arXiv subjects

H. Diamant

Publications and source records attributed to H. Diamant.

16 recordsLinked to original sources

arXiv2006

Surface relaxation of lyotropic lamellar phases

We study the relaxation modes of an interface between a lyotropic lamellar phase and a gas or a simple liquid. The response is found to be qualitatively different from those of both simple liquids and single-component smectic-A liquid crystals. At low rates it is governed by a non-inertial, diffusive mode whose decay rate increases quadratically with wavenumber, $|ω|=Aq^2$. The coefficient $A$ depends on the restoring forces of surface tension, compressibility and bending, while the dissipation is dominated by the so-called slip mechanism, i.e, relative motion of the two components of the phase parallel to the lamellae. This surface mode has a large penetration depth which, for sterically stabilised phases, is of order $(dq^2)^{-1}$, where $d$ is the microscopic lamellar spacing.

cond-mat.soft↗

arXiv2005

Correlated particle dynamics in concentrated quasi-two-dimensional suspensions

We investigate theoretically and experimentally how the hydrodynamically correlated lateral motion of particles in a suspension confined between two surfaces is affected by the suspension concentration. Despite the long range of the correlations (decaying as 1/r^2 with the inter-particle distance r), the concentration effect is present only at short inter-particle distances for which the static pair correlation is nonuniform. This is in sharp contrast with the effect of hydrodynamic screening present in unconfined suspensions, where increasing the concentration changes the prefactor of the large-distance correlation.

cond-mat.soft↗

arXiv2005

From random walk to single-file diffusion

We report an experimental study of diffusion in a quasi-one-dimensional (q1D) colloid suspension which behaves like a Tonks gas. The mean squared displacement as a function of time is described well with an ansatz encompassing a time regime that is both shorter and longer than the mean time between collisions. This ansatz asserts that the inverse mean squared displacement is the sum of the inverse mean squared displacement for short time normal diffusion (random walk) and the inverse mean squared displacement for asymptotic single-file diffusion (SFD). The dependence of the single-file 1D mobility on the concentration of the colloids agrees quantitatively with that derived for a hard rod model, which confirms for the first time the validity of the hard rod SFD theory. We also show that a recent SFD theory by Kollmann leads to the hard rod SFD theory for a Tonks gas.

cond-mat.soft↗

arXiv2005

Hydrodynamic interaction in quasi-two-dimensional suspensions

Confinement between two parallel surfaces is found, theoretically and experimentally, to drastically affect the hydrodynamic interaction between colloid particles, changing the sign of the coupling, its decay with distance and its concentration dependence. In particular, we show that three-body effects do not modify the coupling at large distances as would be expected from hydrodynamic screening.

cond-mat.soft↗

arXiv2004

Chain-reaction cascades in surfactant monolayer buckling

Certain surfactant monolayers at the water-air interface have been found to undergo, at a critical surface pressure, a dynamic instability involving multiple long folds of micron width. We exploit the sharp monolayer translations accompanying folding events to acquire, using a combination of fluorescence microscopy and digital image analysis, detailed statistics concerning the folding dynamics. The motions have a broad distribution of magnitudes and narrow, non-Gaussian distributions of angles and durations. The statistics are consistent with the occurrence of cooperative cascades of folds, implying an autocatalytic process uncommon in the context of mechanical instability.

cond-mat.soft↗

arXiv2001

General criterion for controllable conformational transitions of single and double stranded DNA

Chain-like macromolecules in solution, whether biological or synthetic, transform from a spatially extended conformation to a compact one upon change of temperature or solvent qualities. This sharp transition plays a key role in various phenomena, including DNA condensation, protein folding, and the behaviour of polymer solutions. In biological processes such as DNA condensation the collapse is sensitively induced by a small amount of added molecules. Here we derive a general criterion for the effect of such agents on conformational transitions. We find two different scenarios depending on chain stiffness. If the persistence length --the characteristic distance along which the chain retains its direction-- is smaller than the range of attractive correlations induced by the agent (typically up to several nanometres), the chain contracts gradually. Stiffer chains undergo sharp collapse. We thereby suggest that the enhanced rigidity of double-stranded DNA as compared to the single strand is a prerequisite for sharp, controllable conformational transitions.

