Geckos can run rapidly on walls and ceilings, requiring high friction forces (on walls) and adhesion forces (on ceilings), with typical step intervals of Ϸ20 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 submicronsized 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 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 between the large number of spatulae in contact with the substrate. To detach, the high adhesion͞friction 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 gecko's locomotion mechanism.tape model ͉ pulling angle ͉ lever function ͉ spatula ͉ seta

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 90o . 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 to 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 o 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.

Viscoelasticity-induced particle migration has recently received increasing attention due to its ability to obtain high-quality focusing over a wide range of flow rates. However, its application is limited to low throughput regime since the particles can defocus as flow rate increases. Using an engineered carrier medium with constant and low viscosity and strong elasticity, the sample flow rates are improved to be one order of magnitude higher than those in existing studies. Utilizing differential focusing of particles of different sizes, here we present sheathless particle/cell separation in simple straight microchannels that possess excellent parallelizability for further throughput enhancement. The present method can be implemented over a wide range of particle/cell sizes and flow rates. We successfully separate small particles from larger particles, MCF-7 cells from red blood cells (RBCs), and Escherichia coli (E. coli) bacteria from RBCs in different straight microchannels. The proposed method could broaden the applications of viscoelastic microfluidic devices to particle/cell separation due to the enhanced sample throughput and simple channel design.Continuous manipulation and separation of particles and cells is important for a wide range of applications in biology, 1,2 medicine, 2,3 and industry. [4][5][6] Microfluidic systems have been proven to be promising tools for particle/cell manipulation with higher sensitivity and accuracy than their macroscale counterparts. The last decade has seen extensive development of microfluidic approaches for particle/cell manipulation that resort to immunocapture, 7 externally applied physical fields, [8][9][10][11][12][13][14][15][16][17][18] microfiltration, 19,20 gravitational sedimentation, 21 or deterministic lateral migration. 22,23 More recently, cross-streamline migration induced by the hydrodynamic effects of carrier media, such as inertia 24,25 and viscoelasticity, 26,27 has shown its promise for effective particle/cell manipulation without need of labeling and external force fields. Particles and cells can be separated based on the size-dependent nature of hydrodynamic forces. Briefly, the inertial lift scales as F a ∝ , where a is the particle diameter. There are several cell types of biological and clinical interest with separable size ranges: epithelial tumor cells (15-25 µm in diameter), blood cells (erythrocytes are 6-8 µm biconcave disks and peripheral blood lymphocytes are 7-10 µm in diameter), and bacteria (1-3 µm). Inertial migration in Newtonian fluids has been intensively studied and implemented in high-throughput label-free separation devices for cell separation. [28][29][30][31][32][33][34] Recently, particle migration induced by viscoelasticity has begun to attract increasing attention due to its simple focusing pattern and potential for achieving efficient focusing over a wide range of flow rates. 35,36 In a viscoelastic medium, elasticity coupled with non-negligible inertia will drive particles towards the channel centerline, whic...

Liquid phase exfoliated graphene sheets were modified by oleic acid and dispersed in lubricant oils as additives. The tribological behaviours of graphene-contained oils were investigated using a four-ball tribometer. The lubricant with optimized graphene concentrations of 0.02–0.06 wt% showed enhanced friction and anti-wear performance, with friction coefficient and wear scar diameter reduced by 17% and 14%, respectively.

The extraordinary climbing ability of geckos is partially attributed to the fine structure of their toe pads, which contain arrays consisting of thousands of micrometer-sized stalks (setae) that are in turn terminated by millions of fingerlike pads (spatulae) having nanoscale dimensions. Using a surface forces apparatus (SFA), we have investigated the dynamic sliding characteristics of setal arrays subjected to various loading, unloading, and shearing conditions at different angles. Setal arrays were glued onto silica substrates and, once installed into the SFA, brought toward a polymeric substrate surface and then sheared. Lateral shearing of the arrays was initiated along both the "gripping" and "releasing" directions of the setae on the foot pads. We find that the anisotropic microstructure of the setal arrays gives rise to quite different adhesive and tribological properties when sliding along these two directions, depending also on the angle that the setae subtend with respect to the surface. Thus, dragging the setal arrays along the gripping direction leads to strong adhesion and friction forces (as required during contact and attachment), whereas when shearing along the releasing direction, both forces fall to almost zero (as desired during rapid detachment). The results and analysis provide new insights into the biomechanics of adhesion and friction forces in animals, the coupling between these two forces, and the specialized structures that allow them to optimize these forces along different directions during movement. Our results also have practical implications and criteria for designing reversible and responsive adhesives and articulated robotic mechanisms.

Confined diffusion is ubiquitous in nature. Ever since the “anomalous yet Brownian” motion was observed, the non-Gaussianity in confined diffusion has been unveiled as an important issue. In this Letter, we experimentally investigate the characteristics and source of non-Gaussian behavior in confined diffusion of nanoparticles suspended in polymer solutions. A time-varied and size-dependent non-Gaussianity is reported based on the non-Gaussian parameter and displacement probability distribution, especially when the nanoparticle’s size is smaller than the typical polymer mesh size. This non-Gaussianity does not vanish even at the long-time Brownian stage. By inspecting the displacement autocorrelation, we observe that the nanoparticle–structure interaction, indicated by the anticorrelation, is limited in the short-time stage and makes little contribution to the non-Gaussianity in the long-time stage. The main source of the non-Gaussianity can therefore be attributed to hopping diffusion that results in an exponential probability distribution with the large displacements, which may also explain certain processes dominated by rare events in the biological environment.

Two-dimensional materials having a layered structure comprise a monolayer or multilayers of atomic thickness and ultra-low shear strength. Their high specific surface area, in-plane strength, weak layer-layer interaction, and surface chemical stability result in remarkably low friction and wear-resisting properties. Thus, 2D materials have attracted considerable attention. In recent years, great advances have been made in the scientific research and industrial applications of anti-friction, anti-wear, and lubrication of 2D materials. In this article, the basic nanoscale friction mechanisms of 2D materials including interfacial friction and surface friction mechanisms are summarized. This paper also includes a review of reports on lubrication mechanisms based on the film-formation, self-healing, and ball bearing mechanisms and applications based on lubricant additives, nanoscale lubricating films, and space lubrication materials of 2D materials in detail. Finally, the challenges and potential applications of 2D materials in the field of lubrication were also presented.

Geckos have the extraordinary ability to prevent their sticky feet from fouling while running on dusty walls and ceilings. Understanding gecko adhesion and self-cleaning mechanisms is essential for elucidating animal behaviours and rationally designing gecko-inspired devices. Here we report a unique self-cleaning mechanism possessed by the nano-pads of gecko spatulae. The difference between the velocity-dependent particle-wall adhesion and the velocity-independent spatula-particle dynamic response leads to a robust self-cleaning capability, allowing geckos to efficiently dislodge dirt during their locomotion. Emulating this natural design, we fabricate artificial spatulae and micromanipulators that show similar effects, and that provide a new way to manipulate micro-objects. By simply tuning the pull-off velocity, our gecko-inspired micromanipulators, made of synthetic microfibers with graphene-decorated micro-pads, can easily pick up, transport, and drop-off microparticles for precise assembling. This work should open the door to the development of novel self-cleaning adhesives, smart surfaces, microelectromechanical systems, biomedical devices, and more.