«UNIVERSITY OF CALIFORNIA Santa Barbara Design and Characterization of Fibrillar Adhesives A Dissertation submitted in partial satisfaction of the ...»
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 , 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.
, 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 , 550 kPa from measurements on setal arrays of total area = 0.54 mm2 , 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 :
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 . 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 , and optically observed for tests on both PDMS angled wedge-shaped microﬁbers  and polyvinylsiloxane (PVS) vertical mushroom-tipped microﬁbers . 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 . 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 . 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 , 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 , 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 .
As suggested in  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 . Others, , 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  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 , 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 .
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 , 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 . 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  and .
In , 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  than in both  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 , 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.