cond-mat.soft↗

arXiv2001

Kinetics of Surfactant Adsorption: The Free Energy Approach

We review the free energy approach to the kinetics of surfactant adsorption at fluid/fluid interfaces. The formalism is applied to several systems. For non-ionic surfactant solutions the results coincide with previous models while indicating their limits of validity. We study the case of surfactant mixtures, focusing on the relation between the mixture kinetics and the properties of its individual constituents. Strong electrostatic interactions in salt-free ionic surfactant solutions drastically modify the adsorption kinetics. In this case the theory accounts for experimental results which could not be previously understood. The effect of screening by added salt is studied as well. Our theoretical predictions are compared with available experiments.

cond-mat.soft↗

arXiv2001

Topography and instability of monolayers near domain boundaries

We theoretically study the topography of a biphasic surfactant monolayer in the vicinity of domain boundaries. The differing elastic properties of the two phases generally lead to a nonflat topography of ``mesas'', where domains of one phase are elevated with respect to the other phase. The mesas are steep but low, having heights of up to 10 nm. As the monolayer is laterally compressed, the mesas develop overhangs and eventually become unstable at a surface tension of about K(dc)^2 (dc being the difference in spontaneous curvature and K a bending modulus). In addition, the boundary is found to undergo a topography-induced rippling instability upon compression, if its line tension is smaller than about K(dc). The effect of diffuse boundaries on these features and the topographic behavior near a critical point are also examined. We discuss the relevance of our findings to several experimental observations related to surfactant monolayers: (i) small topographic features recently found near domain boundaries; (ii) folding behavior observed in mixed phospholipid monolayers and model lung surfactants; (iii) roughening of domain boundaries seen under lateral compression; (iv) the absence of biphasic structures in tensionless surfactant films.

cond-mat.soft↗

arXiv2000

Self-Assembly in Mixtures of Polymers and Small Associating Molecules

The interaction between a flexible polymer in good solvent and smaller associating solute molecules such as amphiphiles (surfactants) is considered theoretically. Attractive correlations, induced in the polymer because of the interaction, compete with intra-chain repulsion and eventually drive a joint self-assembly of the two species, accompanied by partial collapse of the chain. Results of the analysis are found to be in good agreement with experiments on the onset of self-assembly in diverse polymer-surfactant systems. The threshold concentration for self-assembly in the mixed system (critical aggregation concentration, cac) is always lower than the one in the polymer-free solution (critical micelle concentration, cmc). Several self-assembly regimes are distinguished, depending on the effective interaction between the two species. For strong interaction, corresponding experimentally to oppositely charged species, the cac is much lower than the cmc. It increases with ionic strength and depends only weakly on polymer charge. For weak interaction, the cac is lower but comparable to the cmc, and the two are roughly proportional over a wide range of cmc values. Association of small molecules with amphiphilic polymers exhibiting intra-chain aggregation (polysoaps) is gradual, having no sharp onset.

cond-mat.soft↗

arXiv2000

Swelling kinetics of the onion phase

A theory is presented for the behavior of an array of multi-lamellar vesicles (the onion phase) upon addition of solvent. A unique feature of this system is the possibility to sustain pressure gradients by tension in the lamellae. Tension enables the onions to remain stable beyond the unbinding point of a flat lamellar stack. The model accounts for various concentration profiles and interfaces developing in the onion as it swells. In particular, densely packed `onion cores' are shown to appear, as observed in experiments. The formation of interfaces and onion cores may represent an unusual example of stabilization of curved interfaces in confined geometry.

cond-mat.soft↗

arXiv2000

Unstable topography of biphasic surfactant monolayers

We study the conformation of a heterogeneous surfactant monolayer at a fluid-fluid interface, near a boundary between two lateral regions of differing elastic properties. The monolayer attains a conformation of shallow, steep `mesas' with a height difference of up to 10 nm. If the monolayer is progressively compressed (e.g. in a Langmuir trough), the profile develops overhangs and finally becomes unstable at a surface tension of about K(delta c_0)^2, where (delta c_0) is the difference in spontaneous curvature and K a bending stiffness. We discuss the relevance of this instability to recently observed folding behavior in lung surfactant monolayers, and to the absence of domain structures in films separating oil and water in emulsions.

