Ferrofluids—liquids seeded with magnetically sensitive ferrous nanoparticles—demonstrate some beautiful and bizarre behaviors when exposed to magnetic fields. This video shows the reaction of a pool of ferrofluid to the magnetic field generated by an alternating current through a simple wire coil. At 1 Hz, the fluid response is not unlike the normal-field instability—the characteristic spikes—the fluid develops when exposed to a permanent magnet. But because field is fluctuating, the spikes pop out and fade again. At 10 Hz, the behavior gets even more interesting. As the frequency of the magnetic field’s oscillation increases, the time the fluid has to respond to changes in the magnetic field decreases. Eventually, one can imagine a point where the magnetic field oscillates faster than the molecules in the fluid can rearrange themselves to respond. It’s unclear if such a mismatch in timescales is the cause of the increasing violence of the ferrofluid’s response in the later clips or whether this results from an unmentioned change to the current through the coil. For something even wilder, check out Nick’s video of the ferrofluid’s response to music. (Video credit: N. Moore)
The eleventh hour is over now [x]
One of HST’s most cited set of observations was the series of images taken in July 1994 as the dishevelled remnants of Comet Shoemaker-Levy 9 crashed into Jupiter. That event highlighted the catastrophic consequences of cosmic impacts, and sparked considerable interest in identifying earth-crossing asteroids and comets. Fifteen years later, on 19 July 2009, an Australian amateur astronomer, Anthony Wesley, noticed a strange dark ‘scar” near Jupiter’s southern pole. The feature was quickly confirmed by other astronomers, both amateur and professional, and was quickly judged as likely to be the result of another, completely unexpected, cometary impact. The SL9 impact was thought to be a once-in-a-lifetime (if not more) event; clearly, the impact rate is higher than previously suspected. At the time of the new event, HST was deeply immersed in the initial on-orbit instrument performance tests, but the science staff were able to interrupt those procedures for a few orbits to obtain images with Wide-Field Camera 3. Further observations will be obtained at a later date to track the evolution of the feature, as Jovian winds disperse the detritus through the atmosphere.