FREE ELECTRONIC LIBRARY - Dissertations, online materials

Pages:     | 1 |   ...   | 9 | 10 || 12 | 13 |   ...   | 17 |

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

-- [ Page 11 ] --

The lateral friction force that may be generated by a synthetic bio-inspired adhesive is very important for applications requiring wall climbing. The adhesive material fabricated generates large and anisotropic in-plane shear forces during sliding along the ±Y-axis (Figure 5.5) for all three preloads tested. The anisotropy in shear forces are 2.0, 1.6, and 1.4 for preloads of 0.025 N, 0.1 N, and 0.18 N respectively. The shear force generated increased with sliding distance until it saturated in a manner similar to that seen with other tests on gecko setal arrays [120], and a large maximum shear pressure of 78±0.1 kPa was supported for a preload of 0.18 N after sliding in the +Y-direction with the tilt of the fibers. Shear force values for the natural system reported in the literature vary, and range from 88 kPa from measurements taken on two whole gecko feet of area = 227 mm2 Chapter 5. Angled Semicircular Fibers Figure 5.5: Shear force along the ±Y-axis as functions of the preload and shear length. The synthetic adhesive, tested over a 12.6 mm2 patch area, was able to sustain a maximum shear pressure of 78±0.1 kPa, approaching the 88–226 kPa range for natural gecko feet and digits. All data points depict average values with ±1 standard deviation error for 6 repeated tests.

[45], 226 kPa from measurements on a single gecko digit of area = 19 mm2 [5, 38], 184±23 kPa from measurements on isolated setal arrays of area = 0.93 mm2 [34], 550 kPa from measurements on setal arrays of total area = 0.54 mm2 [66], up to 4585 kPa from measurements on a single seta of area = 43.6 µm2 [5, 10]. The adhesive in the current study, tested over an area of 12.6 mm2, thus generates a shear pressure approaching the 88–226 kPa range reported for gecko toes and feet.

Chapter 5. Angled Semicircular Fibers

A general equation describing the friction/shear force, Ff, between two adhesive surfaces is [75]:

–  –  –

Where τ is the interfacial shear strength, and Areal is the real area of contact between two surfaces. For non-adhering surfaces, Ff is a linear function of the load, L, and can take the common form Ff = µL, where µ is the coefficient of friction. The friction force is also influenced by the adhesion force between the adhesive material and the substrate. For adhering surfaces, Equation 5.4 remains the same; however, the friction force is no longer a linear function of the load because of the adhesive component of the load. The above equation also demonstrates that a flat surface in contact with a flat substrate is expected to generate higher friction forces than a curved surface against the same substrate.

As is the case with adhesion anisotropy, it is important to note that this approach can be extended to higher modulus materials as well - since the contact area on the curved face is expected to be lower with higher modulus, it should be possible to achieve higher shear force anisotropy ratios.

For PDMS in contact with glass, the interfacial shear strength τ has been experimentally estimated to be ≈0.3 MPa [82]. For a friction/shear force of 0.66 N, as obtained for sliding in the +Y-direction with an initial preload of 0.1 N (Figure 5.5), Equation 5.4 thus predicts a real area of contact of 2.2 mm2. Sliding Chapter 5. Angled Semicircular Fibers in the −Y-direction for the same initial preload resulted in a shear force of 0.41 N, suggesting a real area of contact of 1.4 mm2. However, analysis of the optical microscope images in Figures 5.4(b) and 5.4(c) shows an area of contact of ≈6 mm2 after sliding in the +Y-direction, and ≈1.7 mm2 after sliding in the −Ydirection. A major reason for this discrepancy could be the fact that these optical microscope images were taken at the conclusion of sliding, and just before normal pull-off, whereas the shear forces plotted in Figure 5.5 were measured during sliding. Although sliding was smooth without any macroscopic stick-slip, considering that the sliding/shear lengths for the steady-state friction values in Figure 5.5 far exceeded the dimensions of the fibers, it is possible that random small amplitude stick-slip events of some fraction of the fibers occurred, and thus, despite individual fibers possibly experiencing sliding irregularities, their collective motion is one of smooth sliding. Smooth sliding with stochastic stick-slip of a population of individual fibers has also been previously suggested for tests on gecko setae [35], and optically observed for tests on both PDMS angled wedge-shaped microfibers [35] and polyvinylsiloxane (PVS) vertical mushroom-tipped microfibers [114]. For the PVS microfibers, a significantly lower area of contact was observed during sliding than in the static case. A smaller reduction in real area of contact between polymethylmethacrylate (PMMA) blocks in relative sliding motion with stick-slip characteristics was also observed at the onset of slipping [91, 90]. A reduction in Chapter 5. Angled Semicircular Fibers area of contact during the slip event has also been reported for dissimilar metals in sliding contact [17]. Thus, Figures 5.4(b) and 5.4(c), taken after cessation of sliding, would be expected to only provide an upper bound on the contact area during sliding, although the decrease in contact area may be expected to be less drastic for tilted half-cylinder microfibers than for the vertical mushroom-tipped microfibers in [114]. The test apparatus has since been modified to be able to include continuous video capture of all stages of load-drag-pull testing to verify the behavior of the fibers during both lateral sliding and normal pull-off.

