Michael Urbakh

The nonlinear nature of friction.

Tribology is the study of adhesion, friction, lubrication and wear of surfaces in relative motion. It remains as important today as it was in ancient times, arising in the fields of physics, chemistry, geology, biology and engineering. The more we learn about tribology the more complex it appears. Nevertheless, recent experiments coupled to theoretical modelling have made great advances in unifying apparently diverse phenomena and revealed many subtle and often non-intuitive aspects of matter in motion, which stem from the nonlinear nature of the problem.

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.

Colloquium : Modeling friction: from nanoscale to mesoscale

The physics of sliding friction is gaining impulse from nanoscale and mesoscale experiments, simulations, and theoretical modeling. This Colloquium reviews some recent developments in modeling and in atomistic simulation of friction, covering open-ended directions, unconventional nanofrictional systems, and unsolved problems.

Self-Assembly of Nanoparticle Arrays for Use as Mirrors, Sensors, and Antennas

The self-assembly of nanoparticles (NPs) at liquid-liquid interfaces (LLIs) has recently emerged as a promising platform for tunable optical devices, sensors, and catalysis. There are numerous advantages for such platforms when compared to more conventional solid-state counterparts. For example, they do not need engineering, self-assemble if proper conditions are provided, are self-healing, are practically nondegrading, and are easily renewable. Furthermore, they have the added benefit of being able to facilitate the interactions of analytes dissolved in often-inaccessible environments. In this Perspective, we highlight some important recent developments in understanding the mechanisms and applications of self-assembly of NPs at LLIs for use as mirrors and sensors. Finally, we explore future directions in this field, focusing on NP arrays with electrotunable properties assembled at a LLI, which has been one of the driving forces for developing such technologies.

Atomic scale engines: cars and wheels.

We introduce a new approach to build microscopic engines on the atomic scale that move translationally or rotationally and can perform useful functions such as the pulling of a cargo. Characteristic of these engines is the possibility to determine dynamically the directionality of the motion. The approach is based on the transformation of the fed energy to directed motion through a dynamical competition between the intrinsic lengths of the moving object and the supporting carrier.

Nanotribology: The renaissance of friction

500 years after the first studies on friction, the concepts of superlubricity, wearless sliding and friction control are being realized in laboratories and have become predictable by adequate modelling. The challenge now is to bridge the gap between what is known about these processes on the microscopic and macroscopic scales.

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.

Water transport inside carbon nanotubes mediated by phonon-induced oscillating friction

The emergence of the field of nanofluidics in the last decade has led to the development of important applications including water desalination, ultrafiltration and osmotic energy conversion. Most applications make use of carbon nanotubes, boron nitride nanotubes, graphene and graphene oxide. In particular, understanding water transport in carbon nanotubes is key for designing ultrafiltration devices and energy-efficient water filters. However, although theoretical studies based on molecular dynamics simulations have revealed many mechanistic features of water transport at the molecular level, further advances in this direction are limited by the fact that the lowest flow velocities accessible by simulations are orders of magnitude higher than those measured experimentally. Here, we extend molecular dynamics studies of water transport through carbon nanotubes to flow velocities comparable with experimental ones using massive crowd-sourced computing power. We observe previously undetected oscillations in the friction force between water and carbon nanotubes and show that these oscillations result from the coupling between confined water molecules and the longitudinal phonon modes of the nanotube. This coupling can enhance the diffusion of confined water by more than 300%. Our results may serve as a theoretical framework for the design of new devices for more efficient water filtration and osmotic energy conversion devices.

A model of electrowetting, reversed electrowetting, and contact angle saturation.

While electrowetting has many applications, it is limited at large voltages by contact angle saturation, a phenomenon that is still not well understood. We propose a generalized approach for electrowetting that, among other results, can shed new light on contact angle saturation. The model assumes the existence of a minimum (with respect to the contact angle) in the electric energy and accounts for a quadratic voltage dependence ∼U(2) in the low-voltage limit, compatible with the Young-Lippmann formula, and an ∼U(-2) saturation at the high-voltage limit. Another prediction is the surprising possibility of a reversed electrowetting regime, in which the contact angle increases with applied voltage. By explicitly taking into account the effect of the counter-electrode, our model is shown to be applicable to several AC and DC experimental electrowetting-on-dielectric (EWOD) setups. Several features seen in experiments compare favorably with our results. Furthermore, the AC frequency dependence of EWOD agrees quantitatively with our predictions. Our numerical results are complemented with simple analytical expressions for the saturation angle in two practical limits.

Nonlinear Poisson–Boltzmann theory of a double layer at a rough metal/electrolyte interface: A new look at the capacitance data on solid electrodes

Nonlinear Poisson–Boltzmann theory is developed to extend our previous work [Phys. Rev. E 53, 6192 (1996)] on the case when the potential drop across the double layer is not small compared to the thermal energy. Close to the potentials of zero charge (pzc) the effect of surface roughness on the double-layer capacitance is mainly determined by an interplay between the lateral correlation length of roughness and the Debye length. However, far from the pzc dramatic effects of electrode potential are found which are not reduced to the potential-induced shortening of the diffuse layer thickness.