Electrospray Methodologies for Characterization and Deposition of Nanoparticles

Electrospray is an aerosolization method that generates highly charged droplets from solutions or suspensions and, after a series of solvent evaporation - droplet fission cycles, it results in particles carrying multiple charges. Highly charged particles are used in a variety of applications, including particle characterization, thin film deposition, nanopatterning, and inhalation studies among several others. In this work, a soft X-ray photoionization was coupled with an electrospray to obtain monodisperse, singly charged nanoparticles for applications in online size characterization with electrical mobility analysis. Photoionization with the soft X-ray charger enhanced the diffusion neutralization rate of the highly charged bacteriophages, proteins, and solid particles. The effect of nanoparticle surface charge and nanoparticle agglomeration in liquids on the electrospray process was studied experimentally and a modified expression to calculate the effective electrical conductivity of nanosuspensions was proposed. The effective electrical conductivity of TiO2 nanoparticle suspensions is strongly dependent on the electrical double layer and the agglomeration dynamics of the particles; and such dependence is more remarkable in liquids with low ionic strength. TiO2 nanoparticle agglomerates with nearly monodisperse sizes in the nanometer and submicrometer ranges were generated, by electrospraying suspensions with tuned effective electrical conductivity, and used to deposit photocatalytic films for water-splitting. Nanostructured films of iron oxide with uniform distribution of particles over the entire deposition area were formed with an electrospray system. The micro-Raman spectra of the iron oxide films showed that transverse and longitudinal optical modes are highly sensitive to the crystallize size of the electrospray-deposited films. The fabrication of films of natural light-harvesting complexes, with the aim of designing biohybrid photovoltaic devices, was explored with an electrospray. The ability to charge chlorosomes with large number of charges allowed their ballistic deposition onto TiO2 nanostructured columnar films simultaneously maintaining their light-harvesting properties. Single units of natural light-harvesting complexes were isolated in charged electrospray droplets for subsequent size characterization. The charge distribution of natural light-harvesting complexes, aerosolized with a collision nebulizer, was determined with tandem differential mobility analysis. It was found that nebulized light-harvesting complexes were multiply charged; hence they have potential applications in the deposition of functional films using electric fields. The studies conducted as part of this dissertation addressed fundamental issues in the characterization and deposition of nanoparticle suspensions and elucidated applications of the electrospray technique, particularly for solar energy utilization

Microdynamics of magnetic particles in complex fluids

International audienceThe agregation of magnetic particles in the presence of a magnetic field is the basic phenomenon which underlies all the physcis of magnetorheological (MR) fluids.Although these interactions are well understood when the suspending fluid is a simple liquid, new MR fluids based on dispersions of magnetic microparticles in a ferrofluid or MR elastomers based on dispersion of magnetic paryticles in a rubber matrix, present some unsusual properties which are not well described by conventional theories We analyse in this work, the motion of magnetic particles dispersed in a ferrofluid and submitted to a magnetic field and discuss the possible application

Abrupt contraction flow of magnetorheological fluids

International audienceContraction and expansion flows of magnetorheological fluids occur in a variety of smart devices. It is important therefore to learn how these flows can be controlled by means of applied magnetic fields. This paper presents a first investigation into the axisymmetric flow of a magnetorheological fluid through an orifice so-called abrupt contraction flow. The effect of an external magnetic field, longitudinal or transverse to the flow, is examined. In experiments, the pressure-flow rate curves were measured, and the excess pressure drop associated with entrance and exit losses was derived from experimental data through the Bagley correction procedure. The effect of the longitudinal magnetic field is manifested through a significant increase in the slope of the pressure-flow rate curves, while no discernible yield stress occurs. This behavior, observed at shear Mason numbers 10Mnshear100, is interpreted in terms of an enhanced extensional response of magnetorheological fluids accompanied by shrinkage of the entrance flow into a conical funnel. At the same range of Mason numbers, the transverse magnetic field appears not to influence the pressure drop. This can be explained by a total destruction of magnetic particle aggregates by large hydrodynamic forces acting on them when they are perpendicular to the flow. To support these findings, we have developed a theoretical model connecting the microstructure of the magnetorheological fluid to its extensional rheological properties and predicting the pressure-flow rate relations through the solution of the flow equations. In the case of the longitudinal magnetic field, our model describes the experimental results reasonably well

Normal stresses in a shear flow of magnetorheological suspensions: viscoelastic versus Maxwell stresses

International audienceThis work reports an experimental and theoretical study on the normal force developed by suspensions of magnetic microparticles subjected to magnetic fields. Experimental values of the normal force were obtained using a rotational rheometer, for a broad range of particle concentration in the suspensions. Applied magnetic fields up to 343 kA/m were generated in the plate-plate measuring geometry. It was found that the normal force exhibited a high-value plateau at low shear, followed by a decrease as the suspensions started to flow and a final low-value plateau at high shear. These three regions in the normal force vs. shear rate curve were well correlated with the microscopic regimes in the suspensions: field-aligned structures filling the gap; inclined structures still filling the gap; and structures non-filling the gap. The theoretical model developed is based on the equilibrium between hydrodynamic and magnetostatic torques and forces in a field-induced aggregate of particles subjected to shear. The stress tensor was obtained and the normal force calculated as the integral of the stress over the total surface of the rotational plate. A good correspondence among theoretical and experimental values was obtained

Magnetic and magnetorheological properties of nanofiber suspensions

International audienceIn this work the preparation and characterization of magnetorheological (MR) fluids constituted by CoNi nanofibers (56 nm length, 6.6 nm width) are reported. The properties of these new fluids were characterized by usual techniques (including magnetometry and magnetorheology). The results were compared with those obtained for conventional suspensions constituted by CoNi nanosperes

