arXiv subjects

A. Ajdari

Publications and source records attributed to A. Ajdari.

11 recordsLinked to original sources

arXiv2009

Microchannel flow of a shear-banding fluid: enhanced confinement effect and interfacial instability

Using a micro particle imaging velocity technique, we resolve for the first time the three dimensionnal structure of wormlike shear banding flows in straight microchannels. The study revealed two effects, which should be generic for shear banding flows: the first is a strong amplification of the confinement induced by the edge of the channel, the second is an instability of the interface between the shear bands. A detailed quantitative comparison of our experimental measurements with a theoretical study of the diffusive Johnson Segalman model leads to excellent agreement. Our study clarifies the nature of shear banding flow instabilities, and shows that, despite the challenging complexity of the situation and the uncertainty regarding their molecular structure, shear banding flows in confined geometries are amenable to quantitative modelling, a feature that opens pathways to their practical utilization.

cond-mat.soft↗

arXiv2008

Microfluidic velocimetry reveals spatial cooperativity in soft glassy flows

Amorphous glassy materials of diverse nature -- concentrated emulsions, granular materials, pastes, molecular glasses -- display complex flow properties, intermediate between solid and liquid, which are at the root of their use in many applications. A classical feature, well documented yet not really understood, is the very non-linear nature of the flow rule relating stresses and strain rates. Using a microfluidic velocimetry technique, we characterize the flow of thin layers of concentrated emulsions, confined in gaps of different thicknesses by surfaces of different roughness. Beyond the classical non-linearities of the rheological behaviour, we evidence finite size effects in the flow behaviour and the absence of an intrinsic local flow rule. In contrast, a rather simple non-local flow rule is shown to account for all the velocity profiles. This non-locality of the dynamics is quantified by a length, characteristic of the cooperativity of the flow at these scales, that is unobservable in the liquid state (lower concentrations) and that increases with concentration in the jammed state. Beyond its practical importance for applications involving thin layers, e.g. coatings, our assessment of non-locality and cooperativity echoes observations on other glassy, jammed and granular systems, suggesting a possible fundamental universality.

cond-mat.mtrl-sci↗

arXiv2007

Droplet traffic in microfluidic networks: A simple model for understanding and designing

We propose a simple model to analyze the traffic of droplets in microfluidic ``dual networks''. Such functional networks which consist of two types of channels, namely those accessible or forbidden to droplets, often display a complex behavior characteristic of dynamical systems. By focusing on three recently proposed configurations, we offer an explanation for their remarkable behavior. Additionally, the model allows us to predict the behavior in different parameter regimes. A verification will clarify fundamental issues, such as the network symmetry, the role of the driving conditions, and of the occurrence of reversible behavior. The model lends itself to a fast numerical implementation, thus can help designing devices, identifying parameter windows where the behavior is sufficiently robust for a devices to be practically useful, and exploring new functionalities.

physics.flu-dyn↗

arXiv2007

Designing phoretic micro- and nano-swimmers

Small objects can swim by generating around them fields or gradients which in turn induce fluid motion past their surface by phoretic surface effects. We quantify for arbitrary swimmer shapes and surface patterns, how efficient swimming requires both surface ``activity'' to generate the fields, and surface ``phoretic mobility.'' We show in particular that (i) swimming requires symmetry breaking in either or both of the patterns of "activity" and ``mobility,'' and (ii) for a given geometrical shape and surface pattern, the swimming velocity is size-independent. In addition, for given available surface properties, our calculation framework provides a guide for optimizing the design of swimmers.

cond-mat.soft↗

arXiv2002

A simple model for heterogeneous flows of yield stress fluids

Various experiments evidence spatial heterogeneities in sheared yield stress fluids. To account for heterogeneities in the velocity gradient direction, we use a simple model corresponding to a non-monotonous local constitutive curve and study a simple shear geometry. Different types of boundary conditions are considered. Under controlled macroscopic shear stress $Σ$, we find homogeneous flow in the bulk and a hysteretic macroscopic stress - shear rate curve. Under controlled macroscopic shear rate $\dotΓ$, shear banding is predicted within a range of values of $\dotΓ$. For small shear rates, stick slip can also be observed. These qualitative behaviours are robust when changing the boundary conditions.

