Wrinkling instability of vesicles in steady linear flow

We present experimental observations and numerical simulations of a wrinkling instability that occurs at sufficiently high strain rates in the trembling regime of vesicle dynamics in steady linear flow. Spectral and statistical analysis of the data shows similarities and differences with the wrinkling instability observed earlier for vesicles in transient elongation flow. The critical relevance of thermal fluctuations for this phenomenon is revealed by a simple model using coupled Langevin equations that reproduces the experimental observations quite well.Comment: 6 pages, 9 figures + 2 video

Fluid Vesicles in Flow

We review the dynamical behavior of giant fluid vesicles in various types of external hydrodynamic flow. The interplay between stresses arising from membrane elasticity, hydrodynamic flows, and the ever present thermal fluctuations leads to a rich phenomenology. In linear flows with both rotational and elongational components, the properties of the tank-treading and tumbling motions are now well described by theoretical and numerical models. At the transition between these two regimes, strong shape deformations and amplification of thermal fluctuations generate a new regime called trembling. In this regime, the vesicle orientation oscillates quasi-periodically around the flow direction while asymmetric deformations occur. For strong enough flows, small-wavelength deformations like wrinkles are observed, similar to what happens in a suddenly reversed elongational flow. In steady elongational flow, vesicles with large excess areas deform into dumbbells at large flow rates and pearling occurs for even stronger flows. In capillary flows with parabolic flow profile, single vesicles migrate towards the center of the channel, where they adopt symmetric shapes, for two reasons. First, walls exert a hydrodynamic lift force which pushes them away. Second, shear stresses are minimal at the tip of the flow. However, symmetry is broken for vesicles with large excess areas, which flow off-center and deform asymmetrically. In suspensions, hydrodynamic interactions between vesicles add up to these two effects, making it challenging to deduce rheological properties from the dynamics of individual vesicles. Further investigations of vesicles and similar objects and their suspensions in steady or time-dependent flow will shed light on phenomena such as blood flow.Comment: 13 pages, 13 figures. Adv. Colloid Interface Sci., 201

Characteristic spatial scale of vesicle pair interactions in a plane linear flow

We report the experimental studies on interaction of two vesicles trapped in a microfluidic analog of four-roll mill, where a plane linear flow is realized. We found that the dynamics of a single vesicle is significantly altered by the presence of another vesicle at separation distances up to about 3.2 \div 3.7 times of effective radius of the vesicles. This is supported by direct measurements of a single vesicle back-reaction on the velocity field. Thus, the experiment provides the lower bound for the interaction scale of vesicles and so the corresponding upper bound for the volume fraction \phi=0.08 \div 0.13 of non-interacting vesicle suspensions.Comment: 5 pages, 8 figures, PRE accepted for publicatio

Capital et travail

La recension de l’ouvrageCapital et travail de Victor Levant préparée par Jean Boivin et publiée dans R.I. vol 33, no 2 a donné lieu à une réaction. Respectant le désir de l'auteur, nous publions dans son intégrité la lettre qu'il nous a adressée. Nous publions de plus les remarques des professeurs Laperrière et Noumoff ainsi que la réplique de Jean Boivin

The political economy of Canadian foreign policy in Vietnam

This thesis seeks to determine the nature and character of Canada's foreign policy towards Vietnam.Dependent upon Southeast Asian stability for strategic resources, a merchandise surplus and the expansion of the American economy, Canada also benefitted directly through increased exports of staple goods, automotive parts and war material.Ottawa's record on the International Control Commissions was characterized by partisan voting, complicity in the violation of the Geneva and Paris Agreements and the rationalization of Washington's strategy.Canadian aid, dispensed only to Saigon, was a co-ordinated part of American pacification programs.The purpose of Ottawa's policy was to ensure the permanent division of Vietnam while the ultimate intention and consequence was to legitimate the U.S. intervention. Underlying Ottawa's decision-making was Canada's integration into the world market system and the unequal, albeit voluntary, alliance between the economic elites of Canada and the United States