Adhesive Properties for Taper Nanohairs

We hypothesize that the observed anisotropic behavior arises primarily due to

   

the stresses caused by the moment created when the taper is sheared. This can be understood by analysis of the rotation of the tip during shear loading in each direction (Figure 3.13). The original angle, φ, introduces a moment that is relative to the magnitude of the angle change from its undeformed state. The peeling moment is increased if the tapered pillar is sheared in the releasing direction because it increases the already present tip rotation to a larger angle (φr), increasing Δφ as seen in Figure 3 4.13c. This increased moment peels the leading edge, eventually detaching and overturning the fiber tip. However, when sheared in the gripping direction, the fiber tip begins to return to its original angle, reducing the moment to zero (Figure 3.13a).

When the magnitude of the moment is near zero, the normal stress distribution at the interface is more evenly distributed, reducing the chances of detachment. After this point, if the shearing in the gripping direction is continued, Δφ changes and begins to increase in magnitude, eventually causing the leading edge (left) in detachment. The initial decrease in moment for shearing in the gripping direction increases the allowable displacement before detachment occurs, which means adhesion increased when shear force is applied in a preferred direction, in contrast to the releasing direction where the moment increases immediately. The increased displacement in the gripping direction allows the fibers to stretch and maintain contact, leading to high interfacial shear strength and anisotropy. All the shear and normal forces were recorded at the adhesive‘s failure point within our experiment. Taper shaped pillars we fabricated with slanted angle by pressure technique are shown in Figure 3.14.

The shear force was greatly reduced to 2.1 N/cm² when the sample was pulled in releasing direction, suggesting that the dry adhesive presented here can be used as a smart, directional adhesive patch with strong attachment (~21.5 N/cm²) and easy detachment (~2.1 N/cm²), with the hysteresis close to 13 as shown in Figure 3.15. A simple peeling model can also explain the strong directional adhesion capability.

According to the Kendall peeling model, the critical peel-off force (F_c) of a nanohair can be estimated with an assumption that the tip of slanted taper nanohairs forms intimate contact with the substrate as in an elastic tape, yielding^73-74

. . . ( 1 0 )

where γ = 100 mJ/m² is the adhesive energy, θ is the peel-off angle, and b = 50 nm, t,= 100 nm and E = 19.8 MPa are the width, thickness, and elastic modulus of the tape, respectively. Thus, the peel-off force can be expressed as a function of peeling angle for given parameters and the total peel-off force per unit area can be expressed by

. . . ( 1 1 )

where D = 6.46*10⁸ cm^-2 represented the hair density. Figure 3.15 presented the peel-off force per unit area with varying peeling angles. As shown in Figure 3.15, the peel-off force increased gradually with a stronger shear adhesion force. When the adhesive was pulled in the reverse direction, however, the leaning angle of nanohairs is increased from its initial value of 60° to 180°, and thus the peel-off force is greatly reduced. According to Eq. 11, the peel-off forces are 21.5, 3.4, and 2.1 N/cm² for 0°, 90°, and 180°, respectively. The value of simulation is not quite fit with experimental data at a glance; however, the maximum value of simulation takes place at the angle around 0° which is scarcely possible on average day. Hence if the slanted angle was extended to 54°, the adhesion force will be around 8 N/cm² which agrees fairly well with our experimental results (maximum shear adhesion of ~8 N/cm² in the forward

    

direction and ~1.4 N/cm² in the reverse direction as shown in Figure 3.16.The diagram in Figure 3.17 gave us a comparison between taper shape and pillar shape, then we can find out the higher adhesion of taper shape than pillars‘. There is still remaining a large space that we can improve from 8 N/cm² to 21.5 N/cm² from Figure 3.15. The possible reason which restricts the extension of slanted angle may be the way to measure is not efficient enough.

Gecko with special self-cleaning via walking steps is well known for scientists.

