Strike Forces 2 Free Download
Malka Crickenberger
One unexpected consequence of these extreme strike speeds is the generation of cavitation at the site of impact between the mantis shrimp's heel and the striking surface (Fig. 1; Patek et al., 2004). Cavitation vapor bubbles form in fluids under low pressure. This may be caused by adjacent flow fields moving at drastically different speeds and, at their interface, generating regions of low pressure(Brennen, 1995; Young, 1999). Thus, cavitation often occurs between a solid structure's boundary layer and a rapid flow field over its surface. Vortex cavitation commonly occurs in the vortices shed by pumps and boat propellers, while sheet cavitation often develops in a wake or area of separated flow and is visible along propeller blades and hydrofoils. Cavitation generated during the mantis shrimp's strike(Patek et al., 2004) is most likely caused by a combination of these flow processes, including sheet cavitation along the surfaces of the snail shell and dactyl, and the negative pressure generated during the rapid rebound of the dactyl heel after striking the hard surface (Fig. 1).
When cavitation bubbles collapse, considerable energy is released in the form of heat, luminescence and sound(Brennen, 1995). The shock waves and microjets generated during the collapse of the cavitation bubbles cause stress and fatigue in adjacent surfaces, ultimately leading to failure and flaking of surface materials (Brennen,1995). Remarkably, a 2.7 mmcavitation bubble collapsing near a wall can generate over 9 MPa of impact pressure over a period of approximately 5 μs (Shima et al., 1983; Tomita et al., 1983). Such cavitation forces can destroy rapidly rotating boat propellers, aid in water-based metal cutters, and are even thought to provide the mechanism by which water picks remove dental plaque(Brennen, 1995).
The presence of cavitation is often detected acoustically because the sound of cavitation bubbles collapsing contains greater energy at higher frequencies than similar events without cavitation(Brennen, 1995; Lush and Angell, 1984; Martin et al., 1981). Thus,the acoustic signature of cavitation is the presence of a broadband signal extending, with substantial energy, into the ultrasonic range (above 20 kHz),as compared to events without cavitation that lack power in the ultrasonic acoustic range. This phenomenon has been examined extensively in the engineering literature, including controlled studies in which cavitation is present and absent, as well as correlative studies linking cavitation damage with the acoustic power of the signal in the ultrasonic range(Brennen, 1995; Ceccio and Brennen, 1991; Lush and Angell, 1984; Martin et al., 1981). Cavitation has also been detected via acoustic analyses, especially in the ultrasonic range, in studies of tree xylem cavitation(Ikeda and Ohtsu, 1992; Perks et al., 2004; Tyree et al., 1984) and fern sporangia (Ritman and Milburn,1990).
The presence and dynamics of cavitation can also be detected visually. Extreme high-speed video is necessary to capture the microsecond timescales of cavitation bubble formation, luminescence and collapse. Cavitation bubbles in snapping shrimp were visualized using high-speed video, coupled with the use of a photodector to detect the emission of luminescence(Lohse et al., 2001). The simple presence/absence of cavitation vapor bubbles has been examined in x-rays of joints after knuckle-cracking in humans(Unsworth et al., 1971) and light microscope images of fungal spores(Money et al., 1998).
Despite our rich understanding of crushing forces and their influences on shell evolution, as well as a substantial body of work on the physics of cavitation, little is known about the impact forces generated by biological hammers and biological cavitation. The mantis shrimp's unusual mechanism for breaking shells suggests fundamental questions about the amplitude of the limb impact forces and relative contribution of cavitation forces. Here, through the use of force transducers, acoustic analyses and high-speed video, we report the limb impact and cavitation forces generated by the peacock mantis shrimp Odontodactylus scyllarus. The goals of this study were to (1)visualize limb impact and cavitation while measuring forces, specifically to identify the presence and relative contribution of cavitation to force generation; (2) measure the timing and acoustic signature of impact and cavitation; and (3) measure amplitude of forces across a range of striking surface geometries in order to assess the effects of striking surface on the amplitude of cavitation and impact forces. This study provides the first in-depth examination of a biological hammer and reveals a potent combination of power amplification, extreme impact forces and cavitation dynamics.
