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.

The gecko adhesive system has attracted significant attention since the discovery that van der Waals interactions, which are always present between surfaces, are predominantly responsible for their adhesion. The unique anisotropic frictional–adhesive capabilities of the gecko adhesive system originate from complex hierarchical structures and just as importantly, the anisotropic articulation of the structures. Here, by cleverly engineering asymmetric polymeric microstructures, a reusable switchable gecko‐like adhesive can be fabricated yielding steady high adhesion (F⊥ ≈ 1.25 N/cm2) and friction (F∥ ≈ 2.8 N/cm2) forces when actuated for “gripping”, yet release easily with minimal adhesion (F⊥ ≈ 0.34 N/cm2) and friction (F∥≈ 0.38 N/cm2) forces during detachment or “releasing”, over multiple attachment/detachment cycles, with a relatively small normal preload of 0.16 N/cm2 to initiate the adhesion. These adhesives can also be used to reversibly suspend weights from vertical (e.g., walls), and horizontal (e.g., ceilings) surfaces by simultaneously and judiciously activating anisotropic friction and adhesion forces. This design opens the way for new gecko‐like adhesive surfaces and articulation mechanisms that do not rely on intensive nanofabrication in order to recover the anisotropic tribological property of gecko adhesive pads, albeit with lower adhesive forces compared to geckos.

Self-assembled monolayers (SAMs) are known to form on a variety of substrates either via chemisorption (i.e., through chemical interactions such as a covalent bond) or physisorption (i.e., through physical interactions such as van der Waals forces or "ionic" bonds). We have studied the behavior and effects of water on the structures and surface energies of both chemisorbed octadecanethiol and physisorbed octadecylamine SAMs on GaAs using a number of complementary techniques including "dynamic" contact angle measurements (with important time and rate-dependent effects), AFM, and electron microscopy. We conclude that both molecular overturning and submolecular structural changes occur over different time scales when such SAMs are exposed to water. These results provide new insights into the time-dependent interactions between surfaces and colloids functionalized with SAMs when synthesized in or exposed to high humidity or bulk water or wetted by water. The study has implications for a wide array of phenomena and applications such as adhesion, friction/lubrication and wear (tribology), surfactant-solid surface interactions, the organization of surfactant-coated nanoparticles, etc.

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 ¼ 0.0005 mm s À1 to 158 mm s À1 ) and materials properties on dynamic friction of isolated natural gecko setal arrays. We varied the materials properties (complex modulus) of the setal b-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 À1 ) 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 AE 0.3) nm 3 . 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.

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.