This month’s question:
Why do so many fireplaces send smoke into the room rather than up the chimney?
Last month’s question:
Is it still espresso if there’s no foam on top?
Answer: No, there is no espresso without foam. No human existence either. Afterall, Aphrodite grew from foam.[1]
That espresso foam is one of the three unique properties of espresso that make it espresso: an emulsion of oil droplets in water, suspended solids, and foam. That foam is called crema because of its creamy consistency. This style of coffee beverage was originally called crema caffè before the word espresso caught on in the mid 1940’s.
“Foam” is a word that has probably never appeared on any grade-school vocabulary test. We all come to understand what foam is through life’s experiences and hearing the word in conversation. The dictionary says foam is a light frothy mass of fine bubbles formed in or on the surface of a liquid or from a liquid. Foam specialists – whomever they are – call that a dispersed media.
Maybe, but that sure doesn’t sound like the pink boards I installed on the outside of my house’s foundation wall to keep the heat in: rigid foam insulation.
And it really doesn’t sound like that stuff that just won’t go away in the “I want a weirdly built, super energy efficient home” industry: autoclaved, aerated concrete[2].
Whatever…we’ve got lots of similarly confusing vocabulary in our industry. Why do we say “dead nuts” when we mean level, plumb, and square? Shock absorbers I understand, those are apprentice electricians.
Rigid insulation and aerated concrete aren’t really foam. During their manufacturing process they were foam at one time but, when the liquid turned into a solid, they stopped being foam and became a solid with lots of void spaces filled with gas. Not foam but, it’s expedient to call it foam. Wuzfoam.
All foams start their existence as mixtures of a gas with a liquid. Not just any old mixture, a mixture where they stay separate – meaning, not dissolved into solution – and together they form bubbles that group together. Not all mixtures of gases and liquids make foams, and not all foams are stable. When they are stable, sometimes they’re useful, like espresso and soft serve ice cream[3].
Ask any chef or barista and they’ll tell you there is an art to making stable foam. As a scientist, I appreciate the art but, it comes with a wink and a nod. It’s not really an art. It’s just a unique combination of materials and conditions. Nobody ever called the industrial manufacturing of DuPont’s rigid foam insulation in Midland, Michigan “art.” It’s just a manufacturing process, like making espresso or a souffle.
For espresso, the liquid is water and the gas is mostly carbon dioxide, CO2. Which is why espresso is never served at environmental conferences. The trick, however, with espresso and every other foam, is to get all the other minor ingredients just right AND get the procedure for how to make the bubbles just right. Those other ingredients always include some sort of surfactant – a material that changes the characteristics of the surface of the liquid; it acts on the surface. The special procedure can be darn near anything: temperature, mixing speed, pressure, method of mixing, and more.
Take, for example, pure water. It’s not possible to make stable bubbles with pure water. You can make a bubble with pure water, but it always bursts at once. Add a little soap, however, and the bubbles last. That soap is the surfactant, and some soaps are much better than others. If, like me, you ever put liquid dish soap in your dishwasher this was made obvious: dishwasher soap is specially formulated to be not so foamy, liquid dish soap is too foamy for a dishwasher and comes squirting out from around the door.
Everything is a surfactant, BTW. But some things are much better or more useful surfactants than others. So, we usually reserve the word for those better and more useful materials. It is useful, however, to keep in mind that all liquid surfaces are changed by any second material that interacts with it either from the exterior of the liquid or from the interior of the liquid. And that’s why water goes directly through some wall underlayment sometimes when it seems like it shouldn’t. Sometimes rainwater mixes with something (like resins in the wood used for siding or mystery ingredients added to masonry mortars) that changes the surface characteristics of the water, and the result is it slips through the underlayment rather than being stopped by the underlayment.
That sort of dishwasher foam overflow regularly happens on an industrial scale at all sorts of different manufacturing facilities. Here’s an example from a cheese manufacturing plant in Alabama:
And this sort of thing happens on construction sites also.
Just kidding. Those are geo-blocks. Huge blocks of expanded polystyrene foam insulation used instead of dirt or gravel to fill up a volume below ground.
Ignoring those Friday afternoon Budweisers, a few useful foams are made on site during the construction process. Spray polyurethane foam insulation is the big example. Air entrained concrete is almost another, but it’s not usually considered to be a foam. I think it is, but some losers disagree with me on that.
For any site-manufactured foam, if the mix or the mixing procedure isn’t just right, things can go wrong. It’s a Goldilocks thing. We’re pretty good at doing things correctly these days but, it should be kept in mind that the skill of the chef is important, and on-site formulation is inherently risky. It’s much more likely that factory made extruded polystyrene foam insulation (XPS) will be uniform in every way as opposed to spray polyurethane foam insulation made on site. Likewise, factory made autoclaved, aerated concrete blocks are more likely to be uniform in every way compared to site placed, air entrained concrete. Jobsites are not climate-controlled factories. Often, they don’t even have walls or a roof. Plus, construction workers don’t have full-time access to hot coffee and warm, clean toilets like factory workers. Imagine how your work might suffer if you had to walk outside and use one of those little port-a-johns every time you had the urge to go.
