Tunable Friction Behavior of Photochromic Fibrillar ... - ACS Publications

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Tunable Friction Behavior of Photochromic Fibrillar Surfaces Gabriele Nanni,*,†,‡ Luca Ceseracciu,† Reinier Oropesa-Nuñez,§,⊥ Claudio Canale,§ Princia Salvatore,† Despina Fragouli,† and Athanassia Athanassiou*,† †

Smart Materials, Nanophysics, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova, Italy Dipartimento di Fisica, Università degli Studi di Genova, via Dodecaneso 33, 16146 Genova, Italy § Nanophysics, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova, Italy ⊥ DIBRIS Department, University of Genova, viale Causa 13, 16145, Genova, Italy ‡

S Supporting Information *

ABSTRACT: Grasslike compliant micro/nano crystals made of diarylethene (DAE) photochromic molecules are spontaneously formed on elastomer films after dipping them in a solution containing the photochromic molecules. The frictional forces of such micro- and nanofibrillar surfaces are reversibly tuned upon ultraviolet (UV) irradiation and dark storage cycles. This behavior is attributed to the Young’s modulus variation of the single fibrils due to the photoisomerization process of the DAE molecules, as measured by advanced atomic force microscopy (AFM) techniques. In fact, a significant yet reversible decrease of the stiffness of the outer part of the fibrils in response to the UV light irradiation is demonstrated. The modification of the molecular structure of the fibrils influences their mechanical properties and affects the frictional behavior of the overall fibrillar surfaces. These findings provide the possibility to develop a system that controllably and accurately generates both low and high friction forces.

1. INTRODUCTION Fibrillar surfaces can be found in numerous biological attachment systems. Animals such as lizards, arachnids, and insects rely on such surfaces to climb up smooth vertical surfaces. The fact that unrelated species have evolved independently similar fibrillar architectures reflects the effectiveness of these surfaces to solve certain tribological problems.1−5 Not surprisingly, these structures have raised considerable interest and have become an important research area. The advances in the area of micro- and nanoscale manufacturing have allowed the fabrication of synthetic fibrillar surfaces that match and in some cases surpass the performance of the biological counterparts.6−8 Their attachment behavior is driven by the design of the micro- and nanostructures that compose the fibrillar surfaces. More specifically, geometric parameters such as the fibril tip,9−20 aspect ratio,21,22 tilting angle,23−27 and hierarchy28−30 determine the mechanical response of the individual elements in contact with a surface. Although the use of well-defined micro- and nanoscale geometries has demonstrated considerable potential for high adhesion and friction, the achievement of their controlled tunability is still challenging. Furthermore, available processing approaches suitable for the fabrication of micro- and nanopatterns with precise shape control on polymeric surfaces are based on lithographic techniques that are in general sophisticated and require expensive equipment. To achieve tunability, responsive materials such as photochromic molecules could be very attractive systems since they © 2015 American Chemical Society

change their properties reversibly under the action of an external stimulus.31 Upon light irradiation, photochromic molecules undergo reversible transformation between two forms with different molecular structures, the isomers. The electronic and geometric differences between the isomers are responsible for changes in their physical properties, such as electrical conductivity,32 refractive index,33 or volume.34 Some photochromic molecules, such as diarylethenes35 (DAEs), also have the ability to self-assemble in micro- and nanofibrils of crystalline nature directly on a solid surface. So far, this kind of fibrillar surfaces have been utilized as superhydrophobic systems that undergo topographical modifications upon ultraviolet (UV) light irradiation. In turn, the modification of their topography causes changes in their wettability.36−40 However, according to our knowledge, a tribological investigation of DAE fibrillar surfaces has never been reported before. In this work, we investigate the frictional forces acting on steel spheres sliding on top of DAE micro- and nanofibrillar surfaces as a function of the UV irradiation time (t). At the same time, the topographical and mechanical properties of individual fibrils are investigated. Interestingly, the variation of the Young’s modulus at the nanoscale measured by atomic force microscopy (AFM) techniques demonstrates a significant yet reversible decrease of the stiffness of the outer part of the Received: March 18, 2015 Revised: May 14, 2015 Published: May 27, 2015 6072

