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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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Simply placing the adhesive on top of the glass slide resulted in a non-permanent interface with the possibility for adhesive sample movement. To eliminate any movement between the glass slide and adhesive (PDMS), they were semi-permanently bonded using a corona treater. The oxygen plasma from the corona treater, changes the surface properties of the PDMS and glass, causing silanol groups (– OH) to form. Extended contact of the glass and PDMS causes the silanol groups to condense forming Si-O-Si bonds. The Si-O-Si bonds are covalent in nature and form permanent bonds causing failure of the bulk PDMS when the surfaces are separated [20]. Using spacers on the sides of the glass slide and adhesive to protect them from fouling particles underneath, the adhesive and the glass slide side were placed bonding side up on a non-conducting surface. A hand-held corona treater Chapter 3. Friction and Adhesion Tester was passed over the bonding surfaces at a height of 1/4 inch for 5–15 seconds. The surfaces were aligned and lightly pressed together. Repositioning of the sample should be performed as quickly as possible, within 5 minutes. When good contact without misalignment or bubbles between the surfaces was achieved, the sample was left for an hour to fully bond.

The glass puck testing surface was also cleaned before testing. The glass puck testing surface was submerged in acetone for 3 minutes, isopropyl alcohol for 3 minutes, and then deionized water for 3 minutes. A final submersion in ethanol was performed as was done in other testing between glass and PDMS [58]. The glass puck was dried with compressed air before testing was performed.

3.2.2 Alignment Before testing could commence, the adhesive sample and glass puck had to be aligned. Initial alignment does not have the glass puck holder installed for easier visualization. A 50x objective with a depth of focus of 0.9 µm was installed for the alignment process. The glass slide with the adhesive sample was placed onto the mounting plate with the anisotropic fiber direction roughly aligned to the direction of movement of the motorized in-plane linear stage. The mounting screws were slowly tightened onto the glass slide without large movements to the sample. The adhesive sample was roughly positioned under the microscope using the in-plane Chapter 3. Friction and Adhesion Tester stages. Large rotation adjustments should not be needed, but adjustments could be made by hand on the Z-axis rotation stage. Z-axis alignment was performed by following a column of fibers from one edge of the adhesive sample to the other. Rotational adjustments were made as needed with a micrometer allowing an alignment within a few arc minutes.

The plane of the sample then needs to be made parallel to the plane of movement to avoid unevenly loading the adhesive during in-plane shear displacements.

The general leveling procedure was carried in the following order: X-axis (less important axis) rough alignment, Y-axis (more important axis) rough alignment, X-axis fine alignment, and Y-axis fine alignment. Rough alignment used the microscope objective and visual feedback from the CCD camera to find height differences between the two edges of the adhesive. Corrections were made by adjusting the goniometers for the relevant axis. Fine alignment used a LabVIEW program to incrementally move the sample vertically and take a picture at each height level. The pictures were then analyzed in MATLAB using an autofocusing program and a variety of techniques [86]. Adjustments were made using the goniometers based on the height differences between in-focus images at the two ends of the adhesive. When both axes where aligned, a final check was performed at the center of the sample to ensure a flat adhesive testing plane.

Chapter 3. Friction and Adhesion Tester The final alignment step involved aligning the glass puck to the adhesive surface, and therefore the X- and Y-axes.

Before alignment was performed, the 50x microscope objective was removed and replaced with a 5x objective. The smaller magnification allowed the entire glass puck surface to be viewed when testing.

The glass puck holder with the glass puck was installed for leveling. A LabVIEW program moved the adhesive into contact with the glass puck and then separated the two surfaces. Contact between the two surfaces could be seen with the CCD camera when large enough areas came into contact. The screws on the glass puck alignment component were then adjusted to level the surfaces. When the two surfaces were planar, contact occurred over large areas of the puck very quickly, as opposed to the gradual contact from one edge when there was misalignment present. Alignment tests were also performed with the addition of in-plane displacement to ensure that the in-plane motion was not overloading the fibers in either direction and that forces were symmetrical. With these preliminary alignment test completed, the adhesive could be characterized using the testing procedures described in the next subsection.

3.2.3 Testing Procedures One-dimensional microtribometer tests allow for the testing of either out-ofplane pure adhesion (i.e. adhesion without shear displacement) or in-plane shear Chapter 3. Friction and Adhesion Tester forces. A drawback of using one-dimensional tests is that the relationship between adhesion and shear forces is unable to be captured. To allow for the simultaneous measurements of both friction and adhesion forces simultaneously, other adhesive characterization uses two-dimensional testing procedures.





One-Dimensional Vertical Load-Pull Test To measure adhesion using a vertical load-pull test, shown in Figure 3.3, the two surfaces are moved towards each other perpendicular to both testing planes, also called the vertical direction, until a given condition is met during the load stage. This condition is usually a compressive force, often referred to as the preload, although a vertical distance can also be used. The surfaces can stay in contact for a set time to measure time effects and then are moved vertically away from each other during the pull/retract stage. The pure adhesion force during this test is the maximum tensile force during separation.

One-Dimensional Vertical Load-Drag Test The other one-dimensional microtribometer test measures in-plane shear forces.

To measure shear using a vertical load-drag test, shown in Figure 3.4, the surfaces are brought together vertically, perpendicular to their surfaces, and a small preload is usually applied during the load stage, although some tests use only the Chapter 3. Friction and Adhesion Tester Figure 3.

3: The vertical load-pull test allows one-dimensional characterization of pure adhesion forces. The pure adhesion force is the maximum tensile force between the adhesive and glass puck testing surface during the retract/pull stage.

Chapter 3. Friction and Adhesion Tester Figure 3.

