Noshir S. Pesika

Adhesion and friction in gecko toe attachment and detachment

Geckos can run rapidly on walls and ceilings, requiring high friction forces (on walls) and adhesion forces (on ceilings), with typical step intervals of ≈20 ms. The rapid switching between gecko foot attachment and detachment is analyzed theoretically based on a tape model that incorporates the adhesion and friction forces originating from the van der Waals forces between the submicron-sized spatulae and the substrate, which are controlled by the (macroscopic) actions of the gecko toes. The pulling force of a spatula along its shaft with an angle θ between 0 and 90° to the substrate, has a “normal adhesion force” contribution, produced at the spatula-substrate bifurcation zone, and a “lateral friction force” contribution from the part of spatula still in contact with the substrate. High net friction and adhesion forces on the whole gecko are obtained by rolling down and gripping the toes inward to realize small pulling angles θ between the large number of spatulae in contact with the substrate. To detach, the high adhesion/friction is rapidly reduced to a very low value by rolling the toes upward and backward, which, mediated by the lever function of the setal shaft, peels the spatulae off perpendicularly from the substrates. By these mechanisms, both the adhesion and friction forces of geckos can be changed over three orders of magnitude, allowing for the swift attachment and detachment during gecko motion. The results have obvious implications for the fabrication of dry adhesives and robotic systems inspired by the geckos locomotion mechanism.

Recent advances in the surface forces apparatus (SFA) technique

The surface forces apparatus (SFA) has been used for many years to measure the physical forces between surfaces, such as van der Waals (including Casimir) and electrostatic forces in vapors and liquids, adhesion and capillary forces, forces due to surface and liquid structure (e.g. solvation and hydration forces), polymer, steric and hydrophobic interactions, bio-specific interactions as well as friction and lubrication forces. Here we describe recent developments in the SFA technique, specifically the SFA 2000, its simplicity of operation and its extension into new areas of measurement of both static and dynamic forces as well as both normal and lateral (shear and friction) forces. The main reason for the greater simplicity of the SFA 2000 is that it operates on one central simple-cantilever spring to generate both coarse and fine motions over a total range of seven orders of magnitude (from millimeters to angstroms). In addition, the SFA 2000 is more spacious and modulated so that new attachments and extra parts can easily be fitted for performing more extended types of experiments (e.g. extended strain friction experiments and higher rate dynamic experiments) as well as traditionally non-SFA type experiments (e.g. scanning probe microscopy and atomic force microscopy) and for studying different types of systems.

Peel-Zone Model of Tape Peeling Based on the Gecko Adhesive System

A tape-peeling model based on the geometry of the peel zone (PZ) is derived to predict the peeling behavior of adhesive tapes at peel angles less than or equal to 90°. The PZ model adds an angle-dependent multiplier to the Kendall equation that takes into account the geometrical changes within the peel zone. The model is compared with experimental measurements of the peel force at different angles for a model tape and two commercial tapes, each with different bending moduli, stretch moduli, and adhesive strengths. Good agreement is found for a wide range of peel angles. The PZ model is also applied to the gecko adhesive system and predicts a spatula peel angle of 18.4° to achieve the adhesion forces reported for single setae. The PZ model captures the fact that adhesive forces can be significantly enhanced by peeling at an angle, thereby exploiting high friction forces between the detaching material and the substrate.

Design and Fabrication of Gecko-Inspired Adhesives

Recently, there has been significant interest in developing dry adhesives mimicking the gecko adhesive system, which offers several advantages compared to conventional pressure-sensitive adhesives. Specifically, gecko adhesive pads have anisotropic adhesion properties; the adhesive pads (spatulae) stick strongly when sheared in one direction but are non-adherent when sheared in the opposite direction. This anisotropy property is attributed to the complex topography of the array of fine tilted and curved columnar structures (setae) that bear the spatulae. In this study, we present an easy, scalable method, relying on conventional and unconventional techniques, to incorporate tilt in the fabrication of synthetic polymer-based dry adhesives mimicking the gecko adhesive system, which provides anisotropic adhesion properties. We measured the anisotropic adhesion and friction properties of samples with various tilt angles to test the validity of a nanoscale tape-peeling model of spatular function. Consistent with the peel zone model, samples with lower tilt angles yielded larger adhesion forces. The tribological properties of the synthetic arrays were highly anisotropic, reminiscent of the frictional adhesion behavior of gecko setal arrays. When a 60° tilt sample was actuated in the gripping direction, a static adhesion strength of ~1.4 N/cm(2) and a static friction strength of ~5.4 N/cm(2) were obtained. In contrast, when the dry adhesive was actuated in the releasing direction, we measured an initial repulsive normal force and negligible friction.

Carbon Microspheres as Ball Bearings in Aqueous-Based Lubrication

We present an exploratory study on a suspension of uniform carbon microspheres as a new class of aqueous-based lubricants. The surfactant-functionalized carbon microspheres (∼0.1 wt %) employ a rolling mechanism similar to ball bearings to provide low friction coefficients (μ ≈ 0.03) and minimize surface wear in shear experiments between various surfaces, even at high loads and high contact pressures. The size range, high monodispersity, and large yield stress of the C(μsphere), as well as the minimal environmental impact, are all desirable characteristics for the use of a C(μsphere)-SDS suspension as an alternative to oil-based lubricants in compatible devices and machinery.

