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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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While the fabrication process used to create the angled flaps previously discussed in Section 3.3 is general enough to create a variety of molded angled strucChapter 4. Vertical Semicircular Fibers tures, the durability of the photoresist mold does not allow a high modulus adhesive material to be repeatably molded. For the PDMS angled flaps, the molds were found to survive 15–20 casting cycles. In contrast, the durability of silicon molds with PDMS is practically infinite and does not suffer from the same limitations on elastic moduli. Further, the angled flaps can reach only as high as ≈11 µm due to the fabrication method, limiting the compliance of the flaps. This height restriction could be especially restrictive in future applications focusing on longer lifetime and less fouling with stiffer polymers.

Many complex methods exist for the fabrication of angled fibers and not all fabrication facilities have the capability for many of the methods in the literature. The creation of the vertical half cylinder micro-fibers, however is simple and general enough for most fabrication locations. In this study, large arrays of vertical 10 µm diameter half-cylinder micro-fibers (Figure 4.1) have been fabricated using standard micro-electromechanical systems (MEMS) fabrication techniques.

Highly durable silicon negative molds were created with large patch sizes, up to a full 4-inch diameter silicon wafer, being possible using this technique. The vertical PDMS micro-fibers created from these silicon molds were found to exhibit gecko-like shear adhesion in a manner similar to the pre-angled micro-flaps, with the added advantages of higher adhesion pressure values for the same preloads and increased anisotropy in friction forces.

Chapter 4. Vertical Semicircular Fibers 4.

2 Experimental Methods 4.2.1 Microfabrication In order to fabricate the vertical polymer micro-fibers, an approximately 1.0 µm thick layer of image reversal photoresist was spin coated over a single crystal silicon wafer. The lateral dimensions of the semicircular fibers were defined by lithography in an i-line stepper. Fibers were spaced 25 µm apart in a hexagonal pattern to avoid self-adhering. The wafer was then etched up to a depth of 19 µm using the Bosch Deep Reactive Ion Etching (DRIE) process. For easy separation of the mold and polymer, 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) was vapor deposited and baked onto the silicon wafer before molding. PDMS, mixed in a 1:10 ratio by weight of curing agent to base elastomer, was poured over the negative mold and then was degassed. The mold and PDMS were cured at 100 ◦ C for 10 minutes in a convection oven to fully cure the PDMS with a backing thickness of approximately 2.5 mm. The two materials were then separated by hand, resulting in cm-scale PDMS micro-flaps samples and a reusable silicon mold.

4.2.2 Adhesion and Friction Testing Movement of the sample was generated by two motorized linear stages with incremental movements of 60 nm in the vertical direction and 10 nm in the horiChapter 4. Vertical Semicircular Fibers

–  –  –

Figure 4.1: Scanning electron microscope (SEM) images of vertical half-cylinder PDMS micro-fibers of 19.

0 µm height and 10.0 µm diameter. Articulation of the adhesive system to engage and release occurs in the Y-Z plane. (a) In-plane shear displacement along the Y-axis causes contact between the glass testing surface and the semicircular fibers on the curved or flat side of fiber. The fiber outline is shown in white. (b) Shearing against the flat face results in a higher areal contact for greater adhesion and friction. (c) Shearing against the curved face results in a smaller areal contact for less adhesion and friction.

Chapter 4. Vertical Semicircular Fibers zontal direction.

Force sensing was provided by a load cell with a force resolution of approximately 0.5 mN. A rotational stage was used to align the direction of in-plane motion perpendicular to the flat face of the fiber. Two goniometers with a sensitivity of 1–2 arc seconds oriented the sample parallel to the plane of movement in order to maintain a constant vertical distance during shearing. A three-point leveling mechanism allowed the glass puck to be aligned to the sample, ensuring even force distribution. The sample was viewed during testing by looking through the glass puck onto the top of the sample using a high magnication zoom lens with an attached CCD camera. Control and data acquisition software was all written in LabVIEW. Temperature measurements taken every five minutes during testing showed an average room temperature of 26.4◦ C and a standard deviation of 0.4◦ C. Humidity measurements were not taken during the testing period, however, later measurements taken over a 1 month period showed relative humidity values between 20–60%.

