Pressure-Dependent Friction on Granular Slopes Close to Avalanche

Energy dissipation of a sphere rolling up a granular slope: Slip and deformation of the granular surface

We experimentally investigate the dynamics of a sphere rolling up a granular slope. During the rolling-up motion, the sphere experiences slipping and penetration (groove formation) on the surface of the granular layer. The former relates to the stuck motion of the rolling sphere, and the latter causes energy dissipation due to the deformation of the granular surface. To characterize these phenomena, we measured the motion of a sphere rolling up a granular slope of angle α. The initial velocity v_{0}, initial angular velocity ω_{0}, angle of slope α, and density of the sphere ρ_{s} were varied. As a result, the penetration depth can be scaled solely by the density ratio between the sphere and granular layer. By considering the rotational equation of motion, we estimate the friction due to the slips. Besides, by considering energy conservation, we define and estimate the friction due to groove formation. Moreover, the translational friction is proportional to the penetration depth. Using these results, we can quantitatively predict the sphere's motion including stuck behavior.

Surface instabilities generated by a slider pulled across a granular bed

We report an instability of a slider slowly dragged at the surface of a granular bed in a quasistatic regime. The boat-shaped slider sits on the granular medium under its own weight and is free to translate vertically and to rotate around the pitch axis while a constant horizontal speed is imposed. For a wide range of parameters (mass, length, shape, velocity) a regular pattern of peaks and troughs spontaneously emerges as the slider travels forward. This instability is studied through experiments using a conveyor belt and by means of two-dimensional discrete elements method simulations. We show that the wavelength and amplitude of the pattern scale as the length of the slider. We also observe that the ripples disappear for low and high masses, indicating an optimal confining pressure. The effect of the shape, more specifically the inclination of the front spatula, is studied and found to drastically influence both the wavelength and the amplitude. Finally, we show that the mechanical details (friction, cohesion) of the contact point between the slider and the pulling device is critical and remains to be fully understood.

Substrate-mediated leg interactions play a key role in insect stability on granular slopes

Locomotion on granular inclines is a subject of high relevance in ecological physics as well as in biomimmetics and robotics. Enhancing stability on granular materials represents a huge challenge due to the fluidization transition when inclination approaches the avalanche angle. Our motivating example is the predator-prey system made of the antlion, its pit, and its prey. Recent studies have demonstrated that stability on granular inclines strongly depends on the pressure exerted on the substrate. In this work we show that for multilegged locomotion, along with pressure, the distance between the leg contacts on the substrate also plays a major role in the determination of the stability threshold. Through a set of model experiments using artificial sliders, we determine a critical distance below which stability is importantly affected by the interactions between the perturbed regions generated by each contact point. A simple model based on the Coulomb method of wedges allows us to estimate a stability criterion based on pressure, interleg distance, and substrate characteristics. Our work suggests that mass to leg-length allometric relationships, as the ones observed in ants, may be an important key in determining the locomotion success of multilegged locomotion on granular inclines.

Efficiency of antlion trap design and larval behavior in capture success

Traps constructed by an animal reduce the amount of energy required to seek prey. The main risk of trap-building predators is the greater uncertainty of encountering prey, owing to their immobility. Sometimes environmental characteristics do not allow them to build efficient traps, resulting in lower capture success. We observed prey escape, capture success, and behavior of two antlion species, Cueta lineosa, a habitat specialist, and Myrmeleon hyalinus, a generalist, building geometrically different traps. The traps of C. lineosa are elaborate and deep, consisting of two inverted cones, while M. hyalinus builds simple inverted cones. Prey escape was observed from traps with antlion larvae present and from artificially constructed traps without antlions. We used a 3D printer to create a replica model of both trap types, pressing the model onto the substrate surface to create a trap. The C. lineosa artificial trap slowed prey escape more effectively than the simpler artificial trap of M. hyalinus. Prey escape time was four times longer for two ant species and three times longer for woodlice from C. lineosa traps. Escape time also decreased with increasing prey length. We also found behavioral differences between these two antlion species. The behavior of M. hyalinus is much more efficient in catching prey than that of C. lineosa. The results indicate that both species are efficient trap-building predators; however, it appears that capture success depends not only on trap design but also on larval behavior.

