Dangerous Fieldwork: Reflections on Ethnographic Research with Irregular, Nigerian Streetwalkers and Madams in Spain

This is the final version. Available on open access from SAGE Publications via the DOI in this recordRecent calls have been made to investigate the lived experience of migrant sex workers, to broaden the scope and inclusivity of macro-level conceptualizations, and to develop contextually grounded forms of understanding. Our ethnographic study sought to explore the lived perspectives of an under-researched occupational group: migrant women working as irregular streetwalkers in a European city. Nineteen Nigerian Edo women working as prostitutes and Madams in Spain participated in an ethnographic, longitudinal study spanning five years of data collection. In this article, we focus on some of the key challenges, including ethical considerations, of undertaking ethnographic work in a hazardous fieldwork setting that presents psychological and physical dangers to both participants and researchers, including threats of violence, and researcher burnout

Deformation and flow of a two-dimensional foam under continuous shear

We investigate the flow properties of a two-dimensional aqueous foam submitted to a quasistatic shear in a Couette geometry. A strong localization of the flow (shear banding) at the edge of the moving wall is evidenced, characterized by an exponential decay of the average tangential velocity. Moreover, the analysis of the rapid velocity fluctuations reveals self-similar dynamical structures consisting of clusters of bubbles rolling as rigid bodies. To relate the instantaneous (elastic) and time-averaged (plastic) components of the strain, we develop a stochastic model where irreversible rearrangements are activated by local stress fluctuations originating from the rubbing of the wall. This model gives a complete description of our observations and is also consistent with data obtained on granular shear bands by other groups.Comment: 5 pages, 2 figure

Three-dimensional jamming and flows of soft glassy materials

Various disordered dense systems such as foams, gels, emulsions and colloidal suspensions, exhibit a jamming transition from a liquid state (they flow) to a solid state below a yield stress. Their structure, thoroughly studied with powerful means of 3D characterization, exhibits some analogy with that of glasses which led to call them soft glassy materials. However, despite its importance for geophysical and industrial applications, their rheological behavior, and its microscopic origin, is still poorly known, in particular because of its nonlinear nature. Here we show from two original experiments that a simple 3D continuum description of the behaviour of soft glassy materials can be built. We first show that when a flow is imposed in some direction there is no yield resistance to a secondary flow: these systems are always unjammed simultaneously in all directions of space. The 3D jamming criterion appears to be the plasticity criterion encountered in most solids. We also find that they behave as simple liquids in the direction orthogonal to that of the main flow; their viscosity is inversely proportional to the main flow shear rate, as a signature of shear-induced structural relaxation, in close similarity with the structural relaxations driven by temperature and density in other glassy systems.Comment: http://www.nature.com/nmat/journal/v9/n2/abs/nmat2615.htm

Structure and flow conditions through a colloidal packed bed formed under flow and confinement

International audienceWhen a colloidal suspension flows in a constriction, particles deposit and are able to clog it entirely, this fouling process being followed by the accumulation of particles. The knowledge of the dynamics of formation of such a dense particle assembly behind the clog head and its structural features is of primary importance in many industrial and environmental processes and especially during filtration. While most studies concentrate on the conditions under which pore clogging occurs, i.e., the pore narrowing up to its complete obstruction, this paper focuses on the accumulation of particles that follows pore obstruction. We determine the relative contribution of the confinement dimensions, the ionic strength and the flow conditions on the permeability and particle volume fraction of the resultant accumulation. In high confinement the irreversible deposition of particles on the channel surfaces controls the structure of the accumulation and the flow through it, irrespective of the other conditions, leading to a Darcy flow. Finally, we show that contrarily to the clog head, in which there is cohesion between particles, those in the subsequent accumulation are held together by the fluid and form a dense suspension of repulsive hard spheres

Flow decline during pore clogging by colloidal particles

International audienceThe flow of colloidal suspensions through porous media often leads to the deposition of particles inside the pores which increases the local hydrodynamic resistance by narrowing the pore space available and modifying the flow path of the transported particles. There is a significant flow decline in the extreme case when the entire porous medium becomes clogged. However, there are no experimental studies that determine directly the amplitude of this flow decline when compared with the dynamics of the formation of the particle deposit. This is mainly due to the great challenge of gaining experimental access to the features of the internal structure of the deposit as it grows and thus the ability to determine the flow inside it. In this paper, we show that is possible to monitor the flow decline corresponding to the successive deposition of colloidal particles inside a constriction (pore), ending by its complete blocking. The variation of the flow is determined by the measurement of the velocity of the particles through our channel. Such a technique coupled to the precise knowledge of what is deposited inside the pore, thanks to image analysis, enables us to determine the different contributions to the flow decline. We also use numerical simulations to access the flow inside the porous structure of the deposit as it grows. Together, experiments and simulations demonstrate that the obstruction process and the subsequent limited growth of the clog, corresponding to a few layers of accumulated particles, have a higher impact on the amplitude of the flow decline than the extra growth of the clog