arXiv subjects

A. M. Cazabat

Publications and source records attributed to A. M. Cazabat.

14 recordsLinked to original sources

arXiv2007

Contact line stability of ridges and drops

Within the framework of a semi-microscopic interface displacement model we analyze the linear stability of sessile ridges and drops of a non-volatile liquid on a homogeneous, partially wet substrate, for both signs and arbitrary amplitudes of the three-phase contact line tension. Focusing on perturbations which correspond to deformations of the three-phase contact line, we find that drops are generally stable while ridges are subject only to the long-wavelength Rayleigh-Plateau instability leading to a breakup into droplets, in contrast to the predictions of capillary models which take line tension into account. We argue that the short-wavelength instabilities predicted within the framework of the latter macroscopic capillary theory occur outside its range of validity and thus are spurious.

cond-mat.soft↗

arXiv2003

Intrinsic Friction of Monolayers Adsorbed on Solid Surfaces

We overview recent results on intrinsic frictional properties of adsorbed monolayers, composed of mobile hard-core particles undergoing continuous exchanges with a vapor phase. In terms of a dynamical master equation approach we determine the velocity of a biased impure molecule - the tracer particle (TP), constrained to move inside the adsorbed monolayer probing its frictional properties, define the frictional forces exerted by the monolayer on the TP, as well as the particles density distribution in the monolayer.

cond-mat.stat-mech↗

arXiv2002

Defect-induced perturbations of atomic monolayers on solid surfaces

We study long-range morphological changes in atomic monolayers on solid substrates induced by different types of defects; e.g., by monoatomic steps in the surface, or by the tip of an atomic force microscope (AFM), placed at some distance above the substrate. Representing the monolayer in terms of a suitably extended Frenkel-Kontorova-type model, we calculate the defect-induced density profiles for several possible geometries. In case of an AFM tip, we also determine the extra force exerted on the tip due to the tip-induced de-homogenization of the monolayer.

cond-mat.soft↗

arXiv2001

Binary separation in very thin nematic films: thickness and phase coexistence

The behavior as a function of temperature of very thin films (10 to 200 nm) of pentylcyanobiphenyl (5CB) on silicon substrates is reported. In the vicinity of the nematic/isotropic transition we observe a coexistence of two regions of different thicknesses: thick regions are in the nematic state while thin ones are in the isotropic state. Moreover, the transition temperature is shifted downward following a 1/h^2 law (h is the film thickness). Microscope observations and small angle X-ray scattering allowed us to draw a phase diagram which is explained in terms of a binary first order phase transition where thickness plays the role of an order parameter.

cond-mat.soft↗

arXiv2001

Intrinsic friction of adsorbed monolayers

In the present paper we overview our recent results on intrinsic frictional properties of adsorbed monolayers, composed of mobile hard-core particles undergoing continuous exchanges with a vapor phase. Within the framework of a dynamical master equation approach, describing the time evolution of the system, we determine in the most general form the terminal velocity of some biased impure molecule - the tracer particle (TP), constrained to move inside the adsorbed monolayer probing its frictional properties, define the frictional forces as well as the particles density distribution in the monolayer. Results for one-dimensional solid substrates, appropriate to adsorbtion on polymer chains, are compared against the Monte Carlo simulation data, which confirms our analytical predictions.

cond-mat.soft↗

arXiv2000

Force-velocity relation and density profiles for biased diffusion in an adsorbed monolayer

In this paper, which completes our earlier short publication [Phys. Rev. Lett. 84, 511 (2000)], we study dynamics of a hard-core tracer particle (TP) performing a biased random walk in an adsorbed monolayer, composed of mobile hard-core particles undergoing continuous exchanges with a vapor phase. In terms of an approximate approach, based on the decoupling of the third-order correlation functions, we obtain the density profiles of the monolayer particles around the TP and derive the force-velocity relation, determining the TP terminal velocity, V_{tr}, as the function of the magnitude of external bias and other system's parameters. Asymptotic forms of the monolayer particles density profiles at large separations from the TP, and behavior of V_{tr} in the limit of small external bias are found explicitly.

cond-mat.soft↗

arXiv1999

Stokes formula and density perturbances for driven tracer diffusion in an adsorbed monolayer

We study the intrinsic friction of monolayers adsorbed on solid surfaces from a gas phase or vapor. Within the framework of the Langmuir model of delocalized adsorption, we calculate the resistance offered by the mobile adsorbate's particles to some impure tracer molecule, whose diffusive random motion is biased by a constant external force. We find that for sufficiently small driving forces the force exerted on the tracer shows viscous-like behavior. We derive then the analog of the Stokes formula for two-dimensional adsorbates, calculate the corresponding friction coefficient and determine the stationary particle distribution in the monolayer as seen from the driven impurity.

