Worms show way to efficiently move soil

Tom Shelley reports on an animal inspired way to greatly improve the efficiency of construction equipment and agricultural ploughs

By passing small electric currents between electrodes on a surface, it is possible to attract water lubrication to points which most need it, and reduce resistance to passage for bulldozing blades and agricultural ploughs by a very significant amount.

The discovery has come from a Chinese study of how it is that earthworms and dung beetles ease their path through the soil.

The amounts of electricity required are microscopic - earthworms only need to apply a few tens of millivolts, increased to 12V for machines working on a man made scale.

The discovery is the work of a team of researchers led by Professor Luquan Ren, Vice President of Jilin University in Changchun in North Eastern China.

If electricity is passed through soil, water tends to migrate from positive to negative poles under the action of a surface double layer effect called electrophoresis or electro-osmosis.

Bioelectricity is found in all living creatures. It is of two types. There is a resting potential between the inside and outside of the tissue or cell membrane when the creature is stationary. When moving, there is an additional action potential between the excited part and the resting part of the same tissue or cells. The action potential is of short duration, but larger than the resting membrane potential.

Surface potentials on the outer skin of an earthworm arise from the resting and action potentials. In the study in China, surface potentials on the fore part, middle part and hind parts of an earthworm were measured using silver-silver chloride electrodes and silk electrodes. It was found that the resting potential of the earthworm was zero with respect to earth, but the skin of the moving part rose to as much as -40mV at the fore part of the worm with respect to earth and to the rest of the worm.

In another experiment, a circular silver silver chloride electrode was prepared and fixed inside a 10mm long tube. The earthworm was guided through it and it was found that the worm produced a potential of -35mV at its fore part, -19mV in its middle part and -18mV at its hind part.

In addition to the generation of voltage between the front and rear parts of the worm, it has been found that in various animals which spend much of their time in contact with soil, their skins are ribbed or rough in some way. In each case, surface elements which stick out show a small negative voltage relative to areas in between. The purpose appears to be to cause water to move towards protuberances in order to lubricate them.

Although the voltage differences are very small, the distances are also very small, so that voltage gradients are significant. Water is an extremely good lubricant, as former school students should remember from having to wet glass tubes in order to slide on rubber connecting tubes. Electrophoresis is also known to be the most efficient way of moving fluids on the small scale, and is generally expected to be key to pumping within the emerging generation of lab on a chip devices.

Dung beetles are found to use a similar mechanism to earthworms to help them make their way through soil. It has been concluded that for maximum effectiveness, it is not only necessary for voltage to be generated between different parts of the skin surface, but also that this should be rough. If it rough, water is encouraged to migrate to relatively small areas where lubrication is of maximum benefit.

The Chinese team has been quick to see whether the effects would be useful in larger scale products. Starting with laboratory scale bulldozing experiments, it was found that under otherwise identical conditions, soil stuck onto the surface of smooth or rough plates without electro-osmosis, but little or no soil stuck to a rough plate with energised electrodes. Dome topped electrodes in the test plate were made from carbon steel, placed in holes in the test plate within insulating containments. Dome tops, which stood slightly proud of the flat plate surface were energised at -12V relative to the plate. The team reports the arrangement reduces bulldozing resistance by 15 to 32%.

The team then went onto tests with modified plough mouldboards in the field. It was found that tillage resistance was reduced by 15 to 18%, and fuel consumption reduced by 5.6 to 12.6% under the same conditions of field, tractor, operator and weather. After 500 hours use, the dome tops showed some signs of abrasion while the edge of the mouldboard showed "notable" attrition. Hence it would appear that improving lubrication of the surface by inducing additional water based lubrication also reduces wear. Trials have been undertaken to investigate possible improvements that the method might achieve in the functioning of coal hoppers in electricity generating stations, and steel chain liners in dump trucks and a flexible steel liners in loader buckets.


Further study is now under way at Nottingham Trent University in the hands of Reader in Thermofluids Dynamics Dr Yuying Yan to computer model the process with a view to optimising it. Because of the need for surfaces to be rough and in contact with water film thicknesses, the problem is distinctly non trivial. The study is being supported by the Chinese Government's 973 programme and The Royal Society's UK-China Joint Project 2003-2006. The team at Jilin University is one of the participants in the BIONIS, BIOmimetics Network for Industrial Sustainability based at the University of Reading. More information about this may be found at .

Jilin University
email Professor Luquan Ren ">Professor Luquan Ren</a>
email Dr Yuying Yan


* Application of small electric currents to working parts in contact with soils result in water being transported to negatively charged areas

* Such water films greatly assist lubrication

* Best results are obtained with rough surfaces, with negatively charged protuberances

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