Robert L. Powell

Anarchy in international relations theory: the neorealist-neoliberal debate

Two of the most influential contemporary approaches to international relations theory are neorealism and neoliberalism. The debate between these two approaches has dominated much of international relations theory for the last decade. It is now commonplace for an article about some aspect of international relations theory to begin by locating itself in terms of this debate. These two approaches and the debate between them have failed to contribute as much as they might have to international relations theory. These approaches suffer from serious internal weaknesses and limitations that the neorealistneoliberal debate often has tended to obscure rather than to clarify. Once we have exposed and clarified these weaknesses and limitations, we will be able to see several important directions for future theoretical work. Two books, Neorealism and Its Critics and Neorealism and Neoliberalism: The Contemporary Debate, make significant contributions to this debate. The former offered a wide-ranging critique of neorealism when it was published in 1986. The latter, which has just been published, is more narrowly focused. It takes up where some of the critiques in Neorealism and Its Critics left off. A review of these two complementary volumes affords an excellent opportunity to begin to identify some of the weaknesses and limitations that the neorealist-neoliberal debate frequently has obscured.

Guns, Butter, and Anarchy.

A state in the international system implicit in realism must allocate its limited resources between satisfying its intrinsically valued ends and the means of military power. I formalize this guns-versus-butter problem in a simple infinite-horizon model in which two states must continually decide how to allocate their resources and whether to attack the other state. The analysis establishes sufficient conditions to ensure the existence of an equilibrium in which neither state attacks; shows that there is a strictly Pareto-dominant pair of peaceful equilibrium payoffs; characterizes the unique, peaceful Markov perfect equilibrium that yields them; and describes the comparative statics of the equilibrium allocations. More broadly, the analysis also suggests that the notion of anarchy has little if any substantive significance distinctively related to international politics and that the problem of absolute and relative gains is superfluous.

Effect of particle size distributions on the rheology of concentrated bimodal suspensions

The dependence of the viscosities of highly concentrated bimodal suspensions on the particle size distribution and the relative fractions of the two particle populations is investigated using a parallel‐plate viscometer. For a fixed volume fraction of particles the viscosities of the systems with a bimodal size distribution of solids are less than the viscosity of a suspension with a unimodal size distribution of particulates. The results of two‐dimensional simulations in our previous study [C. Chang and R. L. Powell, J. Fluid Mech. 253, 1 (1993)] show excellent agreement with the experimental data when normalized with respect to their respective maximum packing values. In addition, both experimental and simulation results for the transient shear properties of bimodal suspensions of spherical particles are reported. Qualitatively, both show the same trend: a suspension with a larger relative viscosity needs higher strains to reach the final steady state as compared with suspensions having lower relative viscosities.The dependence of the viscosities of highly concentrated bimodal suspensions on the particle size distribution and the relative fractions of the two particle populations is investigated using a parallel‐plate viscometer. For a fixed volume fraction of particles the viscosities of the systems with a bimodal size distribution of solids are less than the viscosity of a suspension with a unimodal size distribution of particulates. The results of two‐dimensional simulations in our previous study [C. Chang and R. L. Powell, J. Fluid Mech. 253, 1 (1993)] show excellent agreement with the experimental data when normalized with respect to their respective maximum packing values. In addition, both experimental and simulation results for the transient shear properties of bimodal suspensions of spherical particles are reported. Qualitatively, both show the same trend: a suspension with a larger relative viscosity needs higher strains to reach the final steady state as compared with suspensions having lower relative v...

The Relative-Gains Problem for International Cooperation.

Modern realism claims that the fear that others will enjoy relatively greater benefits frequently impedes international cooperation. Recent articles in this Review by Duncan Snidal and Robert Powell modeled conditions under which the impact of relative gains varied. Joseph Grieco criticizes Snidals model as based on assumptions that allow him to avoid, rather than confront, the realist arguments. He also argues that Powells model, while constructive, ignores important additional sources of sensitivity to relative gains. In response, Powell discusses the value of alternative assumptions about preferences and constraints in international relations. Snidal defends his analysis and presents an additional proof to support the independence of his central result—the diminishing impact of relative gains with increasing numbers of states—from assumptions of concern to Grieco. Both responders emphasize their work as contributing to a contextually rich theory of international politics that builds on elements of both realism and neo-liberalism.

Tailoring the Size Distribution of Ultrasound Contrast Agents: Possible Method for Improving Sensitivity in Molecular Imaging

Encapsulated microbubble contrast agents incorporating an adhesion ligand in the microbubble shell are used for molecular imaging with ultrasound. Currently available microbubble agents are produced with techniques that result in a large size variance. Detection of these contrast agents depends on properties related to the microbubble diameter such as resonant frequency, and current ultrasound imaging systems have bandwidth limits that reduce their sensitivity to a polydisperse contrast agent population. For ultrasonic molecular imaging, in which only a limited number of targeted contrast agents may be retained at the site of pathology, it is important to optimize the sensitivity of the imaging system to the entire population of contrast agent. This article presents contrast agents with a narrow size distribution that are targeted for molecular imaging applications. The production of a functionalized, lipid-encapsulated, microbubble contrast agent with a monodisperse population is demonstrated, and we evaluate parameters that influence the size distribution and demonstrate initial acoustic testing.