cond-mat.soft↗

arXiv2002

Rheological Chaos in a Scalar Shear-Thickening Model

We study a simple scalar constitutive equation for a shear-thickening material at zero Reynolds number, in which the shear stress σis driven at a constant shear rate \dotγand relaxes by two parallel decay processes: a nonlinear decay at a nonmonotonic rate R(σ_1) and a linear decay at rate λσ_2. Here σ_{1,2}(t) = τ_{1,2}^{-1}\int_0^tσ(t')\exp[-(t-t')/τ_{1,2}] {\rm d}t' are two retarded stresses. For suitable parameters, the steady state flow curve is monotonic but unstable; this arises when τ_2>τ_1 and 0>R'(σ)>-λso that monotonicity is restored only through the strongly retarded term (which might model a slow evolution of material structure under stress). Within the unstable region we find a period-doubling sequence leading to chaos. Instability, but not chaos, persists even for the case τ_1\to 0. A similar generic mechanism might also arise in shear thinning systems and in some banded flows.

cond-mat.soft↗

arXiv2001

Jamming, hysteresis and oscillation in scalar models for shear thickening

We investigate shear thickening and jamming within the framework of a family of spatially homogeneous, scalar rheological models. These are based on the `soft glassy rheology' model of Sollich et al. [Phys. Rev. Lett. 78, 2020 (1997)], but with an effective temperature x that is a decreasing function of either the global stress σor the local strain l. For appropiate x=x(σ), it is shown that the flow curves include a region of negative slope, around which the stress exhibits hysteresis under a cyclically varying imposed strain rate \gd. A subclass of these x(σ) have flow curves that touch the \gd=0 axis for a finite range of stresses; imposing a stress from this range {\em jams} the system, in the sense that the strain γcreeps only logarithmically with time t, γ(t)\sim\ln t. These same systems may produce a finite asymptotic yield stress under an imposed strain, in a manner that depends on the entire stress history of the sample, a phenomenon we refer to as history--dependent jamming. In contrast, when x=x(l) the flow curves are always monotonic, but we show that some x(l) generate an oscillatory strain response for a range of steady imposed stresses. Similar spontaneous oscillations are observed in a simplified model with fewer degrees of freedom. We discuss this result in relation to the temporal instabilities observed in rheological experiments and stick--slip behaviour found in other contexts, and comment on the possible relationship with `delay differential equations' that are known to produce oscillations and chaos.

cond-mat.soft↗

arXiv2001

Rheological instability in a simple shear thickening model

We study the strain response to steady imposed stress in a spatially homogeneous, scalar model for shear thickening, in which the local rate of yielding Γ(l) of mesoscopic `elastic elements' is not monotonic in the local strain l. Despite this, the macroscopic, steady-state flow curve (stress vs. strain rate) is monotonic. However, for a broad class of Γ(l), the response to steady stress is not in fact steady flow, but spontaneous oscillation. We discuss this finding in relation to other theoretical and experimental flow instabilities. Within the parameter ranges we studied, the model does not exhibit rheo-chaos.

cond-mat.soft↗

arXiv2001

Effective Area-Elasticity and Tension of Micro-manipulated Membranes

We evaluate the effective Hamiltonian governing, at the optically resolved scale, the elastic properties of micro-manipulated membranes. We identify floppy, entropic-tense and stretched-tense regimes, representing different behaviors of the effective area-elasticity of the membrane. The corresponding effective tension depends on the microscopic parameters (total area, bending rigidity) and on the optically visible area, which is controlled by the imposed external constraints. We successfully compare our predictions with recent data on micropipette experiments.

cond-mat.soft↗

arXiv2001

Casimir Torques between Anisotropic Boundaries in Nematic Liquid Crystals

Fluctuation-induced interactions between anisotropic objects immersed in a nematic liquid crystal are shown to depend on the relative orientation of these objects. The resulting long-range ``Casimir'' torques are explicitely calculated for a simple geometry where elastic effects are absent. Our study generalizes previous discussions restricted to the case of isotropic walls, and leads to new proposals for experimental tests of Casimir forces and torques in nematics.

cond-mat.soft↗

arXiv1998

Dynamic Fluctuations of Semiflexible Polymers

We develop a scaling theory to describe dynamic fluctuations of a semiflexible polymer and find several distinct regimes. We performed simulations to characterize the longitudinal and transverse dynamics; using ensemble averaging for a range of different degrees of coarse-graining we avoid the problems of slow equilibration often encountered in simulations. We find that the longitudinal fluctuations of a semiflexible object scales as t^7/8. These fluctuations are correlated over a length which varies as t^1/8. Our results are pertinent to the interpretation of high frequency microrheology experiments in actin solutions.

cond-mat.soft↗