Biomimetic patterned surfaces for controllable friction in micro- and nanoscale devices
© Singh and Suh; licensee Springer. 2013
Received: 21 September 2013
Accepted: 6 December 2013
Published: 18 December 2013
Biomimetics is the study and simulation of biological systems for desired functional properties. It involves the transformation of underlying principles discovered in nature into man-made technologies. In this context, natural surfaces have significantly inspired and motivated new solutions for micro- and nano-scale devices (e.g., Micro/Nano-Electro-Mechanical Systems, MEMS/NEMS) towards controllable friction, during their operation. As a generic solution to reduce friction at small scale, various thin films/coatings have been employed in the last few decades. In recent years, inspiration from ‘Lotus Effect’ has initiated a new research direction for controllable friction with biomimetic patterned surfaces. By exploiting the intrinsic hydrophobicity and ability to reduce contact area, such micro- or nano-patterned surfaces have demonstrated great strength and potential for applications in MEMS/NEMS devices. This review highlights recent advancements on the design, development and performance of these biomimetic patterned surfaces. Also, we present some hybrid approaches to tackle current challenges in biomimetic tribological applications for MEMS/NEMS devices.
KeywordsBiomimetics MEMS/NEMS Silicon Polymer Friction Wear Lithography Patterns Contact area Lubrication
In general, lubrication is the most promising solution to reduce friction. However, lubrication becomes difficult when the dimensions of machine elements decrease from macro- to micro/nano-scale. For example, as the size of the machine elements shrinks from 1 mm to 1 μm, the surface area decreases by a factor of 106 whereas the volume decreases by a factor of 109, yielding the surface area-to-volume ratio of 103. Hence, surface forces such as adhesion and friction become significantly influential at such small scales. Further in miniaturized devices, the gap (clearance) between the elements is small (< 1 μm), which prohibits the use of conventional liquid and/or solid lubricants. For example, a drop of liquid lubricant will be in millimeter size and the size of solid lubricants such as graphite or MoS2 is of the same order as the size of the miniaturized devices. In addition, retention of lubricants at the contact points between the moving elements could be a hurdle for further implementation. Thus, minimizing surface forces in miniaturized devices has remained a real challenge.
At micro/nano-scale, friction is in a regime where the contribution from intrinsic/inherent adhesion outweighs that from asperity deformation . Therefore, friction becomes naturally reduced while minimizing intrinsic adhesion, which arises due to the contribution of various attractive forces such as capillary, electrostatic, van der Waal, and chemical forces under different circumstances . Amongst these forces, the capillary force that causes the formation of meniscus-bridge arising from the condensation of water from the environment is strongest . From a material point of view, silicon is the widely used material for the fabrication of MEMS/NEMS, and in recent years SU-8 polymer has become an attractive material . Silicon is hydrophilic in nature and generates a strong capillary force, resulting in high friction that opposes easy motion of components (Figure 1) [4, 6]. Silicon is also brittle and undergoes wear (Figure 1) [2, 6]. In contrast, SU-8 polymer is hydrophobic, but exhibits high friction/wear due to its soft nature .
In the past, various chemical modifications of surfaces such as boundary lubricant thin films/coatings have been adopted to reduce intrinsic adhesion and friction by increasing hydrophobicity and lubricity of MEMS/NEMS materials. Examples include self-assembled monolayers (SAMs) , diamond-like carbon (DLC) coatings , and also vapor phase lubricants . In order to overcome some of the limitations of chemical modifications, bio-inspired topographical modification of surfaces has recently emerged as an effective route to minimize friction at micro/nano-scale. Biomimetic patterned surfaces lower intrinsic adhesion and friction via hydrophobicity (i.e. reduction in surface energy) and decrease in physical contact area between surfaces. The biomimetic surfaces, when applied with nano-scale boundary lubricants, exhibit even more remarkable capability to reduce surface forces, as well as to enhance durability, i.e. wear resistance.
