Meeting Abstract
Many insects use adhesive footpads to climb on plants. These pads are covered by a thin liquid film, hypothesised to aid surface attachment via viscous and capillary forces. However, as many insects live in environments with considerable daily temperature fluctuations, relying on viscosity may pose significant limitations on their attachment system. So how does the secretion’s viscosity change with temperature? Conventional rheological techniques are unsuitable to answer this question, due to the secretion’s minute volume (~100fL) and its propensity for fast evaporation. Here, we use dewetting, the spontaneous rupture of a thin liquid film due to thermodynamic instabilities, to overcome these challenges. The speed of dewetting can be linked to the liquid’s viscosity via a dimensional argument which balances the capillary and viscous forces at play during film rupture. To quantify the temperature effect on viscosity and decouple it from that on surface tension, dewetting experiments were performed at biologically relevant temperatures (20-50°C) and on conducting coverslips of varying wettability. Joule heating of the coverslips and a custom-built temperature controller were employed to induce and monitor temperature changes, surface wettability was varied via vapour deposition of different silanes, and Indian stick insects were used as a model species. Across the temperature range, the secretion’s viscosity decreased sevenfold, from approximately 55mPas at 20°C to 8mPas at 50°C. Our in vivo viscosity measurements enable further investigation of wet adhesion models by comparing viscosity changes to the temperature dependence of frictional and adhesive forces. These experiments will increase our understanding of the complex viscoelastic pad/secretion/surface interface, and the functional relevance of the pad secretion in general.