Reversible shear thickening at low shear rates of electrorheological fluids under electric fields

By shearing electrorheological (ER) fluids between two concentric cylinders, we show a reversible shear thickening of ER fluids above a low critical shear rate (<1 s(-1)) and a high critical electric field strength (>100 V/mm), which can be characterized by a critical apparent viscosity. Shear thickening and electrostatic particle interaction-induced interparticle friction forces are considered to play an important role in the origin of lateral shear resistance of ER fluids, while the applied electric field controls the extent of shear thickening. The electric-field-controlled reversible shear thickening has implications for high-performance electrorheological-magnetorheological fluid design, clutch fluids with high friction forces triggered by applying a local electric field, other field-responsive materials, and intelligent systems.

Dynamic friction in natural and synthetic gecko setal arrays

Geckos can cling to almost any surface using dense arrays of microscopic, hierarchical setae. The flat, terminal branches of the setae adhere by the van der Waals dispersion force, and the mechanics of the gecko attachment system are a current topic among biologists and researchers of smart materials for adhesion. We studied the interaction between shear velocity (v = 0.0005 mm s−1 to 158 mm s−1) and materials properties on dynamic friction of isolated natural gecko setal arrays. We varied the materials properties (complex modulus) of the setal β-keratin by adjusting atmospheric humidity (RH). Alongside the natural material, we performed similar experiments on synthetic arrays of polyurethane micropillars. Our experiments demonstrate the presence of two regimes in the friction force (F) vs. velocity behavior of the natural adhesives: a materials/RH-dependent domain exists at low v (<1 mm s−1) and a materials/RH-independent domain at higher v. At intermediate velocities, F(v) curves at different RH converge to an RH-independent value. From the dynamic experiments on natural arrays, we calculated a high-v activation volume (V*) of (90.1 ± 0.3) nm3. V* gives an indication of the strength of coupling between sliding elements. Velocity strengthening occurred in synthetic arrays. However, in contrast to the natural material, strengthening of adhesion and friction of synthetic gecko setae occurred at low v and weakened at high v. Activation volumes calculated for the synthetic arrays indicate weaker coupling. These results indicate (i) that the theory of state-rate friction (SRF) can adequately describe the behavior of sliding fibrillar adhesives and (ii) that the macroscopic performance of natural and synthetic setal arrays, when interpreted with an SRF model, provides some insight into the microscopic dynamics of frictional sliding.

Bridging nanocontacts to macroscale gecko adhesion by sliding soft lamellar skin supported setal array

The study of the mechanism of the controlled adhesion of geckos, which is important for the design and fabrication of bio-inspired dry and reversible adhesive surfaces, is widely discussed below the setal level. In this work, the role of the soft lamellar skin in gecko toe adhesion was experimentally revealed. The lamellar skin acting as a soft spring sustains most of the normal deformation during preloading and maintains a wide range of adhesive state rather than a repulsive state. The sequential engagement and peeling off of setal array are responsible for the reliable gecko adhesion and friction control. This soft spring supported pillar structure should be adopted in future bio-inspired adhesives design. A hybrid three-legged spring/setae clamp was developed to transfer a horizontally placed silicon wafer. It indicates the importance of integration and optimization of nanoscale structures as well as the incorporation of their unique, size-dependent properties into functional macroscale devices.

Trilayered Film with Excellent Tribological Performance: A Combination of Graphene Oxide and Perfluoropolyethers

In the current paper, trilayered films (abridged as APS-GO/PFPE) composed of graphene oxide (GO) and perfluoropolyethers (PFPE) were fabricated successfully on the silicon substrate pre-modified with a self-assembled monolayer of 3-aminopropyl triethoxysilane. The so-prepared films were characterized by a range of complementary techniques including Raman spectroscopy, contact angle measurements, X-ray photoelectron spectroscopy, and atomic force microscopy, to reveal the surface chemical compositions and surface morphologies. Furthermore, the microtribological behavior of the so-prepared films was studied on a ball-on-plate tribometer. The APS-GO/PFPE film showed better friction-reducing and wear-resisting properties as compared with the control samples, which was ascribed to the excellent wear-resisting properties of GO and the excellent lubricating nature and low shear strength of PFPE.

Additive-mediated electrochemical synthesis of platelike copper crystals for methanol electrooxidation.

A room-temperature electrochemical approach to synthesizing anisotropic platelike copper microcrystals and nanocrystals in the presence of potassium bromide is presented. Morphological and elemental characterization was performed using SEM, TEM, and XRD to confirm the anisotropic morphology and crystal structure of the synthesized copper particles. A possible mechanism for explaining the anisotropic crystal growth is proposed on the basis of the preferential adsorption of bromide ions to selective crystal faces. The shape-dependent electrocatalytic property of copper particles is demonstrated by its enhanced catalytic activity for methanol oxidation. Further development of such anisotropic copper particles localized on an electrode surface will lead us to find a suitable alternative for noble metal-based electrocatalysts for the methanol oxidation reaction relevant to fuel cells.