Water contact angle (CA) values, and friction properties of various biomimetic patterned surfaces at nano- and micro-scales
Nano-scale friction force (nN)
Micro-scale friction coefficient (μ)
Silicon flat surface
SU-8 thin film
DLC nano-dot surfaces
PMMA micro-patterns (Replicated Surfaces)
Silicon micro-pillars coated with DLC
Silicon micro-pillars coated with ZDOL
SU-8 micro-patterns coated with PFPE
SU-8 micro-patterns treated with Ar and coated with PFPE
SU-8 micro-patterns treated with O2 and coated with PFPE
Inspiration from Lotus Effect
Biomimetic nano-patterned surfaces
Polymers provide a fertile ground for biomimetic approaches as they offer a wide range of materials and easy fabrication routes capable of creating surfaces with controllable properties. In an earlier work, nano-scale polymer patterns resembling the protuberances on lotus leaves were created on poly(methyl methacrylate) (PMMA) films spin-coated on silicon surfaces, using capillary force lithography (CFL) . Figure 2C shows a scanning electron microscope (SEM) image of the patterns fabricated at a holding time of 60 min. Such nano-patterns are hydrophobic when compared to silicon surface. Moreover, approximate calculation of contact area based on simple geometry reveals that the contact area can be lowered by 7 times in the case of nano-patterns as compared to silicon flat surface. By the virtue of their hydrophobicity and the capability to reduce contact area substantially, the nano-patterns reduced friction force by 14–24 times at nano-scale, when the friction force was measured using an atomic force microscope . In these measurements, when the applied normal load was kept zero, friction force in silicon was measured to be 20 nN, whereas for nano-patterns it was ~0.84 nN. It is worth noting that the friction force exists even at the zero applied normal load, at nano-scale. This is due to the influence of intrinsic adhesion on friction force . The large decrease in friction force induced by the nano-patterns in the absence/presence of an applied normal load is indicative of the ability of the patterned surfaces to significantly reduce inherent adhesion and friction.
In general, contact area plays an important role in defining friction at micro/nano-scale. Unlike at macro-scale where friction is independent of contact area between surfaces (Amonton’s law, [4, 14]), at small-scales friction is strongly dependent on contact area [4, 14, 15]. According to Bowden and Tabor, friction force directly depends on the real area of contact for a single asperity contact (Ff = τ Ar, where τ is the shear strength, an interfacial property, and Ar the real area of contact ). This law is effective at micro/nano-scale and any reduction in real area of contact directly reduces friction between surfaces at these scales. As a result, patterned surfaces directly reduce real area of contact as similar to the protuberances on lotus leaves.
Biomimetic micro-patterned surfaces
Taken together, the large scale inter-asperity distance and uneven heights of the asperities in the replicated surfaces retain the μ value at lower levels for the micro-patterns. Further, it is found that the surface topography of replicated surfaces follows the real leaves on a smoother scale (Figure 7), even though the real leaves have hierarchical roughness due to the presence of micro-scale protuberances and nano-scale wax on them. These findings have important implications towards controllable friction with topographically patterned surfaces. Specifically, a simple replication of the surface structures of natural water-repellent leaves would suffice in order to decrease micro-frictional property via the reduction in contact area. If one wants to reproduce the detailed surface topography of real leaves with high accuracy, it would be difficult, costly, and time-consuming, and further may not contribute to lower friction force at small scale.
Combination of nanolubricant and micro-patterned surfaces
In order to achieve low friction and delay the onset of plastic deformation/wear of polymer patterns, chemical anchoring/grafting of nanolubricant molecules onto micro-patterned surfaces is necessary. A recent biomimetic work reporting on the reduced friction for SU-8 polymer supports this argument . In this work, micro-patterns in the form of micro-bumps were fabricated by nano-imprint lithography on SU-8 thin films spin-coated on silicon surfaces. Results showed that the micro-patterns exhibited μ values 2 times lower than that of SU-8 thin film, but underwent plastic deformation in short time duration of t < 0.5 min (100 cycles).