DOI: 10.1021/acs.langmuir.5b01004 Langmuir 2015, 31, 6072−6077

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absolute value of Young’s modulus, each single force curve was fitted with the Hertz model, and the tip was approximated as spherical and with a radius of 20 nm. AFM Imaging. Atomic force microscopy imaging was performed by using a Nanowizard 3 (JPK Instruments, Germany) mounted on an Axio Observer D1 inverted optical microscope (Carl Zeiss, Germany). The images were acquired in noncontact mode, working in air in a vibration-insulated environment. Images of 5 × 5 μm2 were collected with 256 data points per line. PPP-NCHR cantilevers (Nanosensors, USA) with a drive frequency of ∼295 kHz were used. Working set point was kept above 70% of free oscillation amplitude and measurements were taken at scan rate of 0.7 Hz. UV−visible Measurements. UV−visible absorption spectra of DAE crystals grown on PDMS were obtained using a spectrophotometer Cary 6000i from Varian.

fibrils upon UV light irradiation. We demonstrate that the macroscopic friction behavior of DAE fibrillar surfaces can be directly correlated with the stiffness variation of the individual fibrils.

2. EXPERIMENTAL SECTION Materials. 2,3-bis(2,4,5-Trimethyl-3-thienyl)maleimide (>96%) was purchased from TCI Europe N.V. (Zwijndrecht, Belgium), Sylgard 184 silicone was provided by Dow Corning Corporation (Midland, MI, USA), and chloroform (≥99%) was purchased from Sigma-Aldrich. Preparation of the Films. Sylgard 184 polydimethylsiloxane (PDMS) elastomer was supplied as a two-part liquid component kit, a pre-polymer base (part A) and a cross-linking curing agent (part B). Part A and part B were thoroughly mixed in a weight ratio of 10:1. To fabricate bubble-free samples, the uncured PDMS was degassed in a vacuum desiccator. The degassed PDMS mixture was poured into a Petri dish, and it was cured for 1 h on a digitally controlled hot plate at 70 °C. After thermal curing, the PDMS was carefully peeled off and cut into pieces of 0.5 × 1 cm2. The PDMS samples were then dipped for 1 h in a chloroform solution containing DAE molecules in a concentration of 10 mg mL−1. During dipping, the chloroform swelled the PDMS which entrapped DAE molecules in its free volume. Once the PDMS films were removed from the solution and the chloroform evaporated, the samples returned to their initial size and shape. The films were stored at room temperature in the dark for 2 weeks, allowing to the fibrils to grow. SEM Measurements. The morphology of DAE microcrystalline surfaces was investigated by scanning electron microscopy using a JEOL JSM-6490LA microscope (Jeol, Tokyo, Japan) equipped with a tungsten thermionic gun working at an acceleration voltage of 20 kV. Samples were previously coated with a 10-nm-thick gold layer using a Cressington 208HR high-resolution sputter coater (Cressington Scientific Instrument Ltd., U.K.). Contact Angle Measurements. Water contact angles were determined by a video-based OCAH 200 (DataPhysics, Germany), using drop volumes of 3 μL. UV Light Exposure. Irradiation was carried out using a UV−vis fiber light source (spot light source LC8 Lightningcure L9588, Hamamatsu) at λ < 400 nm (Power Density, PD = 0.13 mW mm−2). The samples were irradiated for a time ranging from 30 to 300 s. Friction Force Measurements. Friction forces were measured using a micro scratch tester (CSM Instruments SA, Peseux, Switzerland) equipped with a spherical steel tip with a radius R = 500 μm. Each test started by bringing the tip in contact with the sample with a preload of 30 mN, placed on a translation stage. After applying a normal load ranging from 150 to 300 mN, the stage was moved in the lateral direction at a velocity v = 1 mm min−1 for a distance l = 1 mm, while the friction force resisting the motion of the tip onto the DAE microcrystalline surface was recorded. The samples were exposed to UV light without being removed from the translation stage, so that the measured friction forces were taken on exactly the same points. To exclude modifications of the fibrils upon testing that would affect the measurements in UV-exposed conditions, consecutive tests on the same, non-irradiated location were performed prior to the actual tests. No variations in the frictional response were found. Mechanical Characterization of Single Fibrils. Force distance curves (64 × 64) were acquired by using a NanoWizard 3 AFM system (JPK Instruments, Berlin) in quantitative imaging (QI) mode. An Axio Observer D1 inverted optical microscope (Carl Zeiss, Germany), coupled with the AFM system, was used to choose the areas for the mechanical analysis. Post processing of the force curves allowed extracting information such as height (sample topography), adhesion force, or magnitude of repulsive forces acting on the AFM tip per each pixel. PPP-NCHR cantilevers (Nanosensor, USA) with a spring constant of about 42 mN m−1 and with a nominal tip diameter 300 s) a drastic change of the morphology of the fibrils can be expected.40 In the present study, we focus on levels of irradiation for which the shape of the crystals is preserved. The fact that the morphology of the fibrils did not change upon irradiation for the times investigated in this work was confirmed by SEM analysis (see Figure S1 in SI). Moreover, the measured water contact angle values of the samples used in the present study, which depend on both the surface polarity and morphology, were practically the same for pristine samples (133.6 ± 1.9)° and UV-irradiated samples (134.7 ± 1.6)°.

