Ever heard of Lichtenberg Figures?

My interest in them was sparked by an injury to an acquaintance, who was struck by lightning some months ago. The strike left a remarkable pattern on his skin, almost like the roots of a tree. It looked similar to the picture below, although on his chest rather than on his back.




Investigating, I found that such a pattern was called a 'Lichtenberg Figure'. They're described as follows by Wikipedia:

Lichtenberg figures (Lichtenberg-Figuren, or "Lichtenberg Dust Figures") are branching electric discharges that sometimes appear on the surface or the interior of insulating materials. They are named after the German physicist Georg Christoph Lichtenberg, who originally discovered and studied them. When they were first discovered, it was thought that their characteristic shapes might help to reveal the nature of positive and negative electric "fluids". In 1777, Lichtenberg built a large electrophorus to generate high voltage static electricity through induction. After discharging a high voltage point to the surface of an insulator, he recorded the resulting radial patterns in fixed dust. By then pressing blank sheets of paper onto these patterns, Lichtenberg was able to transfer and record these images, thereby discovering the basic principle of modern Xerography. This discovery was also the forerunner of modern day plasma physics. Although Lichtenberg only studied 2-dimensional (2D) figures, modern high voltage researchers study 2D and 3D figures (electrical trees) on, and within, insulating materials. Lichtenberg figures are now known to be examples of fractals.

I was astonished to find out that a lightning strike can produce a 'printed effect' in (or, rather, on) the human body similar to the electrical charge on the printing drum of a laser printer (see the reference to Xerography above). That's what made the pattern on the skin of the lightning-strike victim above, and on my friend's chest.

Looking further, I found the Web site of Stoneridge Engineering, which produces Lichtenberg figures in plastic blocks for sale as artworks. They describe their creation as follows:

We create our sculptures by injecting specimens with very high velocity electrons using a commercial high power particle accelerator. The energy of the accelerated electrons is measured in millions of electron Volts (or MeV). The LINAC that we use accelerates electrons to a kinetic energy of between three and five MeV. At these energies, electrons leaving the accelerator are traveling at relativistic velocities - between 98.5% and 99.6% of the speed of light.




When a specimen is irradiated, electrons are driven deep inside. The penetration depth is determined by the initial energy of the electron beam, the material's dielectric properties, and its density. The higher the electron beam energy, the deeper the electrons will penetrate. As the specimen is irradiated, huge numbers of electrons accumulate inside the acrylic, creating a cloud-like layer of excess negative electrical charge called a space charge. Since acrylic is an excellent dielectric, most of the injected electrons cannot escape, so they accumulate under continued irradiation, causing a cloud-like region of negative space charge to grow deep inside the specimen. By carefully changing the orientation of the specimens and passing them through the beam in two or more passes, or by rotating them as they are irradiated, complex 3-dimensional space charge regions can be produced.




As the space charge grows, the resulting electrical field also increases. Eventually, the electrical stress from the increasing electrical field overcomes the dielectric strength of the acrylic, and some of the chemical bonds that hold the acrylic molecules together are ripped apart. This strips away additional free electrons (a process called ionization). These newly-freed electrons are also accelerated by the electric field, ionizing even more acrylic molecules, and creating additional free electrons in a runaway process. Within billionths of a second, electrically conductive channels form within the acrylic as the material undergoes dielectric breakdown. Once breakdown occurs, the previously trapped charges suddenly rush out, accompanied by a loud bang(!), and thousands of electrically conductive branches feed current into a main "lightning bolt" that exits the acrylic with a brilliant flash. Although pictures of the discharge seem to suggest that we are injecting high voltage into the block, in reality we are removing the high voltage that was previously trapped within the block. Dielectric breakdown typically occurs within an incredibly short amount of time. For example, the electrical discharge within a 2 inch square specimens may only last for only 20 billionths of a second!




The escaping lightning bolts leave behind permanent fingerprints in the acrylic, forming a branching "lightning fossil" within. The high current electrical discharges may reach hundreds, or even thousands, of amperes. The hot plasma within the discharge causes the acrylic to melt and fracture along each path, and higher current "roots" may slightly char the acrylic. The exit point of the discharge appears as a small hole on the surface of the acrylic. The discharge point is typically located at a surface defect, or where a point of external mechanical stress has weakened the dielectric. The defect concentrates the electric field, creating a weak link where the breakdown process can begin.

That's fascinating - to a geek like me, anyway! The same patterns are visible in the photograph below. Stonebridge Engineering describes it as follows:

The famous photo below is from Sandia National Laboratory's mighty Z Machine, the world's largest pulse generator. After the completion of a high energy experiment, the water breaks down from the huge electrical stress, becoming an electrical conductor that safely dissipates unwanted residual energy from the system, and forming Lichtenberg figures that dance along the water's surface.

Image hosted by Sandia National Laboratories.
Click to view full-size (2,400 x 1,586 pixels).

If you look closely, you'll notice that many of the radial paths actually trace out high voltage electrical field lines along the surface of the water. Although impressive, this display is only dissipating "left over" energy, representing only a very small fraction (perhaps 5%) of the energy that was actually used during the previous pulsed power experiment.

Again, pretty amazing stuff to the inquiring mind. Click on the Z Machine link above to read a description of this mind-boggling apparatus.

On YouTube I found a number of videos of this effect. I selected two to include in this post. The first is a very-high-speed photograph (7,200 frames per second) of a lightning strike, showing that it produces similar patterns, too fast for the unaided eye to detect them.

The second shows a Lichtenberg figure being created by a scientist, Todd Johnson, during a lunchtime meeting. It looks easy, but I'm sure it isn't!

My acquaintance is both fascinated and repelled by what I've learned. His wife tells me he keeps looking down at the pattern on his chest, and making rude remarks about their electrical appliances!



Peter