Surface modifications were conducted to the SU-8 micro-patterns by coating perfluoropolyether (PFPE) nanolubricant in two different ways: (i) direct coating and (ii) coating after their separate exposure to argon (Ar) and oxygen (O2) plasmas. The modified micro-patterns showed a decrease in μ value by > 1.4 times in comparison to that of un-modified micro-patterns, and by > 2.7 times than that of SU-8 thin film. No plastic deformation/wear was observed on the modified micro-patterns when tested for the time duration of t > 8 hours (100,000 cycles), at 0.3 N applied normal load and 15 mm/s sliding speed. The modified micro-patterns exhibited enhanced durability by > 1000 times when compared to that of un-modified micro-patterns and polymer thin film. With an increase in load and speed to 1 N and 42 mm/s, although the modified micro-patterns showed low μ values (≤ 0.15), clear difference was observed in their durability responses. In detail, micro-patterns directly coated with PFPE had durability of 14 min, whereas those pre-treated with Ar and O2 plasmas before being coated with PFPE had durability of 32 min and 60 min, respectively. This is because Ar plasma generates carbonyl groups (C-O and C-O-O groups), while O2 plasma carboxyl groups (COOH groups), each altering the surface properties differently. Thus, the plasma-treated micro-patterns have polar functional groups on their surfaces, to which the hydroxyl groups of PFPE nanolubricant at both their terminal ends get strongly bound, promoting the durability of micro-patterns.
It needs to be further emphasized that the difference in the observed durability amongst the plasma-treated micro-patterns coated with PFPE is attributed to the type of bonding that occurs between PFPE molecules and chemically activated micro-patterned surfaces. In general, hydroxyl groups form covalent bonds with carboxyl groups, whereas with carbonyl groups they form hydrogen bonds. Covalent bonds are stronger than hydrogen bonds and thus when nanolubricant molecules are covalently bonded onto micro-patterned surfaces, they exhibit significant enhancement in their durability in terms of wear resistance capacity. Hence, it was observed that the micro-patterns treated with O2 plasma followed by coating with PFPE showed higher durability when compared to those treated with Ar plasma followed by PFPE coating.
Based on the numerous research outcomes mentioned until now, polymeric micro-patterns with low friction and enhanced durability would be the best candidates for small-scale devices. Also, in other applications such as optoelectronics and photonics, polymer patterns are increasingly employed  for engineering reliable mechanical motion.
As described earlier, the area-to-volume ratio is extremely large at micro/nano-scale, leading us to conclude that friction depends on surface properties rather than bulk material properties. On a comparative note, the difference between thin films/coatings and biomimetic patterned surfaces lies in their approach to reduce/control friction. While the former is based on chemical modification of surfaces, the latter relies on topographical/physical modification.
Contact area in the case of thin films/coatings can be estimated by using the Johnson-Kendall-Roberts (JKR) contact mechanics model : Ar = π[R/K(Fn + 6πγgR + [12πγR Fn + (6πγR)2]1/2)]2/3. Here, R is the size of the counterface ball, K the effective elastic modulus (combination of elastic modulus of coating and counterface), Fn the applied normal load and γ the interfacial energy. For thin films/coatings, SAMs have low elastic modulus  and so the reduction in contact area is not significant. To reduce friction, a complex chemistry has to work such that the suitable selections of end groups, chain length and mixture of monolayers have to be taken into account . Further, achieving uniform coating of SAMs on large surface areas without defects or polymerization poses a great challenge. On the other hand, in DLC coatings, elastic modulus is relatively high so that reduction in contact area plays a supportive role in reducing friction . It is noted that lubrication in DLC coatings mainly occurs by the formation of ‘transfer layer’ . During the formation of transfer layer, not all the material that gets removed from DLC coating is transferred to the counter surface; rather most of it gets released in the form of loose wear particles. Such free wear particles could play an adverse role by interfering physically and/or chemically with the functioning components (DLC has high hardness, electrical insulation, thermal conduction and chemical inertness ). Regarding vapor phase lubrication method , it is necessary to ensure the presence of lubricant vapor at the interfaces for the entire duration time of device operation. Given this situation, it requires a provision to store lubricant vapor within the device and continuously re-supply it to the interfaces. This remains a challenging issue yet to be addressed.
