Viscosity of Colloidal Suspensions

Simple expressions are given for the Newtonian viscosity $\eta_N(\phi)$ as well as the viscoelastic behavior of the viscosity $\eta(\phi,\omega)$ of neutral monodisperse hard sphere colloidal suspensions as a function of volume fraction $\phi$ and frequency $\omega$ over the entire fluid range, i.e., for volume fractions $0 < \phi < 0.55$. These expressions are based on an approximate theory which considers the viscosity as composed as the sum of two relevant physical processes: $\eta (\phi,\omega) = \eta_{\infty}(\phi) + \eta_{cd}(\phi,\omega)$, where $\eta_{\infty}(\phi) = \eta_0 \chi(\phi)$ is the infinite frequency (or very short time) viscosity, with $\eta_0$ the solvent viscosity, $\chi(\phi)$ the equilibrium hard sphere radial distribution function at contact, and $\eta_{cd}(\phi,\omega)$ the contribution due to the diffusion of the colloidal particles out of cages formed by their neighbors, on the P\'{e}clet time scale $\tau_P$, the dominant physical process in concentrated colloidal suspensions. The Newtonian viscosity $\eta_N(\phi) = \eta(\phi,\omega = 0)$ agrees very well with the extensive experiments of Van der Werff et al and others. Also, the asymptotic behavior for large $\omega$ is of the form $\eta_{\infty}(\phi) + A(\phi)(\omega \tau_P)^{-1/2}$, in agreement with these experiments, but the theoretical coefficient $A(\phi)$ differs by a constant factor $2/\chi(\phi)$ from the exact coefficient, computed from the Green-Kubo formula for $\eta(\phi,\omega)$. This still enables us to predict for practical purposes the visco-elastic behavior of monodisperse spherical colloidal suspensions for all volume fractions by a simple time rescaling.Comment: 51 page

Family-based Social Activism: Rethinking the Social Role of Families

This paper sheds light on family-based social activism, defined herein as the broad range of activities undertaken by family members to raise awareness and bring about policy reform about a serious condition or tragic event that affects or affected someone in their family. These activities constitute social activism because they are intended to foster social change. The activism is ‘family-based’ because the social change advocate is a family member responding to the serious or traumatic situation that affects or affected a family member. Mothers Against Drunk Driving, the Westray Families Group, and Family and Friends of Schizophrenics are just a few examples of the many family-based social activist groups which have formed in Canada. Family-based activism is not addressed as a specific subject of study in the sociology literature on families, social movements, social problems, or voluntary association membership. The first goal of this paper is to bring family-based social activism into focus and provide some empirical evidence of this phenomenon in Canada. The second goal is to situate family activism in the sociology literature by drawing theoretical inspirations from social memory literature and suggesting avenues for theoretical development in perspectives on families

The dynamics of superfluid 4He

We present neutron scattering results for the dynamic response by superfluid and normal-fluid 4He and the results of a simple perturbation-theory analysis which allows us to describe all aspects of the observed behavior by considering only density fluctuations. We show that the three key features that are characteristic of the dynamic reponse of superfluid 4He, as well as their dramatic variations with temperature, can be attributed predominantly to the Bose statistics obeyed by all the 4He atoms. It appears that the presence of the Bose condensate exerts at most a minor influence on the dynamic response.Comment: 12 pages, 4 figures. Material closely related to W. Montfrooij and E.C. Svensson, Journal of Low Temperature Physics, Vol. 121, p. 293-302 (2000

A Meso-Level Analysis of the Revitalization of the WUSC Student Refugee Program at St. Francis Xavier University

Although millions of community members have come together in various ways to sponsor and resettle refugees for decades, scholars are just now beginning to study why people get involved and how they organize themselves to accomplish the practical, organizational, and emotional tasks community sponsorship requires. This article contributes to this emerging literature with a meso-level analysis of actions taken by one student-led local committee (LC) to revitalize the World University Service of Canada (WUSC) Student Refugee Program (SRP) nested at its university. Over 2,200 refugee youth have been sponsored through the WUSC SRP to study and settle in Canada since 1978, yet little is known about how campus LCs navigate their responsibilities or evaluate and sustain their program. Analysis of in-depth interviews with executive members was guided by and informs research on community sponsorship and theories of group action, community of practice, and stewardship.Bien que des millions de membres de communautés locales se soient unis de diverses façons pour parrainer et réinstaller des réfugiés pendant des décennies, les chercheurs commencent à peine à étudier les raisons pour lesquelles les gens s’impliquent et comment ils s’organisent pour accomplir les tâches pratiques, organisation- nelles et émotionnelles requises par le parrainage communautaire. Cet article contribue à cette littérature émergente avec une analyse de niveau méso des actions prises par un comité local étudiant pour revitaliser le Programme d’étudiants réfugiés (PÉR) d’Entraide universitaire mondiale du Canada (EUMC). Depuis 1978, plus de 2200 jeunes réfugiés ont été parrainés à travers le PÉR EUMC pour étudier et s’installer au Canada. Cependant, on sait peu de choses sur la manière dont les comités locaux sur les campus s’acquittent de leurs responsabilités ou évaluent et maintiennent leur programme. L’analyse d’entretiens approfondis avec des membres exécutifs a été guidée par et contribue à la recherche sur le parrainage communautaire et les théories de l’action de groupe, de la communauté de pratique et de l’intendance

