ISSN 1744-683X

www.rsc.org/softmatter Volume 8 | Number 32 | 28 August 2012 | Pages 8243–8524

1744-683X(2012)8:32;1-9

COMMUNICATIONHamed Shahsavan and Boxin ZhaoBiologically inspired enhancement of pressure-sensitive adhesives using a thin fi lm-terminated fi brillar interface

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Research in the Polymers & Soft Matter Group of the Materials Physics Center, CFM, (CSIC-UPV/EHU) San Sebastián, Spain.

Title: Neutron scattering and molecular dynamics simulations:

synergetic tools to unravel structure and dynamics in polymers

Over the last years, the Polymers & Soft Matter Group of the CFM has

developed a solid methodology combining diverse experimental

methods (including relaxation and scattering techniques) and

molecular dynamics simulations to investigate the structure and

dynamics of soft materials based on polymers at diff erent length and

time scales. The tutorial review of this issue shows how a strategy

based on the tandem neutron scattering / fully atomistic simulations

can provide unprecedented insight in this kind of problems.

As featured in:

See Arantxa Arbe et al.,

Soft Matter, 2012, 8, 8257.

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Biologically inspired enhancement of pressure-sensitive adhesives using a thinfilm-terminated fibrillar interface

Hamed Shahsavan and Boxin Zhao*

Received 4th April 2012, Accepted 22nd May 2012

DOI: 10.1039/c2sm25795e

A hybrid adhesive structure consisting of an array of elastic

biomimetic micropillars and a thin viscoelastic terminal film was

designed and fabricated. A remarkable adhesion enhancement and

a crack propagation modulation were observed, indicating a syner-

gistic effect between the elastic fibrillar structures and the visco-

elastic terminal film.

The foot pads of many insects and lizards are good dry adhesives,

able to generate sufficient adhesion upon contact without applying

much pressure. The spatulae terminated hair-like structure of gecko

lizards’ foot pads is one typical example. The amazing aptitude of

these insects and lizards to stick readily and rapidly to almost any

surface has attracted extensive research interests on the development

and application of biomimetic structures.1–6 Recent studies have

attributed the adhesive ability of gecko foot pads to the sophisticated

morphology and elasticity coupled with such intermolecular forces as

the universal van der Waals forces and capillary forces. In particular,

thematerial independence, repeatability and flaw tolerance properties

of hair-like structures of the gecko foot pads have made them

a unique prototype for a new generation of bioinspired fibrillar

adhesives (BFAs), able to add new functionalities and smart prop-

erties to the man-made pressure-sensitive adhesives (PSAs).

PSAs are soft materials capable of forming a joint when pressure is

applied to bring the adhesive and its counter-substrate together

without requiring chemical bonding.7 The adhesive strength results

from the good molecular contact with the substrate and the visco-

elastic dissipations through bulk deformations. The design of effec-

tive adhesive commodities is based upon a balance between the

adhesive and cohesive strengths using different bonding parameters

and material properties. The bioinspired fibrillar adhesives (BFAs),

theoretically, can increase the adhesive strength and toughness of

a flat control through mechanisms such as contact splitting,2

enhanced compliance and crack trapping.3However, simple synthetic

fibrils have failed to show good adhesion owing to the detrimental

effect of fibril bundling and a decrease in real contact area. Thus far,

mushroom-shaped or elastic thin film-terminated structures have

been found as the most effective BFAs.

Department of Chemical Engineering and Waterloo Institute forNanotechnology, University of Waterloo, 200 University Avenue West,Waterloo, Ontario, Canada N2L 3G1. E-mail: zhaob@uwaterloo.ca; Tel:+1 519-888-4567 ext. 38666

This journal is ª The Royal Society of Chemistry 2012

In this communication we report a hybrid adhesive structure

consisting of an array of elastic biomimetic micropillars terminated

with a thin viscoelastic or PSA film. Inspired by the recent works in

the field,4–6we hypothesized that a viscoelastic layer can be effectively

combined with the elastic fibrillar surfaces, forming an elastic–

viscoelastic bilayer adhesive. This hybrid structure will be able to

utilize the contributions from both the elastic micropillars and

viscoelastic top layer and their interactions to enhance adhesion. To

the best of our knowledge, there is no systematic study of this type of

structure. We fabricated this type of biomimetic hybrid PSA and

experimentally demonstrated a remarkable adhesion enhancement.

This study has a direct technical implication in industrial applications

of biomimetic microstructures for the manufacturing of novel PSA

products. An interesting result of our work is the local fingering and

cavitations around individual fibrils, which are also of importance for

fundamental understanding of the failure mechanisms of surface-

patterned soft materials.