Self-cleaning ability will occur rapidly as a consequence of energetic disequilibrium;

particles tend to remain attached to the wall rather than to the spatula. For dirt or particle in daily life, equal energy is required to separate the particle from the wall and from the spatula. Unless particles are very small, many spatulas must be attached simultaneously to a single particle to balance the interaction energy between a spherical dirt particle and a planar wall (W_pw). Applying the same assumption from gecko to our taper shape structure, we may conclude the dirt will transfer to wall due to the difference between W_pw and W_pt (the interaction energy between a spherical dirt particle and taper pillar). Approaching this feature, an optimal density is required. It avoids particles for dropping down to the space between pillars but still keep the balance between Wpw and Wpt, and stability which can suffer several times of steps.

Both optimal density and reliable stability could be reached by our taper structure. To get this goal, the super-hydrophobicity as the gecko‘s setal arrays composed of an array of β-keratin pillars is considerable with a water contact angle of 160° and with a contact angle of 93° on flat β-keratin surface but setal arrays from gecko. The water contact angle is usually used to measure self-cleaning ability. Getting the same results from our taper shaped structure, we observe the contact angle is increasing from 65°

(hydrophilic) to 125° (hydrophobic). This is deserved to mention that this level of hydrophobicity is enough for daily using. From Table 3.3, taper shape clearly showed

the advantage of self-cleaning ability over pillar shape. Explanations and reasons are as follows. Generally, high contact angle is induced by nanostructure and low sliding angle caused by nano- and micro-structures such as lotus leaf. Lotus is on behalf of super hydrophobic surfaces in Cassie‘s state, and exhibit a high CA and very low CA hysteresis. In another way, owning to its high adhesive (high CA hysteresis) properties, gecko is the representative of the high CA which is referred to contact area. The nano-slanted taper shape offered enough contact area for adhesion, while the contact area of microstructure or vertical structure is insufficient for high adhesion. Air trapping via nano- or micro-structure is critical for high CA; accordingly, efficiently trapped air is much more important than other factors. Both taper and pillar types are initially in Cassie‘s state and contribute very large fraction of air on the surface. Yet the air pockets of pillar type is in a flow tending condition, which is risky of transferring to Wenzel‘s state because of continuous interface with outside air from top to bottom. For taper type, sealed pockets will lead to flow unfriendly at bottom.

As a demonstration of the adhesion of the taper nanohair arrays, a small area of 1 cm² was evaluated through a frictional adhesion test as shown in Figure 3.18b. Figure 3.18a presented the mold and the tape of PUA after replicating.As to the measurement, a flexible adhesives with slanted nano-taper pillars attached to a glass slide that supported a counter weight in gripping direction under a preload of 0.5 N cm^-2. During the shear-adhesion test, no external normal load was applied. To verify the contamination, Figure 3.18c presented the adhesive after detaching from glass and the water droplet with high CA, which implied the water cleaning ability, on the adhesive, respectively.

3.4 Summary

We successfully presented an approach to fabricate angled taper nanohair arrays

as an excellent directional, reusable and water cleanable gecko-mimicking dry adhesive in large area. From Dahlquist‘s criterion, an ideal taper nanohair of PUA that consist a length of 1.3 μm and a diameter of 380 nm was designed. By using taper PAA mold via decoupling two-step HA process reported firstly by us, taper nanohairs with slanted angle were fabricated. The angled taper nanohair did facilitate the stability and self-cleaning properties compared with pillar nanohairs while still maintain a great directional adhesion. Moreover, remarkably directional force exhibited by angled taper nanohair arrays is showing here with strong shear attachment ( ~8 N/cm²) in the gripping direction and easy releasing( ~1.4 N/cm²) in the reverse direction (pulled against the angled direction of hairs). The smart adhesive presented here would enable the climbing robots, cleaning transport system such as LCD factory and non-residue sticker for future generation. A further study should be done on longer length or stiffer material to improve the adhesion capability against rough surfaces outside the laboratory.

MA HA

Applied voltage (V) 160 190

Current density (mA/cm^-2) 3 20

Growth rate (μm h^-1) 10 60

Interpore distance (D_int; nm) 405 380

Porosity (P; %) 10 3

Table 3.1 MA versus HA in 2.5% H₃PO₄.