Durability is an important concern for synthetic adhesives for most practical applications in climbing robotics and industry. Several synthetic adhesives capable of supporting large adhesion and/or shear forces have been reported in the literature, but either no extended durability testing was done [27, 117, 109, 110], or they exhibited very limited lifetime [50, 61, 89], or no systematic study into durability was performed beyond just 50–100 cycles [47, 78, 56, 13]. In [50], intermediate cleaning of the test probe was required to regain the maximum value of the friction force after just 8 test cycles. In this study, an adhesive has been fabricated that demonstrates very high durability when tested over 10,000 continuous cycles.

10,000 repeated load-drag-pull tests were performed, all at the maximum preload of 0.18 N and maximum sliding distance in the +Y-direction of 0.2 mm, Chapter 5. Angled Semicircular Fibers sufficient for the adhesive to attain large shear adhesion and maximum shear force values. Adhesion and friction forces were recorded in each test, and the results are plotted in Figure 5.6, showing a 23% decrease in adhesion after shearing and an 18% increase in shear force after 10,000 cycles without intermediate cleaning of the glass puck or sample. Although it could be argued that the increase in shear force supported with repeated use is beneficial, the decrease in normal adhesion could be problematic in applications that require secure attachment to both vertical and inverted surfaces. The adhesive material was inspected in an SEM after durability/endurance testing, and no significant damage to individual fibers, or any inter-fiber self-adhesion, was observed over the entire test area, as seen in Figure 5.7. Even though tests were performed in ambient condition, no fouling due to dust particles was observed after two weeks of testing.

The high value of µ′ (= 4.7) observed for this adhesive could have played an important part in enhancing reusability by preventing damage to the fibers during preloading. PDMS being an elastomer with a low Youngs modulus of 1.8 MPa, but with a large elongation at break of 160% and toughness of 4.77 MJ/m3 [22], durability is also a result of the materials ability to bend and deform without fracturing or tearing. Further, as stated earlier in this section, no macroscale stickslip was observed during sliding. Macroscale stick-slip is typically an undesirable behavior that often results in the damage and wear of materials in sliding systems Chapter 5. Angled Semicircular Fibers Figure 5.6: Repeatability tests for shear adhesion and shear forces at a preload of

0.18 N and a shear length of 0.2 mm over 10,000 continuous testing cycles without intermediate cleaning. The adhesive retained 77% of its initial adhesion while shear force increased by 18% from the initial value.

Chapter 5. Angled Semicircular Fibers [46].

As suggested in [35] to explain the high wear resistance of both natural and synthetic fibrillar adhesives, the absence of macroscale stick-slip with just uncorrelated random microscale stick-slip events could explain the durability of the adhesive in this study. The slight decrease in adhesion could be associated with modification of the surface properties of PDMS and the glass puck due to material transfer over a repeated contact [58]. Others, [66], have reported an increase in shear force generated by vertical polypropylene microfiber arrays over repeated test cycles, and suggested that this could be due to increased side contact between the fibers and the substrate due to angling of some of the fibers with use. However, the angling of the polypropylene fibers was reported to be non-permanent, with the fibers recovering to a near-vertical state within several hours of unloading.

In this study, SEM examination of the PDMS fibers after durability testing was performed after a lapse of one week. Although no permanent change in tilt angle was observed, a mechanism similar to the one reported in [66] could be responsible for the small increase in shear force supported over 10,000 test cycles.

The durability of the adhesive in this study is compared with adhesives from two other references that were also tested over 10,000 cycles or more [83, 31] in Table 5.1. While the percentage change in adhesion and shear pressures are greater than those observed for the PDMS wedge shaped adhesive in [83], the material in this study generates 182% the shear adhesion pressure, 312% the Chapter 5. Angled Semicircular Fibers Figure 5.7: No damage to individual fibers, or inter-fiber self-adhesion, was observed in SEM images of the sample test area after durability testing.

Chapter 5. Angled Semicircular Fibers shear pressure, and 224% the value of µ′ reported in [83].

Also, the wedge shaped adhesive material showed no anisotropy in adhesion or friction behavior when sheared towards and against the wedge tilt angle despite structural anisotropy.

