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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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Structures with aspect ratios, height to lateral dimension, greater than ten are often hard to create using many of the techniques listed above. In order to create larger height to lateral dimension ratios, fiber drawing has also been performed.

Chapter 2. Background A combination of imprint lithography combined with drawing of poly(methyl methacrylate) (PMMA) and polystyrene (PS) fibers was used to create an aspect ratio greater than 20 [49].

Despite the large amount of work performed to create the adhesives, a comparison between approaches is difficult due to the differences in size scales, testing geometry, and testing methods. Test areas can also be substantially different, from

0.0006 µm2 [119] to 12 cm2 [4]. Small scale patches, typically using a scanning probe microscope tip [104, 119, 30, 49, 107] to contact one or a few individual fibers, have been tested but fail to evaluate the adhesive performance on size scales applicable to real world use. The small test area simplifies alignment and the experimental pressures obtained with such tests are generally overestimates when performance is extrapolated to larger areas. Testing small patches typically measures only adhesion forces [104, 119, 30, 49], although shear forces have been measured in some cases [107].

Hanging patch tests have been performed against rough surfaces [4], smooth surfaces [61, 48], and patterned surfaces [63]. The hanging patch tests use weight to load the adhesive and are among the simplest to set up. Hanging patch areas are larger than the small scale patches, reaching up to cm2 dimensions [4, 83].

By hanging weights at different angles relative to the adhesive, the performance can be characterized using different loading conditions [59, 95, 88], but the force Chapter 2. Background result for each test is only given for that loading direction. Therefore, simultaneous adhesion and friction force data is not available and multiple tests are needed to fully characterize the adhesive.

Many synthetic adhesives are characterized with microtribometers built specifically for testing adhesion and/or friction forces. The testing area is often smaller than the hanging patch tests and precise control of the motorized positioners allows specific movements to be performed. The testing surfaces can be either curved [54, 37] or flat [33, 98] depending on the microtribometer used. Curved surfaces do not suffer from alignment difficulties, although the area of contact between the two surfaces can be difficult to calculate and may require special techniques [65, 2]. Flat testing surfaces have the opposite problems; the test area is known from the size of the sample or test surface, but aligning the two surfaces to be parallel can be difficult, leading to irregular contact and decreased forces [67].

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Basic background knowledge relating to synthetic adhesives has been presented for the adhesion and friction forces to be measured. The structure and unique features of the gecko have been introduced to illustrate the desired performance of Chapter 2. Background a gecko-inspired adhesive. Additionally, a brief survey of synthetic adhesives and how the forces are measured has been presented show the current state of work in this active research area.

Chapter 3 Friction and Adhesion Tester With the background information presented on adhesion and friction, it is important to show how they will be measured. Commercially available adhesion and friction testers can be very expensive and offer limited modification flexibility due to proprietary software or components. Changes to the substrate type, testing area, test type, or data collected cannot always be accomplished. In order to have flexibility in adhesive characterization, a microtribometer measuring force and torques in each of the three principal axes was designed and assembled from commercial off-the-shelf and custom machined components.

The testing apparatus, given the name Bio-F, was used to characterize the adhesives at the millimeter scale. Section 2.4 introduced the gecko adhesive system and important properties that a gecko-inspired adhesive should possess. Comparisons between the gecko’s values in key areas and those of the synthetic adhesives allow the relative performance to be obtained.

Chapter 3. Friction and Adhesion Tester In this chapter, both the mechanical and electrical components that were chosen for the Bio-F will be introduced.

The process of preparing a synthetic adhesive sample and integrating it with the Bio-F for testing will then be presented. Depending on the adhesive’s property of interest, different testing procedures must be performed. A description of the testing steps and force values obtained from the different tests will be given for reference throughout this document.

