Surface Fluctuations of an Aging Colloidal Suspension: Evidence for Intermittent Quakes

We present measurements of the thermal fluctuations of the free surface of an aging colloidal suspension, Laponite. The technique consists in measuring the fluctuations of the position of a laser beam that reflects from the free surface. Analysing the data statistics, we show that, as the fluid ages, the dynamics becomes intermittent. The intermittent events correspond to large changes in the local slope of the free surface over a few milliseconds. We show that those quakes are uncorrelated, although they are kept in memory by the surface over short time scales

Rejuvenation and overaging in a colloidal glass under shear

We report the modifications of the microscopic dynamics of a colloidal glass submitted to shear. We use multispeckle diffusing wave spectroscopy to monitor the evolution of the spontaneous slow relaxation processes after the sample have been submitted to various straining. We show that high shear rejuvenates the system and accelerates its dynamics whereas moderate shear overage the system. We analyze this phenomena within the frame of the Bouchaud's trap model.Comment: 4 pages, 4 figures, to be published in PR

Slow flows of yield stress fluids: complex spatio-temporal behaviour within a simple elasto-plastic model

A minimal athermal model for the flow of dense disordered materials is proposed, based on two generic ingredients: local plastic events occuring above a microscopic yield stress, and the non-local elastic release of the stress these events induce in the material. A complex spatio-temporal rheological behaviour results, with features in line with recent experimental observations. At low shear rates, macroscopic flow actually originates from collective correlated bursts of plastic events, taking place in dynamically generated fragile zones. The related correlation length diverges algebraically at small shear rates. In confined geometries bursts occur preferentially close to the walls yielding an intermittent form of flow localization.Comment: 4 pages, 4 figure

Dip coating with colloids and evaporation

International audienceThere is a growing interest in coating hard and soft substrates with colloids, with numerous applications to optics and microelectronics [1]. A possibility to realize these substrates is to use dip coating with evaporation [2], i.e. to remove at constant speed a plate from a bath of colloids while drying occurs. This leads to several undesired effects: defects, heterogeneous deposition, fracture and de-lamination [1,3]. The problem is also difficult to model as three divergences may coexist at the contact line (CL) receding on the substrate [4-5] (and even in a advancing case [6]) : (1) divergence of viscous stresses, (2) divergence of evaporation as in the well known "coffee stain" effect [7-9], (3) and divergence of colloid concentration. In a recent paper we modeled the hydrodynamics in the vicinity of a moving, evaporating, contact line [4], and we found that in the dip coating case there should exist two different regimes at respectively low and high plate velocity, in which the deposed mean thickness should respectively decrease and increase with the plate velocity. This should lead to a minimum of the deposed thickness for a critical intermediate velocity. Up to a recent thesis in our group [5], this effect has never been evidenced in a dip coating experiment, though similar behaviors were found for deposition of phospholipids [10], and for colloids in a rather specific two-plate geometry (meniscus receding in a Hele-Shaw cell) [11-12]. We present here evidences in favour of this effect, revealed by this work, and we correct the model of ref.[4] which contained a mistake. A sketch of the experimental set up is suggested on Fig.1. A clean glass plate is plunged inside a colloidal suspension and removed from this bath at constant speed (V ranging between 50 µm/s and 5 cm/s), while deposition and evaporation takes place on the glass. We used silica suspensions (Klebosol silica sluries 50R50, 30R25 and 30R12) with three different particle diameters (12 nm, 25 nm and 50 nm), and two different volume concentrations (φ 0 =5% and 10%). The glass plate is cleaned and prepared before each experiment by the following protocole. First the glass surface is rub with a abrasive cerium oxid suspension (concentration 20%), cleaned with pure water, ethanol, and again pure water, and then let to dry. A plasma treatment is then imposed to the glass

Dip coating with colloids and evaporation

International audienceThere is a growing interest in coating hard and soft substrates with colloids, with numerous applications to optics and microelectronics [1]. A possibility to realize these substrates is to use dip coating with evaporation [2], i.e. to remove at constant speed a plate from a bath of colloids while drying occurs. This leads to several undesired effects: defects, heterogeneous deposition, fracture and de-lamination [1,3]. The problem is also difficult to model as three divergences may coexist at the contact line (CL) receding on the substrate [4-5] (and even in a advancing case [6]) : (1) divergence of viscous stresses, (2) divergence of evaporation as in the well known "coffee stain" effect [7-9], (3) and divergence of colloid concentration. In a recent paper we modeled the hydrodynamics in the vicinity of a moving, evaporating, contact line [4], and we found that in the dip coating case there should exist two different regimes at respectively low and high plate velocity, in which the deposed mean thickness should respectively decrease and increase with the plate velocity. This should lead to a minimum of the deposed thickness for a critical intermediate velocity. Up to a recent thesis in our group [5], this effect has never been evidenced in a dip coating experiment, though similar behaviors were found for deposition of phospholipids [10], and for colloids in a rather specific two-plate geometry (meniscus receding in a Hele-Shaw cell) [11-12]. We present here evidences in favour of this effect, revealed by this work, and we correct the model of ref.[4] which contained a mistake. A sketch of the experimental set up is suggested on Fig.1. A clean glass plate is plunged inside a colloidal suspension and removed from this bath at constant speed (V ranging between 50 µm/s and 5 cm/s), while deposition and evaporation takes place on the glass. We used silica suspensions (Klebosol silica sluries 50R50, 30R25 and 30R12) with three different particle diameters (12 nm, 25 nm and 50 nm), and two different volume concentrations (φ 0 =5% and 10%). The glass plate is cleaned and prepared before each experiment by the following protocole. First the glass surface is rub with a abrasive cerium oxid suspension (concentration 20%), cleaned with pure water, ethanol, and again pure water, and then let to dry. A plasma treatment is then imposed to the glass

