Sensitive force measurements are performed on sheared granular layers with fast time resolution and simultaneous optical imaging. Stick-slip dynamics occurs for low imposed velocity gradients across the layer, with a transition to continuous motion at a higher gradient. The frictional force is a multi-valued function of the instantaneous velocity during slip. Localized microscopic rearrangement events are found to precede (and follow) macroscopic slip events; the accumulation of these events leads to macroscopic creep.
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Figure 1: Schematic diagram of the apparatus, showing the granular layer GL and a transparent cover plate CP pushed by a leaf spring SP connected to a translating stage TS. An inductive sensor PS detects the deflection of the leaf spring. The microscope objective MO is also shown.
Figure 2: (A) Spring deflection dx(t) as a function of time for periodic stick-slip motion at pushing speed V=113.33 mm/s, spring constant k=135 N/m, and mass M=1.09 x10-2 kg. (B) Irregular stick-slip motion at V=11.33 m/s and k=3636 N/m.
Figure 3: (A) Instantaneous velocity v(t) of the cover plate during slippage for various pushing speeds in the stick-slip regime (k=135 Nm^-1; M=1.09 x 10^-2 kg). The time origins of the pulses are forced to agree at the end of each event. (B) The instantaneous normalized frictional force F(t)/Mg as a function of v(t), for 3 slip events at V=113 mm/s.
Figure 4: (A) Image of a portion of a granular layer (about 1 mm across), and (B) difference image (Dt =1 s) showing localized particle rearrangements (circled) between major slip events.
Figure 5: (A) Average number of microscopic slip sites (per second in a 4 mm2 area) as a function of time before or after a major slip event, in units of the period T of the stick-slip cycle. (B) Displacement creep (of variable amount) before several major slip events.
The figures on this page are adapted from "Friction in Granular Layers: Hysteresis and Precursors", S. Nasuno, A. Kudrolli, and J.P. Gollub, Phys. Rev. Lett. Vol. 79, pp. 949-952, Aug. 4, 1997. Note: Permission to reproduce the figures requires the consent of the authors. Please write to: jgollub@haverford.edu.
This work was also highlighted in Physics Today, September 1997, p. 17.
See the reprints page for a downloadable postscript file.