M. Dubé

Thermal effects on atomic friction.

We model friction acting on the tip of an atomic force microscope as it is dragged across a surface at nonzero temperatures. We find that stick-slip motion occurs and that the average frictional force follows (absolute value lnv)(2/3), where v is the tip velocity. This compares well to recent experimental work, permitting the quantitative extraction of all microscopic parameters. We calculate the scaled form of the average frictional forces dependence on both temperature and tip speed as well as the form of the friction-force distribution function.

Imbibition in disordered media

The physics of liquids in porous media gives rise to many interesting phenomena, including imbibition where a viscous fluid displaces a less viscous one. Here we discuss the theoretical and experimental progress made in recent years in this field. The emphasis is on an interfacial description, akin to the focus of a statistical physics approach. Coarse-grained equations of motion have been recently presented in the literature. These contain terms that take into account the pertinent features of imbibition: non-locality and the quenched noise that arises from the random environment, fluctuations of the fluid flow and capillary forces. The theoretical progress has highlighted the presence of intrinsic length-scales that invalidate scale invariance often assumed to be present in kinetic roughening processes such as that of a two-phase boundary in liquid penetration. Another important fact is that the macroscopic fluid flow, the kinetic roughening properties, and the effective noise in the problem are all coupled. Many possible deviations from simple scaling behaviour exist, and we outline the experimental evidence. Finally, prospects for further work, both theoretical and experimental, are discussed.

Synthesis of cationic polymer-grafted cellulose by aqueous ATRP

We describe the synthesis of cellulose fibers densely grafted with the cationic polymer poly[2-(methacryloyloxy)ethyl]-trimethylammoniumchloride (PMeDMA) through aqueous ATRP. The hydroxyl groups present on the cellulose surface were exploited to initiate the ATRP polymerization of MeDMA. We first grafted a bromide initiator, known to be an efficient initiator for ATRP, on the cellulose surface from which the polymer was then directly grown. The resulting fibers/PMeDMA complex was analyzed with infra-red, XPS and SEM techniques and present clear evidences that the polymer is present on the cellulose surface. In order to better characterize the polymer, sacrificial initiators were also added in the mixture and subsequently recovered for analysis. Size exclusion chromatography shows that the polymerization in this heterogeneous medium was controlled. Finally, we show that the mechanical properties of test hand sheets made from modified pulp are markedly improved by the grafting of the cationic PMeDMA.

Liquid conservation and nonlocal interface dynamics in imbition

(February 1, 2008)The propagation and roughening of a liquid-gas interface moving through a disordered mediumunder the influence of capillary forces is considered. The system is described by a phase-field modelwith conserved dynamics and spatial disorder is introduced through a quenched random field. Liquidconservation leads to slowing down of the average interface position H and imposes an intrinsiccorrelation length ξ

Conserved dynamics and interface roughening in spontaneous imbibition: A critical overview

Abstract:Imbibition phenomena have been widely used experimentally and theoretically to study the kinetic roughening of interfaces. We critically discuss the existing experiments and some associated theoretical approaches on the scaling properties of the imbibition front, with particular attention to the conservation law associated to the fluid, to problems arising from the actual structure of the embedding medium, and to external influences such as evaporation and gravity. Our main conclusion is that the scaling of moving interfaces includes many crossover phenomena, with competition between the average capillary pressure gradient and its fluctuations setting the maximal lengthscale for roughening. We discuss the physics of both pinned and moving interfaces and the ability of the existing models to account for their properties.

Stochastic resonance and diffusion in periodic potentials

We study the diffusive motion of a particle in a periodic adiabatic potential under the influence of a time-periodic bias, concentrating on the case of high friction and small bias with respect to the potential barriers. We find that the distribution of escape times shows a series of peaks, whose amplitude varies with temperature, in analogy to stochastic resonance in standard double-well systems. However, the hydrodynamic diffusion coefficient does not show a stochastic resonance type of maximum.

Adatom dynamics in a periodic potential under time-periodic bias

Abstract We study the dynamics of a Brownian particle in a 1D external potential under the influence of a time-periodic bias with an amplitude small with respect to the potential barriers. We consider both a periodic potential corresponding to a smooth crystal surface and a regular array of steps with an extra Ehrlich–Schwoebel barrier for step crossing. For the smooth surface, we extend our previous work in the high friction limit to the low friction case and find that the oscillating bias enhances the diffusion coefficient D T due to the broadening of the jump length distribution. In the case of a stepped surface with terraces of length L , the bias induces a non-zero average current J ave in the direction of descending steps as long as the driving frequency is smaller than a threshold frequency Ω T ≈ L −1 . The current shows a maximum as a function of temperature for fixed L . However, no evidence of stochastic resonance type of enhancement can be found either in D T or J ave .

Effects of phonons and nuclear spins on the tunneling of a domain wall

We consider the quantum dynamics of a magnetic domain wall at low temperatures, where dissipative couplings to magnons and electrons are very small. The wall motion is then determined by its coupling to phonons and nuclear spins, and any pinning potentials. In the absence of nuclear spins there is a dominant superOhmic l-phonon coupling to the wall velocity, plus a strongly T-dependent Ohmic coupling to pairs of phonons. There is also a T-independent Ohmic coupling between single phonons and the wall chirality, which suppresses “chirality tunneling”. We calculate the effect of these couplings on the T-dependent tunneling rate of a wall out of a pinning potential. Nuclear spins have a very strong and hitherto unsuspected influence on domain wall dynamics, coming from a hyperfine-mediated coupling to the domain wall position. For kBT »ω0this coupling yields a spatially random potential, fluctuating at a rate governed by the nuclear T2. When kBT «ω0, the hyperfine potential fluctuates around a linear binding potential. The wall dynamics is influenced by the fluctuations of this potential, ie., by the nuclear spin dynamics. Wall tunneling can occur when fluctuations open an occasional “tunneling window”. This changes the crossover to tunneling and also causes a slow “wandering”, in time, of the energy levels associated with domain wall motion inside the pinning potential. This effect is fairly weak in Ni- and Fe-based magnets, and we give an approximate treatment of its effect on the tunneling dynamics, as well as a discussion of the relationship to recent domain wall tunneling experiments.

Quantum Measurement Operations with Nanomagnets and SQUIDs

The low-energy behaviour of 2 coupled nanomagnets or 2 coupled SQUIDs, interacting with their environment, can be described by the model of a “Pair or Interacting Spins Coupled to an Environmental Sea” (PISCES). These physical systems can then be used for a measurement operation in which system, apparatus and environment are all treated quantum mechanically. We design a “Bell/Coleman-Hepp” measuring system, and show that in principle one may design a situation in which quantum interference between system and apparatus can upset the usual measurement operation.

Droplet spreading and pinning on heterogeneous substrates.

The contact angle of a fluid droplet on an heterogeneous surface is analyzed using the statistical dynamics of the spreading contact line. The statistical properties of the final droplet radius and contact angle are obtained through applications of depinning transitions of contact lines with nonlocal elasticity and features of pinning-depinning dynamics. Such properties not only depend on disorder strength and surface details, but also on the droplet volume and disorder correlation length. Deviations from Wenzel or Cassie-Baxter behavior are particularly apparent in the case of small droplet volumes and small contact angles.