arXiv subjects

Armand Ajdari

Publications and source records attributed to Armand Ajdari.

26 records · Page 2Linked to original sources

arXiv2003

Effective interactions between inclusions in complex fluids driven out of equilibrium

The concept of fluctuation-induced effective interactions is extended to systems driven out of equilibrium. We compute the forces experienced by macroscopic objects immersed in a soft material driven by external shaking sources. We show that, in contrast with equilibrium Casimir forces induced by thermal fluctuations, their sign, range and amplitude depends on specifics of the shaking and can thus be tuned. We also comment upon the dispersion of these shaking-induced forces, and discuss their potential application to phase ordering in soft-materials.

cond-mat.soft↗

arXiv2003

Mechanical aging and non-linear rheology of concentrated colloidal suspensions: experimental facts and simple models

Many colloidal systems display very non-Newtonian and solid-like behaviour when concentrated, a striking feature being the apparition of a yield stress. After recalling some basics about the interactions between colloidal particles, I present a few experimental facts commonly observed in these systems: aging and non-linear rheology. A simple phenomenological model is then introduced, in which the local state of the system is described by a single scalar parameter, the fluidity. I proceed with comments on heterogeneous flows in some of these systems. These notes are not intended to be a comprehensive review, and the reader is directed to the references for further reading

cond-mat.soft↗

arXiv2002

Fluctuations of Fluctuation-Induced "Casimir" Forces

The force experienced by objects embedded in a correlated medium undergoing thermal fluctuations--the so-called fluctuation--induced force--is actually itself a fluctuating quantity. We compute the corresponding probability distribution and show that it is a Gaussian centered on the well-known Casimir force, with a non-universal standard deviation that can be typically as large as the mean force itself. The relevance of these results to the experimental measurement of fluctuation-induced forces is discussed, as well as the influence of the finite temporal resolution of the measuring apparatus.

cond-mat.soft↗

arXiv2002

Tracer diffusivity in a time or space dependent temperature field

The conventional assumption that the self-diffusion coefficient of a small tracer can be obtained by a local and instantaneous application of Einstein's relation in a temperature field with spatial and temporal heterogeneity is revisited. It is shown that hydrodynamic fluctuations contribute to the self-diffusion tensor in a universal way, i.e. independent of the size and shape of the tracer. The hydrodynamic contribution is anisotropic--it reflects knowledge of the global anisotropy in the temperature profile, leading to anisotropic self-diffusion tensor for a spherical tracer. It is also retarded--it creates memory effects during the diffusion process due to hydrodynamic interactions.

cond-mat.soft↗

arXiv2001

Dynamic Response of Adhesion Complexes: Beyond the Single-Path Picture

We analyze the response of molecular adhesion complexes to increasing pulling forces (dynamic force spectroscopy) when dissociation can occur along either one of two alternative trajectories in the underlying multidimensional energy landscape. A great diversity of behaviors (e.g. non-monotonicity) is found for the unbinding force and time as a function of the rate at which the pulling force is increased. We highlight an intrinsic difficulty in unambiguously determining the features of the energy landscape from single-molecule pulling experiments. We also suggest a class of ``harpoon'' stickers that bind easily but resist strong pulling efficiently.

cond-mat.stat-mech↗

arXiv2001

Transverse electrokinetic and microfluidic effects in micro-patterned channels: lubrication analysis for slab geometries

Off-diagonal (transverse) effects in micro-patterned geometries are predicted and analyzed within the general frame of linear response theory, relating applied presure gradient and electric field to flow and electric current. These effects could contribute to the design of pumps, mixers or flow detectors. Shape and charge density modulations are proposed as a means to obtain sizeable transverse effects, as demonstrated by focusing on simple geometries and using the lubrication approximation.

cond-mat.soft↗

arXiv2000

Averaging rheological quantities in descriptions of soft glassy materials

Many mean-field models have been introduced to describe the mechanical behavior of glassy materials. They often rely on averages performed over distributions of elements or states. We here underline that averaging is a more intricate procedure in mechanics than in more classical situations such as phase transitions in magnetic systems. This leads us to modify the predictions of the recently proposed SGR model for soft glassy materials, for which we suggest that the viscosity should diverge at the glass transition temperature $T_g$ with an exponential form $η\sim \exp(\frac{A}{T-T_g})$.

cond-mat.soft↗

arXiv1999

Energy Transduction of Isothermal Ratchets: Generic Aspects and Specific Examples Close to and Far from Equilibrium

We study the energetics of isothermal ratchets which are driven by a chemical reaction between two states and operate in contact with a single heat bath of constant temperature. We discuss generic aspects of energy transduction such as Onsager relations in the linear response regime as well as the efficiency and dissipation close to and far from equilibrium. In the linear response regime where the system operates reversibly the efficiency is in general nonzero. Studying the properties for specific examples of energy landscapes and transitions, we observe in the linear response regime that the efficiency can have a maximum as a function of temperature. Far from equilibrium in the fully irreversible regime, we find a maximum of the efficiency with values larger than in the linear regime for an optimal choice of the chemical driving force. We show that corresponding efficiencies can be of the order of 50%. A simple analytic argument allows us to estimate the efficiency in this irreversible regime for small external forces.

cond-mat.stat-mech↗