i like coffee, i like tea

Jazzedjenluc The Spousal Unit gave a mind-bendingly good — and incredibly persuasive — talk on Sunday to the Caltech Skeptics Society on "Particles and People," outlining the term he coined recently to describe his intriguing new science-based worldview: dysteleological physicalism. Google it today! How persuasive was he? Let's just say Jen-Luc Piquant had a "Saul on the Road to Tarsus" experience and will be seeking to convince her fellow avatars across Cyberspace to embrace supervenience (it's akin to my recent blog post on introducing the physics concept of duality into our public discourse). And I was reminded all over again why I fell for the guy in the first place. He's smart, articulate, witty, charming, and always uses coasters to avoid staining the coffee table.

Trust me, that last one can be a deal-breaker if you're the Coaster Queen. I've amassed quite the collection of novelty coasters over the years; my current faves are tumbled marble squares imprinted with classic winery labels. And now I covet my very own set of "interactive coasters," after reading about them in Technology Review. A couple of postdocs at Newcastle University came up with the idea after attending a science conference in Germany and noticing far too many people sitting in isolated groups, not interacting. (They should have hung out with the science writing contingent. Those people know how to party.) Apparently there's been at least one attempt at designing "interactive badges" at conferences as an icebreaking strategy, but users understandably balked at entering their personal information.

The coaster scheme invented by Tom Bartindale and Jack Weeden involves a partially transparent "smart bar" surface, with an infrared light source, a camera and a projector just underneath. So the bar surface can detect when one of the special coasters is placed on the bar, and automatically assigns that coaster a gender and sexual preference. Now the coaster has been activated — evidenced by a halo around it created by the projector that displays lines of text — and it will try to "chat up" any nearby coasters with a series of very bad pick up lines. ("Are you a parking ticket? Because you've got 'fine' written all over you.") Apparently Bartindale and Weeden Googled "bad chat-up lines" for material. They have a sense of humor. (Why yes, there's a list of bad physics pickup lines, many of which are very bad indeed: "What's your resonance frequency?") The receiving coaster then rates the pick-up line; if the sending coaster scores badly, the receiving coaster will refuse to talk to it anymore. Nice to see these guys programmed rejection into their science experiment. At least the rejected coaster doesn't get a drink tossed in its face.

3076684141_03e3d6481f_o Why am I such a fan of coasters? Probably because a tiny part of me dies inside whenever I see a nice tabletop permanently stained with wine rings — or, for that matter, coffee rings. (Image at right by Michael Naples, from his A Painting a Day blog.) On the upside, turns out there's some interesting physics going on with the latter.

I wrote a couple of blog posts back in November about nifty papers presented at the APS Division of Fluid Dynamics meeting, but never got around to blogging about one of the coolest: three physicists who have been studying the physics of coffee ring formation (a.k.a. "the coffee ring effect") with an eye towards developing useful tools for "microphysics" — a rather vague term that seems to incorporate such areas as industrial coatings, electronic fabrication, and drug development.

Fundamentally these are all deposition processes. So it all comes down to being able to control particles suspended in fluids at tiny size scales to ensure — if we're talking about coatings, for example — that they are deposited uniformly across a given surface. And as any physicist working in this area can tell you, that's easier said than done. I mean, how complicated can coffee rings be, really? The answer is, not very, if all you care about is not staining the table.

But if you're trying to understand what's going on at the micro-level so well that you can make the micro-droplets do whatever you want — well, then it gets complicated, because you're dealing with a complex system. To figure out what causes those coffee stains, you need (a) a scanning electron microscope, and (b) high-performance computers capable of doing some pretty advanced mathematical modeling (or you've got to be pretty fast with a pencil and paper). Those were the tools of Shreyas Mandre (Brown University), Ning Wu (Colorado School of Mines), and L. Mahadevan and Joanna Aizenberg, both from Harvard. They used an SEM to study microscopic glass particles in a solution in the lab, and combined those findings with mathematical models to describe characteristics of the ring patterns that formed. What did they find?

The team found that during ring deposition, a particle layer of uniform thickness is deposited if the concentration is above a certain threshold. Below that threshold the deposits form non-uniform bands. The threshold is formed because evaporation at the solid-liquid interface of the rim occurs faster than a replenishing flow of water from the center of the droplet can replace the evaporating rim fluid. This leaves the particles on the rim high, dry — and deposited.

