# Mechanical Characterization of Stick Insect Tarsal Attachment Fluid Using Atomic Force Microscopy (AFM)

## TL;DR

This paper uses atomic force microscopy to study the mechanical properties of fluid from stick insect tarsal pads, revealing three distinct droplet types with different viscoelastic behaviors.

## Contribution

The study introduces a novel classification of tarsal fluid droplets based on their viscoelastic properties, using stress–relaxation nanoindentation and modeling.

## Key findings

- Three droplet categories were identified: 'almost inviscid', 'viscous', and 'rigid'.
- Droplet properties are determined at secretion and remain stable for days.
- Non-uniform composition of droplets suggests adaptive adhesion mechanisms.

## Abstract

Most insects secrete special fluids from their tarsal pads which are essential for the function of their attachment systems. Previous studies investigated several physical and chemical characteristics of this pad fluid in different insect species. However, there is not much known about the mechanical properties of fluid from smooth adhesive pads. In this study, we used the stress–relaxation nanoindentation method to examine the viscoelastic properties of pad fluid from Sungaya aeta. Force–displacement and stress–relaxation curves on single fluid droplets were recorded with an atomic force microscope (AFM) and analyzed using Johnson–Kendall–Roberts (JKR) and generalized Maxwell models for determination of effective elastic modulus (E), work of adhesion (Δγ) and dynamic viscosity (η). In addition, we used white light interferometry (WLI) to measure the maximal height of freshly acquired droplets. Our results revealed three different categories of droplets, which we named “almost inviscid”, “viscous” and “rigid”. They are presumably determined at the moment of secretion and retain their characteristics even for several days. The observed mechanical properties suggest a non-uniform composition of different droplets. These findings provide a basis for advancing our understanding about the requirements for adaptive adhesion-mediating fluids and, hence, aid in advancing technical solutions for soft or liquid temporal adhesives and gripping devices.