«UNIVERSITY OF CALIFORNIA Santa Barbara Design and Characterization of Fibrillar Adhesives A Dissertation submitted in partial satisfaction of the ...»
Adhesion forces were a maximum with no shearing of the ﬁbers, up to a pressure of 19.2±0.1 kPa, and shearing in either direction caused decreasing shear adhesion values. Similar behavior can also be seen in . At shear lengths around 100–110 µm, the adhesion forces suddenly jump upwards, almost reaching the initial value. This change in force corresponds to stick-slip motion between the surfaces causing a large loss of contact area across the puck. The stick-slip motion continued at larger shear lengths (data not shown), but was omitted because these slip events represent a failure of interface between the materials. The behavior was in qualitative agreement with tests by others,  showing repeated pronounced stick-slip motion. Varenberg  used mushroom tipped structures lacking directionality but having high initial adhesion and used increasing shear lengths to decrease the shear adhesion. Smooth sliding of the entire surface due to random small amplitude single stick-slip events was also observed. Here, similar smooth sliding behavior was also seen on the semicircular ﬁbers; however, shear displacements were used to increase shear adhesion in one direction and decrease it in the other.
The shear force for ﬂat PDMS was at a minimum with no shear and increases with shear length. It can be seen that the maximum shear force leveled oﬀ at the same shear length as the large stick-slip event. Since the interface was ruptured, Chapter 4. Vertical Semicircular Fibers Figure 4.
6: Shear adhesion forces along the ±Y-axis as a functions of shear length and preloads for a ﬂat PDMS control sample. Roughly equal shear adhesion forces are seen in both directions at similar shear lengths. After the stick-slip event at a shear length of 100–110 µm, the values jump towards their initial value. 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 additional shearing did not further increase the shear forces.
The ﬂat PDMS had higher values for shear forces, a pressure of up to 69.4±0.2 kPa was possible, than the patterned surface due to the higher contact area available. Murarash  saw drops of ≈30% in friction force for area densities between 20–80% with small aspect ratio pillars when compared to the ﬂat sample. Here, the smallest change for an equivalent preload was 500% suggesting that combining smaller area densities and larger aspect ratios do not follow the same trend. Although higher force values were seen in adhesion and shear, ﬂat PDMS lacks the anisotropy based on direction of shear found both on the gecko and the adhesive described here. Flat PDMS has been seen here and elsewhere to lose contact with the opposing surface when sheared possibly leading to failure of a robot when climbing. Furthermore, patterned structures have the ability to perform better in shear and adhesion when surface roughness is present .
Recent approaches by groups fabricating gecko-like adhesives have shown many desirable features, although none has been able to rival the properties of the gecko in all aspects. In , testing performed on setal arrays, 0.93 mm2 in area, generated 48 kPa in shear adhesion and 184 kPa in shear using a load-dragpull test. In , an angled wedge structure was tested in a similar manner to this experiment showing a long lifetime of 30,000 cycles with a maximum shear pressure of 25 kPa and shear adhesion pressure of 5.1 kPa, although no adhesion Chapter 4. Vertical Semicircular Fibers Figure 4.
7: Shear forces along the ±Y-axis as functions of shear length and preload. Preloads only seem to aﬀect the force at which the maximum shear force levels oﬀ. A slight misalignment between the sample and ﬂat PDMS is responsible for the lack of symmetry. 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 or friction anisotropy was shown despite structural anisotropy.
Triangular ﬁbers topped with a triangular pad have shown an adhesion energy anisotropy between 1.6–1.9 using a peel test. Here the triangular pad, not the ﬁber, restricted crack propagation from the edge of the triangle while the crack was free to propagate from the tip of the triangle . High adhesion values have been realized using mushroom tipped structures against both ﬂat, 56 kPa  and hemispherical, 180 kPa, surfaces . However, the structures would likely behave in an isotropic manner when tested for shear and shear adhesion. The ﬁbers and mushroom tips were later angled to increase the anisotropy and controllability of the adhesive .
High shear forces up to 300 kPa were reported for polypropylene ﬁbers although adhesion forces could not be supported . It can be seen that certain materials perform very well for a given property, however this usually is at the expense of another property. Here we have created an anisotropic material with values on the same order as a recently published balanced gecko-like adhesive , but with a simple, straightforward, inexpensive fabrication process.