4: The vertical load-drag test allows one-dimensional characterization of shear forces. The shear force is the maximum in-plane force between the adhesive and glass puck testing surface during the drag/shear stage.

weight of the adhesive sample as the preload. During the drag stage, the two surfaces are then moved in-plane relative to each other. The maximum in-plane force during the shear displacement of the two surfaces is called the shear force.

Two-Dimensional Vertical Load-Drag-Pull Test Two different types of two-dimensional microtribometer tests have been performed on the fibrillar adhesives in order to investigate the effects of different articulation mechanisms. The most common test used for determining the adhesion and friction force is the vertical load-drag-pull (vLDP) test, shown in Figure Chapter 3. Friction and Adhesion Tester

3.5. In the vLDP test, the two surfaces approach each other in the out-of-plane vertical direction until a given condition is met, typically a preload force or distance, during the load stage. The out-of-plane approach motion then stops and the surfaces are moved in-plane parallel to each other for a set distance, the shear length, during the drag/shear stage. Once the shear length has been reached, the surfaces are separated from each other in the out-of-plane vertical direction during the pull/retract stage. The maximum tensile force supported is then defined as the shear adhesion force, to emphasize the effect of prior shearing on the adhesive pulloff force, and the maximum in-plane force supported is referred to as the shear force.

Two-Dimensional Angled Load-Drag-Pull Test The angled load-drag-pull (aLDP) test, shown in Figure 3.6 uses an approach carried out at an angle, instead of a vertical approach, to bring the surfaces in contact. Unlike the vertical test, a single preload cannot be specified to stop the approach for a range of approach angles because the maximum compressive vertical force supported by the fibers can lead to either buckling or insufficient surface contact, depending on approach angle. An approach depth, zapproach, after contact must therefore be used to stop the approach. The two surfaces are then moved in-plane parallel to each other over the shear length distance. After Chapter 3. Friction and Adhesion Tester Figure 3.

5: The vertical load-pull test allows two-dimensional characterization of shear adhesion and shear forces. The shear adhesion force is the maximum tensile force between the adhesive and glass puck testing surface and the shear force is the maximum in-plane force between the adhesive and glass puck testing surface.

Chapter 3. Friction and Adhesion Tester Figure 3.

6: The angled load-pull test allows two-dimensional characterization of shear adhesion and shear forces for approach and retraction angles between 0– 180◦. The shear adhesion force is the maximum tensile force between the adhesive and glass puck testing surface and the shear force is the maximum in-plane force between the adhesive and glass puck testing surface.

shearing is completed, the two surfaces are separated at the retraction angle. The same conventions are used for calculating the shear adhesion and shear forces.

Endurance Test Synthetic adhesives must be able to retain adhesion and shear forces over tens of thousands of uses to compete with the adhesive found on the animal and for realistic use in robotic and commercial applications. Endurance tests Chapter 3. Friction and Adhesion Tester determine the adhesive’s performance over time by repeating single tests for a given number of cycles. The lifetime for gecko-inspired adhesives, if endurance tests are presented at all, can be limited to few tests or reach up to tens of thousands. Only a few adhesives have been able to show force retention for high numbers of test cycles and none has equaled the gecko. Setal arrays showed a 25 percent increase in adhesion and a 5 percent decrease in friction when tested over 30,000 load-drag-pull tests with a total drag distance of 300 meters [35].

3.3 Initial Characterization of Patterned Sur

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3.3.1 Introduction A synthetic adhesive inspired by the gecko should replicate the important properties found on the gecko, but does not need to be a direct copy of its system. This section describes the testing of micron-sized vertical and tilted rectangular flaps composed of polydimethylsiloxane (PDMS) that successfully mimic key characteristics found on the Tokay gecko. Rectangular flaps were chosen in order to create large areas of contact on the side face of the structures after shearing and are similar to the animal’s terminal triangular flap structures, spatulae. Additionally, Chapter 3. Friction and Adhesion Tester an angle was implemented on similar rectangular flaps to generate anisotropy in the system and offer comparisons with the vertical counterparts.

While some research aims to create either high adhesion [30] or friction [56], a true gecko-inspired adhesive must be able to achieve both for placement on vertical and inverted surfaces. Among other properties elucidated by Autumn [6], anisotropic attachment, non-sticky default state, and easy detachment are essential for gecko-like materials. Many synthetic adhesives have used cylindrical geometries with varying degrees of success, but rectangular geometries have yet to be fully explored and past results on the flat faces of adhesive structures have been encouraging [83]. A cylinder’s contact area without in-plane shearing is limited to the top face of the structure. The addition of a capped top element has been successful at creating high adhesion [33]. When cylindrical structures are sheared, they produce only a line contact on the rounded face, thus severely limiting the contact area achieved and the forces they can sustain. To add anisotropy to the shear and adhesion forces, angling of the fibers and the top capped surface has resulted in larger contact areas and directionality at the cost of fabrication complexity [78]. Rectangular structures do not suffer from line contacts, except between faces, and a large vertical or angled face could offer more contact area than the top face, if the flap can sustain high bending.

Chapter 3. Friction and Adhesion Tester

–  –  –

Figure 3.7: Scanning electron microscope (SEM) images of (a) vertical PDMS flaps and (b) tilted (20◦ from vertical) PDMS flaps.

The two sets of flaps are both 10 µm wide and ≈ 4 µm thick. The vertical flaps are 15 µm tall and the angled flaps are 10.7 µm tall. The vertical flaps have twice the density of the angled flaps due to differences in microfabrication.

Chapter 3. Friction and Adhesion Tester

–  –  –

Table 3.1: Geometric Properties of Vertical and Angled Flaps The two different flap structures tested are shown in Figure 3.



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