cond-mat.soft↗

arXiv1999

Kinetics of Surfactant Adsorption at Fluid-Fluid Interfaces: Surfactant Mixtures

The adsorption at the interface between an aqueous solution of several surface-active agents and another fluid (air or oil) phase is addressed theoretically. We derive the kinetic equations from a variation of the interfacial free energy, solve them numerically and provide an analytic solution for the simple case of a linear adsorption isotherm. Calculating asymptotic solutions analytically, we find the characteristic time scales of the adsorption process and observe the behavior of the system at various temporal stages. In particular, we relate the kinetic behavior of the mixture to the properties of its individual constituents and find good agreement with experiments. In the case of kinetically limited adsorption, the mixture kinetics is found to be considerably different from that of the single-surfactant solutions because of strong coupling between the species.

cond-mat.soft↗

arXiv1998

Self-Assembly in Mixtures of Amphiphilic Polymers and Surfactants

We present a model for the joint self-assembly of amphiphilic polymers and small amphiphilic molecules (surfactants) in a dilute aqueous solution. The polymer is assumed to consist of a hydrophilic backbone and a large number of hydrophobic side groups. Preference of the surfactant to bind to hydrophobic microdomains along the polymer induces an effective attraction between bound surfactants. This leads to two distinct binding regimes depending on a single physical parameter, ε, which represents the ratio between surfactant-polymer affinity and polymer hydrophobicity. For small εthe binding is non-cooperative, whereas for large εit becomes strongly cooperative at a well-defined critical aggregation concentration. Our findings are in accord with observations on diverse experimental systems.

cond-mat.soft↗

arXiv1998

Adsorption Kinetics of Surfactants at Fluid-Fluid Interfaces

We review a new theoretical approach to the kinetics of surfactant adsorption at fluid-fluid interfaces. It yields a more complete description of the kinetics both in the aqueous solution and at the interface, deriving all equations from a free-energy functional. It also provides a general method to calculate dynamic surface tensions. For non-ionic surfactants the results coincide with previous models. Common non-ionic surfactants are shown to undergo diffusion-limited adsorption, in agreement with experiments. Strong electrostatic interactions in salt-free ionic surfactant solutions are found to lead to kinetically limited adsorption. In this case the theory accounts for unusual experimental results which could not be understood using previous approaches. Added salt screens the electrostatic interactions and makes the ionic surfactant adsorption similar to the non-ionic case. The departure from the non-ionic behavior as the salt concentration is decreased is calculated perturbatively.

cond-mat.soft↗

arXiv1998

Kinetics of Surfactant Adsorption at Fluid/Fluid Interfaces: Non-ionic Surfactants

We present a model treating the kinetics of adsorption of soluble surface-active molecules at the interface between an aqueous solution and another fluid phase. The model accounts for both the diffusive transport inside the solution and the kinetics taking place at the interface using a free-energy formulation. In addition, it offers a general method of calculating dynamic surface tensions. Non-ionic surfactants are shown, in general, to undergo a diffusion-limited adsorption, in accord with experimental findings.

cond-mat.soft↗

arXiv1996

Kinetics of Surfactant Adsorption at Fluid-Fluid Interfaces

We present a theory for the kinetics of surfactant adsorption at the interface between an aqueous solution and another fluid (air, oil) phase. The model relies on a free-energy formulation. It describes both the diffusive transport of surfactant molecules from the bulk solution to the interface, and the kinetics taking place at the interface itself. When applied to non-ionic surfactant systems, the theory recovers results of previous models, justify their assumptions and predicts a diffusion-limited adsorption, in accord with experiments. For salt-free ionic surfactant solutions, electrostatic interactions are shown to drastically affect the kinetics. The adsorption in this case is predicted to be kinetically limited, and the theory accounts for unusual experimental results obtained recently for the dynamic surface tension of such systems. Addition of salt to an ionic surfactant solution leads to screening of the electrostatic interactions and to a diffusion-limited adsorption. In addition, the free-energy formulation offers a general method for relating the dynamic surface tension to surface coverage without relying on equilibrium relations.

cond-mat↗