A novel and simple approach for creating a responsive ﬁbrillar adhesive system inspired by the gecko has been presented. The experimental results demonstrate Chapter 4. Vertical Semicircular Fibers how adhesion and friction force anisotropy for reversible adhesives can be generated through the use of a single material with asymmetric vertical ﬁbers having no tip modiﬁcation. When these ﬁbers were articulated in a speciﬁc manner, they created varying amounts of intimate surface contact with a substrate leading to a diﬀerence in the forces they can support. Future work will incorporate a tilt angle using a previously developed fabrication method to further improve the anisotropy. It is also desired to increase the strength of the adhesive through diﬀering the material modulus and ﬁber geometry as well as incorporating higher microﬁber areal density designs.
Chapter 5 Angled Semicircular Fibers The behavior of the vertical semicircular ﬁbers discussed in Chapter 4 and the angled rectangular ﬂaps discussed in Chapter 3 each had anisotropic adhesion and shear forces. The angled semicircular ﬁbers presented here in Chapter 5 are a result of combining the two design elements, semicircular ﬁber shape and tilted structures. It was unclear if the anisotropy from each element would combine for even greater anisotropy values. The ﬁber density was increased roughly 3.5 fold from the vertical semicircular ﬁbers to see if higher force values were possible. The overall performance of the ﬁbers was tested using two separate tests. A load-dragpull test characterized the adhesion, shear, and anisotropy values. A systematic endurance test was also performed to see how performance changed with repeated testing.
Chapter 5. Angled Semicircular Fibers
5.1 Introduction Key characteristics of the Tokay gecko  include high and anisotropic normal adhesion and lateral shear forces during attachment, low detachment force, a high adhesion to initial preload force ratio (µ′ ), lack of inter-ﬁber self-adhesion, and operation over more than 30,000 cycles without signiﬁcant loss of adhesion performance . The highly reversible and controllable adhesion and friction in the gecko adhesive system enables both sticking and easy, rapid peeling. Recent experimental and theoretical ﬁndings have demonstrated that the anisotropic structure of gecko setae results in anisotropy in their adhesion and friction forces when engaged and displaced along opposing directions [120, 121]. Depending on articulation direction, either large numbers of nano-scale spatulae are brought into contact with the substrate for strong adhesion and friction, or the natural adhesive is able to peel oﬀ with very low pull-oﬀ forces. Further, unlike conventional pressure-sensitive adhesives which are easily fouled after a few uses, the natural gecko adhesive survives a huge number cycles between successive skin molts.
To design dry, responsive adhesive systems inspired by the gecko, various kinds of patterned surfaces with arrays of vertical and tilted ﬁbers, with tips of diﬀerent shapes and materials, and with sizes spanning both the micrometer and nanometer scales, have been fabricated [47, 97, 27, 117, 109, 50, 61, 89, 78, 56, 83, 31].
Chapter 5. Angled Semicircular Fibers In addition to high and controllable adhesion and friction, most practical applications for reversible synthetic adhesives require them to also demonstrate high reusability.
Although several synthetic adhesives capable of supporting large adhesion and/or shear forces have been reported, 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 this chapter, vast arrays of angled half-cylinder polydimethylsiloxane (PDMS) microﬁbers (Figure 5.1) were used to create an adhesive material with high adhesion anisotropy, high µ′, high shear force, and high reusability. In order to rigorously test the adhesive for durability, it was subjected to 10,000 repeated test cycles without intermediate cleaning, and the material was found to maintain high adhesion and friction forces without any signiﬁcant wear of the ﬁbers. Although a bio-inspired synthetic ﬁbrillar adhesive system with a long lifetime comparable to gecko setae has been reported in , the wedge-shaped ﬁbers used did not exhibit any gecko-like adhesion or friction anisotropy on sliding, and supported maximum shear loads far inferior to the gecko. The only other study thus far with tests over 10,000 cycles reported a large reduction in normal adhesion for high-density polyethylene (HDPE) ﬁbers, and a complete loss of adhesion for polypropylene (PP) ﬁbers . Large reductions in shear performance were also reported over these repeated tests. The ratio of adhesion to initial preload Chapter 5. Angled Semicircular Fibers force (µ′ ) is also especially important for climbing applications, with higher values resulting in increased operational stability on walls and ceilings due to a low corresponding reaction force from the substrate surface, as well as enhancing reusability by preventing damage to ﬁbers during preloading. Results from the current study also show a large improvement in the value of µ′ over other high-cycle adhesives.