Truth be told, making a proper foam, fit for purpose, is about as difficult as passing a camel through the eye of a needle. Mother nature pulls it off occasionally[4], like with sea foam, but for humans, it’s a neat trick that we’ve only perfected in the last several hundred years. The reason it’s difficult is that bubbles are delicate and gravity’s a bitch. It’s not hard to make bubbles. It’s hard to make just the right amount and size of bubbles that also remain stable.
Believe it or not, Nature doesn’t care about you and me. It just is what it is and does what it does. We’re just along for the ride and, if we’re lucky, we might recognize some patterns that allow us to make predictions: dark clouds, probably going to rain later; spinning baseball, probably going to curve; three stage rocket with the perfect quantity of fuel and ignition time, probably going to send a spaceship to the Moon; small plastic loop dipped into soapy water then swung through the air, probably going to make round bubbles.
The physics behind those round bubble shapes, and the various shapes made when they bunch together is both frighteningly complex and down-to-the-bone level fundamental. In short, bubbles, alone or bunched together, always take a shape that minimizes the total potential energy of the mass. That potential energy is mostly in the surface of each bubble – the surface tension. Physicists call this the Minimum Energy Principle, which is not the Second Law of Thermodynamics but a close neighbor.
One single bubble is pretty easy to wrap your head around so long as you ignore gravity. We all grew up blowing bubbles and knowing that they’re always round. Because of that experience we’ve internalized that as obvious like we’re all as smart as Pierre Luis Maupertuis.[5] It’s not obvious, and I’m not as smart as Maupertuis but, Nature does what it does. I dare you to go study isoperimetric inequality. It’s hard to even say those words.
When groups of bubbles come together – like in any foam on a building site or espresso cup – understanding really goes off the rails. Lord Kelvin worked on this back in the 1800’s and the problem of partitioning space into equal volume cells with minimal surface area is even called The Kelvin Problem. However, this problem is so challenging that neither Kelvin, nor any physicists since has been able to prove that a particular shape provides the minimal surface area. Mother Nature does it in the blink of an eye every time you wash your hands with soap and water. She is a bad ass.
So, what’s the big deal? The big deal is that there are A LOT of variables. Correctly predicting what Nature is going to do is very difficult when there are a lot of variables. Think climate modeling, or WUFI modeling, or stock price modeling. Mathematical models can be useful, and occasionally, like a blind squirrel finding a nut, they even provide a correct prediction, but they’re about as delicate as hot house orchids.
Foams are like mathematical models: it’s exceptionally difficult to know which unique mix of ingredients and conditions will result in a useful foam. Some of the variables are easy to point to: temperature, pressure, mixing speed, and the like. Others are not so easy: viscosity, chemical concentration, forces of attraction between different chemicals, and the like. And that’s why there aren’t so many different foam materials used in building or even in our everyday lives. We just haven’t figured out the right combination of variables to produce many useful foams.
Even espresso wasn’t easy. We’ve been making coffee for about 800 years now, but espresso wasn’t invented until 1884. It requires a specialized piece of equipment that operates at industrial pressures: 130 PSI. A pressure cooker operates at about 15 PSI and they scare the bejeebers out of me. A can of soda out in the Sun on a hot summer day is about 90 PSI and should only be opened by your favorite uncle. Steam pressurized to 130 PSI inside a coffee maker on your kitchen counter is a death wish. In fact, death by exploding espresso machine is a real thing and happens every few years somewhere around the world. Most recently in Sardinia where the explosion caused the house to collapse. Esploded!? Espress-Oh No!?
Back to the construction site: air entrained concrete is made at pedestrian temperatures and pressures, so, no worries. Spray polyurethane foam is another story: 1,200 PSI. That’s a big number. I love spray foam, but I don’t really want to be anywhere near that truck when it’s being installed. Off-site production of autoclaved, aerated concrete is an industrial process: high temperature (375oF) and high pressure (150 PSI).
That’s about it for foam. Good luck with that fireplace question.
[1] The historical record on this creation from foam is sketchy but, regardless, she, and her Roman version Venus, is the goddess of love, lust, beauty, pleasure, passion, procreation, desire, sex, fertility, property, and victory.
[2] Don’t get me wrong, there are a few great uses for autoclaved, aerated concrete. Mass wall construction of buildings, using only AAC, anyplace were it rains, just isn’t one of them. (No Don Ho, “Tiny Bubbles” jokes, please.)
[3] Little known fact: Margaret Thatcher was a chemist before she was the Prime Minister of England and worked for J. Lyons & Company on their soft serve ice cream products. Soft serve ice cream is a foam. No relation to me but, I do like soft serve ice cream.
[4] Actually, everything in the Universe is in a foam structure but that’s way too much for today.
[5] Pierre Luis Maupartis was a man of letters who first stated the Least Action Principle in 1741. Everyone thought he was an incorrect idiot. Eventually, his work evolved into our current, most basic understanding of how things work. Like F=ma and potential + kinetic energy = constant. He’s one of the original gangsters.
