Surface force spectroscopic point load measurements and viscoelastic modelling of the micromechanical properties of air flow sensitive hairs of a spider (Cupiennius salei)

The micromechanical properties of spider air flow hair sensilla (trichobothria) were characterized with nanometre resolution using surface force spectroscopy (SFS) under conditions of different constant deflection angular velocities (theta)over dot (rad s(-1)) for hairs 900-950 mu m long prior to shortening for measurement purposes. In the range of angular velocities examined (4 x 10(-4)-2.6 x 10(-1) rad s(-1)), the torque T (Nm) resisting hair motion and its time rate of change (T)over dot (Nm s(-1)) were found to vary with deflection velocity according to power functions. In this range of angular velocities, the motion of the hair is most accurately captured by a three-parameter solid model, which numerically describes the properties of the hair suspension. A fit of the three-parameter model (3p) to the experimental data yielded the two torsional restoring parameters, S-3p = 2.91 x 10(-11) Nm rad(-1) and S'(3p) = 2.77 x 10(-11) Nm rad(-1) and the damping parameter R-3p = 1.46 x 10(-12) Nm s rad(-1). For angular velocities larger than 0.05 rad s(-1), which are common under natural conditions, a more accurate angular momentum equation was found to be given by a two-parameter Kelvin solid model. For this case, the multiple regression fit yielded S-2p = 4.89 x 10(-11) Nm rad(-1) and R-2p = 2.83 x 10(-14) Nm s rad(-1) for the model parameters. While the two-parameter model has been used extensively in earlier work primarily at high hair angular velocities, to correctly capture the motion of the hair at both low and high angular velocities it is necessary to employ the three-parameter model. It is suggested that the viscoelastic mechanical properties of the hair suspension work to promote the phasic response behaviour of the sensilla.