4.3 Results and Discussion The fibers exhibited adhesion even without shearing due to the availability of the approximately flat top of each fiber for contact with the substrate (Figure 4.2). The adhesion force of the fibers increased with preload due to more fibers Chapter 4. Vertical Semicircular Fibers 0.05

–  –  –

0.03 0.02 0.01

–  –  –

Figure 4.2: Pure adhesion forces of the vertical half-cylinder micro-fibers as a function of the preload force.





Points plotted in all figures represent the average values observed over 5 tests in the same sample region, and the error bars represent plus and minus one standard deviation.

coming into close contact with the flat glass substrate. At a preload of 0.13 N, the adhesion force decreased due to fiber buckling leading to smaller contact areas.

The maximum adhesive pressure observed without shearing was 3.6±0.3 kPa.

The vertical fibers exhibited anisotropic shear adhesion and shear forces in the +Y- (towards flat face) versus the −Y- (towards curved face) direction (Figures

4.3 and 4.4). The shear adhesion force dropped rapidly in both +Y- and −Ydirections for small shear lengths as the real contact area decreased from the flat top face of the fibers to a line contact along the top edge. Further shearing resulted Chapter 4. Vertical Semicircular Fibers in an increase in shear adhesion and shear forces for all preloads and both shear directions, due to larger lateral contact area per fiber. At high preloads, increased fiber deflection enabled a larger contact area for higher shear and shear adhesion force values.

The shear adhesion forces reached a maximum pressure of 5.6±0.1 kPa, considerably higher than the adhesion value without shear. For preloads of 20, 60, and 100 mN, the average steady state shear adhesion value after shearing in the +Y-direction was respectively 4.1±1.7, 4.9±1.1, and 2.9±0.2 times the steady state value after shearing in the −Y-direction. Shearing in the +Y-direction resulted in a larger contact area along the flat face of the half cylinder fibers (Figure 4.1(b)) than along the curved surface in the −Y-direction (Figure 4.1(c)). The adhesion value without in-plane movement fell in between the positive and negative direction shear adhesion values. This anisotropy based on direction of shear demonstrated the high controllability and reversibility of the adhesive system.

Such a high anisotropy in the shear adhesion force using a system with simple vertical fibers had not been reported in the literature prior to this study.

The measured anisotropy was close to the value of 1.8 calculated using the Kendall peel theory represented by Equation (4.4). Differences between the calculated and experimental values could be caused by the assumption that the flat and curved faces detach at the same angle. The lack of bending energy and the Chapter 4. Vertical Semicircular Fibers

–  –  –

Figure 4.3: Shear adhesion forces along the ±Y-axis as functions of the preload and shear length.

Shear adhesion pressure reached a maximum pressure of 5.6±0.1 kPa and the shear adhesion forces were anisotropic for all preloads tested. Points plotted in all figures represent the average values observed over 5 tests in the same sample region, and the error bars represent plus and minus one standard deviation.

Chapter 4. Vertical Semicircular Fibers fibers moving before being separated could also account for the differences.

Values in the literature for the elastic modulus of PDMS and work of adhesion between PDMS and glass vary and those chosen here might not perfectly match the system. The model also assumes a constant tape thickness whereas the fibers do not have this property. A tape thickness equal to twice the distance from the flat face to the centroid of the fiber was chosen as an equivalent tape thickness.

For the preloads of 20, 60, and 100 mN, the peel angles fitting the experimental results were 85, 75, and 28◦, respectively. The theoretical forces calculated for the shear adhesion against the flat and curved sides were 26 mN and 12 mN for the 20 mN preload, 30 mN and 14 mN for the 60 mN preload, and 70 mN and 32 mN for the 100 mN preload. These values can be compared with the experimental values in Table 4.1.