Fine sand particles enable antlions to build pitfall traps with advanced three-dimensional geometry

Pit-building antlion larvae are predators that construct pitfall traps in fine sand. We used three-dimensional laser scanning and geometric morphometrics to reveal the shape of antlion pits of two antlion species, analysed the particle size composition of sands from the different natural habitats, and measured the slope angles of the pits of the two species. In most antlions, the pits are structured as a simple inverted cone, as in Myrmeleon hyalinus, studied here. The other antlion studied, Cueta lineosa, constructs a unique pit composed of two inverted truncated cones inserted into one another, which feature substantially steeper walls than the pits of any other antlion studied to date. Pit stability depends on the slope inclination, which oscillates between the maximum angle of stability and the angle of repose. The angles in C. linosa substrates were larger than those in M. hyalinus substrates. One reason for the steeper walls is the greater proportion of fine sand in the natural sand inhabited by C. lineosa However, video-recording revealed that both the natural sand of C. lineosa and the finest sand tested had a higher maximum angle of stability than any of the other substrates studied here. Furthermore, experiments with pits built in different substrates revealed that the shape of the pit is variable and depends on the structure of the sand. Myrmeleonhyalinus displayed a more flexible pit construction behaviour than C. lineosa The present demonstration of such differences in pit characteristics contributes to understanding how these two species co-exist in the same habitat.

Antlions are sensitive to subnanometer amplitude vibrations carried by sand substrates

The antlion larvae (Myrmeleontidae) are ambush predators. They detect substrate-borne vibrations induced by the movement of the prey. European pit-building antlions (Myrmeleon inconspicuus) are studied for their ability to perceive vibrations generated by the locomotion of an ant (Cataglyphis cursor) outside the pit. These strides have been recorded and copied in detail in their time sequences. The signal created was emitted by piezoelectric transducers placed several centimeters outside the peripheries of the pits: the ant movements create waves with particle accelerations that are three orders of magnitude less than g, alleviating any possibility of sand avalanche towards the bottom of the pit. Depending on the amplitude of the vibrations, the antlions answer back, generally by sand tossing. One remarkable feature is the time delay between the start of the cue and the predatory behaviour induced by this cue. This time delay is studied versus the cue amplitude. We found that antlions answer back within minutes to cues with amplitudes of nanometer range, and within seconds to these same cues if they are preceded by a sequence of signals at the Ångström amplitude. This difference in latency is used to evidence the sensitivity to vibrations at an extremely low level.

Digging the optimum pit: antlions, spirals and spontaneous stratification

Most animal traps are constructed from self-secreted silk, so antlions are rare among trap builders because they use only materials found in the environment. We show how antlions exploit the properties of the substrate to produce very effective structures in the minimum amount of time. Our modelling demonstrates how antlions: (i) exploit self-stratification in granular media differentially to expose deleterious large grains at the bottom of the construction trench where they can be ejected preferentially, and (ii) minimize completion time by spiral rather than central digging. Both phenomena are confirmed by our experiments. Spiral digging saves time because it enables the antlion to eject material initially from the periphery of the pit where it is less likely to topple back into the centre. As a result, antlions can produce their pits—lined almost exclusively with small slippery grains to maximize powerful avalanches and hence prey capture—much more quickly than if they simply dig at the pit's centre. Our demonstration, for the first time to our knowledge, of an animal using self-stratification in granular media exemplifies the sophistication of extended phenotypes even if they are only formed from material found in the animal's environment.

Locomotion of Ants Walking up Slippery Slopes of Granular Materials

Many insects encounter locomotory difficulties in walking up sand inclines. This is masterfully exploited by some species for building traps from which prey are rarely able to escape, as the antlion and its deadly pit. The aim of this work is to tear apart the relative roles of granular material properties and slope steepness on the insect leg kinematics, gait patterns, and locomotory stability. For this, we used factorial manipulative experiments with different granular media inclines and the ant Aphaenogaster subterranea. Our results show that its locomotion is similar on granular and solid media, while for granular inclined slopes we observe a loss of stability followed by a gait pattern transition from tripod to metachronal. This implies that neither the discrete nature nor the roughness properties of sand alone are sufficient to explain the struggling of ants on sandy slopes: the interaction between sand properties and slope is key. We define an abnormality index that allows us to quantify the locomotory difficulties of insects walking up a granular incline. The probability of its occurrence reveals the local slipping of the granular media as a consequence of the pressure exerted by the ant's legs. Our findings can be extended to other models presenting locomotory difficulties for insects, such as slippery walls of urns of pitcher plants. How small arthropods walking on granular and brittle materials solve their unique stability trade-off will require a thorough understanding of the transfer of energy from leg to substrate at the particle level.

Resonantly Induced Friction and Frequency Combs in Driven Nanomechanical Systems

We propose a new mechanism of friction in resonantly driven vibrational systems. The form of the friction force follows from the time- and spatial-symmetry arguments. We consider a microscopic mechanism of this resonant force in nanomechanical systems. The friction can be negative, leading to the onset of self-sustained oscillations of the amplitude and phase of forced vibrations, which result in a frequency comb in the power spectrum.