simple way, without the use of challenging microfabrication techniques. The photochromic behavior of the DAE fibrils on PDMS films is shown in Figure 1b by comparing the UV−visible absorption spectra before and after exposure to UV light, and after 24 h of dark storage. From the spectra, there is the clear appearance of two bands centered at 375 and 525 nm that can be assigned to the ring-closing reaction as depicted in the inset of Figure 1b. Figure 2 shows the changes in friction force measured on a fibrillar surface upon UV light irradiation for different times

Figure 2. Friction force as a function of the irradiation time of a DAE fibrillar surface for a normal load of 150 mN (diamonds), 200 mN (circles), and 250 mN (triangles).

within a range 0 < t < 300 s (λ < 400 nm, PD = 0.13 mW mm−2) for normal loads F = 150 mN (diamonds), 200 mN (circles) and 250 mN (triangles). As it can be seen, when the fibrils are exposed to UV light for a time 0 < t < 60 s, there is a significant increase of the friction force with the irradiation time for any applied normal load. The increase is in the range between 17% (for F = 250 mN) and 37% (for F = 150 mN), whereas for irradiation time 60 < t < 300 s, the friction force remains unaltered. For prolonged irradiation (t > 300 s), a gradual decrease of the friction force is observed (results not shown).

Figure 3. Histograms of the Young’s modulus density obtained (a) for a pristine sample, (b) after UV light irradiation (t = 60 s), and (c) after 24 h of storage in the dark at room temperature. 6074

DOI: 10.1021/acs.langmuir.5b01004 Langmuir 2015, 31, 6072−6077

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Langmuir To rationalize the way modifications of the local stiffness affect the tribological properties of the system, the deformation of a fibril in contact with the rigid sphere is analyzed using the Hertz theory.44 We consider the fibril pressed against the sphere as a cylinder of diameter df, completely clamped at its base, making contact with rigid half−plane over a length L (the sphere is considered infinite in radius). The area of contact A upon the application of a load F is a rectangle of width 2b and length L. The half-width b is given by the equation44 b=