5.2 Experimental Methods 5.2.1 Microfabrication To fabricate large arrays of tilted PDMS half-cylinder microﬁbers over cm-scale patch areas, a negative mold with angled slots was fabricated out of photoresist polymer material using a previously developed angled photolithography technique .
This method required the use of a non-reﬂective glass substrate wafer, which was then spin coated with a photoresist bilayer - ﬁrst with a ≈12 µm thick layer of polymethylglutarimide (PMGI) positive tone photoresist (PMGI SF-15, MicroChem Corp., Newton, MA), and then with a top layer of AZ 5214 image reversal photoresist (AZ Electronic Materials, Branchburg, NJ) approximately 1.4 µm thick. Since the two photoresist materials are sensitive to diﬀerent wavelengths of UV light, it was possible to independently deﬁne ﬁrst the ﬁber lateral dimensions Chapter 5. Angled Semicircular Fibers
Figure 5.1: Scanning electron microscope (SEM) images of angled semicircular ﬁbers.
Movement in the +Y-direction contacted the ﬂat face of the ﬁbers, while −Y-direction movement contacted the curved face. The ±X-axis was parallel to the straight edge of the top face. The +Z-axis was the normal loading direction, while the −Z-axis was the normal unloading direction. Articulation of the adhesive occurred in the Y-Z plane. (a) The ﬂat face of the ﬁber was tilted at 28◦ from the vertical and the curved face of the ﬁber was tilted at 15◦ from the vertical. The ﬁber outline is also shown. (b) The diameter of the ﬁber changed over the length of the ﬁber and the average diameter was found to be 8.5 µm.
Chapter 5. Angled Semicircular Fibers (as semicircles of 10 µm diameter) in the AZ 5214 layer using an i-line UV stepper aligner, and then the angle of tilt in the PMGI layer underneath in a separate patterning stage using a deep UV ﬂood exposure system.
In the angled exposure step, the wafer was mounted so as to ensure that the ﬁnal PDMS structures fabricated from this mold would have the ﬂat side of the half-cylinder ﬁbers tilted so as to face towards any contacting substrate, and the curved side of the ﬁbers face away, in order to maximize anisotropy in contact area on lateral shearing. The PMGI exposure and PMGI development were required to be repeated 12 times each in order to fully develop the photoresist, with each exposure step being for 5 minutes at a constant deep UV lamp power of 1000 W, and each develop step being for 25 seconds in PMGI 101 developer.
It was observed that the durability of the patterned PMGI molds used to fabricate arrays of PDMS microﬁbers could be increased (to facilitate reuse) by hard baking the molds at 110 ◦ C for 20 minutes before casting. The molds were then exposed to an oxygen plasma for 3 minutes, followed by vapor deposition of a layer of 1H,1H,2H,2H-perﬂuorodecyltrichlorosilane (FDTS) to facilitate easy peeloﬀ. PDMS (Sylgard 184, Dow Corning, Midland, MI) was mixed in a 1:10 ratio by weight of curing agent to base elastomer, and poured over the negative mold. This was ﬁrst degassed and then cured at 100 ◦ C for 15 minutes in a convection oven to fully cure the PDMS. The two materials were then separated by carefully peeling Chapter 5. Angled Semicircular Fibers Figure 5.2: Schematic diagram showing a glass wafer with a top AZ 5214 photoresist layer (pre-patterned with semicircular features) being mounted and exposed at an angle to create angled slots in the underlying deep UV photoresist layer (PMGI SF-15). The wafer was mounted so as to ensure that the ﬁnal PDMS half-cylinder microﬁbers fabricated from the photoresist negative mold would have the ﬂat side tilted to face towards any contacting substrate, and the curved side facing away, in order to maximize anisotropy in contact area on lateral shearing.