blow it out and it continues to smoke. As the wick cools and the convection currents stop, the heavy smoke starts to fall down the candle.

I had never heard of back flow incense burners and now I want one. lol A thought I had while looking at this was that maybe the shape helped to compress the smoke a little bit so it would be more visible. Angling the outflow probably causes more of the smoke particles to bounce off of the sides of the tube. I would suspect that the one that flows out into the glass container is probably slightly narrower at the bottom to make the smoke more visible and then the glass keeps air currents in the room from immediately disrupting the effect. For the one with those petals on it, I think the shape of the petals themselves acts to further funnel the smoke into a smaller area and maintain its thickness relative to the smoke exiting from the top. I also wonder if the very fact that the top end is the one that's burning creates some kind of pressure phenomenon that draws the smoke down the inner part of the incense cone. Fascinating!

I wonder how many smokefall incense burners you just sold.

4:14 "Ryan *Cummings"* **insert dank trollface**

Another great explanation, thank you!!

Just love your channel always such an interesting watch!

7:40 Don't say it Don't say it Don't say it Don't say it Don't say it Don't say it Don't say it Don't say it Don't say it Don't say it Don't say it Don't say it!

😮. Does this mean that it is possible to create torches that don’t create soot on the ceilings and walls?

Years back I picked up a wine bottle that was modified to be an incense burner by having a couple holes drilled in the bottom. An incense stick would be suspended in the neck of the bottle by a key ring, and the smoke would drift out the holes at the bottom. Unfortunately, the oils would also thoroughly stain the surface it was on.

Thank you for pointing out the pomegranate juice... I was watching it thinking it was a complex chemical reaction in lab glassware, as soon as you mentioned it I could see the Pom bottle.

Fill a medium to large glass up with warm soapy water and shake it around while your hand covers the lid with a seal. You will find a significant amount of air pressure whooshing out when you remove your hand. Do the same thing without the soap and you will find that there is not any significant air pressure created. It seems to me that bubbles are able to create pressure through tensile forces in a way that I don't think has been fully detailed by physicists. I think this could have implications on the scale of the cosmos. I know this sounds asinine, but I hope you will humor me if you are running out of ideas for more videos.

This is the first time I've ever heard of it being called a "Mexican Wave." Which I think is peculiar... cause I'm Mexican.

Loved the podcast btw, MORE MOULD!

This is very interesting. Thank you for sharing!

Well, I can tell you right away why it's so weird, smoke usually goes up.

I noticed the video didn't actually give a real answer but if you dig around in the comments you'll find one!!! 🤨

The world needs a video from you on Ball screws

Really interesting! I wonder how much the dynamics of the shape if the incense holder effects the oscillation of the turbulent flow crating a harmonic lateral rotation effect.

you just unexpectedly explained what fire is. god bless you!

I'm glad you mentioned that KiwiCo doesn't just do subscriptions. I've considered getting something like this as a gift for my nieces/nephews, but the subscription bit always turned me off. I'm sure the subscription is great, but I imagine after a while boxes of these stack up and become clutter. A one-off box seems like a nice way to give them (and their parents) a taste of what KiwiCo has to offer, before committing to a monthly subscription.

A curious note on this... if you make a cylindrical wick (i.e. stick a piece of paper as a hollow cylinder in some wax) the smoke will get caught in the centre... to the point about the smoke fall in a stickey note working I wonder if its a matter of the heavy particles having somewhere too fall before they are caught by those fast moving convective currents...

Several things I hadn't considered before. Thanks very much. I'd imagine that the smoke falls because its not being caught up in the draft flow to the flames, partly because there aren't any flames, and partly because the smoke is released far enough from the combustion. As always, interesting stuff, told clearly, concisely, and with a sense of humor and wonder. I'll be back. 🙂

That “really bad line” joke was seriously the best joke I’ve heard in a long time. Brilliant, mate!

When I was a kid we'd make them all the time with cigarette packets. If you set a pack of smokes, lid down, and lift the bottom part of the plastic wrapping to create a square see through cellophane housing. Then we'd roll a piece of paper just like you did, use the burning tip to poke through the cellophane, leave it in, and watch the smoke fall.