2Fdf ⎛ 1 − νf2 1 − νs2 ⎞ ⎜ ⎟ + πL ⎜⎝ Ef Es ⎟⎠

(1)

where Ef, Es are the elastic moduli and νf, νs are the Poisson’s ratios associated with the fibrils and sphere materials, respectively. By looking at eq 1, it is clear that the contact area A = 2bL, and therefore the friction force, increases by increasing the external load F and by reducing Young’s modulus of the fibrils, Ef. An additional contribution to the increase of the frictional force can be attributed to the reduced bending stiffness of the DAE fibrils upon UV light irradiation. The bending stiffness of a fibril is equal to the product of the elastic modulus Ef and the area moment of inertia I of the fibril cross−section. For stable load F, as the Young’s modulus Ef decreases, the fibrils deform more due to the reduction of bending stiffness, and consequently they increase the length of side contact with the sphere. It should be pointed out that the area moment of inertia is increased with the square power of the distance from the neutral axis of bending. Therefore, changes in stiffness even of a thin layer of the fibrils can affect their bending stiffness greatly. Details of both models are included in the SI. Photo-induced dimensional changes of crystalline structures of photochromic molecules and systems containing photochromic molecules have been reported in the literature.45−50 Thus, a role of the dimensional change of the fibrils in the bending stiffness modification cannot be excluded. To investigate this aspect, the morphology of individual DAE fibrils was monitored before and after UV light irradiation by AFM imaging. Figure 4a−b shows the AFM images of two DAE fibrils (insets) and the line profiles of their sections (dotted white line) before and after 60 s of UV light irradiation, respectively. The profiles of the fibrils indicate that the UV light does not affect their dimension and, therefore, it cannot affect their bending stiffness. The above-demonstrated reversibility of the softening phenomenon induced on DAE fibril crystals upon UV light irradiation and dark storage immediately provokes the question whether the friction force of the DAE crystalline surfaces would follow the same trend. To investigate this possibility, the friction properties of DAE fibrillar surfaces were tested before irradiation, after 60 s of irradiation, and after storage in the dark for 24 h, for three repeating cycles. Figure 5 shows the friction force measured for normal loads ranging from 150 to 250 mN. As it can be seen, after the increase of the friction force upon UV light exposure, the DAE crystals restore their initial friction properties after 24 h of storage in the dark. The return to the initial frictional behavior can be safely correlated with the reversible change of the Young’s modulus of the crystals. Therefore, this system allows to maximize or minimize the friction force generated by a fibrillar surface by exposing it to UV light or by keeping it in the dark, respectively.

Figure 4. Line profiles of fibrils section (a) before and (b) after UV light irradiation (t = 60 s). Insets are a 3D rendering of the AFM images. The line sections are indicated in the insets with a dotted white line.

Figure 5. Changes in friction force upon irradiation with UV light for t = 60 s (shaded regions) and recovery in the dark for 24 h (non-shaded regions) for a normal load of 150 mN (diamonds), 200 mN (circles), and 250 mN (triangles).

4. CONCLUSIONS In summary, we reported on the friction properties of DAE crystalline fibrillar surfaces. The crystals were formed on PDMS surfaces after simply dipping the elastomer in a solution containing the photochromic molecules. The friction force of such fibrillar surfaces gradually increased when exposed to UV light for short irradiation times and then returned to the initial value upon dark storage for 24 h. The increase of friction was ascribed to the isomerization of the DAE molecules that are located at the outer part of the fibrils, provoking a change of 6075

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their outer layer from a crystal to an amorphous state, and resulting in a reduced Young’s modulus of the DAE crystals. The change of the Young’s modulus was reversible since its initial value was recovered after dark storage, providing the possibility to develop a system with photo-tunable and reversible friction properties. These findings are of particular interest from both scientific and technological points of view, and they could help in developing devices requiring quantitative and dynamic control of friction.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

SEM images of DAE fibrillar surfaces irradiated for t = 30 s, t = 180 s, and t = 300 s (Figure S1), fit to the data of the Young’s modulus values, expanded discussion on the contact area increase of the UV−irradiated fibrils (Figure S2), discussion on the bending stiffness of cylindrical core−shell systems. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01004.

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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