Research

Future Research Interests:  Mechanics of Nanostructures and Interfaces

By altering geometry, nanostructured surfaces have been shown to exhibit remarkable tribological and wetting properties, such as low stiction and hydrophobicity. My recent work exploits geometry in order to produce an ultra-high friction surface from otherwise low friction polymer.  Controlling surface forces through nanostructure might also allow materials to be adhesive, self-cleaning, or commensurate with particles for fluidic self-assembly. These systems could be fabricated from a vast range of materials and processing methods, and thus have the potential to be cheap, bio-compatible, durable, and temperature resistant. I plan to pursue such systems through a research agenda that emphasizes the governing role of mechanics and surface forces.

I've identified three specific lines of research that exploit geometry to control the interaction of contacting nanostructures. They concern systems governed by mechanical, surface and intermolecular forces and can be modeled with analytical techniques presented in my previous works on rod and membrane delamination. 

Self-Cleaning has been demonstrated experimentally in gecko foot pads and, when properly understood, can be extended to gecko-inspired microfiber arrays for adhesion and enhanced friction over repeated loading cycles or in contaminated environments. 


Self-Repairing is an essential property of plants and other living organisms.  Recently, biologists have discovered a passive mechanical process that seals fissures and ruptures in the pipe vine .  Such systems are potential inspiration for a new family of self-repairing, microstructured materials. 


Nanoscale Self-Assembly is an exciting new area of investigation inspired by the spontaneous aggregation of molecules in nature, and is an essential tool for high throughput fabrication of mesoscale structures. Despite the growing popularity of self-assembly methods, however, mechanical insights, design principles, and scaling limitations remain to be established. 


Attachment Methods for Miniaturized Sensors have applications in endovascular cardiac diagnositics and structural health monitoring. I'm concerned with methods for attaching sensors with fibrillar or thin film adhesives that are mechanically compatible and which faithfully transfer information on substrate deformation. 

Current Research:  Design of Synthetic Gecko Adhesive

My primary research involves the design, fabrication, and testing of synthetic gecko adhesives.  These are nano- and micro-scale structures composed of millions of elastic fibers that mimic the setal arrays on the digits of natural geckos.  A comparison of setal arrays and our synthetic products are shown in the figure below.
Figure 1  (left) scanning electron micrograph of setal array from the Anolis lizard;  (right) array of vertically aligned polyimide fibers;  both structures are composed of 20 micron long, 0.6 micron diameter polymeric fibers

Design

Designs for micro fiber adhesives are inspired by the setal arrays in gecko lizards.  Individual setal stalks independently bend to conform to a surface.  In this way, the array achieves intimate contact and forms millions of bonds at the setal tips through weak surface forces.  By selecting the appropriate geometry, this same effect can be achieved with synthetic microstructures for a wide range of materials.  Of particular interest are stiff, hydrophobic materials since they are resistant to wear and fouling.

An important design principle is the global compliance of a micro fiber array and the ability of fibers to uniformly share load during attachment and detachment.  One mechanism that allows uniform load sharing is side contact, in which individual fibers make contact with a surface along there side and must be peeled off during detachment [1,2].  This mechanism has been postulated to explain the adhesion of high aspect ratio structures such as multi-walled carbon nanotubes [3], silicon nanowires [4], and polyimide nano fibers (unpublished).

Figure 2  (left) array of 60 micron long, 200 nanometer diameter polyimide fibers;  (right) postulated mode of contact for nano fibers

Nano fibers are susceptible to adhering to their neighbors and this may lead to the formation of large clumps.  Based on a mathematical model similar to that used to study side contact, we can predict the occurrence and extent of clumping in an array of nano fibers [5].

Figure 3  (left) clumps of 20 micron long, 0.6 micron diameter polyimide fibers;  (right) model of adhesion between two adjacent fibers

Fabrication

Arrays of polymer micro fibers are fabricated by casting uncured polymer in a commercially available mold.  We typically use a nuclear-track etched porous polycarbonate membrane for our mold. 

Testing

We have measured adhesion and high friction with arrays of polypropylene micro fibers.  Friction measurements are obtained with a traditional pulley apparatus as well as with a two degree-of-freedom optical force sensor.  Remarkably, high friction is maintained even under larger pressures of >100 kPa.  A mathematical model of the structure suggests behavior that is consistent with the experimental result for high friction [6] and adhesion.

Figure 4  (left) array of 20 micron long, 0.6 micron diameter polypropylene fibers;  (right) US quarter suspended on an 82 degree incline with the array of polypropylene micro fibers.

 

[1]  C. Majidi, R. E. Groff, R. S. Fearing, "Attachement of fiber array adhesive through side contact," J. Applied Physics 98 103521 (2005).

[2]  C. Majidi, "Remarks on formulating an adhesion problem using Euler's elastica," Mechanics Research Communications 34 85-90 (2007).

[3]  Y. Zhao, et al.  "Interfacial energy and strength of multiwalled-carbon-nanotube-based dry adhesive," J. Vacuum Sci. & Tech. B 24 331-335 (2006).

[4]  R. Dubrow, US Patent No. US2004/0250950 A1 (2004).

[5]  C. Majidi, R. E. Groff, R. S. Fearing, "Clumping and Packing of Hair Arrays Manufactured by Nanocasting," Proc. IMECE, ASME, Anaheim (2004).

[6]  C. Majidi, et al.  "High Friction from a Stiff Polymer using Micro-Fiber Arrays,"  Physical Review Letters 97 076103 (2006).

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Last Updated April 18, 2007