The agreement between the theory and experiments was seen to also extend to the force values. On the flat face, where the contact width is known, the values are almost the same for the higher preloads. On the curved face, where the contact width is estimated, the theoretical values across all preloads are within 300% of those measured. It is not surprising that at lower preloads, where sufficient contact is least likely to form, that the difference between the theoretical and experimental values are the largest.

Chapter 4. Vertical Semicircular Fibers

–  –  –

20 26 12 12 3.5 Table 4.1: Comparison using the Kendall peel model between the theoretical and experimental adhesion forces predicted for the flat and curved faces of the semicircular fibers.

The shear force, reaching a maximum shear pressure of 12.6±0.4 kPa, along the Y-axis was also anisotropic (Figure 4.4), crucial for repeated sticking and peeling on vertical surfaces. Similar behavior has also been found in friction tests on a gecko setal arrays [120]. For preloads of 20, 60, and 100 mN, the ratio of shear forces in the +Y-direction to the −Y-direction was 1.8±0.2, 2.3±0.2, and

2.4±0.1. These are higher than the anisotropy ratio of 1.7 for SFA tests [117] and 1.6 for Bio-F tests at 100 mN preload previously reported with angled microaps having similar dimensions [110]. It is important to note that very little shear force anisotropy (ratio = 1.1) was seen for symmetric rectangular vertical micro-flaps displaced along the ±Y-axes alone [110]. A simple change to the fiber geometry of these symmetric rectangular vertical micro-flaps fibers was used to Chapter 4. Vertical Semicircular Fibers

–  –  –

Figure 4.4: Shear forces along the ±Y-axis as functions of the preload and shear length.

The shear pressure reached a maximum of 12.6±0.4 kPa and shear forces were also anisotropic for all preloads tested. Points plotted in all figures represent the average values observed over 5 tests in the same sample region, and the error bars represent plus and minus one standard deviation.

gain anisotropy in shear and shear adhesion forces as shown here with the results from semicircular fibers. It is important to note that this approach could thus be combined with both vertical and angled structures that currently use purely rectangular or cylindrical designs to further increase force anisotropy.

Durability of adhesives is often a substantial concern due to the ability of polymer fibers to become fouled or self-adhere. A systematic fatigue study was not performed not performed on the vertical semicircular fibers, but the adhesive Chapter 4. Vertical Semicircular Fibers survived over 1,080 tests in order to collect the data shown in Figures 4.2, 4.3, and 4.4. The average percentage change between the first and last test at 100 mN preload for the 20 largest shear lengths was 5.6%. When taken with the small error bars and level saturation values, it can be seen that significant changes in forces were not occurring throughout the testing life. Some damage to the adhesive was seen over small areas from airborne particles during testing causing pillars to become attached to the substrate and each other. In load-drag-pull tests with preloads greater than 200 mN (results not presented) gradual reduction in supported forces was seen due to fibers self-adhering to the substrate.

For comparison to the semicircular fibers, testing was also performed on an unpatterned portion of the same piece of PDMS. The flat PDMS showed higher values for adhesion forces, with adhesion pressures around 21 kPa being observed.

This high adhesion pressure should not be surprising since the fibers are spaced far apart to avoid self matting and the fiber tops cover only ≈7% of the available area. The 3.6 kPa adhesion pressure using the semicircular fibers is surprisingly high when compared to the 1.5 kPa pressure expected using the adhesive pressure from the unpatterned PDMS and fiber density from the semicircular fibers. The higher compliance of the fibers likely allowed better conformation to the small scale roughness of the glass puck and resulted in higher than expected forces.

Chapter 4. Vertical Semicircular Fibers Figure 4.

5: Adhesion forces of a flat PDMS control sample as a function of the preload force shows very little dependence on preload. Points plotted represent the average values observed over 5 tests in the same sample region, and the error bars represent plus and minus one standard deviation.

Chapter 4. Vertical Semicircular Fibers The load-drag-pull tests on flat PDMS unsurprisingly showed higher values for both shear adhesion and shear forces.



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