Thirteen peacock mantis shrimp Odontodactylus scyllarus L.(Crustacea, Stomatopoda, Gonodactyloidea, Odontodactylidae), ranging in size from 27 to 36 mm carapace length, were purchased from commercial collectors. Animals were held at 25C in recirculating artificial saltwater, and were fed a diet of fresh snails and freeze-dried and frozen shrimp. Because of their unpredictable molt cycles, different combinations of individuals were used in each of the experiments. During a molt, animals were unable to strike for several days, and only gradually recovered full striking strength. We therefore tested animals only when they were in an intermolt period. Animals regularly struck objects coated with shrimp paste and most animals were willing to strike objects under bright video lights after a period of training. In natural conditions, peacock mantis shrimp carefully position a snail on a firm surface or anvil-like rock, and then deliver a blow that typically causes little movement of the snail. In this study, a force sensor(load cell) was mounted at the base of an aluminum beam that was manually presented to the mantis shrimp. This arrangement permitted minimal movement of the apparatus when struck.
We used a one-axis force sensor to measure the relative contributions of limb impact force and cavitation force (force range 444.8 N, upper frequency limit 75 kHz, Model 200B02, PCB Piezotronics, NY, USA). The stainless steel force sensor had a 12.7 mm diameter load surface and a stiffness of 1.9 kNμm-1. Data were collected at 500,000 samples s-1using a data acquisition board (NIDAQ 6062E, National Instruments, Austin, TX,USA). Peak forces (amplitude of force trace) and force impulse (integrated area under a force curve; Caldwell et al.,2004; Ozkaya and Nordin,1999) were analyzed using custom-designed computer analysis tools(Matlab v7.0.1). The onset of the first peak was set as an increase of 0.05 V above the average value of a 100-sample window. The second peak onset was set at 0.08 V above a 40-sample average window after the first peak. The ends of the first and second peaks were set to the same value as the onset voltage for each peak.
Video recordings (60 frames s-1, Sony DCR-VX2100, Sony Corp.,New York, NY, USA) were simultaneously collected in order to establish whether the force sensor was struck by one or both raptorial appendages. Both peaks were analyzed if only one raptorial appendage struck the force sensor. If two raptorial appendages struck the force sensor in close succession, four force peaks were logged, leading to potential ambiguity as to the source of each of the four peaks. In these cases, only the first peak was included in the analysis. Some individuals exceeded the capacity of the load cell, thus any force data that exceeded the linear range of the load cell (>445 N)wereremoved. After the overloaded data had been removed, the final dataset reported here included four individuals with 6, 12, 22 and 25 strikes per individual.
We measured the effects of surface geometry on force generation through the use of curved and flat surfaces. For the curved sensor, we measured the radius of curvature of a range of snails typically consumed by these mantis shrimp and machined a curved cap for the force sensor with the average measured radius of curvature (9.7 mm curvature; 28.5 mmsolid, 300-series stainless steel from strike surface to sensor surface). The flat sensor was 24.1 mm24.1 mm, with 18.1 mm solid, 300-series stainless steel from strike surface to sensor surface.
Strike forces on curved and flat surfaces were compared using a waterproof,three-axis, piezoelectronic force sensor designed for measuring impact forces(force range 1334 N in each axis, 90 kHz upper frequency limit,
The peak amplitude of forces in the three axes was measured using a custom,automated computer program and forces from each axis were summed using standard vector calculations (Matlab v7.0.1). The onset threshold of the first force peak was set as 0.02 V above the average value of a 100-sample window;the onset of the second force peak was set as 0.05 V over a 40-sample average window following the first peak.
Values are means s.d. One-way analysis of variance(ANOVA) was used to assess individual variation in the temporal aspects of force generation. The scaling of force with carapace length and dactyl heel width was evaluated with a linear regression. Statistical software was used for these calculations (JMP 5.0.1, SAS Institute, Inc., Cary, NC, USA).
Cavitation vapor bubble formation, collapse and rebound were visible with ultra-high speed imaging. We analyzed the temporal correlation between force generation, limb impact and cavitation bubble collapse using the video and force data. A single strike by a single appendage generated two force peaks in rapid succession (Figs 2, 3). The individual mantis shrimp used in this study only had one raptorial appendage, thus allowing us to rule out fast double-strikes as the cause of the two force peaks. In all strike sequences, the first force peak corresponded with limb impact and the second force peak occurred during cavitation bubble collapse (Figs 2, 3). Videos are available online as supplementary material.
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