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Effect of fracture behaviour of crystalline plant waxes on insect pad contamination
Elena Gorb
Last modified: 2009-12-28
Abstract
Elena V. Gorb1, Feodor M. Borodich2 and Stanislav N. Gorb3
1 Evolutionary Biomaterials Group, Department of Thin Films and Biological Systems, Max Planck Institute for Metals Research, Stuttgart, Germany
o.gorb@mf.mpg.de
2 School of Engineering, Cardiff University, Cardiff, UK
3 Department of Functional Morphology and Biomechanics, Zoological Institute at the University of Kiel, Kiel, Germany
It is known that attachment abilities of insects depend on various factors, including the texture of the substrate. On plant surfaces covered with crystalline epicuticular waxes, insect attachment was found to be highly reduced in many insect species. Such effect of minute wax crystals, ranging from hundred nanometres to several micrometers in length, on insect adhesion may be explained by (1) the decrease of the real contact area between the substrate and insect adhesive pads caused by plant surface microroughness; (2) the absorption of the pad fluid from the setal surface by the structured wax coverage; (3) the dissolving of the wax in the pad secretion, resulting in substrate slipperiness; and (4) the contamination of pads by wax crystals.
In this study, mechanics of the plant wax crystals fracture during contact formation between insect adhesive organs and plant surface is examined in order to explain the contamination of insect pads by wax crystals. It is shown that mechanisms of the wax crystal fracture may be rather different, depending on the aspect ratio of the crystals. If the crystals have high values of the aspect ratio, they may buckle elastically or in an elastic-plastic way. For five plant species under consideration, the critical value for elastic buckling is 26.5, while for elastic-plastic buckling it is 18.7. The calculations showed that from all considered plant wax crystals having tubular shapes, only crystals of Aristolochia fimbriata can buckle elastically. Rather short crystals may be bent under the weight of insects as elastic-plastic beams. The bending moment in the crystal at the fixed end is proportional to the crystal length. These conclusions are in agreement with our previous observations that the degree of pad contamination by wax crystals is related to both the length and aspect ratio of crystals. Although this study considered only crystals of a tubular shape, the general approach is valid also for crystals of other shapes.
1 Evolutionary Biomaterials Group, Department of Thin Films and Biological Systems, Max Planck Institute for Metals Research, Stuttgart, Germany
o.gorb@mf.mpg.de
2 School of Engineering, Cardiff University, Cardiff, UK
3 Department of Functional Morphology and Biomechanics, Zoological Institute at the University of Kiel, Kiel, Germany
It is known that attachment abilities of insects depend on various factors, including the texture of the substrate. On plant surfaces covered with crystalline epicuticular waxes, insect attachment was found to be highly reduced in many insect species. Such effect of minute wax crystals, ranging from hundred nanometres to several micrometers in length, on insect adhesion may be explained by (1) the decrease of the real contact area between the substrate and insect adhesive pads caused by plant surface microroughness; (2) the absorption of the pad fluid from the setal surface by the structured wax coverage; (3) the dissolving of the wax in the pad secretion, resulting in substrate slipperiness; and (4) the contamination of pads by wax crystals.
In this study, mechanics of the plant wax crystals fracture during contact formation between insect adhesive organs and plant surface is examined in order to explain the contamination of insect pads by wax crystals. It is shown that mechanisms of the wax crystal fracture may be rather different, depending on the aspect ratio of the crystals. If the crystals have high values of the aspect ratio, they may buckle elastically or in an elastic-plastic way. For five plant species under consideration, the critical value for elastic buckling is 26.5, while for elastic-plastic buckling it is 18.7. The calculations showed that from all considered plant wax crystals having tubular shapes, only crystals of Aristolochia fimbriata can buckle elastically. Rather short crystals may be bent under the weight of insects as elastic-plastic beams. The bending moment in the crystal at the fixed end is proportional to the crystal length. These conclusions are in agreement with our previous observations that the degree of pad contamination by wax crystals is related to both the length and aspect ratio of crystals. Although this study considered only crystals of a tubular shape, the general approach is valid also for crystals of other shapes.