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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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6.2 Experimental Methods 6.2.1 Microfabrication The fibers were created using microfabrication and molding techniques similar to those described in [109]. To create the negative mold, photoresist was spin coated onto a clean silicon wafer. The fiber pattern was transferred onto the photoresist using projection lithography. After developing the photoresist to expose semicircular holes, the wafer was etched anisotropically using the Bosch process until a given etch depth was achieved. The photoresist was then removed from the silicon wafer using a stripping agent followed by an oxygen plasma. To facilitate separation after complete curing of PDMS, 1H,1H,2H,2Hperfluorodecyltrichlorosilane (FDTS) was vapor-deposited and baked onto the silicon wafer before molding.

The PDMS (Sylgard 184, Dow Corning, Midland, MI) was prepared by mixing a 1:5 ratio by weight of curing agent to base elastomer. The PDMS was first degassed to remove any pockets of air introduced during mixing and then poured onto the silicon mold. The wafer and polymer were then degassed together to fill any voids created in the mold during pouring. To cure the PDMS, the wafer and polymer were place in a convection oven for 15 minutes at 100 ◦ C. The polymer was then carefully separated from the silicon wafer by hand to produce Chapter 6. Fiber Articulation a reusable mold and a synthetic adhesive composed of semicircular fibers. A scanning electron microscope (SEM) image of the fibers can be seen in Figure 6.2.

6.2.2 Adhesion and Shear Testing Testing was performed using a home-built microtribometer, as previously described in Chapter 3 and [109], with the flat face of a 4 mm glass puck as the opposing surface (contact area = 0.126 cm2 ). The glass puck was measured with white-light interferometry (Wyko NT1100 Optical Profiling System) to find the surface characteristics. Over the area likely to contact the adhesive, a circular central area with a diameter of 3.9 mm, the puck was found to have a RMS roughness of ≈70 nm and a maximum height difference of 1.3 µm. Over the smoother central region of the puck, 3 mm diameter, the RMS roughness was ≈40 nm and the maximum height difference was ≈230 nm. The puck would be best described as having small amplitude waves over large areas of the surface.

Humidity and temperature measurements were taken every 5 minutes while tests were being performed. The controlled temperate ranged from 21.6 to 22.4 ◦ C with an average temperature and standard deviation of 21.9±0.2 ◦ C. The uncontrolled relative humidity ranged from 51.2 to 67.5% with the average and standard deviation being 62.9±2.8%.

Chapter 6. Fiber Articulation In the vLDP tests, the approach and retraction speed was 1 µm/s, the inplane shear speed was 3 µm/s, preloads varied between 60 and 180 mN, and shear lengths varied between −150 and 150 µm.

For the aLDP tests, a schematic of the test procedure is shown in Figure 6.1, the vertical component of the approach and retraction speed was 0.3 µm/s, the in-plane shear speed was 5 µm/s, the approach angle varied between 2.5 and 177.5◦, the approach depth, zapproach, was

6.5 µm, the shear lengths varied between −40 and 40 µm, and the retraction angle varied between 2.5 and 177.5◦. Testing parameters controlling the movements were set to be the same values for the vertical and angled tests; although the speeds achieved differed slightly between the two tests. The difference in vertical speed during approach and retraction is likely due to the additional horizontal motion when performing angled tests. Differences in force values for the horizontal speeds used in these tests are likely small [35]. It should also be noted that these speed magnitudes and differences are small when compared to the wide range of speeds used to characterize synthetic adhesives.

6.3 Results and Discussion The purpose of the experiments performed was not to highlight a new adhesive design, but to show how testing procedure can influence key force values of the Chapter 6. Fiber Articulation adhesive. Initial characterization of vertical semicircular fibers can be found in [109] or Chapter 4. However, the difference between these and previous vertical semicircular fibers are likely to generate interest in how well this adhesive performs relative to a flat surface of the same material. For this reason, the shear adhesion and shear forces against an unpatterned portion of the 1:5 curing agent to base elastomer are shown in Figures 6.4 and 6.5. Very little difference is seen in the adhesion forces, Figure 6.4, for the four preloads tested. Maximum shear adhesion values reached as high as 286±1 mN, an adhesion pressure of 23 kPa, and as low as 1 mN at and above 90 µm shear lengths in either direction. At these higher shear lengths the adhesive was not observed to reattach to the glass testing puck and no large scale stick-slip events were seen. The shear adhesion forces were roughly symmetric when equal shear lengths are applied in the positive and negative directions.

The shear forces, Figure 6.5, were also symmetric and, for all preloads, differ very little at shear lengths of 70 µm and below. At higher shear lengths, the shear forces separated from each other and each preload plateaued to its maximum value. The maximum shear forces were as high as 780 mN, a shear pressure of 62 kPa, for the 180 mN preload. The maximum shear force for the 60 mN preload was 640 mN. The maximum shear force for the other two preloads fell between these two values. The adhesion and shear forces of the unpatterned Chapter 6. Fiber Articulation Figure 6.





4: The maximum shear adhesion forces on an unpatterned PDMS sample were symmetric in both the positive and negative Y-directions for shear lengths between 0–190 µm and preloads between 60–180 mN using the vertical load-dragpull test. Across all shear lengths and preloads tested, a similar maximum adhesion value of 286±1 mN was achieved when there was no in-plane shear displacement.

The shear adhesion values are reduced with increasing shear displacements until a shear length of 90 µm is reached. At and above 90 µm shear lengths, there is almost no shear adhesion. Error bars show one standard deviation above and below the mean value based on 5 tests.

Chapter 6. Fiber Articulation PDMS were higher than the values of the fibrillar adhesive which will be shown later.

However, fibrillar adhesives offer significant advantages such as anisotropic attachment, lower sensitivity to roughness, self cleaning, and non-sticky default state.