In [31], large adhesion and shear pressures were supported by both polypropylene (PP) and HDPE fiber adhesives. However, tests were performed with all fiber samples mounted in a loop-against-flat configuration to increase compliance. For a similar PP fibrillar material, this configuration was found to yield a 3 to 4 fold increase in maximum shear stress generated when compared with a previously used flat-on-flat configuration [66]. Thus, a direct comparison with the adhesive in the current study, tested in a flat-on-flat configuration, is not straightforward.

However, despite the increased compliance of the loop, the value of µ′ for HDPE fibers was 49% lower than the current study. Large losses in adhesion and shear forces were observed over 10,000 test cycles, with significant wear seen after just 300 cycles. For the PP fiber adhesive, a complete loss of adhesion was reported, along with poor repeated shear force performance. Further, the value of µ′ for PP fibers was very low at 0.2.

Since no inter-fiber self-adhesion was observed after 10,000 cycles in this study, it is therefore possible to increase the fiber areal density in future designs to further increase both adhesion and friction forces without compromising fiber integrity. The current microfiber density is 5,128 mm−2, chosen as a conservative Chapter 5. Angled Semicircular Fibers

–  –  –

Table 5.1: A comparison of the performance of gecko-inspired adhesives from the current study and from references [83] and [31].

In [31], the durability was worse than the current study for both polypropylene (PP) and high-density polyethylene (HDPE) fibers. While higher shear pressures were reported for both HDPE and PP fiber loops, this was at significantly lower values of µ′ than the current study.

Higher durability was reported in [83] than in both [31] and the current study, but maximum adhesion, shear and µ′ values were all lower than in the current study.

Chapter 5. Angled Semicircular Fibers first iteration, and significantly less than 14,400 mm−2 for gecko setae [40, 12].

However, the durability of the synthetic adhesive in this study is still lower than that of natural gecko setae as reported in [35], where a 25% increase in adhesion and just 5% decrease in shear force were observed after 30,000 load-drag-pull tests.

In addition to its effect on adhesion and friction anisotropy, the use of polymers of varying elastic moduli in future designs will be studied for its effect on, and to maximize, adhesive durability.

5.4 Conclusion Large arrays of half-cylinder tilted PDMS microfibers have been reliably fabricated at low unit cost over cm-scale patch sizes using an angled photolithography and molding process to create a gecko-inspired synthetic dry adhesive that displays high and anisotropic friction and adhesion forces, a high ratio of normal adhesion to initial preload, and very high durability when tested over 10,000 continuous cycles. The normal adhesion forces obtained experimentally are close to theoretical approximations from the Kendall peel model, quantitatively demonstrating that both fiber anisotropic shape and tilt angle are crucial in obtaining highly anisotropic adhesion after lateral sliding with synthetic adhesives, thereby offering clear guidelines for future design of adhesives with even higher anisotropy. The Chapter 5. Angled Semicircular Fibers lack of wear or inter-fiber self-adhesion makes this adhesive a good candidate for future use in supporting large payloads in high-cycle applications such as robotics, industrial grippers, and mobile sensor platforms. Ongoing work is aimed at further increasing the forces obtained with higher fiber areal density designs, fully characterizing the performance of the adhesive by varying the material modulus, and incorporating nano-scale structural hierarchy to be able to conform to several scales of roughness like its natural counterpart.

Pages:     | 1 |   ...   | 9 | 10 || 12 | 13 |   ...   | 17 |

Similar works:

«2013 NHB Set A Bee Round 1 BEE ROUND 1 1. Jacques-Benigne Bossuet (BUH-sway) was the foremost French exponent of this doctrine. Robert Filmer's defense of this idea was demolished by the argument that Adam only has one true heir in John Locke's first Treatises of Government. The Stuart Dynasty's belief in this idea provoked Parliamentary resistance in the English Civil War and Glorious Revolution. For the point, name this philosophy which held that a monarch received his authority to rule from...»

«“Heidegger and Marcuse: On Reification and Concrete Philosophy,” The Bloomsbury Companion to Heidegger,” F. Raffoul and E. Nelson, eds., Bloomsbury Press, 2013, pp. 171-176 Andrew Feenberg Introduction Herbert Marcuse (1898–1979) completed his doctorate in 1922 but decided not to pursue the habilitation which would have qualified him for an academic career. Instead, he returned to Berlin where he established an antiquarian bookstore with a partner. When he read Being and Time shortly...»

«ALGORITHMS AND METHODOLOGY TO DESIGN ASYNCHRONOUS CIRCUITS USING SYNCHRONOUS CAD TOOLS AND FLOWS by Vikas S. Vij A dissertation submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Electrical and Computer Engineering The University of Utah December 2013 Copyright c Vikas S. Vij 2013 All Rights Reserved The University of Utah Graduate School STATEMENT OF DISSERTATION APPROVAL Vikas S. Vij This...»

