Insect Adhesion on Micro-Rough Surfaces
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To protect crops and buildings against pest insects, insecticides are widely used, raising alarming environmental and health concerns. There is a need to find environmentally friendly ways to control insects, and nature can provide solutions. Insects can easily climb on most vertical surfaces thanks to their adhesives devices. However, they are unable to climb on some plant surfaces, which have a micro-‐rough coating of wax crystals. Producing synthetic coatings with similar non-‐adhesive properties would reduce the need to use toxic chemicals. So far, the detailed mechanisms of how micro-‐rough surfaces prevent insect attachment are still unclear. An intuitive explanation would be that both friction and adhesion forces are lower on micro-‐rough surfaces. We tested the performance of stick insects on micro-‐rough and smooth surfaces by comparing freely climbing insects and single-‐pad force measurements. Freely climbing stick insects frequently fell off vertical micro-‐rough substrates but could climb without difficulty on the smooth surface. However, we found that single-‐pad friction forces (with feedback-‐ controlled normal load) on micro-‐rough substrates were higher than on smooth surfaces. Only the adhesion forces on micro-‐rough surfaces were slightly smaller than those measured on smooth surfaces but the forces were still too high to explain a fall. This result was independent of the initial normal preload, its duration and the pull-‐off speed. This finding suggests that the strict dependence of adhesion on friction forces (‘frictional adhesion’) which has been shown for smooth surfaces, does not hold for micro-‐rough surfaces. However, adhesion forces still remained too high to explain slipperiness. We discovered that pads exhibited stick-‐slip only on micro-‐rough but never on smooth surfaces when sliding without feedback-‐controlled normal load. Stick slips induce repeated partial, or complete, loss of adhesion and could explain the insects’ falling. Moreover, the pattern of stick-‐slip depends on the slipperiness of the surface, changing in frequency and amplitude. I found that slides performed without feedback-‐controlled normal load (letting the friction induce negative normal forces) showed results which were perfectly consistent with the insect climbing tests, suggesting that they are most representative of the natural condition. Further research is required to determine whether loss of contact for simultaneous shear and negative normal forces is the key mechanism causing the slipperiness of micro-‐rough surfaces. The results of this work will provide important information for the development of artificial slippery surfaces used for pest control. |