Hexagonal arrays of polydimethylsiloxane (PDMS) elastic

micropillars having a diameter of 50 mm, a height of 150 mm and

a center-to-center spacing of 115 mmwere fabricated through the soft-

lithography technique. A mixture of PDMS (Sylgard� 184, Dow

Corning) resin (90 wt%) and curing agent (10 wt%) was poured on

a master-mold of the microholes created by the DRIE process. The

PDMS coated master-mold was cured at 90 �C for 1 h. Fabricated

micropillars were toppedwith thin layers of PDMSfilms according to

the procedure described in ref. 3. The elastic and viscoelastic PDMS

terminal films were prepared using 10 wt% curing agents and 4 wt%

curing agents, respectively. The thickness of the terminal films was

determined by optical interferometry. Structures of the fabricated

samples are shown in Fig. 1A–C. For brevity and clarity, we desig-

nate the samples by the type of elasticity (E: elastic, and VE: visco-

elastic), geometry (S: smooth, P: pillars, and T: thin film-terminated),

and terminal film thickness.

Frequency sweep rheology tests at room temperature were per-

formed using a RA2000 model rheometer (TA Instruments) to

characterize the thermomechanical properties of samples with

10 wt% and 4 wt% curing agents. As shown in Fig. 2, both storage

and loss components of modulus (E0 and E0 0) of the PDMS con-

taining 10 wt% curing agent are higher than those of the sample with

4 wt% curing agent. The damping factor tan d¼ E0 0/E0 of the sample

with 4wt% curing agent is higher than that of the samplewith 10wt%

curing agent, indicating the more viscoelastic or dissipative nature of

the sample with 4% curing agent.

Soft Matter, 2012, 8, 8281–8284 | 8281

Fig. 1 (A) Schematic view of the viscoelastic film-terminated fibrillar

interface. SEM micrographs for (B) simple micropillars of diameter

50 mm and height 150 mm, and (C) 8 mm thick film-terminated structure.

Fig. 2 Plots of the storage (E0)and loss (E0 0) modulus of the PDMS

samples containing 10% and 4% curing agent vs. angular frequency and

plots of the tan d against angular frequency (the inset curve).

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Adhesive properties of the biomimetic samples and flat controls

were characterized by the indentation of a 6 mm diameter hemi-

spherical fused silica probe (Ispoptics Co., NY, USA) using a micro-

indenter assembled on top of an inverted optical microscope as

shown in Fig. 3A. The load–displacement data were measured with

a 0–25 g load cell (GSO-25, Transducer Techniques); the contact area

and deformations during loading and unloading processes were

observed from the bottom-view image (Fig. 3B) acquired through the

inverted microscope. The set-up was equipped with a side-view

camera to monitor the possible necking during separation and the

Fig. 3 (A) Indentation set-up equipped with bottom and side view

cameras, (B) a typical bottom view image of the indentation on a visco-

elastic thin film-terminated fibrillar structure, and (C) a typical side view

image of the indentation probe and adhesive during an unloading indi-

cating fibrillation for a viscoelastic thin film-terminated fibrillar structure.

8282 | Soft Matter, 2012, 8, 8281–8284

mode of separation (Fig. 3C). Indentation tests were performed with

a constant velocity of 1 mm s�1 under ambient conditions. Visual

examination of the adhesive surface and glass probe showed no

adhesive residues on the probe surface after separation, indicating an

interfacial separation mechanism.

Fig. 4A shows typical load versus displacement curves of the

viscoelastic film-terminated fibrillar structures (VE-T-8 andVE-T-24)

in comparison with five control samples: elastic smooth control

(E–S), simple micropillar samples (E–P), viscoelastic smooth controls

(VE-S-8 and VE-S-24), and elastic film-terminated structures (E-T-

24). The pull-off force for E–P is lower than that for elastic control

(E–S) mainly owing to the decrease in the real contact area. Coating

of the elastic smooth control with a thin layer of viscoelastic PDMS

resulted in a pressure-sensitive adhesive with enhanced displacement

at failure. Incorporation of an elastic thin film on top of the micro-

pillars results in an increment of the real contact area and further

energy dissipation due to the crack trapping mechanism.3 Benefiting

from both of these advantages, the viscoelastic film-terminated

structure (VE-T-24) displayed an outstanding improvement in both

the pull-off force and loading–unloading hysteresis.

We visualized the debonding processes to gain qualitative insights

into the debonding mechanism. The bottom-view images of contact

deformations at three points of the indentation tests for VE-S-8 are

shown in Fig. 4B and those for VE-T-8 in Fig. 4C. At the same

preload, the contact radius of VE-T-8 is much larger than that of the

corresponding smooth sample VE-S-8, indicating that the interme-

diate fibrillar structure enhanced the compliance of the layered

structures. At the pull-off point, VE-S-8 developed Safman–Taylor

Fig. 4 (A) Typical load vs. displacement curves for the indentation test

for different samples, (B) a bottom-view image of the indentation test on

sample VE-S-8, and (C) a bottom-view image of the indentation test on

VE-T-8. The experiments were performed at 5 mN preload.