Having trouble visualizing it? Go on, you folks who don't use coasters: pick up your coffee mug and check the inevitable ring pattern left behind. Now wait until the liquid has evaporated and note the darker ring around the perimeter. It's darker because the particles in that ring are much more concentrated than those in the center.

Coffee-Ring-UCLA-News-Release2-prv The effect occurs with other liquids too, which means it might be possible to use that telltale ring as a disease marker in biosensing, using blood, salive or other bodily fluids — which contain microscale and nanoscale molecules or particles that can be indicators of disease (or lack thereof). Being able to control the effect at such small scales would mean being able to pack thousands of biosensors on a single lab-on-a-chip, possibly even testing for multiple diseases at once. And since the technique relies on the natural process of evaporation, there's fewer complicating factors to deal with, like electrical power sources or moving parts.

Six months before the DFD meeting (that would be May 2010), PhysOrg.com ran a story about a UCLA team studying the coffee ring effect with an eye towards applying it to biosensing. Lead researcher Chih-Ming Ho told PhysOrg.com that "Before we can engineer biosensing devices to do these applications, we need to know the definitive limits of this phenomenon. So our research turned to physical chemistry to find the lowest limits of coffee-ring formation." The idea here is that particles move to the perimeter of a droplet as the water evaporates, resulting in a ring pattern, but if the droplet is small enough, the water evaporates so fast that the particles don't have time to move to the perimeter, and you end up with a concentrated center stain.

Ho's team conducted an experiment with latex particles of various (small) sizes suspended in water and poured it on a specially designed surface with checkerboard squares of alternating water-loving and water-repellant squares. They found that the threshold for 100-nanometer particles is a droplet measuring around 10 micrometers (10 times smaller than a human hair, to use the classic science writing analogy). That's the point where the water evaporated so quickly that the particles didn't have time to drift to the perimeter. (Check out the image above for a visual representation of their results.) The whole coffee ring effect fascination has been around since 1997, incidentally, when graduate students in Sid Nagel's lab at the University of Chicago started investigating after Nagel broached the topic to them over lunch one day. (I'm guessing Nagel doesn't use a coaster.)

There's tons of science involved in brewing coffee, too. Until quite recently, the Spousal Unit had a rather inelegant approach to making his morning cup of coffee. Let's just say that while he always ground the coffee beans fresh each morning — a must for anyone who cares about good coffee — the rest of the process involved placing the grounds into a loose coffee filter inside a plastic funnel and positioning it over a mug, then pouring hot water into it.

It worked fine, but the Spousal Unit also loves fancy gadgets, and started scouring the Internet for the perfect coffee making machine, combining aesthetic form with function at a reasonable price. That's when he came across syphon coffee makers, quite possibly the most labor-intensive, gadgetry-obsessed, snobbish approach to making coffee ever invented. But the gadget in question is oh-so-pretty, so very scientific in its operating principles, and — by all accounts — results in one hell of a brew.

The syphon method (also known as a vacuum brewer, vac pot, siphon, or syphon coffee maker) has been around since the 1930s, when (Wikipedia tells me, without providing further details) someone named Loeff of Berlin invented it. Its use was a bit too complicated for the average household coffee drinker, but hipster coffee enthusiasts were around even then; siphon coffee has always had its fans, usually of the well-heeled variety. Listen to the folks at Coffee Geek rhapsodize about it:

"Almost everything about using a vacuum coffee maker is sensory involved: aromas, fragrance, motion, touch, action. Grind the coffee, add it to the top vessel. Add cold (or hot) water to the bottom. Put the bottom on a heat source. Add the top vessel with its attached siphon. Watch. Liquids defy gravity. The brew gurgles, but it's not boiling. Remove from the heat source. Watch the coffee move back down, or "south." Watch the bottom vessel's brewed coffee gurgle as air is drawn through the spent grounds to release the built up vacuum. Remove top vessel. Smell. Ahhhh. Pour. Taste. More ahhh."

As the name implies, the method relies on vapor pressure and creating a vacuum to brew the coffee; it exploits the expansion and contraction of gases. There are two chambers: a bottom container where you put the plain water, and where the final brew will come to rest, and a top container with a siphon tube attached, where the actual brewing will take place. You also need something like a rubber gasket to act as a seal so you can create a partial vacuum, a filter, and a heating source (usually a cloth-wick alcohol burner, gas or electric stovetop, or, if you're a true purist, a specialty butane burner).

