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 :)

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.

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.

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 😬

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.

Once again Steve, always with these interesting science topics.

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

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.

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.

The questions we didn't know we needed answered!

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!

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.

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

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

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!

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.

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!

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?

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!

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!

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

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.

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!

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."

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

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

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!

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.

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.

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).

The world needs a video from you on Ball screws

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.

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

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

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

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

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

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.

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

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

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.

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

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

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!

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

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!

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

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

I wonder how many smokefall incense burners you just sold.

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.

Just love your channel always such an interesting watch!

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.

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).

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?

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.

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

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.

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

I love how Steve went out of his way to describe the Post-it note - the same size and shape of a cigarette, with smoke at one end and a flame at the other - as just a rolled up Post-it note with the diameter of a pencil.

greatest patreon plug ive seen so far

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.

Pretty sure my videos prompted the invention of these things. I did a few videos on early KZfaq 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.

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 .

Loved the podcast btw, MORE MOULD!

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.

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

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.

Finally a video on one of these that isn’t weird clickbait advertisement from a dubiously legal source.

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

Another great explanation, thank you!!

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

Weird thing is... I feel like this entire episode is a promotional commercial on Back-flow incense burners. lol I've never heard of these until now and now I want one!!!

I learned how to make a smoke fall by rolling up the paper and tin foil from the inside of a cigarette pack then slipping the cellophane up on the pack so that the pack is only half in it cellophane burning a tiny hole in the cellophane and placing the role of tin foil and paper in the hole and lighting it... I learned this trick in the 80s... This is the first I've seen anybody try to give any kind of scientific breakdown of it. Awesome job, thanx. Also I didn't know they made incent burners that use that and I burn a lot of incense... this is an amazing find to me.

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.

This is very interesting. Thank you for sharing!

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.

I used to create smokefall's all the time when I was younger and smoked cigarettes' about 10 years ago. I'd take a small, long, piece of a discarded cigarette box and roll it up really tight into a small paper tube, leaving just a small hole thru the roll. Then, I'd light one end on fire and blow it out to form a small tinder. Using that lit end, I'd make a small hole into the center-bottom cellophane wrapper still attached to the discarded pack of cigarettes, inserting the un-lit end of the paper tube into the small hole and upright. I'd then turn the whole assembly with the bottom of the box facing upwards, and slowly raise the delicate cellophane wrapper as it held the previously created paper tube, then stop just before it came off the box and watch as the smokefall hit the box and collected and the bottom of the pack of cigarettes. They were always fun to watch and interact with.

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. 🙂

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 lit a paper straw from a box I bought on fire at the top and the smoke poured out the bottom :O It's amazing how you're making a video on the exact same thing I was thinking about :P

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'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.

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!)

Very cool! Always wondered about these.

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.

Discovered this when young. Instead of rolling, you can make a paper airplane, point the nose up, and light it, and watch the smoke fall down the center V. The smoke is also flammable, touch flame to it and it will flash and be gone.

I remember where I almost saw this phenomenon: it happened whilst I was burning a heap of bamboo stalks! I simply chucked them in a fire, only to be spooked soon thereafter by literal wooden bombs as the pressure inside the sealed cavities made the stalks burst vigorously. So, for the next batch, I poked holes throughout the pieces which had sealed cavities, basically creating a long tube of very oily wood. While burning, which wasn't easy at all, lots of dense white smoke oozed lazily out of the top, barely rising; I'm sure that, had I lifted the cane out of the fire, I'd have witnessed a smokefall out of its bottom!

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.