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type fingering instabilities (Fig. 4B-2).8 In contrast, the fibrils of the

sample VE-T-8 facilitate bulging of the crack front but significantly

impede the development of the fingering. Furthermore, cavitations

were observed at the edge of fibrils inside the contact area (Fig. 4C-2).

As the separation progressed, the finger instabilities continued to

grow toward the center of the contact for VE-S-8 (Fig. 4B-3). For

VE-T-8, subtle air fingers occurred on top of individual fibrils within

the contact area while the crack front became relatively smooth. We

estimated the crack propagation velocity as the average velocity of

the contact line moved from the preload to the pull-off point. The

velocity is 3.04 mm s�1 for E-T-24, 0.70 mm s�1 for VE-T-24, and

0.15 mm s�1 for VE-T-8, confirming that the crack propagation was

increasingly hampered by the dissipative nature of terminal films.

Fig. 5A plots the pull-off forces vs. preload. The dependence of the

pull-off on the preload is much more pronounced for the viscoelastic

thin film-terminated fibrillar structures (VE-T-24) than the other

ones.Moreover, the pull-off forces for theVE-T-24 aremore sensitive

to the preload at the low preload regime than at the high preload

regime, indicating its pressure-sensitive nature. We further take into

account the important influence of the degree of confinement of the

adhesive on the adhesion and pattern of crack growth by varying the

thickness of the terminal layer. According to Fig. 5A, the thickest film

(VE-T-24) always has higher pull-off forces than the other two (VE-

T-12 and 8); and, at the low preloads, the pressure sensitivity of

Fig. 5 (A) Variation of the pull-off forces against preload for the

different samples and (B) adhesion energy with a 5 mN preload for

different samples.

This journal is ª The Royal Society of Chemistry 2012

thicker films is higher than the thinner films. Furthermore, we

compared the adhesion energy (WA) with a preload of 5 mN, defined

as the total dissipated energy in a loading–unloading cycle, per

maximum contact area for each sample in Fig. 5B. The adhesion

energy and pull-off force in our viscoelastic film-terminated structure

increase with the thickness of the terminal thin film, which is in

agreement with the results in ref. 7 and 9. Evidently, bulk energy

dissipation is greater in thicker layers. Note that the energy release

rate of elastic film-terminated structures slightly increases with

a decrease of thickness with a factor of h�3.3

To obtain insights into the debonding instabilities, we model our

adhesive system as a three layered structure as illustrated in Fig. 1A

consisting of a top thin film, an intermediate fibrillar structure and an

elastic backing material. Since we have maintained the material

properties of the intermediate and bottom backing materials the

same, we presume that the nature of the terminal film accounts for

the observed different adhesion and debonding behaviours. In

indentation, the interfacial interaction between the probe and the

terminal film controls the interfacial crack propagation mechanism,

while the viscoelastic modulus of the thin films and its interaction

with the fibrils determine the bulk deformation mechanism.10 The

separation of an interface is controlled by a competition between

interfacial crack propagation and bulk deformation. The former is

governed by the interfacial adhesion or the fracture energy release

rate (Gc); the latter is essentially controlled by the elastic modulus (E)

of the bonded materials. For the lower values of Gc/E, the interfacial

crack propagation mechanism becomes more pronounced, while for

the higher values of Gc/E the separation is controlled by bulk insta-

bilities such as cavitation, fingering, and fibrillation. For the visco-

elastic materials, the situation is more complex as Gc depends on the

temperature, T, and crack growth velocity, v, according to the

following correlation: Gc z G0(1 + f(aTv)).11 G0 is the quasi-equi-

librium energy release rate at v z 0, and aT is the temperature

dependent shift factor. It has been proposed that f(aTv) is linearly

proportional to the damping factor tan d.12

The type of bulk deformation depends on the confinement ratio,

defined as the ratio of the maximum contact radius to the initial

thickness of the adhesive (a ¼ amax/h). We calculated a critical

confinement above which shear deformations are more beneficial for

the system’s energy than normal deformations, leading to undula-

tions of crack front. We examined the bottom-view images of the

contact deformation for samples VE-S-24, VE-S-12, and VE-S-8 and

found that the undulations of the crack front became significant when

the thin film thickness was less than 12 mm. Thus, we estimated the

critical confinement ratio ac z 24 for our system, which is in good

agreement with the values in the literature.13,14 Furthermore, we used

a modified competition model for the prediction of the debonding

mechanisms:

Gc

EzG0tan d

E0h

In this model, the elastic modulus has been replaced by the storage

component of the modulus and Gc is replaced by G0tan d. The effect

of confinement ratio on the debonding mechanism has been taken

into account by incorporating the thickness of the adhesive in the

denominator. Since G0 is deemed as a constant value, we compared

tan d/E0h for samples E-T-24, VE-T-24, VE-T-12, and VE-T-8. Fig. 6

shows that the thinner viscoelastic terminated structures have higher

Soft Matter, 2012, 8, 8281–8284 | 8283

Fig. 6 Variation of tan d/E0h for samples with different terminal film

thicknesses and viscoelastic properties vs. angular frequency.

