A. E. Filippov

Beyond the conventional description of dynamic force spectroscopy of adhesion bonds

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.

Dynamic force spectroscopy: a Fokker–Planck approach

In this Letter we show that recent observations using dynamic force spectroscopy can be described by a generalized Tomlinson model which includes the contribution of an external noise. We show that the measured friction forces depend on the microscopic potential and dissipation inherent to the system as well as on the mechanical properties of the setup (i.e. spring constant) and the external noise. Tuning the noise and spring constant offers ways to extract information about the microscopic properties.

Effect of tip flexibility on stick–slip motion in friction force microscopy experiments

We have investigated the effect of tip flexibility on stick–slip dynamics, treating the motion of the tip apex and the cantilever within a Langevin description. We have found that the dynamical system, which includes the cantilever and the tip apex, exhibits a rich variety of regimes of motion depending on the values of the dissipation constants associated with the translational motion of the apex and the bending motion of the tip. The proposed model explains the fine structure of the stick–slip patterns and the wide variation of slip durations, between microseconds and milliseconds, observed in recent experiments with a friction force microscope (Maier et al 2005 Phys. Rev. B 72 245418). The results of our full Langevin description are compared with the predictions of the single-spring Prandtl–Tomlinson model and the hybrid Langevin–Monte Carlo approach, which has recently been proposed.

Chemical control of friction: mixed lubricant monolayers

Controlling frictional behavior in nanoscale sheared systems can be made possible when the relationship between the macroscopic frictional response and the microscopic properties of the sheared systems is established. Here, a new approach is proposed for tuning the frictional response and obtaining desirable frictional properties. This tuning is achieved through shear-induced phase transitions in a mixed lubricant monolayer consisting of a base solvent and an additive. The interaction between the solvent and additive molecules and their relative concentrations are shown to be the major parameters in determining the magnitude of the friction force and the nature of the response (stick–slip or sliding).

Single-Molecule Tribology: Force Microscopy Manipulation of a Porphyrin Derivative on a Copper Surface

The low-temperature mechanical response of a single porphyrin molecule attached to the apex of an atomic force microscope (AFM) tip during vertical and lateral manipulations is studied. We find that approach-retraction cycles as well as surface scanning with the terminated tip result in atomic-scale friction patterns induced by the internal reorientations of the molecule. With a joint experimental and computational effort, we identify the dicyanophenyl side groups of the molecule interacting with the surface as the dominant factor determining the observed frictional behavior. To this end, we developed a generalized Prandtl-Tomlinson model parametrized using density functional theory calculations that includes the internal degrees of freedom of the side group with respect to the core and its interactions with the underlying surface. We demonstrate that the friction pattern results from the variations of the bond length and bond angles between the dicyanophenyl side group and the porphyrin backbone as well as those of the CN group facing the surface during the lateral and vertical motion of the AFM tip.

Inverted stick-slip friction: What is the mechanism?

A mechanism is proposed for an observed inverted stick–slip motion and a relationship between the macroscopic mechanical response and the dynamics of the embedded system in this regime is established. It is shown that the requirement for the occurrence of inverted stick–slip is the existence of two sliding regimes in the system. The inverted stick–slip stems from a bifurcation from one sliding regime to another. The mechanism suggests that the inverted stick–slip behavior reflects a transition of the embedded system from nonslip to slip boundary conditions.

Manipulations of Individual Molecules by Scanning Probes

In this letter, we suggest a new method of manipulating individual molecules with scanning probes using a “pick-up-and-put-down” mode. We demonstrate that the number of molecules picked up by the tip and deposited in a different location can be controlled by adjusting the pulling velocity of the tip and the distance of closest approach of the tip to the surface.

Effects of molecule anchoring and dispersion on nanoscopic friction under electrochemical control

The application of electric fields is a promising strategy for in situ control of friction. While there have recently been many experimental studies on friction under the influence of electric fields, theoretical understanding is very limited. Recently, we introduced a simple theoretical model for friction under electrochemical conditions that focused on the interaction of a force microscope tip with adsorbed molecules whose orientation was dependent on the applied electric field. Here we focus on the effects of anchoring of the molecules on friction. We show that anchoring affects the intensity and width of the peak in the friction that occurs near a reorientation transition of adsorbed molecules, and explain this by comparing the strength of molecule-molecule and molecule-tip interactions. We derive a dispersion relation for phonons in the layer of adsorbed molecules and demonstrate that it can be used to understand important features of the frictional response.

Actin-based motility: cooperative symmetry-breaking and phases of motion

A microscopic model is proposed for the motility of a bead driven by the polymerization of actin filaments. The model exhibits a rich spectrum of behaviours similar to those observed in biomimetic experiments, which include spontaneous symmetry-breaking, various regimes of the beads motion and correlations between the structure of the actin tail which propels the bead and the bead dynamics. The dependences of the dynamical properties (such as symmetry-breaking time, regimes of motion, mean velocity, and tail asymmetry) on the physical parameters (the bead radius and viscosity) agree well with the experimental observations. We find that most experimental observations can be reproduced taking into account only one type of filaments interacting with the bead: the detached filaments that push the bead. Our calculations suggest that the analysis of mean characteristics only (velocities, symmetry-breaking times, etc) does not always provide meaningful information about the mechanism of motility. The aim should be to obtain the corresponding distributions, which might be extremely broad and therefore not well represented by their mean only. Our findings suggest a simple coarse-grained description, which captures the main features obtained within the microscopic model.

Molecular pumping and separation in a symmetric channel

A mechanism responsible for directed transport and molecular separation in a symmetric channel is proposed. We found that under the action of spatial harmonic oscillations of the channel, the system exhibits a directed transport in either direction, presenting multiple current reversals as the amplitude and/or frequency of the oscillations are varied. The particles of different masses may be forced to move with different velocities in the same or in the opposite directions by properly adjusting driving parameters. The directed transport can be produced in both directions even in the absence of thermal noise, the latter can speed up or slow down the transport depending on the system parameters.