No abstract

Dynamic force spectroscopy of single molecules is described by a model that predicts a distribution of rupture forces, the corresponding mean rupture force, and variance, which are all amenable to experimental tests. The distribution has a pronounced asymmetry, which has recently been observed experimentally. The mean rupture force follows a (lnV) 2/3 dependence on the pulling velocity, V, and differs from earlier predictions. Interestingly, at low pulling velocities, a rebinding process is obtained whose signature is an intermittent behavior of the spring force, which delays the rupture. An extension to include conformational changes of the adhesion complex is proposed, which leads to the possibility of bimodal distributions of rupture forces.S ingle molecule spectroscopy is by now an established approach that can report on distributions of molecular properties, and can provide kinetic information on conformational changes, such as folding and unfolding of molecules without the ''scrambling'' that occurs due to ensemble averaging (1). Such information could be valuable in particular for biomolecules, where rare events might have functional significance, but which can be masked in an ensemble approach. Dynamic force spectroscopy (DFS) has been introduced as a spectroscopic tool to probe the complex relationships between ''force-lifetime and chemistry'' in single molecules bound in an adhesion complex (2-4), and to reveal details of molecular scale energy landscapes and adhesion strength.The rupture force in DFS is quantified by the maximum extension of a spring, the linker, which is followed by a rapid recoil of the spring to its rest position. This behavior resembles the stick-to-slip transition in studies on friction. The unbinding process of a single molecule is studied one molecule at a time, which means that one measures a collection of independent random rupture events. This type of measurement leads to a distribution of rupture forces. In addition, measurements of rupture forces over a wide range of pulling velocities, from very slow to extremely fast, are used to explore the energy landscape of the bound complex.In DFS experiments, an adhesion bond is driven away from its equilibrium by a spring pulled at a given velocity. Rupture of adhesion bonds occurs by means of thermally assisted escape from the bound state across an activation barrier. The latter diminishes as the applied force increases, so the rupture force is determined by an interplay between the rate of escape in the absence of the external force and the pulling velocity (loading rate). Thus, the measured forces are not an intrinsic property of the bound complex, but rather, they depend on the mechanical setup and loading rate applied to the system.In this article, we discuss an approach to describe unbinding processes measured by DFS, which goes beyond previous models and methods of analysis (2-11). As we show, our approach (i) proposes a possible mechanism of rupture; (ii) emphasizes the importance of investigating the distribution function...

Influenza epidemics are associated with a rise in autopsy-confirmed coronary deaths. Influenza vaccination should be advocated for patients at high risk of developing cardiovascular events.

Superlubricity between incommensurate surfaces provides a desired low-friction state essential for the function of small-scale machines. Here we demonstrate experimentally and theoretically that superlubricity in contacts lubricated by lamellar solids might be eliminated due to torque-induced reorientation coupled to lateral motion. We find that the possibility of reorientation always leads to stabilization of a high frictional state which corresponds to a commensurate configuration. DOI: 10.1103/PhysRevLett.100.046102 PACS numbers: 68.35.Af, 07.79.Sp, 46.55.+d, 68.37.Ps The practical importance and the relevance to basic scientific questions have motivated studies towards understanding the conditions which lead to superlow friction. A mechanism for superlow dry friction, which arises from the structural incompatibility of two contacting solids, was first suggested by Hirano and Shinjo [1,2]. This phenomenon is also referred to as superlubricity. However, incommensurability of the interfaces results in a cancellation of only one of the channels of energy dissipation, which originates from a stick-slip instability. Other dissipative processes, such as electronic and phononic friction, etc., still persist, and therefore even in the case of complete incommensurability the net friction force will not be identical to zero. Nonetheless, typical to the superlubricity state is a reduction of the friction force by orders of magnitude.Detailed experimental studies of superlubricity have been performed recently by , who measured friction between a graphite flake attached to the tip of a frictional force microscope (FFM) and an atomically flat graphite surface. Superlow friction forces (<50 pN) have been found for most relative orientations of the flake and the substrate, for which the contacting surfaces find themselves in incommensurate states. For narrow ranges of orientations corresponding to commensurate contacts, stick-slip motion was observed and friction was high (typically 250 pN). A few earlier experiments [6,7] also provided indications of the superlubricity phenomenon in dry friction.Measuring friction between graphite flakes and an underlying graphite surface we found that the ''lifetime'' of a superlubric state can be finite and therefore, superlow friction does not persist. We demonstrate that the dominating contribution to such residual friction in contacts lubricated by lamellar solids may stem from torque-induced reorientation of the flakes attached to the tip. Dynamics of the flake reorientation result in irregular sharp peaks of the lateral force shown in Fig. 1, which are observed during the restoration of the high friction state. The lifetime may depend on the structure of the tip and the density of defects on the graphite surface. Our calculations show that the possibility of flake rotation always stabilizes the high frictional state which corresponds to the commensurate configuration of the contacting surfaces in relative motion. Reorganization of a sliding layer leading to a locking in a ''comme...

Nature has successfully evolved the mushroom-shaped contact geometry in many organisms in order to solve the attachment problem. We studied the detachment process of individual bioinspired artificial mushroom-shaped adhesive microstructures (MSAMSs) resolving the failure dynamics at high spatiotemporal resolution. The experimental data provide strong evidence for a homogeneous stress distribution in MSAMS, which was recently proposed. Our results allow us to explain the advantage of such contact geometry and provide a suggestion for the widely observed mushroom-shaped contact geometry.