Why do insects have such a high density of flow-sensing hairs? Insights from the hydromechanics of biomimetic MEMS sensors

Abstract

Insects and arachnids are often quite hairy. The reasons for this high density of sensory hairs are unknown. Previous studies have predicted strong hydrodynamic coupling between densely packed airflow-sensitive hairs. Flow perturbation owing to single hairs and between tandem hairs, however, has never been experimentally measured. This paper aims to quantify the extent of flow perturbation by single and tandem hairs directly, using biomimetic microelectromechanical system (MEMS) hairs as physical models and particle image velocimetry (PIV) for flow visualization. Single and tandem MEMS hairs of varying interhair distances were subjected to oscillatory flows of varying frequency. Decreasing hair-to-hair distance markedly reduced flow velocity amplitude and increased the phase shift between the far-field flow and the flow between hairs. These effects were stronger for lower flow frequencies. We predict strong hydrodynamic coupling within whole natural hair canopies exposed to natural stimuli, depending on arthropod and hair sizes, and hair density. Thus, rather than asking why arthropods have so many hairs, it may be useful to address why hairs are packed together at such high densities, particularly given the exquisite sensitivity of a single hair.

Bio-inspired hair flow-sensor arrays : from nature to MEMS

“…Although flow is a very common phenomenon, the understanding of flow and flow signatures are far from trivial. Scientifics (in various disciplines) try to understand and model the dynamic representation of fluid flow around objects by devising measurement setups and sensory systems along with their developing theories [Shimozawa et al, 1984b& Casas et al, 2008. Determining the flow rate of fluids is a common example for flow measurement in industrial and (bio-)medical applications [Ho et al, 1998& Viarani et al, 2006.…”

“…Determining the flow rate of fluids is a common example for flow measurement in industrial and (bio-)medical applications [Ho et al, 1998& Viarani et al, 2006. Another example is the detection of flow patterns which can be used for detecting specific events as generated by the relative flow to object movement [Barth et al, 1995, Casas et al, 2008& Kant et al, 2009. The characteristics of the measured flow patterns can be exploited to determine size, position and number of flow sources & Casas et al, 2008.…”

“…The airflow patterns produced by spider motion are complex with small-scale high flow velocity content at the legs and more extended flow patterns due to the spider's body [Casas et al, 2008]. The cricket senses, perceives and responds to these airflow patterns efficiently.…”

“…Spiders use at least two attacking strategies to catch crickets; either they apply an ambush strike or they hunt at high velocity. Figure 6.1 (a) illustrates the airflow field around a running spider as measured by particle image velocimetry (PIV) (from [Casas et al, 2008]). …”

“…A nice literature survey of the detection of transient flow fields as generated by approaching predators or moving prey is presented in [Kant et al, 2009]. Figure 6.1: Close-up view of airflow velocity field around (a) running spider and (b) suddenly moved cylindrical piston imitating spider motion [Casas et al, 2008].…”