Given the above-mentioned limitations/drawbacks of thin films/coatings and vapor phase lubrication, biomimetic patterned surfaces have some noteworthy salient features: (i) they provide a simple and easy route to reduce friction via the direct reduction in contact area. In addition, their performance can be further enhanced by applying boundary lubricant thin films/coatings. (ii) Lithography techniques used to make patterns are the same as those used to fabricate MEMS/NEMS, indicating that patterning the surfaces of device components can be easily conducted during their fabrication. This makes the process even more convenient and cost- and time-effective compared to other methods. (iii) The main aim to reduce friction in small-scale devices is not to eliminate friction completely, since with zero friction sliding components cannot engage each other. Instead, friction should be controlled in such a way that it should be low enough to support smooth motion, but sufficiently high to engage relative mechanical motion between components, e.g. micro-gears. Taken together, making patterns on component surfaces with varying sizes and shapes along with the application of a variety of boundary lubricants provides greater flexibility and strength to control friction at micro/nano-scale, for desired smooth operations of MEMS/NEMS.
Considering the theoretical modelling of biomimetic surfaces, it is focused towards understanding the effect of hierarchical roughness on adhesion force [36–40], especially ‘roughness induced superhydrophobicity’. The studies aim to support the design of low-adhesion, low-friction and non-wetting surfaces. Wetting involves interactions at different scale levels/sizes, namely macro-scale (water drop), micro-scale (surface texture) and nano-scale (molecules). Hence, understanding the role of hierarchical surface roughness, the transition between various wetting regimes and optimization of roughness etc., becomes important. Modelling has given some insights on solid–liquid interactions. As an example, studies on wetting of micro-patterned surfaces have shown that: (i) the wetting behavior depends on the spacing factor (a micro-scale non-dimensional parameter), and (ii) the contact angle hysteresis and Cassie-Wenzel transition are governed by micro- and nano-scale effects. These insights on the fundamental mechanisms of wetting are very useful in designing non-adhesive surfaces. It is to be noted that when compared to solid–liquid adhesion, dry solid-solid friction is a complex phenomenon that at times is influenced by wear, and so is rather difficult to model.
We have presented that the underlying principles in natural surfaces provide valuable insights towards solving intricate technological issues. Biomimetic engineering of surfaces is an exciting and practical way to control friction at small-scales. Examples presented in this review clearly indicate that by the design and development of biomimetic patterned surfaces, in combination with boundary lubrication, better solutions can be pursued to control friction at small-scales, for the efficient operation and application of miniaturized devices.
Arvind Singh received his doctoral degree from the Indian Institute of Science (IISc), India in 2003. In his doctoral research work, he consolidated a new approach to understand metallic wear based on dynamically evolved microstructure. As a Materials Scientist at the John F. Welch Technology Center (GE India), he investigated tribological systems in aircraft engines and on super-abrasives. During his tenure as a Visiting Scientist at the Korea Institute of Science and Technology (KIST), South Korea, he proposed micro/nano-patterns as biomimetic surfaces for tribological application in MEMS/NEMS. He continued his research on biomimetics at the National University of Singapore (NUS), where he formulated novel surface modification methods to functionalize SU-8 polymer for tribological applications in MEMS/Bio-MEMS and for indwelling biomedical devices. At Vestas Global R&D, he investigated tribology of wind energy systems. Currently, at the Energy Research Institute (ERIAN), Singapore, he is leading tribology and surface engineering R&D projects related to wind & marine renewables.
Kahp-Yang Suh received his doctoral degree from the Seoul National University (SNU), Korea in 2002. He specialized in micro/nano-patterning of polymeric surfaces using capillary force lithography/nanoimprint lithography, and made significant contributions to the field. He was a postdoctoral researcher at the Massachusetts Institute of Technology (MIT), USA. In 2004 he joined as an Assistant Professor at SNU. He was an outstanding faculty and researcher in the field of fabrication of micro/nano-polymeric patterns, whose applications ranged from tribology to lab-on-chip devices. He won a number of awards, including TR100 Young Innovator Award by MIT Technology Review, Best Graduate Student Award by the Brain Korea (BK) 21 Program of the Korean Government, Korea Presidential Young Scientist Award and Young Professor Award by College of Engineering of SNU.
Arvind Singh and Kahp-Yang Suh, shared the ‘First Surface Engineering Best Paper Award’, given by the Society of Tribologists & Lubrication Engineers (STLE), USA (2007), for their pioneering work on biomimetic approach as a robust solution for tribological issues in micro/nano-scale devices.
Arvind Singh dedicates this review article to Kahp-Yang Suh, for his memorable contribution to the field.
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