Maintaining monodispersity in a microbubble population formed by flow-focusing.

The dynamic processes impacting the size distributions of lipid-encapsulated microbubbles formed by flow-focusing were observed by video optical microscopy. Parameters studied included the filling gas, gas saturating the surrounding solution, and microbubble size (initial size 2-12 microm) to simulate typical laboratory conditions. Typically, dissolution or growth, followed by Ostwald ripening at a collection cover glass, were observed and quantified. However, in the case of small nitrogen-filled microbubbles surrounded by an air-saturated solution, Ostwald ripening was avoided for at least 9 h. These bubbles had a final size distribution of 1.5 +/- 0.1 microm. This work suggests that lipid-encapsulated microbubbles formed by flow-focusing should be given sufficient time to reach a terminal size before coming into contact with each other. These long-lived mondisperse microbubbles should be of interest in ultrasound contrast agents, microfabrication, food, and research applications.

Dynamic simulation of bimodal suspensions of hydrodynamically interacting spherical particles

Stokesian dynamics is used to simulate the dynamics of a monolayer of a suspension of bimodally distributed spherical particles subjected to simple shearing flow. Hydrodynamic forces only are considered. Many-body far-field effects are calculated using the inverse of the grand mobility matrix. Near-field effects are calculated from the exact equations for the interaction between two unequal-sized spheres. Both the detailed microstructure (e.g. pair-distribution function and cluster formation) and the relative viscosity are determined for bimodal suspensions having particle size ratios of 2 and 4. The flow of an ‘infinite’ suspension is simulated by considering 25, 49, 64, and 100 particles to be ‘one’ cell of an infinite periodic array. The effects of both the size ratio and the relative fractions of the different-sized particles are examined. When the area fraction, ϕ a , is less than 0.4 the particle size distribution does not affect the calculated viscosity. For ϕ a > 0.4, and for a fixed fraction of small spheres, the bimodal suspensions generally have lower viscosities than monodispersed suspensions, with the size of this effect increasing with ϕ a . These results compare favourably with experiment when ϕ a and the volume fraction, ϕ v , are normalized by the maximum packing values in two and three dimensions, respectively. At the microstructural level, viscosity reduction is related to the influence of particle size distribution on the average number of particles in clusters. At a fixed area fraction, the presence of smaller particles tends to reduce average cluster size, particularly at larger ϕ a , where significant viscosity reductions are observed. Since the presence of large clusters in monodispersed suspensions has been directly linked to high viscosities, this provides a dynamic mechanism for the viscosity reduction in bimodal suspensions.

Suspensions of Rodlike Particles: Literature Review and Data Correlations

Recent experimental work on the macroscopic rheological properties of suspensions of rodlike particles in Newtonian and non-Newtonian fluids is reviewed. Correlations are proposed for the shear viscosity-shear rate relationship for semi-concentrated and concentrated suspensions. In the former case, shear thinning behavior is found with the slope of the power law region depending upon fiber aspect ratio. For the concen trated systems, the data are substantially independent of volume fraction and aspect ratio and show slight shear thinning.

The Macroeconomic Challenges of Scaling Up Aid to Africa

This paper surveys the economic literature on the scaling-up of aid to Africa. It provides a checklist of issues that need to be considered when preparing a long term macroeconomic projection for a country involving the assumption of a significant increase in aid. Such scaling-up scenarios are most likely to be developed in the context of a countrys efforts to achieve the Millennium Development Goals (MDGs) with the support of the international donor community. The paper stresses that when preparing a scaling-up scenario it is critical to have a detailed understanding of the likely use of additional aid flows.

The viscosity-volume fraction relation for suspensions of rod-like particles by falling-ball rheometry

The relative viscosities of suspensions of randomly oriented rods in a Newtonian fluid were measured using falling-ball rheometry. The rods were monodisperse and sufficiently large to render colloidal and Brownian forces negligible. Steel and brass ball bearings were dropped along the centreline of cylindrical columns containing the suspensions. The terminal velocities of the falling balls were measured and used to determine the average viscosities of the suspensions. The suspensions behaved as Newtonian fluids in that they were characterized by a constant viscosity. They exhibited a linear relative viscosity-volume fraction relationship for volume fractions less than 0.125, and, for volume fractions between 0.125 and 0.2315, the specific viscosity increased with the cube of the volume fraction. The relative viscosity was found to be independent of falling-ball size for a ratio of falling ball to fibre length greater than 0.3. It was found to be independent of the diameter of the containing cylindrical column for a ratio of column diameter to fibre length greater than 3.2. The value determined for the intrinsic viscosity is in good agreement with theoretical predictions for suspensions of randomly oriented rods.