Why HOT water freezes quicker than COLD water

HOT water sometimes freezes faster than cold water - but why? This peculiar phenomenon has baffled scientists for generations, but now there is evidence that the effect may depend on random impurities in the water.

Fast-freezing of hot water is known as the Mpemba effect, after a Tanzanian schoolboy called Erasto Mpemba (see "How the Mpemba effect got its name"). Physicists have come up with several possible explanations, including faster evaporation reducing the volume of hot water, a layer of frost insulating the cooler water, and differing concentration of solutes. But the answer has been very hard to pin down because the effect is unreliable - cold water is just as likely to freeze faster.

James Brownridge, who is radiation safety officer for the State University of New York at Binghamton, believes that this randomness is crucial. Over the past 10 years he has carried out hundreds of experiments on the Mpemba effect in his spare time, and has evidence that the effect is based on the shifty phenomenon of supercooling.

"Water hardly ever freezes at 0 °C," says Brownridge. "It usually supercools, and only begins freezing at a lower temperature." The freezing point depends on impurities in the water which seed the formation of ice crystals. Typically, water may contain several types of impurity, from dust particles to dissolved salts and bacteria, each of which triggers freezing at a characteristic temperature. The impurity with the highest nucleation temperature determines the temperature at which the water freezes.

Brownridge starts with two samples of water at the same temperature - say, tap water at 20 °C - in covered test tubes and cools them in a freezer. One will freeze first, presumably because its random mix of impurities give it a higher freezing point.

If the difference is large enough, the Mpemba effect will appear. Brownridge selects the sample with the higher natural freezing temperature to heat to 80 °C, warming the other to only room temperature, then puts the test tubes back in the freezer. The hot water will always freeze faster than the cold water if its freezing point is at least 5 °C higher, Brownridge says.

It may seem surprising that moving the finish line by only 5 °C makes enough of a difference, when the hotter sample starts out 60 °C behind in the race. But the bigger the temperature difference between an object and its surroundings - in this case, the freezer - the faster it cools. So the hot sample will do most of its cooling very quickly, helping it to reach its own freezing point of -2 °C, say, before the cooler water gets to its freezing point of -7 °C.

Wireless 'thought-to-text' cap

LONDON — A team of researchers from IMEC, the Holst Center and the lab of neuro- and psychophysiology at the Katholieke Universiteit Leuven has presented Mind Speller, a thought-to-text device intended to help people with motor disabilities.

A number of research institutes are working on similar devices which make use of electro-encephalogram (EEG) brainwaves and positive biological feedback so that individuals can use thought processes alone to control a cursor or a computer action.

The Mind Speller,uses a portable device, the size of a matchbox, connected to a cap that contains electrodes located at specific positions on the head to capture the relevant EEG-signals. The electronics in the matchbox are developed by IMEC and the Holst Center. It contains IMEC's and Holst Center's proprietary eight-channel EEG-chip to process the EEG signals, a commercially available microcontroller that digitizes the EEG signals and a 2.4-GHz radio that transmits the EEG signals wirelessly to a nearby PC. The data is interpreted on the PC by signal processing algorithms developed by the team of Professor Marc Van Hulle at the lab of neuro- and psychophysiology of the Katholieke Universiteit Leuven.

"The Mind Speller is a generic device that can be easily adjusted to different users. Therefore, it could be a cost-efficient communication solution for people with temporal impairments for whom the existing solutions are too expensive. Moreover, the Mind Speller may help those patients that are not helped with the existing devices driven by motor activity, as the Mind Speller is based on a different principle, using P300 EEG potentials to read people's thoughts," said Professor Van Hulle.

IMEC is adapting the electronics to allow it to work with dry electrodes, thus making the system less difficult to use.

Your Life: The Shirt-Pocket Movie

by Eirc A Taub

Think of it as Twitter with pictures. With a miniature wearable camcorder called the uCorder, you can document every mundane aspect of your life, from tying your shoes to going into the subway.
uCorder

The uCorder is about the size of a stick of gum. You can attach it to your belt, put it on a lanyard around your neck or slip it into a shirt pocket. Additional flash memory can be added to either of the two models, the $80 IRDC150 and $100 IRDC250, which come with 1GB and 2GB of memory, respectively, giving you either 36 or 72 minutes of video. The larger-capacity model can also double as a Web cam, using the included mount.

The uCorder records in 640 by 480 resolution using the AVI format. A built-in LED can help in low-light situations.

Playing videos from the device was simple. I just connected the uCorder to my Mac and double-clicked on the file that appeared on the desktop. In my tests, I found the video to be of good quality, but the audio was filled with lots of background noise and hum. While the device comes with a lanyard, don’t dangle the uCorder from it while you’re walking around, or you’ll be taking a lot of pictures of your shirt as the device twirls around your neck.

While the video quality was certainly acceptable watching in a small computer screen, don’t expect to use the uCorder to record life’s important events. Leaving a camcorder to find its own way around your neck or in your pocket will not make for the most compelling imagery. However, if you’re interested in recording a college lecture or the interaction between you and your date, it might do the trick.

THE NEW DORK

It is now a well established fact that geeks and dorks are cool so here we hear it loud and clear with the a spoof on the Jay-Z/Alicia Keys "New York".

The Blue Brain Project