Material (Young’s modulus)

Shear

adhesion Durability Directional adhesion

Self- cleaning Natural Gecko⁷ β-keratin (2 GPa) 10 > 30000 Yes Excellent

Our work PUA (19.8 MPa) 8 > 100 Yes Good

Angled pillar¹⁴ PUA (19.8 MPa) 11 N/A Yes No

Slanted Pad²⁷ PP (1 GPa) 4.5 N/A Yes No

Pillar¹⁹ h-PDMS (11 MPa) 0.05 N/A No Good

Hierarchical Tip²³ PDMS (1.8 MPa) 2 N/A No No

Angled spatulata²⁵ PU (3 MPa) 10 > 100 Yes No

Table 3.2 The comparison between different types of polymer-based gecko adhesives

(N/A represents ―no information‖).

Plane PUA Pillar PUA Taper PUA

CA = 65° CA = 94° CA = 125°

Table 3.3 Measurement of contact angle for various cases with a dry adhesive pad.

Figure 3.1 SEM image of our taper shape PAA. (a) The cross-sectional image showed a length of 1.4 μm. (b) The magnified image of channel tips. Top-view images of the pore-widening time in (c) 3 min and (d) 15 min.

Figure 3.2 Chronoamperical curves during HA in 0.25 M H3PO4. A conventional MA (0.25 M H3PO4, 160 V) is also plotted (blue line) for comparison.

Figure 3.3 SEM images of taper PAA with different lengths: (a) 600 nm, (b) 1.1 μm, (c) 1.8 μm, and (d) 2.3 μm.

Figure 3.4 Processing anodization in H3PO4 with different concentration. (a) 1% which cannot offer enough ionic species. (b) 5% and (c) 10% with excess jeoul heat from current densities.

Figure 3.5 (a)-(d) Demonstrated that the ordered condition increased as voltage parameter increased. (e) The relationship between interpore distances, current densities and voltage.

Figure 3.6 SEM images of decoupling system. (a)-(b) Optimal processing widows in both first and second step. Unmatched voltage cause a bad result because the applied voltage at second step is (c) insufficient and (d) excess to fit the optimal condition.

Figure 3.7 (a) Taper shaped pillars profile sketch map. (b) Pillar shape profile sketch map. (c) SEM image of taper shaped pillars and (d) Illustration of taper‘s advantage.

Figure 3.8 Force measurements versus cycles of attachment and detachment, and the force remained the same for over hundreds of time.

Figure 3.9 Taper pillars with different lengths. (a) 600 nm from tilted SEM image (b) 600 nm from cross SEM image. (c) 1.4 μm from cross SEM image. (d) 1.4 μm from cross SEM image. The insets showed the molds of replicating or SEM images of high magnification, respectively.

Figure 3.10 SEM images of taper and pillar nanohairs. (a) Low magnification of our structure and the inset is the top view image that displays the taper edge and hexagonal arrays. (b) Tilted SEM image of the pillar shape showing this type cannot support the same height as the taper shape and (c) SEM image from cross-section. (d) Stable pillar with decreasing the length.

Figure 3.11 Illustrated hierarchy (a) of gecko and (b) of taper shape as a hierarchy-like by ―cake‖ model. (c) Illustration of Eq. 8.

Figure 3.12 Displays the simulation of E_eff versus slanted angle. Clearly, the E_eff drops below 100 kPa, which fits the Dahlquist criterion, with decreasing slanted angle after 73°.

Figure 3.13 Theoretical analysis of directional adhesion mechanism of the slanted taper shaped pillars. An illustration showed the change of leaning angle of the slanted taper nanohairs when the adhesive is pulled in (a) the gripping, (b) initial state and (c) releasing direction.

Figure 3.14 Taper shaped pillars with slanted angle we fabricated by pressure technique. (a) Low magnification. (b) High magnification of tilted SEM image of the structure. (c) Low magnification and (d) high magnification from cross-sectional view.

Figure 3.15 Simulation of critical peeling-off forces as a function of peeling angle.

Figure 3.16 Measurement of shear force for various cases with an adhesive patch of 1.0 cm². The taper nanohairs were composed of soft PUA.

Figure 3.17 Giving a comparison between taper shape and pillar shape, we can find out the higher adhesion of taper shape than pillars‘ can account for the higher density, longer length or adhere efficiently we have discussed previously.

Figure 3.18Photographs of (a) tape of PUA after replicating and the mold. (b) Counter weitht measured system. (c) The high CA of tape after detaching from the glass.