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Register a new userGravity-dependent stick-slip dynamics in penetration experiments on simulated regolith
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The surfaces of many planetary bodies, including asteroids and small moons, are covered with dust to pebble-sized regolith held weakly to the surface by gravity and contact forces. Understanding the reaction of regolith to an external perturbation will allow for instruments including sensors and anchoring mechanisms for use on such surfaces to implement optimized design principles. We analyze the behavior of a flexible probe inserted into loose regolith simulant as a function of probe speed and ambient gravitational acceleration to explore the relevant dynamics. The EMPANADA experiment (Ejecta-Minimizing Protocols for Applications Needing Anchoring or Digging on Asteroids) flew on several parabolic flights. It employs a classic granular physics technique, photoelasticity, to quantify the dynamics of a flexible probe during its insertion into a system of bi-disperse, cm-sized model grains. We identify the grain-scale forces throughout the system for probe insertion at a variety of speeds and for four different levels of gravity: terrestrial, martian, lunar, and microgravity. We identify discrete, stick-slip failure events that increase in both magnitude and frequency as a function of the gravitational acceleration. In microgravity environments, stick-slip behaviors are negligible, and we identify that faster probe insertion can suppress stick-slip behaviors where they are present. We conclude that the mechanical response of regolith on rubble pile asteroids is likely quite distinct from that found on larger planetary objects. Techniques borrowed from Earth-based granular physics experiments provide a promising set of methods for future work analyzing both local and global particle rearrangement events in microgravity conditions.
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Since the Apollo program or earlier it has been widely believed that the lunar regolith was compacted through vibrations including nearby impact events, thermal stress release in the regolith, deep moon quakes, and shallow moon quakes. Experiments have shown that vibrations both compact and re-loosen regolith as a function of depth in the lunar soil column and amplitude of the vibrational acceleration. Experiments have also identified another process that is extremely effective at compacting regolith: the expansion and contraction of individual regolith grains due to thermal cycling in the upper part of the regolith where the diurnal thermal wave exists. Remote sensing data sets from the Moon suggest that the soil is less compacted in regions where there is less thermal cycling, including infrared emissions measured by the Diviner radiometer on the Lunar Reconnaissance Orbiter (LRO). Here, we performed additional experiments in thermal cycling simulated lunar regolith and confirm that it is an effective compaction mechanism and may explain the remote sensing data. This creates a consistent picture that the soil really is looser in the upper layers in polar regions, which may be a challenge for rovers that must drive in the looser soil.
We discuss the stick-slip motion of an elastic block sliding along a rigid substrate. We argue that for a given external shear stress this system shows a discontinuous nonequilibrium transition from a uniform stick state to uniform sliding at some critical stress which is nothing but the Griffith threshold for crack propagation. An inhomogeneous mode of sliding occurs, when the driving velocity is prescribed instead of the external stress. A transition to homogeneous sliding occurs at a critical velocity, which is related to the critical stress. We solve the elastic problem for a steady-state motion of a periodic stick-slip pattern and derive equations of motion for the tip and resticking end of the slip pulses. In the slip regions we use the linear viscous friction law and do not assume any intrinsic instabilities even at small sliding velocities. We find that, as in many other pattern forming system, the steady-state analysis itself does not select uniquely all the internal parameters of the pattern, especially the primary wavelength. Using some plausible analogy to first order phase transitions we discuss a ``soft selection mechanism. This allows to estimate internal parameters such as crack velocities, primary wavelength and relative fraction of the slip phase as function of the driving velocity. The relevance of our results to recent experiments is discussed.
We reconsider the treatment of Lise and Jensen (Phys. Rev. Lett. 76, 2326 (1996)) on the random neighbor Olami-Feder-Christensen stik-slip model, and examine the strong dependence of the results on the approximations used for the distribution of states p(E).
The growth dynamics of an air finger injected in a visco-elastic gel (a PVA/borax aqueous solution) is studied in a linear Hele-Shaw cell. Besides the standard Saffmann-Taylor instability, we observe - with increasing finger velocities - the existence of two new regimes: (a) a stick-slip regime for which the finger tip velocity oscillates between 2 different values, producing local pinching of the finger at regular intervals, (b) a ``tadpole regime where a fracture-type propagation is observed. A scaling argument is proposed to interpret the dependence of the stick-slip frequency with the measured rheological properties of the gel.
We calculate the friction of fully mobile graphene flakes sliding on graphite. For incommensurately stacked flakes, we find a sudden and reversible increase in friction with load, in agreement with experimental observations. The transition from smooth sliding to stick-slip and the corresponding increase in friction is neither due to rotations to commensurate contact nor to dislocations but to a pinning caused by vertical distortions of edge atoms also when they are saturated by Hydrogen. This behavior should apply to all layered materials with strong in-plane bonding.
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