Statistics of Bubble Rearrangements in a Slowly Sheared Two-dimensional Foam

Many physical systems exhibit plastic flow when subjected to slow steady shear. A unified picture of plastic flow is still lacking; however, there is an emerging theoretical understanding of such flows based on irreversible motions of the constituent ``particles'' of the material. Depending on the specific system, various irreversible events have been studied, such as T1 events in foam and shear transformation zones (STZ's) in amorphous solids. This paper presents an experimental study of the T1 events in a model, two-dimensional foam: bubble rafts. In particular, I report on the connection between the distribution of T1 events and the behavior of the average stress and average velocity profiles during both the initial elastic response of the bubble raft and the subsequent plastic flow at sufficiently high strains

Impact of boundaries on velocity profiles in bubble rafts

Under conditions of sufficiently slow flow, foams, colloids, granular matter, and various pastes have been observed to exhibit shear localization, i.e. regions of flow coexisting with regions of solid-like behavior. The details of such shear localization can vary depending on the system being studied. A number of the systems of interest are confined so as to be quasi-two dimensional, and an important issue in these systems is the role of the confining boundaries. For foams, three basic systems have been studied with very different boundary conditions: Hele-Shaw cells (bubbles confined between two solid plates); bubble rafts (a single layer of bubbles freely floating on a surface of water); and confined bubble rafts (bubbles confined between the surface of water below and a glass plate on top). Often, it is assumed that the impact of the boundaries is not significant in the ``quasi-static limit'', i.e. when externally imposed rates of strain are sufficiently smaller than internal kinematic relaxation times. In this paper, we directly test this assumption for rates of strain ranging from $10^{-3}$ to $10^{-2} {\rm s^{-1}}$. This corresponds to the quoted quasi-static limit in a number of previous experiments. It is found that the top plate dramatically alters both the velocity profile and the distribution of nonlinear rearrangements, even at these slow rates of strain.Comment: New figures added, revised version accepted for publication in Phys. Rev.

Forced oscillations dynamic tribometer with real-time insights of lubricated interfaces

This paper presents an innovative forced oscillations dynamic tribometer, the CHRONOS tribometer, with a lubricated ball-on-flat contact configuration fitted out with an in situ optical visualization system and a triggered high-speed camera. The CHRONOS tribometer generates controlled oscillating kinematics by means of a shaker with a range of strokes from 5 μm to 2.5 mm and an oscillation frequency which can be adjusted from 5 Hz to 250 Hz. Displacement and velocity are measured using a vibrometer. The ball-on-flat mean contact pressure is set between 200 MPa and 600 MPa. During motion, the instantaneous normal and friction forces and the interfacial film thickness distribution (in the nanometer scale) are simultaneously measured. In addition to this instantaneous approach, a more macroscopic approach is developed in terms of moving averages of friction and velocity. Another parameter, the friction-velocity tilt angle, is also introduced. This last parameter may give information on the friction-velocity dependence. Eventually, the experiments performed on the CHRONOS device lead to the representation of synchronized temporal signals of displacement/velocity, friction, and lubricant central film thickness. This superimposition of key parameters reveals time effects introduced by the periodical fluid squeeze and flow in the contact

Shear-Induced Stress Relaxation in a Two-Dimensional Wet Foam

We report on experimental measurements of the flow behavior of a wet, two-dimensional foam under conditions of slow, steady shear. The initial response of the foam is elastic. Above the yield strain, the foam begins to flow. The flow consists of irregular intervals of elastic stretch followed by sudden reductions of the stress, i.e. stress drops. We report on the distribution of the stress drops as a function of the applied shear rate. We also comment on our results in the context of various two-dimensional models of foams

IJTC2007-44273 DRAINAGE OF A WETTING LIQUID: SLIP EFFECT OR MOLECULARLY ORGANISED LAYERS?

