Nickel Hair

August 18, 2014

Folk wisdom gave about the same results as medical science until a few hundred years ago. The use of herbs to treat disease goes back three millennia, and some of the proposed treatments are strange. One working concept was Similia Similibus Curantur ("Like cures like") that is the principle behind the contemporary expression, "hair of the dog."

Hair of the dog, as practiced today, is to use an alcoholic beverage as a palliative for an alcoholic hangover. From what I've read, this isn't recommended, and it's also not recommended that you drink so much to have a hangover in the first place. As they say, an ounce of prevention is worth a pound of cure.

Hair of the dog apparently refers to the folk practice of putting a few hairs of the dog that bit you into the bite wound. How that came into practice sounds more like witchcraft than anything else, but the idea of Similia Similibus Curantur goes back at least as far as Aristophanes (446 BC - c. 386 BC).

Although I haven't found the passage (it's a fairly long book), Aristophanes, in "The Deipnosophistae of Athenaeus," supposedly tells the tale of a man who, having been blinded by thorns, is cured by throwing himself back into the brier patch.[1] Aristophanes, as was his style, was poking fun at Similia Similibus Curantur.

Br'er Rabbit at the table. Br'er Rabbit's favorite hiding place was a brier patch. (Via Wikimedia Commons.)[2]

In physics, hair is referenced in the no-hair theorem, which is stated simply as "black holes have no hair." This conjecture by John Archibald Wheeler is that, except for its mass, electric charge, and angular momentum, all information about an object is lost after its falling into a black hole.

In electronics, another type of hair is important. Tin whiskers are small metal hairs (dendrites) that grow from solder joints as a result of stress. This problem is rarely seen in lead-tin solder, but tin whiskers became a big problem with the move to lead-free solder. A few patents have been issued on lead-free solder compositions that don't engender tin whiskers.[3]

Engineering has its informal hair's breadth unit, the width of a human hair. This is about 50 micrometers, but the width of a human hair varies over a wide range, so a hair's breadth is usually intended as a figure of speech. Since the engineering profession was male dominated in its early years, engineers sometimes used another hair unit which is not repeated in polite company.

Although powdered wigs were popular in his era, the young Isaac Newton (1642-1727) didn't appear to need hair extensions in this portrait. (Via Wikimedia Commons.)

Nanotechnology has given scientists the ability to produce arrays of very small objects, and now a team of engineers from the Massachusetts Institute of Technology (MIT, Cambridge, Massachusetts) has produced dense arrays of nickel hairs on thin, elastic, transparent layers of silicone. Each "microhair" is about 70 micrometers high and 25 micrometers wide, so it's a fraction of a hair's breadth.

A paper in the journal, Advanced Materials, about this research is authored by Evelyn N. Wang, an associate professor of Department of Mechanical Engineering at MIT, graduate student Yangying Zhu, former graduate student Rong Xiao, and postdoc Dion Antao.[4-6]

These hairs are actually an attempt to mimic such natural structures as the small nasal hairs called cilia that sway back and forth to remove dust from the nose. Other scientists have made moving hairlike structures by embedding magnetic particles in polymers with limited success. The MIT team made the pure nickel pillars by creating a mould, electroplating nickel, stripping away the moulds, and bonding the resultant nickel pillars to a layer of soft, transparent silicone. The pillars align themselves with an applied magnetic field (see figure).[5]

Response of nickel hairs in a magnetic field. Because of a magnetic property called shape anisotropy, the hairs have a lower energy state when they are aligned parallel with the magnetic field. As can be seen in one image, the elastic mounting resists bending at too high an angle. (Screen captures from an MIT YouTube video.)[6]

Says Zhu, "We can apply the field in any direction, and the pillars will follow the field, in real time."[5] Experimentally, the pillars could be moved over a tilt angle from upright (0°) to 57°.[4] An applied fluid only flowed in the direction of the tilted microhairs, while it was pinned in all other directions. The tilted microhairs form a path through which fluid can flow, so they might be useful for lab-on-a-chip devices. These would magnetically direct the flow of cells and analytes through such a chip's microchannels.[5] The nickel hair array has interesting optical properties, also. Since the silicone layer to which they are attached is transparent, they can modulate transmitted light, as shown in the photo.[5]

Optical response of nickel hairs in a magnetic field. (Screen captures from an MIT YouTube video.)[6]

Although the experiments were done with a uniform magnetic field, a more complex field would yield a textured surface. Its flexibility may have an advantage, also. Says Wang,

"A nice thing about this substrate is that you can attach it to something with interesting contours... or, depending on how you design the magnetic field, you could get the pillars to close in like a flower. You could do a lot of things with the same platform."[5]

This research was funded by the Air Force Office of Scientific Research.[4-5]

References:

1. Aristophanes, "The Deipnosophistae of Athenaeus," from the Loeb Classical Library, 1930.
2. Joel Chandler Harris, "Br'er Rabbit at the table from Uncle Remus, His Songs and His Sayings: The Folk-Lore of the Old Plantation," Illustrations by Frederick S. Church and James H. Moser, (D. Appleton and Company, New York), 1881.
3. Iver E. Anderson, Frederick G. Yost, John F. Smith, Chad M. Miller and Robert L. Terpstra, "Pb-free Sn-Ag-Cu ternary eutectic solder, US Patent No. 5,527,628, June 18, 1996.
4. Yangying Zhu, Dion S. Antao, Rong Xiao and Evelyn N. Wang, "Real-Time Manipulation with Magnetically Tunable Structures," Advanced Materials, Early View (July 22, 2014), DOI: 10.1002/adma.201401515.
5. Jennifer Chu, "New material structures bend like microscopic hair," MIT Press Release, August 6, 2014.
6. Melanie Gonick, "MIT engineers show their magnetic microhairs in action," Massachusetts Institute of Technology YouTube video, August 4, 2014.