The Friction Properties of Firebrat Scales

Abstract

Friction is an important subject for sustainability due to problems that are associated with energy loss. In recent years, micro- and nanostructured surfaces have attracted much attention to reduce friction; however, suitable structures are still under consideration. Many functional surfaces are present in nature, such as the friction reduction surfaces of snake skins. In this study, we focused on firebrats, Thermobia domestica, which temporary live in narrow spaces, such as piled papers, so their body surface (integument) is frequently in contact with surrounding substrates. We speculate that, in addition to optical, cleaning effects, protection against desiccation and enemies, their body surface may be also adapted to reduce friction. To investigate the functional effects of the firebrat scales, firebrat surfaces were observed using a field-emission scanning electron microscope (FE-SEM) and a colloidal probe atomic force microscope (AFM). Results of surface observations by FE-SEM revealed that adult firebrats are entirely covered with scales, whose surfaces have microgroove structures. Scale groove wavelengths around the firebrat's head are almost uniform within a scale but they vary between scales. At the level of single scales, AFM friction force measurements revealed that the firebrat scale reduces friction by decreasing the contact area between scales and a colloidal probe. The heterogeneity of the scales' groove wavelengths suggests that it is difficult to fix the whole body on critical rough surfaces and may result in a "fail-safe" mechanism.

Keywords: AFM; colloidal probe; firebrat; friction; microstructure; scale.

Figure 1

Surface observations of a firebrat by using a light microscope and FE-SEM. (A) Light microscopy image of an adult male firebrat. (B–E) FE-SEM images of firebrat surfaces for confirming alignments of scales on the firebrat integument: (B) head (I); (C) prothorax (II); (D) anterior abdominal region (III); and (E) 9th abdominal tergum (V). White arrows indicate the growth directions of scales. (F–I) Higher magnification images from (B–E), respectively. (J–L) FE-SEM images of backside of the scales. (K,L) Higher magnification images from (J).

Figure 2

Graphs of structural variations of firebrat scales. (A) Groove wavelengths of firebrat scales (mean ± standard deviation) for the different body regions: head (I); pronotum (II); metanotum (III); 8th abdominal tergum (IV); and 9th abdominal tergum (V). (B) Groove height measured from the scale base to the apex. Data shown present the average height of three scales with a groove wavelength of ca. 3.5 µm (black) and three scales with a groove wavelength of ca. 2.0 µm (gray). Error bars represent the standard deviation. The inset shows a stitched image of AFM topographies. White lines indicate the positions of the grooves’ measuring height.

Figure 3

Surface analysis of firebrat scale surface before and after chloroform treatment. (A,B) FTIR spectra and (C,D) FE-SEM images and photographs of water droplets on the firebrat dorsal surface. (A,C) Bare surface and (B,D) chloroform-treated surface.

Figure 4

Topography and friction force images obtained by AFM with a needle probe. White arrows show the scanning direction. The body part measured was the dorsal pronotum, and the scanning area was 50 µm × 50 µm.

Figure 5

Topographies and friction force images obtained by AFM with three types of colloidal probe. Scanning direction was scale base to apex only. The body part measured was the pronotum.

Figure 6

Friction force images obtained by AFM. Black dotted lines indicate the top of the grooves and white broken lines indicate the bottom of grooves. White lines are sampling points of the height profile and the friction force as shown in Figure 7. The scanning direction was the same (left to right), and the scanning area was 15 µm square. Within this figure, same scale spot was measured by four different probes.

Figure 7

Graph and schematic illustrations showing the relationship between groove structures, friction force and colloidal probes. (A) Height profiles of the groove structures (black line) and friction forces (colored lines). The data were selected from the white lines on the friction force images in Figure 4. (B–D) Schematic of the relationship between groove structures and the (B) 2, (C) 3.5, and (D) 6.6 µm diameter colloidal probes. Black arrows indicate the contact area between groove structures and colloidal probes.