ABSTRACT A Surface Force Apparatus has been used to investigate the drainage of a blend of copolymer viscosity improver additive molecules in a base oil. A quasi-static squeeze of the interface is used to characterize the first repulsion layer and the confined adsorbed layer. Moreover, dynamical measurements enlighten a significant negative value of the immobile layer thickness (-50 nm). The occurrence of slip at the wall that may result in such effect, is discussed in terms of liquid-solid interface wettability, surface roughness and cleanliness, and friction experiments. As a consequence, a modeling of the interface consisting of two layers of different viscosity is proposed. This model of interfacial molecular organization allows us to explain the drainage behaviour of the wetting liquid

Vibration analysis of rotating shaft on textured hydrodynamic journal bearings

The aim of this study is to analyse the influence of surface texturing on the dynamic behavior of a shaft on two hydrodynamic bearings. A prediction algorithm is presented with Fluid Structure Interaction (FSI) between the rotating shaft and the lubricant based on finite element method. Cavitation in bearings is taken into account. The hydrodynamic flow in the texturing pattern is modeled by two multi-scale algorithms. Static, modal and transient analyses are performed in order to predict the impact of several texturing patterns on the system vibrations

Influence of Humidity on Microtribology of Vertically Aligned Carbon Nanotube Film

The aim of this study is to probe the influence of water vapor environment on the microtribological properties of a forestlike vertically aligned carbon nanotube (VACNT) film, deposited on a silicon (001) substrate by chemical vapor deposition. Tribological experiments were performed using a gold tip under relative humidity varying from 0 to 100%. Very low adhesion forces and high friction coefficients of 0.6 to 1.3 resulted. The adhesion and friction forces were independent of humidity, due probably to the high hydrophobicity of VACNT. These tribological characteristics were compared to those of a diamond like carbon (DLC) sample

Shear behavior of an amorphous film with bubble soap raft model

Wear and friction in the boundary regime are often governed by the mechanical behavior of very thin layers separating the two solids in sliding contact. Sometimes, these layers are amorphous. Thanks to a bubble soap raft model, we simulate the sliding of two homogeneous rectangular crystalline rafts separated by an amorphous layer. The thickness and the length of this layer are variable. The kinematic field of the plastic flow with large displacements is experimentally determined at the same time as the tangential stress is recorded. Experimental results show that the sliding stress depends on the thickness-length ratio of the amorphous layer. When the ratio is high, solid mechanical theories explain the behavior, but when it is low, the effect of microscopic instabilities in the amorphous layer is dominant.La présence d'un film mince amorphe dans un contact glissant, influence de façon importante son comportement en frottement et à l'usure. Nous utilisons ici le modèle plan de bulles de savon dû à Bragg, pour modéliser le glissement d'un massif homogène cristallisé sur un autre massif recouvert d'une couche de matériau amorphe, et pour simuler les effets physiques et mécaniques gouvernant le comportement de l'interface amorphe. L'analyse de l'évolution de la force de frottement en fonction du déplacement relatif des corps est reliée à l'étude du champ cinématique de l'écoulement à l'intérieur de l'interface amorphe. Nous mettons en évidence l'existence de valeurs critiques dans le rapport de l'épaisseur du film sur sa longueur cisaillée. Ses lois de comportement sont influencées par ces effets d'échelle

An Interfacial Friction Law for a Circular EHL Contact Under Free Sliding Oscillating Motion

International audienceThe friction response of a lubricated interface under free sliding oscillating motion is investigated as a function of the contact pressure and the rheology of the lubricant in terms of viscosity and piezoviscosity. For loaded contacts, both velocity dependent friction, referred to as viscous damping, and friction independent of the instantaneous sliding velocity contribute to the energy dissipation. Viscous damping mainly corresponds to the dissipation in the lubricant meniscus surrounding the contact, while dissipation within the confined lubricated interface is mainly independent of the instantaneous sliding velocity. The friction coefficient independent of the instantaneous sliding velocity falls on a master curve for the wide range of tested operating conditions and lubricant rheological properties. The master curve is a logarithmic function of a dimensionless parameter corresponding to the ratio of the viscosity of the confined lubricant to the product of the pressure and a characteristic time. The physical meaning of this latter and the friction law are discussed considering the confined interface as a viscoelastic fluid or a non-Newtonian Eyring fluid