cond-mat.stat-mech↗

arXiv1999

Directed random walk in adsorbed monolayer

We study the dynamics of a tracer particle, which performs a totally directed random walk in an adsorbed monolayer composed of mobile hard-core particles undergoing continuous exchanges with a vapour phase. In terms of a mean-field-type approach, based on the decoupling of the tracer-particle-particle correlation functions into the product of pairwise, tracer-particle correlations, we determine the density profiles of the monolayer particles, as seen from the stationary moving tracer, and calculate its terminal velocity, V_{tr}. In the general case the latter is determined implicitly, as the solution of a certain transcendental equation. In two extreme limits of slow and fast monolayer particles diffusion, we obtain explicit asymptotic forms of V_{tr}. We show next that the density profile in the monolayer is strongly inhomogeneous: In front of the stationary moving tracer the local density is higher than the average value, ρ_L, and approaches ρ_L as an exponential function of the distance from the tracer. Past the tracer the local density is lower than ρ_L and the approach to ρ_L may proceed differently depending whether the particles number in the monolayer is not or is explicitly conserved. In the former case the approach is described by an exponential dependence with a different characteristic length, compared to the behavior in front of the tracer; in the latter case, the density tends to ρ_L algebraically. The characteristic lengths and the amplitudes of the density relaxation functions are also determined explicitly

cond-mat.stat-mech↗

arXiv1998

Biased diffusion in a one-dimensional adsorbed monolayer

We study dynamics of a probe particle, which performs biased diffusive motion in a one-dimensional adsorbed monolayer of mobile hard-core particles undergoing continuous exchanges with a vapor phase. In terms of a mean-field-type approach, based on the decoupling of the third-order correlation functions into a product of the pairwise correlations, we determine analytically the density profiles of the monolayer particles, as seen from the stationary moving probe, and calculate the terminal velocity, mobility and the self-diffusion coefficient of the probe. Our analytical results are confirmed by Monte Carlo simulations.

cond-mat.stat-mech↗

arXiv1998

Kinetics of Anchoring of Polymer Chains on Substrates with Chemically Active Sites

We consider dynamics of an isolated polymer chain with a chemically active end-bead on a 2D solid substrate containing immobile, randomly placed chemically active sites (traps). For a particular situation when the end-bead can be irreversibly trapped by any of these sites, which results in a complete anchoring of the whole chain, we calculate the time evolution of the probability $P_{ch}(t)$ that the initially non-anchored chain remains mobile until time $t$. We find that for relatively short chains $P_{ch}(t)$ follows at intermediate times a standard-form 2D Smoluchowski-type decay law $ln P_{ch}(t) \sim - t/ln(t)$, which crosses over at very large times to the fluctuation-induced dependence $ln P_{ch}(t) \sim - t^{1/2}$, associated with fluctuations in the spatial distribution of traps. We show next that for long chains the kinetic behavior is quite different; here the intermediate-time decay is of the form $ln P_{ch}(t) \sim - t^{1/2}$, which is the Smoluchowski-type law associated with subdiffusive motion of the end-bead, while the long-time fluctuation-induced decay is described by the dependence $ln P_{ch}(t) \sim - t^{1/4}$, stemming out of the interplay between fluctuations in traps distribution and internal relaxations of the chain.

cond-mat.soft↗

arXiv1998

Dewetting, partial wetting and spreading of a two-dimensional monolayer on solid surface

We study the behavior of a semi-infinite monolayer, which is placed initially on a half of an infinite in both directions, ideal crystalline surface, and then evolves in time due to random motion of the monolayer particles. Particles dynamics is modeled as the Kawasaki particle-vacancy exchange process in the presence of long-range attractive particle-particle interactions. In terms of an analytically solvable mean-field-type approximation we calculate the mean displacement X(t) of the monolayer edge and discuss the conditions under which such a monolayer spreads (X(t) > 0), partially wets (X(t) = 0) or dewets from the solid surface (X(t) < 0).

cond-mat.soft↗

arXiv1998

Microscopic model for spreading of a two-dimensional monolayer

We study the behavior of a monolayer, which occupies initially a bounded region on an ideal crystalline surface and then evolves in time due to random hopping motion of the monolayer particles. In the case when the initially occupied region is the half-plane $X \leq 0$, we determine explicitly, in terms of an analytically solvable mean-field-type approximation, the mean displacement $X(t)$ of the monolayer edge. We find that $X(t) \approx A \sqrt{D_{0} t}$, in which law $D_{0}$ denotes the bare diffusion coefficient and the prefactor $A$ is a function of the temperature and of the particle-particle interactions parameters. We show that $A$ can be greater, equal or less than zero, and specify the critical parameter which distinguishes between the regimes of spreading ($A > 0)$, partial wetting ($A = 0$) and dewetting ($A < 0$).

cond-mat.soft↗

arXiv1998

Molecular Weight Dependence of Spreading Rates of Ultrathin Polymeric Films

We study experimentally the molecular weight $M$ dependence of spreading rates of molecularly thin precursor films, growing at the bottom of droplets of polymer liquids. In accord with previous observations, we find that the radial extension R(t) of the film grows with time as R(t) = (D_{exp} t)^{1/2}. Our data substantiate the M-dependence of D_{exp}; we show that it follows D_{exp} \sim M^{-γ}, where the exponent γis dependent on the chemical composition of the solid surface, determining its frictional properties with respect to the molecular transport. In the specific case of hydrophilic substrates, the frictional properties can be modified by the change of the relative humidity (RH). We find that γ\approx 1 at low RH and tends to zero when RH gets progressively increased. We propose simple theoretical arguments which explain the observed behavior in the limits of low and high RH.

cond-mat.soft↗

arXiv1996

Spreading of molecularly thin wetting films on solid interfaces

In this paper we study kinetics of spreading of thin liquid films on solid interfaces. We present an overview of current experimental picture and discuss available theoretical approaches and their limitations. We report some new experimental results on spreading of molecularly thin liquid films and propose an analytically solvable microscopic model, which reproduces experimentally observed behaviors and provides a seemingly plausible explanation of the underlying physical processes.

cond-mat↗