... in 2000 or 2001 I had some insence. They were "match insence" they were a match with an insence wrapped around the match stick. I noticed I could slide most of the insence off the matches. Which was great because the match added a wood smell that wasn't as pleasant as the insence by itself. Needless to say I ended up with a hollow insence. I needed something that I could burn it on. A flat surface wouldn't let the insence burn all the way. I enede up finding a rock. It had a crevice in it that perfectly fit the insence. When I placed the insence on the rock the smoke would flow down the rocks crevices like a tiny river. At the time I thought it was just kool to look at. I didn't realize this wasn't a thing yet. I've thought about it ever since. Then a few years ago they come out with these "down flowing" insence... been kicking myself in the arse ever since. I have a couple more great ideas that I'm too broke to do anything with. Sooo disappointing.

I really appreciated that, "a lot of smoke is coming out of the top; it's just moving faster, because of convection, so it's harder to see." was reiterated into "for a fixed smoke flux a stream of smoke moving faster must be less dense."

My favorite way to do a burner is a mason jar. Large volume mason jar with a few holes poked in the lid allows you to observe the unperturbed flow from those cones. You need multiple holes in the lid to allow enough air to get a good flow

Air cooling and picking up moisture is used in downdraft solar dehydrators, and in a rocket masonry heater that uses a bell system. A very modern invention using simple materials and physics. Some masonry heaters make use of gasses speeding up as the chamber tapers like that incense cone, but larger. I think they’re more finnish or something. I don’t know if they utilize the down draft to re-burn the smoke, but they definitely use the taper to speed up gasses and influence draft direction. They’re more common in Europe.

I got to this video through my watch later playlist (having previously marked it as something I wanted to watch), but when it started I for a moment thought that I was hearing the sounds of a of a Firefly documentary I also had queued. The incense burner looks very Inara, and the music is just about right for Firefly! (Also, awesome video!)

I am reminded of the experiment where you get an egg into a bottle by lighting a match and dropping it in and than putting the egg quickly over the mouth of the bottle. I guess this experiment teaches you about high and low air pressure. The expanding and contracting of the hot and cold air is what creates the vacuum effect in the experiment.... I am also certain that the burning of the fuel also consumes oxygen which probably helps create the vacuum. The incense holder you said has a nice metal heat sink type thing seems to have this going on. I believe it creates a kind of vacuum chamber which is drawing cold "air" through the porous incense. 🤔

Hey Steve, I have seen smoke from chimneys go down, instead of rising. only on certain days, I think those days are generally very cold (below -20C) and calm, no wind. It's odd when it happens, and I have never been able to explain it. I suppose the cooling of the smoke going up the chimney would account for the falling smoke? Really interesting video, thank you

Reminds me of: When out of lighters and matches, a tightly rolled sheet of paper, and rubbing one end against a red hot stove top will light it, and I noticed the small fall. Also reminds me of: Ear candling. the use of a flammable tube covered in a type of wax, which people think removes ear-wax. So, is all the smoke fall being allowed to enter directly into peple's ears as well as claiming the tube's wax left over is supposed to be your ear wax removed.

I watch a lot of spaceflight videos on RU-vid, and in rocket engines one important metric is the fuel to oxidizer ratio. If there is more oxidizer (i.e. oxygen) in a combustion, the combustion gets hotter and the exhaust (smoke) gets clearer, while if there is less oxidizer the combustion gets cooler and the exhaust darker. Is it possible that what we are seeing here is also at least partially due to a different fuel to oxidizer ratio at the top vs. inside the incense cone? At the top you have a lot of surface area exposed to a large body of air, so oxygen can easily reach the combustion, while inside the porous cone and in the small hole you have a lot of incense (fuel) which is burning, but oxygen needs to first filter through the porous material or pass through the small opening at the bottom to get to the combustion. This might make the smoke at the bottom thicker and cooler, and hence heavier. It can't be the only factor though, as the experiment with the cone in his hand shows.

Used to do that trick using the cellophane on a cigarette pack for the smoke fall chamber.