Transport in a highly asymmetric binary fluid mixture

We present molecular dynamics calculations of the thermal conductivity and viscosities of a model colloidal suspension with colloidal particles roughly one order of magnitude larger than the suspending liquid molecules. The results are compared with estimates based on the Enskog transport theory and effective medium theories (EMT) for thermal and viscous transport. We find, in particular, that EMT remains well applicable for predicting both the shear viscosity and thermal conductivity of such suspensions when the colloidal particles have a ``typical'' mass, i.e. much larger than the liquid molecules. Very light colloidal particles on the other hand yield higher thermal conductivities, in disagreement with EMT. We also discuss the consequences of these results to some proposed mechanisms for thermal conduction in nanocolloidal suspensions.Comment: 13 pages, 6 figures, to appear in Physical Review E (2007

Offering Sexual Health Fairs to Supplement Existing Sex Education Programs: An Evaluation of Adolescent Students' Knowledge Needs

A health fair called Choices Not Chances has been designed by the public health nurses in a southern Ontario city to increase the knowledge of young high school students about sexual health and healthy relationships. Our purpose in designing this study in cooperation with the public health nurses ivas to assess the existing level of knowledge among grades 9 and 10 students to determine if the content presented through this fair was appropriate for these students. Forty-five grades 9-10 students completed one survey during class time in their sex-segregated physical education class. Our findings show that although students had basic knowledge about STDs, anatomy, and pregnancy prevention, their knowledge of healthy communication and behavior in relationships was much less developed, which may put students at risk of becoming involved in unsafe sex and abusive relationships. Overall, the health fair appears to provide a valuable supplement for sex education.Dans le but d'accroître les connaissances qu'ont les élèves du secondaire au sujet de la santé en matière de sexualité et des relations interpersonnelles saines, des infirmières de la santé publique ont organisé, dans une ville du sud de l'Ontario, une foire sur la santé intitulée Choices Not Chances (Des choix plutôt que des risques). Nous avons entrepris cette étude avec la coopération de ces infirmières de la santé publique afin d'évaluer ce que savent les élèves en 9e et en 10e années pour ensuite déterminer si le contenu de la foire leur convenait. Quarante-cinq élèves en 9e et en 10e années ont complété un sondage pendant leur cours d'éducation physique (pendant lequel ils sont regroupés selon leur sexe). Les résultats indiquent que, quoique les étudiants aient des connaissances de base au sujet des MTS, de l'anatomie et de la prévention de la grossesse, ils en savent beaucoup moins sur la communication et le comportement sains dans les relations interpersonnelles, ce qui pourrait indiquer qu'ils courrent le risque de se retrouver dans des relations abusives ou d'adopter des pratiques sexuelles risquées. De façon globale, la foire sur la santé semble constituer un supplément valable au cours d'éducation en matière de sexualité

Mesoscopic two-phase model for describing apparent slip in micro-channel flows

The phenomenon of apparent slip in micro-channel flows is analyzed by means of a two-phase mesoscopic lattice Boltzmann model including non-ideal fluid-fluid and fluid-wall interactins. The weakly-inhomogeneous limit of this model is solved analytically. The present mesoscopic approach permits to access much larger scales than molecular dynamics, and comparable with those attained by continuum methods. However, at variance with the continuum approach, the existence of a gas layer near the wall does not need to be postulated a priori, but emerges naturally from the underlying non-ideal mesoscopic dynamics. It is therefore argued that a mesoscopic Lattice Boltzmann approach with non-ideal fluid-fluid and fluid-wall interactions might achieve an optimal compromise between physical realism and computational efficiency for the study of channel micro-flows.Comment: 5 pages, 3 figure