The shear adhesion forces supported by the fibers during a vLDP test can be seen for preloads of 60, 100, 140, and 180 mN in Figure 6.6. Greater preloads compressed the fibers a larger amount, leading to higher shear adhesion and shear forces when sheared due to more contact along the side of the fibers. The shear lengths tested during each set of preloads varied from 0 to 150 µm in both the positive and negative directions perpendicular to the flat face of the fiber. Contact without shearing resulted in adhesion pressures of around 3 kPa. The pressure can be reduced to a minimum of 0.5 kPa for easy detachment at small negative shear lengths. Large positive shear lengths resulted in longer contact lengths along the side of the fibers until the maximum length had been reached. Depending on the preload, shear adhesion values reached maximum values for shear lengths between 20–50 µm, although values at shear lengths of 40 µm, the maximum shear length for aLDP tests, were quite similar to the maximum shear adhesion values for the same preload. The maximum shear adhesion force supported was 73.6±0.3 mN, a shear adhesion pressure of 6 kPa, and occurred at the maximum preload of 180 mN, µ′ =0.41, and shear length of 50 µm. The µ′ values at maximal adhesion Chapter 6. Fiber Articulation Figure 6.

5: The maximum shear forces on an unpatterned PDMS sample were symmetric in both the positive and negative Y-directions for shear lengths between 0–190 µm and preloads between 60–180 mN using the vertical load-drag-pull test.

For the shear lengths tested, the 180, 140, 100, and 60 mN preloads had maximum shear values of 775±1 mN, 729±1 mN, 682±0.4 mN, and 635±1 mN, respectively.

The plateaued shear force values depended on the preload applied and occurred at shear lengths between 70–90 µm. Error bars show one standard deviation above and below the mean value based on 5 tests.

Chapter 6. Fiber Articulation values across all preloads tested using a vLDP test fell between 0.

33–0.43 because of the high preloads necessary to compress the fibers.

Shear adhesion anisotropy, the ratio of forces in the positive direction to those in the negative direction, for the fibers is between 3 and 7.3, higher than the 3–4.9 anisotropy values seen with similarly shaped 1:10 curing agent to base elastomer ratio fibers [109]. The maximum shear adhesion values in the +Y-direction are approximately the same for the two different polymer mixtures, but the shear adhesion values in the −Y-direction values are smaller for the 1:5 curing agent to base elastomer ratio polymer. This behavior was predicted for semicircular fibers in [109, 19] since a higher modulus polymer should have a smaller contact width during pulloff, as was shown for an initially vertical cylinder contacting with a flat surface [71].

The shear forces supported by the fibers for the various preloads reached a maximum of 237±4 mN, a shear pressure of 19 kPa over the area of the glass puck, with a 180 mN preload as shown in Figure 6.7. Higher preloads, through increased compression of the fibers, again led to greater contact areas and therefore larger shear forces. This was in agreement with the current data which also shows increasing shear forces with larger preloads. The anisotropy ratio in shear forces in the +Y-direction to −Y-direction of 1.8–2.7 was slightly higher than the 1.8–

2.4 value obtained with 1:10 curing agent to base elastomer ratio fibers [109].

Chapter 6. Fiber Articulation Figure 6.

6: The maximum shear adhesion forces for shear lengths between 0– 150 µm and preloads between 60–180 mN using the vertical load-drag-pull test were less than the unpatterned samples. Across all shear lengths tested, the 180, 140, 100, and 60 mN preloads had maximum shear adhesion values of 73.6±0.3 mN, 60.2±1.7 mN, 33.1±1.2 mN, and 20.2±1.2 mN, respectively. The maximum µ′ values for the preloads tested ranged from 0.33–0.43 when using a vLDP test because of high preloads necessary for large areas of contact. Error bars show one standard deviation above and below the mean value based on 5 tests.

Chapter 6. Fiber Articulation At similar preloads, the curved face deformation for the higher modulus material should be less than the lower modulus due to its ability to resist the applied forces.

For this reason, there was a small difference with the upper values in the shear force anisotropy.

Tests on top contact fibrillar synthetic adhesives have been able to reach µ′ values up to 40 [25], although most works fall very short of this impressive value.

Angled side contact fibers have reached µ′ values as high as 13 [98] while vertical side contact fibers have only achieved µ′ values between 0.25 [101]–2 [31]. To increase µ′ values for side contact adhesives, by reducing the preload forces during attachment, an angled approach, instead of a vertical approach, was investigated experimentally with vertical fibers. By loading a fiber at an angle, the high compression forces during attachment should be able to be reduced for side contact adhesives. The approach and retraction angles tested were 2.5, 5, 10, 30, 50, 70, 90, 110, 130, 150, 170, 175, and 177.5◦, where angles less than 90◦ caused +Ydirection movement toward the flat face of the fiber and angles greater than 90◦ caused −Y-direction movement toward the curved face of the fiber as shown in Figure 6.1.

Using an angled approach and retraction, the adhesion forces supported by the adhesive without any additional shearing can be seen in Figure 6.8. Three different regions, high, middle and low adhesion values, corresponded to contacts with Chapter 6. Fiber Articulation Figure 6.

7: The maximum shear forces for shear lengths between 0–150 µm and preloads between 60–180 mN using the vertical load-drag-pull test were less than the unpatterned samples. For the shear lengths tested, the 180, 140, 100, and 60 mN preloads had maximum shear values of 237±4 mN, 163.6±3.1 mN, 86.2±2.5 mN, and 49.1±0.7 mN, respectively. The highest shear force values occurred when sufficient shear lengths had created large areas of contact with the flat face of the fiber. Error bars show one standard deviation above and below the mean value based on 5 tests.

Chapter 6. Fiber Articulation different parts of the fiber.



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