As the water in the bottom container heats up, it starts converting to a vapor — a phase transition — which expands and starts to compress. Since it can only compress so much, it needs to relieve that pressure, and the only route available is through the siphon tube — but there's a bunch of water in the way. So the gas pushes the water through the tube into the top chamber, where the coffee grounds live, and starts brewing. At some point the water level in the bottom container will be lower than the siphon tube, and vapor pours into the top chamber, creating ideal brewing temperatures of around 90-95 degrees Celsius (185-204 degrees F). When the coffee is done brewing, just remove the heat source, so that the water vapor starts to contract. This creates negative pressure in the bottom container and the brewed coffee travels back down the siphon tube the bottom container, through the filter.

Voila! You have what many consider to be the perfect cup of coffee. Needless to say, while admiring the artistry and gadgetry involved in the syphon coffee process, the Spousal Unit also is not a morning person. He needed something a bit simpler for his morning cup of joe. So instead we have a sleek, shiny, stainless steel contraption that is "Morning-Person Proof." It's still a vast improvement over a coffee filter stuffed inside a plastic funnel, and should the Spousal Unit have a craving for siphon-brewed coffee, well, there's an Intelligentsia coffee house a mile or so away.


Syphon, Intelligentsia from The D4D on Vimeo.

And just in case tea drinkers are feeling left out, there's science to the art of brewing the perfect cup of tea, too. Christopher Hitchens has weighed in on this recently, citing no less an authority than George Orwell, who published "A Nice Cup of Tea" in The Evening Standard on January  12, 1946 outlining 11 "golden rules" to follow. But Hitchens can reduce those to one: make sure the water is boiling.

Ground [coffee] beans are heaver and denser, and in any case many good coffees require water that is just fractionally off the boi. Whereas tea is a herb (or an herb if you insist) that has been thoroughly dried. In order for it to release its innate qualities, it requires to be infused. And an infusion, by definition, needs the water to be boiling when it hits the tea. Grasp only this, and you hold the root of the matter."

Ives-tea-flow Lest you question how much Orwell or Hitchens knows about the science of brewing tea, let me point you to an excellent PDF paper by a young grad student in mechanical engineering named Daniel Ives, describing an experiment he conducted as part of a flow visualization class.

He placed a clear glass of hot water against a white paper background, and then sprinkled a teaspoon of hibiscus tea leaves onto the surface. Hitchens would object right here: the tea should already be in the cup (or teapot), and the water should be boiling at contact. But Ives needed to take "before", "during" and "after" pictures to document the diffusion process (as dictated by Fick's laws of diffusion). He's doing science, not making the perfect cuppa — although an interesting follow-up experiment might be a multiple "taste test" of various brewed teas to test Hitchens' hypothesis about how the water must be boiling.

Anyway, Ives' photos capture both the aesthetics and the fluid flows of a brewing cup of tea quite nicely. His anaylsis invokes the Archimedes Principle describing buoyancy:

The hibiscus flowers, like tea leaves, contain various water- soluble particles. When placed in contact with water, the hibiscus flowers become hydrated and swell. The absorbed water acts as a solvent, pulling various solutes from the flowers into the water. the water that closely surrounds the flowers becomes saturated with a high concentration of solutes compared to the rest of the water in the glass…. As the streams of water containing a high concentration of solute particles flow downward, they mix with the bulk water. Over time, the solutes will diffuse throughout the glass until there is an equilibrium concentration of solutes in the water.

Here's where Archimedes comes in. The tea leaves have a higher density than water at 85 °C, so they will have an upward buoyancy force equal to the weight of water they displace. This is countered by a downward gravitational force for each particle in the tea leaves, and that gravitational force is greater than the buoyancy force. So the water saturated with "leached solutes" sinks in the glass. Ives concludes: "However, because the volume of each particle is so small, the net downward force is very small, leading to a very small downward acceleration of each particle by Newton’s Second Law of Physics."

Eureka! There you have it: the science behind the perfect cup of tea. Why not brew yourself a nice cuppa this evening? Don't forget to use a coaster.


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