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First published online October 5, 2007
Journal of Experimental Biology 210, 3538-3546 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.009084

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The acoustic mechanics of stick–slip friction in the California spiny lobster (Panulirus interruptus)

S. N. Patek^*, and J. E. Baio^*

Department of Integrative Biology, University of California, Berkeley, CA 94720-3140, USA

View larger version (21K): [in this window] [in a new window]   Fig. 1. The sound-producing morphology of the California spiny lobster (Panulirus interruptus). A pair of plectrums (pink), an extension off each antenna, rub posteriorly over the files (dark red) to produce sound. The green arrow indicates the direction of the plectrum's movement when producing sound. Inset: a mass-and-spring representation of the acoustic stick–slip mechanism found in spiny lobsters (Patek, 2002; Patek, 2001). The plectrum (pink) is modeled as a moveable component with a mass sandwiched between two springs. The plectrum moves across the file (dark red), which is fixed in place. Each time the plectrum slips, a pulse of sound is produced. Movie 1 and Movie 2 in supplementary material show sound production and associated morphology, respectively.

View larger version (9K): [in this window] [in a new window]   Fig. 2. Sliding friction dynamics in steady sliding (A) and periodic stick–slip motion (B). During the static phase (orange), frictional forces increase linearly until they reach the critical frictional force value (Fstatic) when the slip phase (blue) starts. (A) In steady sliding systems, the kinetic friction force (Fkinetic) is measured once the system reaches a constant sliding velocity. (B) In stick–slip motion, however, the slip phase is immediately followed by another stick phase. Thus, stick–slip systems often never reach a constant velocity during the slip phase, thereby making calculations of Fkinetic difficult. In the present study, we calculated the maximum change in the coefficient of friction (µ) per stick–slip cycle by using the time points at which maximum and minimum frictional forces occurred during each stick–slip cycle, which correspond to Fstatic and the minimum frictional force (Fmin), respectively. (Adapted from Persson, 2000.)

View larger version (13K): [in this window] [in a new window]   Fig. 3. A schematic of the device used for measuring frictional forces between the file (dark red) and plectrum (pink). The plectrum was mounted on a linear stage, which moved the plectrum over the file at velocity (V) (directed into/out of the page). A force beam was mounted on the vertical beam to measure the frictional force, Ff, which is directed opposite to the movement of the plectrum. A second force beam was mounted on a manual translation stage with which the normal force (Fn), applied orthogonally between the two surfaces, could be varied. The inset shows the directions of the force vectors; note that this drawing is rotated 90° relative to the apparatus diagram.

View larger version (132K): [in this window] [in a new window]   Fig. 4. Scanning electron micrographs of the file and plectrum surfaces. Anterior is to the right of the page in all images. (A) The dorsal file surface is covered with microscopic shingles with their leading edges oriented anteriorly. A groove on the medial edge of the file (medial to bottom of page) guides a knob on the plectrum such that the plectrum slides anteriorly and posteriorly in a controlled alignment, similar to a sliding door in its track. Scale bar, 100 µm. (B) An oblique view of the file shingles shows the distinct separation of the shingles and the additional ridge running medio-laterally on most shingles. Scale bar, 5 µm. (C) A dorsal view of file shingles (scale bar, 10 µm; inset, 2 µm) shows the more prominent frictional edges of the shingles on the anterior side, analogous to shingles on the roof of a house. (D) The ventral surface of the plectrum is composed of soft-tissue plectrum ridges that run parallel to their antero-posterior movement over the file. Scale bar, 100 µm. (E) The leading edges (posterior) of the plectrum ridges fuse with the setae on the ventral surface of the plectrum. Scale bar, 50 µm. (F) The anterior edge of the hemisphere of plectrum ridges ultimately attaches to the ventral cuticle of the antennal base. Scale bar, 100 µm.

View larger version (13K): [in this window] [in a new window]   Fig. 5. Synchronous measurements of plectrum kinematics and acoustic signals. Grey regions indicate the slip phases of the stick–slip cycling. A pulse of sound is produced each time the plectrum slips over the file.

View larger version (22K): [in this window] [in a new window]   Fig. 6. Synchronous measurements of plectrum displacement (A) frictional forces (B, Ff; C, Fn) and coefficient of friction (D, µ). The oscillating force and coefficient of friction waveforms are a product of the stick–slip friction dynamics of the plectrum moving over the file. Each force trace (Ff and Fn) is correlated with the movement of the plectrum. In B, the raw data are depicted in blue and the filtered data (low pass filter at 280 Hz) in red. Grey regions label the slip phases of the stick–slip cycling. In D (coefficient of friction), the linear fits of the static friction regions of the waveform are depicted in red.