Oil filters - inspired by nature

Bees possess a highly effective technique to store and transport oil by using closely spaced hairs at their back legs. The Institute for Numerical Simulation is investigating how this natural process could be used by humans for technical applications.

Close up shot of a bee

The research area of bionics examines how mechanisms from nature that have been perfected over millions of years could be translated into modern technology. To this end, a great number of mathematical methods are applied which help to analyze and describe biological materials and processes to serve as examples for innovative technologies.

An especially interesting natural example for industrial applications are bees that store and transport floral oils instead of or in addition to nectar. These bees, which mainly live in South America, collect the floral oils mixed with pollen from flowers and feed it to their larvae. The floral oil has an energy content eight times higher than the usual nectar and is therefore particularly nutritious for the animals. To transport the oil, the bees have developed special hair structures at their back legs. These structures enable the bees to absorb thirty times the weight of their hair in oil and to release the oil without loss. The hair can be used to transport the oil not just once but as often as they like. In contrast, man-made components in technical machines that serve the same purpose have to be exchanged frequently. That is one of the reasons why, for instance, the operation of oil filters is very expensive.

Alexander Rüttgers and Michael Griebel from the Institute for Numerical Simulation at the Hausdorff Center have analyzed which factors give the excellent oil binding properties to the bees’ hair using complex computer models. To identify the physical processes and parameters that are responsible for the bees’ hair being such an efficient storage and transportation medium for oil, the scientists conducted a numerical simulation on a parallel high-performance computer cluster. For their simulation, the researchers measured a bee with a micro-CT scan and derived a three-dimensional computer model from the obtained data; also capturing the microscopic branching of the hair network. Next to the floral oil’s flow in the hair at the back leg, the simulation also includes the turbulent air flow around the hair at the back leg during the flight. Therefore, the simulation is a two-phase flow simulation. The researchers from Bonn created new numerical methods specifically for this problem that enable them to replicate the physics at the contact angle of oil, air, and the bees’ hair as well as possible with the computer.

“For the first time, we were able to describe the deformation of an oil droplet in the hair network of the bees completely mathematically”, says Alexander Rüttgers. The results of the numerical simulation make obvious that there exists a high circulation of air in the hairy network at the bees’ back legs during their flight. The simulation also identifies the areas of the structure where the oil most easily could get lost. Furthermore, the surface tension of the oil and the complexity of the hair’s structure play an essential role. Only the small branching in the microstructure of the hair makes it possible for the bees’ hair to store that much oil. The simulations have clearly improved our understanding of the underlying physical mechanisms.

Further work is based on these results that, among other things, could lead to more efficient oil filters in the future. The mathematicians’ findings have already been practically applied in cooperation with biologists from the University of Bonn, engineers from the Textile Research Center Denkendorf, partners from the economy, and the Fraunhofer Institute for Algorithms and Scientific Computing: At the end of a cleaning process, the wash solution which is used to clean internal combustion engines in cars or agricultural machines contains a lot of oil. The wash solution can only go into the wastewater, if an oil separator eliminates the oil. Inspired by the bees, a fiber based oil filter has been created for that matter. This filter has been tested in industrial applications and soon could improve and replace the conventional procedures for oil separation. “We hope that in the future our work could maybe contribute to clean the sea after oil spill disasters and thereby minimize the ecological damage”, say the scientists.

Original paper

ALEXANDER RÜTTGERS, MICHAEL GRIEBEL, L. PASTRIK, H. SCHMIED, D. WITTMANN, A. SCHERRIEBLE,A. DINKELMANN, T. STEGMAIER (2015): Simulation of the oil storage process in the scopa of specialized bees. Computers & Fluids 119: 115-130.