Triboelectrification and Razorbacks: Geophysical Patterns Produced in Dry Grains

Electrostatic interactions between particles can dramatically affect granular flows, creating industrial safety and handling problems [K. N. Palmer, Dust Explosions and Fires (Chapman and Hall, London, 1973), pp. 388–389]. We present experimental data demonstrating that charging of grains can also cause spontaneous self-assembly that may generate lasting geological patterns under arid conditions. Paradoxically, we find that grains that tribocharge enough to produce small explosions, ejecting grains meters into the air, leave little net charge on grains. Rather, grains charge into strongly heterogeneous polar clusters. These assemble into stereotyped residual structures that resemble geological features, for example, razorbacks observed on Mars [“The Razorback Mystery,” July 16, 2004, http://www.jpl.nasa.gov/missions/mer/images.cfm?id=701].

Figure 1

“Razorbacks.” (a) Experimental flow consisting of a cascading stream of art sand (with a mean diameter of $200\pm 50\mu \mathrm{m}$) bounded by razorbacks. The scale bar is about 10 cm long. The heights of the boundary structures are measured by taking snapshots illuminated by the laser sheet shown. The inclination angle of the acrylic sheet here is $28\pm 1°$ as measured with a digital level. The inset shows an enlargement of the bounding razorbacks after flow has stopped and the bed has fully come to rest. Each jagged tip is one grain wide. (b) Immediately after flow has stopped, the bed remains active, ejecting individual grains with high velocity (curved streaks in boxed region are individual grains). The sheet inclination in (b) is $32\pm 1°$. Both (a) the razorbacks and (b) the discharging grains disappear in the presence of a static eliminator (see text). (c) Photo from the NASA rover Opportunity, obtained on sol 160 of its mission in Meridiani Planum. Photo credit: NASA/JPL/Cornell. For scale, the spherical concretions scattered throughout the snapshot are about 4 mm in diameter.

Figure 2

Quantitative confirmation that laboratory razorbacks are produced by electrostatic influences. Main plot: Average over 3 trials of the dependence of the standard error of the local slope of razorbacks taken from side view snapshots of the flow. Smaller plot: Slopes of a laser line [cf. Fig. 1a] from front view snapshots of the flow. Error bars here are standard errors from many razorbacks in each snapshot and several ($\sim 5$) repetitions of the experiment at each static eliminator position.

Figure 3

Charged cluster transport [20]. (a)–(d) This sequence shows a cluster of grains, encircled, that (a) detaches from the razorback alongside the flow wake, (b)–(c) becomes airborne, and (d) reattaches further downstream to elongate the razorback at its final location. The time lapse between successive frames here is $1/30\sec$, and the scale bar in (a) is about 1 cm long. (e)–(h) This sequence shows a less common, but still frequent, transport mechanism, in which a monofilament of grains travels intact down the inclined surface. This filament travels smoothly and steadily the length of the surface (more than 10 cm). The time lapse in this case between frames is about 0.25 sec. The filament shown is very elongated and appears to consist of a stalk of single grains attached in a linear chain [see detail in Fig. 4, inset (b)]. Thicker clusters have also been seen that hop periodically downhill in a saltatory motion. (i)–(j) This sequence shows in a filament that becomes airborne and finally disintegrates (circled) into individual grains. (i) Digital differences in brightness between successive frames enlarged from boxed region in (j). Humidity in these experiments is $28\pm 2\%$, and the acrylic sheet is inclined at $28\pm 1°$.

Figure 4

Net charge of grains measured with two methods vs distance to static eliminator. Note that the net charge grows as the static eliminator is brought closer. Circles are data taken by receiving 4 kg from output of chute into a large Faraday cup; squares are obtained by sampling $\sim 400\mathrm{g}$ and transferring it into a smaller Faraday cup. The thick line is to aid the eye, and error bars are standard errors over 3 replicates. Inset (a) shows an enlargement of the detached cluster from Fig. 3b, and inset (b) shows the monofilament seen in Figs. 3e, 3f, 3g, 3h. The filamentary structure is very thin but is readily apparent in moving video frames [20]. To make the filament visible, in (b) we have superimposed all of Figs. 3e, 3f, 3g, 3h, centered at the filament, and have adjusted the brightness and contrast. We also sketch the shape of the filament and its shadow in inset (c).