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«UNIVERSITY OF CALIFORNIA Santa Barbara Design and Characterization of Fibrillar Adhesives A Dissertation submitted in partial satisfaction of the ...»

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66. High FOM values were only possible with small retraction angles or large positive shear lengths, both of which caused large areas of contact on the flat face of the semicircular fibers. Contact with the top or curved face of the fiber further reduced the FOM from 0.66 because of low shear adhesion, low shear, or low force values for both.

By using angled testing instead of the common vertical testing, high adhesion forces, high shear forces, and high µ′ values have been shown to be simultaneously possible for the vertical fibers resulting in a large FOM. The optimal articulation for this involved a shallow approach angle causing contact with the flat face of the fiber, a non negative shear length, and a retraction angle close to vertical but not 90◦. Other synthetic adhesive will require individual characterization, but the advantages of angled testing for side contact fibers can be realized by applying the general principles presented here.

Although the focus of this study was the importance articulation plays in achieving maximal force values, durability is often a problem for synthetic adhesives. Typical durability results use a repeated test to show the performance of the adhesive over a given number of cycles. The highest durability tests have reached ten thousand cycles [19, 31] or more [83]. Although a quantitative study of durability was not performed, the adhesive survived over 8,300 testing cycles Chapter 6. Fiber Articulation

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Figure 6.15: The figure of merit (FOM) for an approach angle of 90◦ is calculated as the average of the normalized values (across all tests) of adhesion, shear, and µ′.

A score of 1 means that the articulation simultaneously had the highest values of adhesion, shear, and µ′ across all 1,521 articulations. High preload forces significantly reduced the FOM values. A maximum FOM equal to 0.64 was achieved when contact was made with the flat face at small retraction angles or large positive shear lengths. The FOM values for a 90◦ approach were less than those achieved with a 2.5◦ angled approach for most retraction angles and shear lengths.

Chapter 6. Fiber Articulation without visible damage or noticeable drops to the adhesion or shear forces.

The improvement in lifetime is significant when compared to previous testing of vertical semicircular fibers which survived 1,080 tests [109]. Both sets of fibers were only tested over the length of the experiment, not until decreased performance or failure of the fibers occurred. Future work aims to explicitly investigate the durability of the adhesive by using both vertical and angled testing procedures to compare how the articulation scheme influences durability.

Fiber clumping did not occur during testing because of the fiber-to-fiber distance of 25 µm. This suggests that the density of fibers can be increased. A higher fiber density should increase the forces the adhesive can support to reach the values achieved by other adhesives and close the gap relative to the forces a flat sample can support. Previous work using higher density angled semicircular fibers [19] showed the force values approaching those of the gecko and a similar fiber density could be tested with these fibers. Likewise, the angled testing procedure used here could be used on the higher density angled semicircular fibers to test for increased fiber performance with higher µ′ values. Fiber geometry and polymer material will also be investigated to further increase force values in future designs.

Chapter 6. Fiber Articulation

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The importance of articulation when testing dry adhesives has been experimentally investigated over a large range of approach angles, retraction angles, and shear lengths, a systematic study not previously performed. ABAQUS simulations modeling the approach predicted preload force dependence on approach angles, with certain articulations leading to significantly lower preload forces during attachment. The use of an angled approach, specifically the angled approaches shown in the simulations to reduce the preload forces on the adhesive, and retraction increased µ′ values to previously unachievable levels, even surpassing the gecko’s range. Adding in-plane motion after attachment to the articulation, retained high µ′ values and lead to nearly maximum shear adhesion and shear values for a larger range of approach and retraction angles. The maximum shear adhesion and shear values were very similar for both the vertical and angled testing methods used, further evidence of the advantages of angled testing methods for certain synthetic adhesives. The experiments and simulations suggest that approach angles resulting in a large bending component, instead of compression, are part of an optimal articulation scheme for the vertical fibers tested due to their ability to produce high shear adhesion forces, high shear forces, and high µ′ values.

Chapter 7 Conclusions and Future Work The advances made in gecko-inspired adhesives over the last decade have been substantial. Many designs have been experimentally characterized, each with their own strengths and weaknesses. None to date has achieved all the important properties at levels equal to or greater than the gecko. The results shown here focused on two main areas important in the development of synthetic adhesives, fiber geometry and fiber articulation. Fiber geometry was explored in two manners. Comparisons between two types of angled and vertical fibers have shown the advantages of tilted structures. Fiber shape was explored by fabricating semicircular fibers and comparing the force values when contacting the flat face and curved face of the fibers. Fiber articulation investigated a vertical and angled testing technique. Angled testing procedures were shown to have additional benefits when compared to vertical testing, yet they are not commonly used to characterize Chapter 7. Conclusions and Future Work synthetic adhesives. These results can be of use to others researching gecko-like adhesives.

Section 7.2.

1 discusses the important conclusion from the work presented in this dissertation. Section 7.2 offers suggestions for future work and is divided into two parts. The first part, Section 7.2.1, outlines adhesive improvements for more gecko-like behavior. The second part, Section 7.2.2, discusses necessary modifications for integration with climbing robots.