3.1 Experimental Apparatus The Bio-F test apparatus, with its name given for the biologically inspired forces it was designed to measure, was constructed to quantify friction and adhesion at size scales relevant to real world applications. Tests on gecko-inspired adhesives have been performed over very small areas [119, 62, 104], leading to high adhesion and shear pressures. However, tests on the gecko reveal a general trend of decreasing pressures with increasing test areas [10, 66, 34, 45, 5]. This has generally been attributed to the difficulty of making equally good contact over the entire test area when sizes are increased. This behavior was also seen over centimeter sized areas with gecko-inspired adhesives [83]. The mechanical and electrical components were therefore selected to have their ranges cover reasonChapter 3. Friction and Adhesion Tester able values based on previous testing of other synthetic adhesives at relevant size scales.





3.1.1 Mechanical Setup In order to find the friction and adhesion properties of test samples, a testing apparatus was designed and constructed. The system was required to perform the load-pull, load-drag-pull, or a variant test using a flat-on-flat testing geometry. A glass puck with a diameter of four millimeters was chosen to be small enough for decreased alignment difficulty and large enough to compare with other largescale test results. The mechanical components are all attached to metal structure sitting on a vibration isolation table. The tester can be seen in Figure 3.1.

Two motorized and one manual linear stage provide movement for aligning the sample and testing. The manual stage is used for rough horizontal positioning of the sample and is controlled by a knob with a resolution of 3 mm per turn. The other in-plane stage (Newport Corporation; XMS50) is motorized and has a travel distance of 50 mm, a resolution of 1 nm, and a minimum incremental motion of 10 nm. Vertical motion is provided by a motorized linear stage (Newport Corporation; VP-5ZA) with a travel distance of 4.8 mm, a resolution of 20 nm, and a minimum incremental motion of 60 nm. Additional settings such as stage velocity Chapter 3. Friction and Adhesion Tester Figure 3.

1: The main components of the friction and adhesion tester allow for the sample and testing surface to be aligned so that uneven loading conditions don’t occur. Motorized linear stages in the Y- and Z-directions and a load cell for measuring forces in the three principal axes allow different testing procedures to characterize the properties of adhesive samples.

Chapter 3. Friction and Adhesion Tester or stage accelerations are regulated by a motion controller (Newport Corporation;

XPS-C4).

Leveling of the sample is provided by two goniometers, adjusting the roll (θX ) and pitch (θY ), and orientation is adjusted using a rotation stage, changing the yaw (θZ ). The upper goniometer (Newport Corporation; GON65-U) has a travel range of ±5◦ and a sensitivity of 3 arc sec while the lower goniometer (Newport Corporation; GON65-L) has a travel range of ±3◦ and a sensitivity of 2 arc sec.

Rotating the sample is accomplished using a rotation stage (Newport Corporation;

RS65) with 360◦ of travel range for coarse adjustments and 10◦ of travel range and a sensitivity of 11 arc sec for fine adjustments.

The sensing is performed by a six-axis force and torque sensor (ATI Industrial Automation; Nano17 Transducer, SI-12-0.12) using silicon strain gauges. The system has a force resolution of ≈ 0.5 mN in FX, FY, and FZ and a torque resolution of ≈ 2 Nµm in TX, TY, and TZ. The rated sensing ranges are ±12 N in FX and FY, ±17 N in FZ, and ±120 Nmm in TX, TY, and TZ. The resonant frequency of the sensor is 7200 Hz for all force and torque directions.

In order to have the correct orientation of the glass testing puck, a three-point leveling system was integrated onto the large cantilever over the adhesive sample.

The leveling system allows the plane of the puck to be changed so that it can match that of the sample. Misalignment of the puck leads to uneven loading Chapter 3. Friction and Adhesion Tester of the sample and smaller supported forces since the adhesive is slowly peeled from the glass puck over time instead of losing contact quickly. The glass puck was lapped and polished to remove curvature and smooth the contacting surface.

Depending on the puck used, the RMS roughness value of the glass pucks were ≈ 70–160 nm over the area likely to contact the adhesive.

