Earthworm epidermal mucus: Rheological behavior reveals drag-reducing characteristics in soil

摘要

Soil adhesion occurs on the surfaces of soil-tillage implements during use. This phenomenon increases energy consumption and decreases tillage quality. Nevertheless, earthworms can comfortably move in moist or adhesive soil, although with soil particles seldom sticking to bodies. The influence of earthworm movement on epidermal mucus provides perspective for the interpretation of the results on drag reducing characteristics. The amino acid composition and rheological behavior of epidermal mucus samples from three earthworm ecological species were studied in the laboratory. These earthworm species were represented by Eisenia fetida, Aporrectodea trapezoides, and Amynthas pingi, which were collected from the Typic Hapludolls with loam textures. An electro-stimulation device was developed to extract and collect approximately 5 g of mucus (field weight) from several individuals of 30 g in total of each earthworm species. Mucus samples from the three earthworm species were found to contain 16 amino acids. Aspartic acid, glutamic acid, and valine were present in relatively high concentrations in all the samples. The rheological behavior of earthworm epidermal mucus was investigated via continuous shear, thixotropy, and oscillation tests. The apparent viscosity initially increased and then decreased with the increase in mucus shear rate. The thixotropic behavior was demonstrated as the apparent viscosity of epidermal mucus recovered gradually with the decrease in shear rate. Oscillatory tests showed that the viscoelastic behavior of epidermal mucus can be altered with changes in oscillation frequency. A five layer interface contact model revealed that the apparent viscosity of mucus changes with earthworm movement, thereby reducing the soil adhesion and friction resistance toward the earthworm body surface. Overall, this study revealed the drag-reducing characteristics of earthworm epidermal mucus and inspired the development of some potential bionic applications with lubricating functions for soil tillage implements. (C) 2015 Elsevier B.V. All rights reserved.