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MPhil in Engineering for Sustainable Development

global challenges, engineering solutions

Studying at Cambridge

Pauline Chone

Insect Adhesion on Micro-Rough Surfaces

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.