Does Hot Water Freeze Faster Than Cold? Debate Continues Over The Mpemba Effect

July 2, 2022 by Donald Papp 92 Comments

Does hot water freeze faster than cold water? On its face this idea seems like it should be ridiculously simple to test, and even easier to intuit, but this question has in fact had physicists arguing for decades.

Erasto Mpemba’s observations initiated decades of research into the Mpemba effect: whether a liquid (typically water) which is initially hot can freeze faster than the same liquid which begins cold.

There’s a name for the phenomenon of something hot freezing faster than something cold: the Mpemba effect, named for Erasto Mpemba (pictured above) who as a teenager in Tanzania witnessed something strange in high school in the 1960s. His class was making ice cream, and in a rush to secure the last available ice tray, Mpemba skipped waiting for his boiled milk-and-sugar mixture to cool to room temperature first, like everyone else had done. An hour and a half later, his mixture had frozen into ice cream whereas the other students’ samples remained a thick liquid slurry.

Puzzled by this result, Mpemba asked his physics teacher what was going on. He was told “You were confused. That cannot happen.” Mpemba wasn’t convinced by that answer, and his observations ultimately led to decades of research.

What makes this question so hard to nail down? Among many of the issues complicating exactly how to measure such a thing is that water frankly has some odd properties; it is less dense as a solid, and it is also possible for its solid and liquid phases to exist at the same temperature. Also, water in the process of freezing is not in equilibrium, and how exactly things act as they relax into equilibrium is a process for which — physics-wise — we lack a good theory. Practically speaking, it’s also a challenge how to even accurately and meaningfully measure the temperature of a system that is not in equilibrium.

But there is experimental evidence showing that the Mpemba effect can occur, at least in principle. How this can happen seems to come down to the idea that a hot system (having more energy) is able to occupy and explore more configurations, potentially triggering states that act as a kind of shortcut or bypass to a final equilibrium. In this way, something that starts further away from final equilibrium could overtake something starting from closer.

But does the Mpemba effect actually exist — for example, in water — in a meaningful way? Not everyone is convinced, but if nothing else, it has sure driven a lot of research into nonequilibrium systems.

Why not try your own hand at investigating the Mpemba effect? After all, working to prove someone wrong is a time-honored pastime of humanity, surpassed only in popularity by the tradition of dismissing others’ findings, observations, or results without lifting a finger of your own. Just remember to stick to the scientific method. After all, people have already put time and effort into seriously determining whether magnets clean clothes better than soap, so surely the Mpemba effect is worth some attention.

An Anodiser That Does Gradients

July 2, 2022 by Jenny List 9 Comments

Anodizing aluminium, the process of electrolytic build up of the metal’s the oxide layer in the presence of dyes to create colored effects, is such a well-established process that we probably all have anodized items within sight. It’s usually an industrial mass-production process that creates a uniform result, but there’s an anodizing machine from a Dutch design studio which promises to place anodized aluminium in a new light. Studio Loop Loop’s Magic Color Machine enacts a small-scale automated anodizing process driven by a microcontroller, and is capable of effects such as gradated colors.

Unfortunately their website is long on marketing and short on technical details, but the basic function of a line of chemical baths with a pulley to lower and lift the item being anodized shouldn’t be too difficult for any Hackaday reader to understand. There’s a short video clip posted on Instagram which also gives some idea. It’s a powerful idea that should lead to some eye-catching work for their studio, but its interest here lies in the techniques it might inspire others to try. We look forward to an open-source version of a gradated anodize. Meanwhile if anodizing takes your fancy, it’s a subject we’ve visited before.

Mining And Refining: Helium

June 15, 2022 by Dan Maloney 46 Comments

With a seemingly endless list of shortages of basic items trotted across newsfeeds on a daily basis, you’d be pardoned for not noticing any one shortage in particular. But in among the shortages of everything from eggs to fertilizers to sriracha sauce has been a growing realization that we may actually be running out of something so fundamental that it could have repercussions that will be felt across all aspects of our technological society: helium.

The degree to which helium is central to almost every aspect of daily life is hard to overstate. Helium’s unique properties, like the fact that it remains liquid at just a few degrees above absolute zero, contribute to its use in countless industrial processes. From leak detection and welding to silicon wafer production and cooling the superconducting magnets that make magnetic resonance imaging possible, helium has become entrenched in technology in a way that belies its relative scarcity.

But where does helium come from? As we’ll see, the second lightest element on the periodic table is not easy to come by, and considerable effort goes into extracting and purifying it enough for industrial use. While great strides are being made toward improved methods of extraction and the discovery of new deposits, for all practical purposes helium is a non-renewable resource for which there are no substitutes. So it pays to know a thing or two about how we get our hands on it.

