100 research outputs found
Influence of packing density and surface roughness of vertically-aligned carbon nanotubes on adhesive properties of gecko-inspired mimetics.
We have systematically studied the macroscopic adhesive properties of vertically aligned nanotube arrays with various packing density and roughness. Using a tensile setup in shear and normal adhesion, we find that there exists a maximum packing density for nanotube arrays to have adhesive properties. Too highly packed tubes do not offer intertube space for tube bending and side-wall contact to surfaces, thus exhibiting no adhesive properties. Likewise, we also show that the surface roughness of the arrays strongly influences the adhesion properties and the reusability of the tubes. Increasing the surface roughness of the array strengthens the adhesion in the normal direction, but weakens it in the shear direction. Altogether, these results allow progress toward mimicking the gecko's vertical mobility.The authors acknowledge funding from the EC project Technotubes.This is the accepted manuscript. The final version is available at http://pubs.acs.org/doi/abs/10.1021/am507822b
Use of opposite frictional forces by animals to increase their attachment reliability during movement
Recent advances in gecko adhesion and friction mechanisms and development of gecko-inspired dry adhesive surfaces
Origin of the Contact Angle Hysteresis of Water on Chemisorbed and Physisorbed Self-Assembled Monolayers
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
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 ångstroms). 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.<br /
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 gecko's locomotion mechanism
Anomalous Potential-Dependent Friction on Au(111) Measured by AFM
We present an exploratory
study of the tribological properties
between an AFM probe and a Au(111) surface in an aqueous environment
while subjected to applied surface potentials. Using a three-electrode
setup, the electrical potential and interfacial electric field on
a Au(111) working electrode are controlled. Lateral force microscopy
is used to measure the friction forces between the AFM probe and the
Au surface. As the AFM probe approaches the surface, normal forces
are also measured to gain insight into the interfacial forces. When
a positive potential is applied to the Au surface, the friction is
found to rise sharply at a critical potential and level off at a relatively
high value. However, when a negative potential is applied, the friction
forces are low, even lower compared to the open circuit potential
case. These changes in friction, by a factor of approximately 35,
as a function of the applied potential are found to be reversible
over multiple cycles. We attribute the origin of the high friction
at positive potentials to the formation of a highly confined, ordered
icelike water layer at the Au/electrolyte interface that results in
effective hydrogen bonding with the AFM probe. At negative potentials,
the icelike water layer is disrupted, resulting in the water molecules
acting as boundary lubricants and providing low friction. Such friction
experiments can provide valuable insight into the structure and properties
of water at charged surfaces under various conditions and can potentially
impact a variety of technologies relying on molecular-level friction
such as MEMs
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