טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentMurarash Boris
SubjectTribology of Contact Surface Patterns Evolved in Biological
Microsystems
DepartmentDepartment of Mechanical Engineering
Supervisors Assistant Professor Michael Varenberg
Dr. Gregory Halperin
Full Thesis textFull thesis text - English Version


Abstract

Nowadays elastomers are widely used in various mechanical devices ranging from printing rolls to microelectromechanical systems. Highly deformable elastomer usually forms a large real area of contact with its mating surface, which inevitably leads to high adhesive friction and often prevents sliding in the accepted sense. Instead, the displacement of contacting bodies is accommodated by stick-slip or detachment waves which lead to uncontrollable and unstable surface behavior resulting in vibration, noise, diminished accuracy, energy loss and increased wear that affect the function and shorten the service life of machine components. These technical problems may however be overcome by employing certain surface textures originally evolved in the biological world.

One of the most promising biological surface textures is the hexagonal one found in smooth attachment pads of some tree frogs and bush crickets. To test this pattern experimentally, a cricket-inspired hexagonal elastomeric surface texture was recently fabricated and examined. Interestingly, this texture prevented hydroplaning in wet conditions and eliminated stick-slip instabilities in dry conditions. To advance the subject we studied the role of area density (contact area over apparent area) and aspect (height over diameter) ratio of the texture elements.

The work was performed using a special tribometer we built to operate inside an environmental scanning electron microscope enabling charge-free imaging of non-conductive and/or hydrated materials. This device was employed for simultaneous testing and in situ visual inspection of patterned elastomer surfaces loaded against a smooth rigid counterface.

Here we show that the hexagonal surface texture not only stabilizes the sliding behavior of elastomer surface but also allows tuning its friction force from as low as 50% to nearly 100% of that of an unmodified surface by adjusting the aspect ratio of the texture elements. Changing the texture area density does not affect the friction force which allows choosing this parameter independently to support different normal loads. Both effects are explained using an in situ scanning electron microscopy of steady-sliding contacts.