3.1.2 Electrical Setup The electrical setup, shown in Figure 3.2, performs four separate functions important for characterizing gecko-inspired adhesives. The force sensing is performed using a six-axis load cell (ATI Industrial Automation; Nano17 Transducer, SI-12which converts force and torque loads in to electrical signals to be transmitted over the transducer cable to the Network Force/Torque Interface (Net Box).

The Net Box contains the power supplies and network interfaces. It processes and then communicates the force and torque values to the computer using a standard Ethernet cable.

The motion control and position sensing components are comprised of a motion controller and two linear stages operating in the Y- and Z-axes. The motion controller (Newport Corporation; XPS-C4) controls the trajectory of the adhesive sample during testing through connections with the two motorized linear stages. The in-plane stage (Newport Corporation; XMS50) uses a three-phase Chapter 3. Friction and Adhesion Tester Figure 3.

2: The Bio-F friction and adhesion tester uses four separate systems to record force and torque values, control and record the positions of two linear stages, visualize and record contact between the glass puck and adhesive, and record wirelessly the temperature and relative humidity. Each of these tasks is controlled by a central computer, where the data is collected.

Chapter 3. Friction and Adhesion Tester synchronous ironless linear motor to provide fine motion control.

Vertical motion is provided by a motorized linear stage (Newport Corporation; VP-5ZA) using a high torque DC servo motor and tachometer along with a ball screw drive.

Position feedback for both stages is supplied by high accuracy linear scales.

Visualization of the contact between the glass puck and adhesive structures uses a CCD color camera (Sony Corporation; DXC-107) and image acquisition board (National Instruments Corporation; PCI-1405). The CCD camera has a resolution of 768x494, 4 white balance modes, 8 shutter settings, and camera identification adjustments. The image acquisition board allows analog video output from the camera to be acquired on the computer. LabVIEW software was programmed to save movies of the entire test procedure or save individual frames at important testing events.

Sensing of the temperature and relative humidity during testing is performed with an atmospheric sensor (Computer Aided Solutions; A1-05). Temperatures between −40◦ C and +70◦ C can be measured with a resolution of ±0.1◦ C. Temperature is controlled in the laboratory where the tests were conducted which resulted in steady value with small fluctuations of less than a few degrees. The relative humidity can be measured between 5% and 95% with a resolution of 0.03%. The relative humidity in the lab is not controlled and reached as high as 70.5% and as low as 14.9% over a one year period.

Chapter 3. Friction and Adhesion Tester

3.2 Experimental Procedure After the adhesive sample has been created using a variety of microfabrication and molding techniques, it must be able to be integrated with the Bio-F tester.

The polymer adhesives cannot be used in their post-molding state and the modications listed in this section were performed for the adhesives characterized here.

The method of attaching the adhesive to the tester and leveling the system is first presented. With these steps performed, the adhesive was characterized using one of the tests presented at the end of this section.

3.2.1 Sample Preparation The adhesive samples resulting from molding processes are not typically the correct size. The photolithography parameters defining the fiber pattern can leave large unpatterned areas between test areas. The molding technique itself can also produce large unpatterned edges around the test sample when a secondary container is used to hold the negative mold. Adhesive samples are cut to the correct size using an X-Acto knife or razor blade. All untested samples are stored in a container protecting the adhesive from airborne fouling particles.

Since the adhesive could not be attached directly to the load cell, a rigid connection capable of transmitting the forces was needed. A mounting plate above Chapter 3. Friction and Adhesion Tester the load cell was machined with mounting holes for connection to the load cell as well a screw pattern allowing 1-inch glass slides to be held in place. The glass slide was first scribed and broke to the correct length based on the sample size. The glass slide was then cleaned using acetone, isopropyl alcohol, and deionized water.

A flat white background, usually a labeling sticker, was then applied to the backside of the glass slide to create a uniform background without strong reflections from the metal mounting plate underneath. Custom sized glass mounting surfaces could also be created using the dicing saw located in the UCSB Nanofabrication Facility.



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