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Growing Silver Nanoprisms With Light

May 9, 2022 by Dave Rowntree 1 Comment

Nanoparticles sound a bit like science fiction to minds of your average hacker — too esoteric and out of reach to be something we might get to work with in our own lairs — but [Ben Krasnow] of [Applied Science] over on YouTube has proven that they most definitely can be made by mere mortals, and importantly they can be tuned. With light. That’s right, nano particle growth appears to be affected very strongly by being illuminated with specific wavelengths, which locks-in their size, and thus defines their light-bending properties. This is the concept of photo mediated synthesis, which causes nanoparticles to clump together into different configurations depending on the wavelength. The idea is to start with a stock solution of Silver Nitrate, which is then reduced to form silver nanospheres which are then converted to larger silver nanoprisms, sized according to the wavelength of the illuminating source.

The process seems simple enough, with a solution of Silver Nitrate and Sodium Citrate being vacuum degassed to remove oxygen, and then purged by bubbling argon or nitrogen. Sodium Borohydride acts as a reducing agent, producing silver metal nanoparticles from the Silver Nitrate solution. The Sodium Citrate coats the silver nanoparticles, as they are produced, preventing them clumping together into a mushy precipitate. PVP (Polyvinylpyrrolidone) is added, acting as a colloiding agent preventing the coated nanoparticles from clumping together, and helping keep the solution stable long enough for the photo mediated synthesis process to complete. Finally, the pH is adjusted up to 11 using sodium hydroxide. The resulting silver nanoparticle stock solution has a pale yellow colour, and is ready for the final particle size selection using the light source.

The light source was custom made because [Ben] says he couldn’t find something suitable off the shelf. This is a simple design using a Teensy to drive an array of PAM2804 LED drivers, with each one of those driving its own medium power LED, one for each of the different wavelengths of interest. As [Ben] stresses, the naïve approach of trying to approximate a specific colour with an RGB LED setup would not work, as although the human eye perceives the colour, the actual wavelength peak will be totally wrong, and the reaction will not proceed as intended. The hardware design is available on MultiSpectLED GitHub for your viewing pleasure.

Nanoparticles have all kinds of weird and wonderful properties, such as making the unweldable, weldable, enabling aluminium to be 3D printed, and even enabling the production of one of our favourite liquid toys, ferrofluid.

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Big Chemistry: Synthetic Oil

May 2, 2022 by Dan Maloney 49 Comments

For as long as I’ve been driving, I’ve been changing oil. Longer than that, actually — before I even got my license, I did a lot of the maintenance and repair work on the family car. It seemed natural to do it back then, and it continues today, despite the fact that it would probably be cheaper overall to farm the job out. I keep doing it mainly because I like keeping in touch with what’s going on with my cars.

Oil changes require supplies, but the last few times I made the trip to BigBoxMart I came back empty-handed. I don’t know whether it’s one of the seemingly endless supply chain problems or something else, but the aisle that usually has an abundance of oil was severely understocked. And what was there was mostly synthetic oil, which I’ve never tried before.

I’ve resisted the move to synthetic motor oil because it just seemed like a gimmick to relieve me of more of my hard-earned money than necessary. But now that it seems like I might have little choice but to use synthetic oil, I thought I’d do what normally do: look into the details of synthetic oils, and share what I’ve found with all of you.

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The State Of Play In Solid State Batteries

April 27, 2022 by Lewin Day 141 Comments

Electric vehicles are slowly but surely snatching market share from their combustion-engined forbearers. However, range and charging speed remain major sticking points for customers, and are a prime selling point for any modern EV. Battery technology is front and center when it comes to improving these numbers.

Solid-state batteries could mark a step-change in performance in these areas, and the race to get them to market is starting to heat up. Let’s take a look at the current state of play.

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Blu-ray Microscope Uses Blood Cells As Lenses

April 22, 2022 by Anne Ogborn 12 Comments

When you think of high-throughput ptychographic cytometry (wait, you do think about high throughput ptychographic cytometry, right?) does it bring to mind something you can hack together from an old Blu-ray player, an Arduino, and, er, some blood? Apparently so for [Shaowei Jiang] and some of his buddies in this ACS Sensors Article.

For those of you who haven’t had a paper accepted by the American Chemical Society, we should probably clarify things a bit. Ptychography is a computational method of microscopic imaging, and cytometry has to do with measuring the characteristics of cells. Obviously.

This is definitely what science looks like.

Anyway, if you shoot a laser through a sample, it diffracts. If you then move the sample slightly, the diffraction pattern shifts. If you capture the diffraction pattern in each position with a CCD sensor, you can reconstruct the shape of the sample using breathtaking amounts of math.

One hitch – the CCD sensor needs a bunch of tiny lenses, and by tiny we mean six to eight microns. Red blood cells are just that size, and they’re lens shaped. So the researcher puts a drop of their own blood on the surface of the CCD and covers it with a bit of polyvinyl film, leaving a bit of CCD bloodless for reference. There’s an absolutely wild video of it in action here.

Don’t have a Blu-ray player handy? We’ve recently covered a promising attempt at building a homebrew scanning electron microscope which might be more your speed. It doesn’t even require any bodily fluids.

[Thanks jhart99]