DYNAMICS OF A COMPLETE WETTING LIQUID UNDER EVAPORATION

International audienceWetting phenomena are extensively studied from a experimental to a theoretical point of view (see Refs. [1, 2] for reviews) and much attention has been drawn recently to the case of the dynamics of liquid droplet under evaporation [3-9]. In this paper, we propose a model of a contact line under evaporation and total wetting conditions taking into account van der Waals interactions and the divergent nature of evaporation near the border of the liquid evidenced by Deegan et al. [3]. We apply this result to study the dynamics of an evaporating droplet in complete wetting situation

Weak non-linearities of amorphous polymer under creep

The creep behavior of an amorphous poly(etherimide) (PEI) polymer is investigated in the vicinity of its glass transition in a weakly non linear regime where the acceleration of the creep response is driven by local configurational rearrangements. From the time shifts of the creep compliance curves under increasing applied stresses in the range 1-15~\si{\mega\pascal}, we determine a macroscopic acceleration factor. At the start of creep, the stress is homogeneous and the macroscopic acceleration can be assimilated to that of the local rearrangements which is shown to vary as $f=e^{-(\sigma/Y)^n}$ with $n=2 \pm 0.2$, where $\sigma$ is the local stress and $Y$ is a decreasing function of compliance. This experimental result is in agreement with the recent theory of Long \textit{et al.} (\textit{Phys. Rev. Mat.} (2018) \textbf{2}, 105601 ) which predicts $n=2$. From a mean field approximation, we interpret the variation of $Y$ with compliance as the result of the development of stress heterogneities during creep

Dynamics of Liquid Contact Line on Visco-Elastic Gels

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Light induced flows opposing drainage in foams and thin-films using photosurfactants

International audienceWe study the influence of UV light on the drainage flows of foams and thin-liquid films stabilized by photoswitchable azobenzene surfactants, whose shape and hydrophobicity can be modified using UV illumination. This model system, the dynamics of which was well characterized in a previous study, enables us to trigger a controlled variation of the surface excess and surface tension. In both geometries we observe light-induced flows which are able to suppress the drainage flow induced by gravity. However, we show that the physical origin of the flows is different in both geometries. At the scale of a few films in the so-called 'two-bubble' experiment the comparisons of the physical length scales, i.e. the radius of the meniscus and the film thickness, to the chemical "reservoir length" (Γ/c) show that the flux of the surfactant at the interface in the presence of UV light is different in the films and in the meniscus, inducing a Marangoni flow from the meniscus to the film, which is stronger than gravity and capillary suction. The velocity of this flow can be tuned by the light intensity and the surfactant concentration. In the real foams, however, we show that the above mechanism is not relevant because the radii of curvature of the Plateau borders are orders of magnitude lower than in the two-bubble experiment, thus the capillary suction prevents such transfer between the films and the Plateau borders. Instead, the decrease of the drainage velocity is shown to be due to a gradient of the surface tension in the illuminated zone hence to a local variation of the capillary pressure. This study underlines the importance of characterizing the radius of the Plateau borders for the understanding of foams, as this key parameter sets the order of magnitude of capillary pressure, film thickness and amount of available surfactant. We also show that this photosurfactant is a new toolbox for the understanding of foam stability

Multispeckle diffusing-wave spectroscopy: a tool to study slow relaxation and time-dependent dynamics

A multispeckle technique for efficiently measuring correctly ensemble-averaged intensity autocorrelation functions of scattered light from non-ergodic and/or non-stationary systems is described. The method employs a CCD camera as a multispeckle light detector and a computer-based correlator, and permits the simultaneous calculation of up to 500 correlation functions, where each correlation function is started at a different time. The correlation functions are calculated in real time and are referenced to a unique starting time. The multispeckle nature of the CCD camera detector means that a true ensemble average is calculated; no time averaging is necessary. The technique thus provides a "snapshot" of the dynamics, making it particularly useful for non-stationary systems where the dynamics are changing with time. Delay times spanning the range from 1 ms to 1000 s are readily achieved with this method. The technique is demonstrated in the multiple scattering limit where diffusing-wave spectroscopy theory applies. The technique can also be combined with a recently-developed two-cell technique that can measure faster decay times. The combined technique can measure delay times from 10 ns to 1000 s. The method is peculiarly well suited for studying aging processes in soft glassy materials, which exhibit both short and long relaxation times, non-ergodic dynamics, and slowly-evolving transient behavior.Comment: 11 pages 13 figures Accepted in Review of Scientific Instrument (june 02