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The design of a flat-on-flat microtribometer with a millimeter-sized testing surface and the testing procedures performed were presented. Initial characterization on rectangular flap structures showed that fiber designs beyond symmetric circular fibers were possible alternatives for synthetic adhesives. They also revealed that the PDMS structures could support large deflections enabling contact with the side faces of the flap. Similar contact behavior resulted in anisotropic friction and adhesion when an angled lithography technique was used to create tilted rectangular flaps. Roughening the top surface of the flap resulted in a non-sticky default state, an interesting design feature for future designs.

Chapter 7. Conclusions and Future Work The angled lithography fabrication process achieved force anisotropy, but at the expense of a more complicated fabrication process and a small mold lifetime.

The creation of a simple fiber design with a long mold lifetime was achieved with a vertical semicircular fiber pattern in a silicon mold. The process was simple enough to be performed at most microfabrication facilities and was the first reported vertical anisotropic fiber design without tip modification or secondary material addition. The vertical semicircular fiber exhibited higher anisotropy in adhesion and shear than the vertical flaps and the shear anisotropy even outperformed the angled rectangular flaps.

Further increases in force values and force anisotropy were required to reach performance closer to the gecko. A higher density of fibers was combined with the anisotropic fiber shape and fiber tilt to achieve higher performance. The adhesion and shear force values reached higher levels than previous designs, with the shear pressure nearing the range of the gecko. Adhesion anisotropy was also higher than previous designs due to fiber density causing higher adhesion forces and fiber tilt reducing the preload forces during attachment. The lifetime of the adhesive was found to be greater than 10,000 cycles with 118% of the shear forces and 77% of the adhesion forces retained when compared to the initial values. Comparing the angled semicircular fibers to other high lifetime (10,000 cycles) adhesives, showed better performance characteristics in important properties.

Chapter 7. Conclusions and Future Work A final investigation using the vertical semicircular fiber was performed to determine what effect articulation, i.

e. how the fibers are moved during testing, had on the adhesive. Experiments on the gecko’s adhesive structures have revealed the importance of articulation, but synthetic adhesives had not been characterized in the same manner. Systematically varying the approach angle, shear length, and retraction angle showed that similar force values for shear and adhesion forces could be achieved using angled testing as with vertical testing, if a sufficient shear length was included in the test. Angled testing had the additional advantage of low preload forces for specific approach angles, resulting in µ′ values exceeding those of the gecko. The similar force values compared to vertical testing procedures and high µ′ values offered motivation for the use of non-vertical testing procedures for certain types of synthetic adhesives.

The results presented here have shown the importance of fiber geometry and fiber articulation. The important properties of the different adhesives can be seen and compared with the gecko in Table 7.1. Each design had one or two areas where it outperformed the other designs. The angled rectangular flaps had a nonsticky default state and the highest adhesion anisotropy. The angled semicircular fibers had the longest lifetime and highest forces. The vertical semicircular fibers had the highest µ′ value when using an angled testing procedure. Future designs, Chapter 7. Conclusions and Future Work discussed in the following section, will need to combine these different elements for improved adhesives.

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The progress made so far with fiber geometry and fiber articulation leaves many interesting opportunities for continued research. Although the adhesives developed show a large improvement in important parameters, they can’t yet rival the gecko in all measures. Improvements to the adhesive for better performance are clearly necessary for commercial and industrial applications. In the first subsection, improvement of gecko-like properties for synthetic adhesive will be suggested. The second subsection will discuss required development for integration with climbing robots. The work carried out in either of these topics has the potential to benefit both areas and the two subsections are not as separate as they may appear.

7.2.1 Gecko-like Properties The adhesives developed thus far have been single-level designs without any hierarchy. Tests on both the gecko and other adhesives have shown advantages of hierarchy such as increased compliance and adaptability to multiple scales of Chapter 7. Conclusions and Future Work

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Table 7.1: A comparison of the performance of the gecko and the gecko-inspired adhesives presented in this work.

Gecko setal arrays (a single seta for µ′ values) use a much smaller testing area for the comparison, contributing to the large pressure differences. The non-sticky default state and high adhesion anisotropy seen in the angled flaps are missing in the other adhesive designs. The angled semicircular fibers had a lifetime of over 10,000 cycles and the shear forces are close to the gecko. The vertical semicircular fibers when tested with an angled procedure had a µ′ value higher than the gecko.

Chapter 7. Conclusions and Future Work Figure 7.

1: The hierarchical design with nanofibers atop the angled flaps can be used in future designs to adapt to multiple scales of roughness and possibly self clean.

roughness [15]. Larger backing layer structures or smaller nanoscale fibers [81] can both be integrated with the current fabrication scheme allowing hierarchy to be incorporated into the adhesive. A hierarchical design, shown in Figure 7.1, uses angled rectangular flaps with nanofibers. The creation of these nanofibers is compatible with a PDMS and can be integrated with current and future designs.

The self-cleaning property of the gecko and synthetic adhesives has yet to be fully explored. A model has been developed [38] but cannot fully explain the mechanism. Experimental validation of self-cleaning [64] occurred only with Chapter 7. Conclusions and Future Work particles of a certain size using nano-sized fibers. For gecko-like behavior, the adhesives must self-clean all particle sizes likely to be encountered outside of a controlled environment. Future adhesive designs will need to test both different particle sizes and different types of contaminants in addition to fiber geometry and fiber articulation to fully develop an understanding of key parameters governing self-cleaning.

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