«IMAGINING THE DECOLONIAL SPIRIT: Ecowomanist Literature and Criticism in the Chinese Diaspora A DISSERATION SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY Xiumei Pu IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Edén Torres, Adviser July 2013 ©Xiumei Pu 2013 Acknowledgements My sincere thanks go to my committee. Your insightful feedback and continuous support have made my project possible. Thank you to the Department of...»

«379 yva/j M0e 3mg AMBIGUITY OF LOSS, ANTICIPATORY GRIEF, AND BOUNDARY AMBIGUITY IN CAREGIVER SPOUSES AND PARENTS DISSERTATION Presented to the Graduate Council of the University of North Texas in Partial Fulfillment of the Requirements For the Degree of DOCTOR OF PHILOSOPHY by Jan K. Rider, B.F.A., M.S. Denton, Texas August, 1993 yva/j M0e 3mg AMBIGUITY OF LOSS, ANTICIPATORY GRIEF, AND BOUNDARY AMBIGUITY IN CAREGIVER SPOUSES AND PARENTS DISSERTATION Presented to the Graduate Council of the...»

«From Greenhouse to Icehouse: Understanding Earth’s Climate Extremes Through Models and Proxies Clay Richard Tabor A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Geology) in the University of Michigan Doctoral Committee: Professor Chris J. Poulsen, Chair Associate Professor Jeremy N. Bassis Associate Professor Brian K. Arbic Senior Scientist David Pollard Acknowledgements Foremost, I would like to thank my advisor Chris Poulsen for...»

«360 BC SOPHIST by Plato translated by Benjamin Jowett SOPHIST PERSONS OF THE DIALOGUE: THEODORUS; THEAETETUS; SOCRATES An ELEATIC. STRANGER, whom Theodorus and Theaetetus bring with them. The younger SOCRATES, who is a silent auditor. Theodorus. Here we are, Socrates, true to our agreement of yesterday; and we bring with us a stranger from Elea, who is a disciple of Parmenides and Zeno, and a true philosopher. Socrates. Is he not rather a god, Theodorus, who comes to us in the disguise of a...»

«ANALYTICAL ESTIMATION OF THROUGHPUT DISTRIBUTION FOR SERIAL MANUFACTURING SYSTEMS WITH MULTI-STATE MACHINES AND ITS APPLICATION by Saumil Mahendra Ambani A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Mechanical Engineering) in The University of Michigan 2011 Doctoral Committee: Professor Jun Ni, Co-Chair Assistant Research Scientist Lin Li, Co-Chair Professor Kazuhiro Saitou Associate Professor Amy Ellen Mainville Cohn ⃝ c Saumil...»

«TEMPLATE BASED ASYNCHRONOUS DESIGN By Recep Ozgur Ozdag A Dissertation Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (ELECTRICAL ENGINEERING) November 2003 Copyright 2003 Recep Ozgur Ozdag Contents List of Tables v List of Figures vi Abstract ix 1. Introduction 1.1 Asynchronous Circuit Design Flow 1.2 Expected Contributions of the Thesis 1.3 Thesis Organization 2. Background 2.1...»

«Group Information Behavioural Norms and the Effective Use of a Collaborative Information System: A Case Study by Colin David Furness A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Faculty of Information University of Toronto © Copyright by Colin David Furness 2010 ii Group Information Behavioural Norms and the Effective Use of a Collaborative Information System: A Case Study Colin David Furness Doctor of Philosophy Faculty of Information...»

«VERIFIABLE SEARCHABLE SYMMETRIC ENCRYPTION BY ZACHARY A. KISSEL B.S. MERRIMACK COLLEGE (2005) M.S. NORTHEASTERN UNIVERSITY (2007) SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY COMPUTER SCIENCE UNIVERSITY OF MASSACHUSETTES LOWELL Signature of Author: Date: Signature of Dissertation Chair: Dr. Jie Wang Signatures of Other Dissertation Committee Members Committee Member Signature: Dr. Xinwen Fu Committee Member Signature: Dr. Tingjian Ge Committee...»

«Model Studies of Proposed Intermediates in Homogeneous Gold(I) Catalysis by Timothy Justin Brown Department of Chemistry Duke University Date:_Approved: _ Ross A. Widenhoefer, Supervisor _ Katherine J. Franz _ Jiyong Hong _ Qiu Wang Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry in the Graduate School of Duke University i v ABSTRACT Model Studies of Proposed Intermediates in Homogeneous Gold(I) Catalysis by...»

<<  HOME   |    CONTACTS
2016 www.dissertation.xlibx.info - Dissertations, online materials

Materials of this site are available for review, all rights belong to their respective owners.
If you do not agree with the fact that your material is placed on this site, please, email us, we will within 1-2 business days delete him.