Stick slip contact mechanics between dissimilar materials: effect of charging and large friction

Influence of Lateral Movement on Level Behavior of Adhesion Force Measured Repeatedly by an Atomic Force Microscope (AFM) Colloid Probe in Dry Conditions

An atomic force microscope (AFM) was operated to repeatedly measure the adhesion forces between a polystyrene colloid probe and a gold film, with and without lateral movement in dry conditions. Experimental results show that the adhesion force shows a level behavior without lateral movement and with a small scan distance: the data points are grouped into several levels, and the adhesion force jumps between different levels frequently. This was attributed to the fact that when the cantilever pulls off the sample, the contact area of the sample is not exactly the same between successive contacts and jumps randomly from one to another. Both lateral velocity and material wear have little influence on level behavior. However, with a medium scan distance, level behavior is observed only for some measurements, and adhesion forces are randomly distributed for the other measurements. With a large scan distance, adhesion forces are randomly distributed for all measurements. This was attributed to the fact that the cantilever pulls off the sample in many different contact areas on the scanning path for large distances. These results may help understand the influence of lateral movement and imply the contribution of asperities to adhesion force.

Understanding Surface Adhesion in Nature: A Peeling Model

Nature often exhibits various interesting and unique adhesive surfaces. The attempt to understand the natural adhesion phenomena can continuously guide the design of artificial adhesive surfaces by proposing simplified models of surface adhesion. Among those models, a peeling model can often effectively reflect the adhesive property between two surfaces during their attachment and detachment processes. In the context, this review summarizes the recent advances about the peeling model in understanding unique adhesive properties on natural and artificial surfaces. It mainly includes four parts: a brief introduction to natural surface adhesion, the theoretical basis and progress of the peeling model, application of the peeling model, and finally, conclusions. It is believed that this review is helpful to various fields, such as surface engineering, biomedicine, microelectronics, and so on.

Role of contact electrification and electrostatic interactions in gecko adhesion

Geckos, which are capable of walking on walls and hanging from ceilings with the help of micro-/nano-scale hierarchical fibrils (setae) on their toe pads, have become the main prototype in the design and fabrication of fibrillar dry adhesives. As the unique fibrillar feature of the toe pads of geckos allows them to develop an intimate contact with the substrate the animal is walking on or clinging to, it is expected that the toe setae exchange significant numbers of electric charges with the contacted substrate via the contact electrification (CE) phenomenon. Even so, the possibility of the occurrence of CE and the contribution of the resulting electrostatic interactions to the dry adhesion of geckos have been overlooked for several decades. In this study, by measuring the magnitude of the electric charges, together with the adhesion forces, that gecko foot pads develop in contact with different materials, we have clarified for the first time that CE does contribute effectively to gecko adhesion. More importantly, we have demonstrated that it is the CE-driven electrostatic interactions which dictate the strength of gecko adhesion, and not the van der Waals or capillary forces which are conventionally considered as the main source of gecko adhesion.

Instabilities of Teflon AF thin films in alumina nanochannels and adhesion of bi-level Teflon AF nanopillars

In this paper, a novel replica-molding technique for fabrication of bi-level Teflon AF nanopillars, as an electrostatic-based dry adhesive, is reported. The technique reported herein relies on the concurrent heating and cooling of the Teflon AF melt which filled vertically aligned alumina nanochannels as the mold. Unlike conventional polymer infiltration methods which consist of filling the mold by only heating the polymer above its glass transition temperature, in the current method, the polymer melt was also simultaneously cooled down during the infiltration process. Concurrent cooling of the Teflon AF melt allowed control over the interfacial instabilities of the polymer thin film, which formed ahead of the polymer melt upon its infiltration into the alumina nanochannels. By doing so, the geometrical properties of the peculiar fluffy nanostructure which was subsequently developed-after removal of the mold-on top of the extremely high aspect-ratio Teflon AF nanopillars (200 nm in diameter, ~25 μm tall) were modified. The height of the base nanopillars was measured and the structural properties (i.e., surface area fraction and roughness) of the fluffy nanostructure terminating the base nanopillars at the tip were quantified. Next, the effects of the topographical properties of the bi-level Teflon AF nanopillars on their adhesion, in both the normal and shear directions, were investigated. Tribological results were discussed in detail to clarify the contribution of the structural properties of the fabricated dry adhesive toward its remarkable adhesion and friction forces generated via contact electrification. It is worthwhile to mention that bi-level Teflon AF nanopillars with these specific structural properties have generated enhanced adhesion and friction strengths, up to ~2.1 and 13 N cm(-2), respectively.

Interfacial water structure at polymer gel/quartz interfaces investigated by sum frequency generation spectroscopy

Interfacial structures of water at polyvinyl alcohol (PVA) and poly(2-acrylamido-2-methypropane) sulfonic acid sodium salt (PNaAMPS)/quartz interfaces were investigated by sum frequency generation (SFG) spectroscopy. Two broad peaks were observed in OH stretching region at 3200 and 3400 cm(-1), corresponding to the symmetric OH stretching of tetrahedrally coordinated, i.e., strongly hydrogen bonded "ice-like" water, and the asymmetric OH stretching of water in a more random arrangement, i.e., weakly hydrogen bonded "liquid-like" water, respectively, in both cases. The "liquid-like" water became dominant when the PVA gel was pressed against the quartz surface. The relative intensity of the SFG signal due to the "liquid-like" water to that due to the "ice-like water" at the quartz surface modified with a self-assembled monolayer of aminopropyltrimethoxysilane (APS) became higher when the negatively charged PNaMPS gel was contacted to the APS modified quartz surface in a solution of pH = 12, where the surface was negatively charged and electrostatic repulsive interaction and low friction were present between the PNaMPS gel and the APS modified surface. It, however, did not change in a solution of pH = 2, where the surface was positively charged and electrostatic attractive interaction and very high friction were present between the PNaMPS gel and the APS modified surface. These results suggest the important role of water structure for small friction at the polymer gel/solid interface.