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values of tan d/E0h so as to be more inclined to experience bulk

deformations. This prediction is consistent with our observation that

the VE-T-8 experienced the severest bulk deformation (air fingers and

cavitations). Moreover, the tan d increases with the frequency or the

crack growth velocity; thus, the Gc/E should increase with the

velocity. We did notice the increase of the pull-off force with inden-

tation velocity in several preliminary tests and are investigating the

effect of velocity on the pull-off force and interfacial deformations

in details.

Finally, it is worthwhile to speculate the determining influence of

the fibrils (intermediate sub-layer) on the properties of our adhesive.

As shown in Fig. 4C (1–3), all of the bulk instabilities take place

around or on top of the fibrils. Thus, the confinement assumption of

the terminal film might be only valid on top of the fibrils; the other

portions of the terminal film are not confined. The fibrils pinned the

contact line causing more deformation at the interfaces in shape of

the air fingers, cavitations and fibrillation. In addition to cavities

around fibrils, we see some cavitations in the center of the fibrils for

sample VE-T-24. This defect could be considered as a penny-shape

interfacial defect rendering an adhesive mechanism of separation

instead of a cohesive one.15 By adding the elastic biomimetic fibrils

beneath the viscoelastic thin film, we take advantage of the dissipative

crack trapping process in the non-confined region and the modifi-

cation of the stress field in the viscoelastic layer by the fibrils, which

8284 | Soft Matter, 2012, 8, 8281–8284

consequently decrease the extent of viscoelastic deformation of the

thin film and then limit the cohesive failure. This hybrid design of

pressure sensitive adhesives benefits from both strong adhesion and

controlled bulk and interfacial deformations. Although this work

represents a novel and effective approach to increase adhesive

properties of PSAs using a BFA structure, future research is needed

to investigate the failure mechanism of such adhesives. Moreover,

common issues of the PSAs such as reusability and optimization of

the fibrillar structure are open questions yet to be tackled.

Acknowledgements

This work was supported by the Natural Sciences and Engineering

Research Council of Canada (NSERC) and Ontario Centre of

Excellences (OCE). The authors appreciate the help of Professor

Costas Tzoganakis and Dr Shui Han Zhu with the rheological

characterizations and Mr Yougun Han for his technical assistance

with the indentation instrumentation.

Notes and references

1 S. N. Gorb, M. Sinha, A. Peressadko, K. A. Daltorio andR. D. Quinn, Bioinspir. Biomimetics, 2007, 2, S117–S125.

2 E. Arzt, S. Gorb and R. Spolenak, Proc. Natl. Acad. Sci. U. S. A.,2003, 100, 10603–10606.

3 N. J. Glassmaker, A. Jagota, C.-Y. Hui, W. L. Noderer andM. K. Chaudhury, Proc. Natl. Acad. Sci. U. S. A., 2007, 104,10786–10789.

4 E. Cheung and M. Sitti, J. Adhes., 2011, 87, 547–557.5 G. Castellanos, E. Arzt and M. Kamperman, Langmuir, 2011, 27,7752–7759.

6 H. Shahsavan and B. Zhao, Langmuir, 2011, 27, 7732–7742.7 A. J. Crosby and K. R. Shull, J. Polym. Sci., Part B: Polym. Phys.,1999, 37, 3455–3472.

8 B. Zhao, H. Zeng, Y. Tian and J. Israelachvili, Proc. Natl. Acad. Sci.U. S. A., 2006, 103, 19624–19629.

9 A. N. Gent and R. G. Hamed, Polym. Eng. Sci., 1977, 17, 462–466.10 C. Carelli, F. D�eplace, L. Boissonnet and C. Creton, J. Adhes., 2007,

83, 491–505.11 A. N. Gent and A. J. Kinloch, J. Polym. Sci., Polym. Phys. Ed., 1971,

9, 659–668.12 D. Maugis and M. Barquins, J. Phys. D: Appl. Phys., 1978, 11, 1989–

2202.13 A. Ghatak, M. Chaudhury, V. Shenoy and A. Sharma, Phys. Rev.

Lett., 2000, 85, 4329–4332.14 J. Nase, A. Lindner and C. Creton, Phys. Rev. Lett., 2008, 101, 1–4.15 A. J. Crosby, K. R. Shull and H. Lakrout, J. Appl. Phys., 2000, 88,

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