Steady shear flow of magnetic fiber suspensions: theory and comparison with experiments

International audienceThis paper is focused on the rheology of magnetic fiber suspensions in the presence of a magnetic field applied perpendicular to the flow. At low Mason numbers, Mn<0.1, the experimental flow curves show a steep initial section corresponding to the inclination and stretching of the gap-spanning aggregates formed upon magnetic field application. At higher Mason numbers, aggregates no longer stick to the walls and the flow curves reach a Bingham regime, with the dynamic yield stress growing with the magnetic field intensity. This yield stress appears to be about three times higher for the fiber suspensions than for the suspensions of spherical particles. Such difference, measured at relatively low magnetic field intensities, H0<30 kA/m, is explained in terms of the enhanced magnetic susceptibility of the aggregates composed of fibers compared to the aggregates composed of spherical particles. For weak magnetic fields, the forces of solid friction between fibers are expected to play a minor role on the stress level of the suspension. In order to confirm these findings, we propose a new theoretical model, taking into account hydrodynamic interactions. The flow curve and the yield stress predictions are in a good agreement with the experimental results for semi-diluted suspensions

Isolated cutaneous Rosai-Dorfman disease: a case report

Background:Rosai-Dorfman also known as sinus histiocytosis with massive lymphadenopathy is a benign, idiopathic lymphoproliferative disorder that usually affects the lymph nodes. Cutaneous Rosai-Dorfman disease is a rare extranodal variant that occurs as histiocyte-rich inflammatory infiltrates, manifesting with a variable clinical morphology. Usually it appears as erythematous to brown papules, plaques, or nodules, with no predilection for site. The histological picture shows abnormal lymph node architecture, reactive germinal centers, fibrosis and emperipolesis in the dermis. On immunophenotypic analysis, S100 protein and CD68 are usually present on dendritic cells. Case Report: We report a case of purely cutaneous Rosai-Dorfman disease. A 55-year-old male presented to our clinic for an indurated nodule on the left malar region. He reported a slow and progressive growth of 2-year history without systemic symptoms. A cutaneous biopsy showed a nodular inflammatory infiltrate within the dermis consisting of histiocytes, local aggregates of plasma cells and lymphocytes. Histiocytes were enlarged with vesicular nuclei, and emperipolesis was observed. Furthermore, histiocytes stained positively for S-100 and CD68. Owing to local involvement, the patient received a surgery to exsect the lesion completely. Conclusions: Sinus histiocytosis is a rare inflammatory disease mainly affecting the cervical lymph nodes, presenting with skin lesions in 10% of cases. The diagnosis of cutaneous RDD is differentiated from other histiocytic conditions by the combination of clinical findings accompanied by histopathologic and immunohistochemical confirmationUniversidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Yield stress in magnetorheological suspensions near the limit of maximum-packing fraction

International audienceThis work deals with the magnetic field-induced static yield stress of magnetorheological (MR) suspensions with concentration near the limit of maximum-packing fraction. With this aim, homogeneous suspensions of iron microparticles with 50 vol.% concentration were prepared, and their yield stress measured as a function of the applied magnetic field. In view of the failure of existing models to predict, on the basis of realistic hypotheses, the values of the yield stress of highly concentrated MR suspensions, we developed a new model. Our model considers that field application induces body-centered tetragonal (BCT) structures. Upon shearing, these structures deform in such a way that interparticle gaps appear between neighboring particles of the same chain, whereas the approach of particles of parallel chains ensures the mechanical stability of the whole multi-chain structure. Based on this hypothesis, and using finite element method simulations of interparticle magnetic interactions, our model is able to quantitatively predict the yield stress of highly concentrated MR suspensions. Furthermore, estimations show that the main contribution to the field-dependent part of the yield stress comes from the change in the permeability of the structures as interparticle gaps are enlarged by the shear

Stick-slip instabilities in the shear flow of magnetorheological suspensions

International audienceThis work is devoted to the stick-slip instabilities that appear in the shear flow of highly concentrated suspensions of magnetic microparticles. The effect of the applied magnetic field strength was analyzed in details. With this aim, homogeneous suspensions of iron microparticles with concentration near the limit of maximum-packing fraction were prepared, and shear-flow measurements were performed in a controlled-rate mode using a rheometer provided with a rough parallel-plate geometry. For each given value of the shear rate, the time evolution of the shear stress was monitored for at least 20 min. Saw-tooth-like stress oscillations, typical of stick-slip instabilities, were obtained at low enough shear rate values. The measurements were restricted to small enough oscillations, at which the rheometer was still able to maintain the shear rate constant. From the microscopic viewpoint, these stick-slip instabilities principally appear due to the periodic failure and healing of the field-induced particle structures, as inferred from experimental observations. This hypothesis is corroborated by a theoretical model developed on the basis of the balance of the magnetic and hydrodynamic torques over the particle structures, allows us to predict the correct order of magnitude of the main parameters of the stick-slip instabilities, including the amplitude and period of the stress oscillations

Fabrication and Actuation of Magnetic Shape-Memory Materials

Soft actuators are deformable materials that change their dimensions and/or shape in response to external stimuli. Among the various stimuli, remote magnetic fields are one of the most attractive forms of actuation, due to their ease of use, fast response and safety in biological systems. Composites of magnetic particles with polymer matrices are the most common material for magnetic soft actuators. In this paper, we demonstrate the fabrication and actuation of magnetic shape-memory materials based on hydrogels containing field-structured magnetic particles. These actuators are formed by placing the pregel dispersion into a mold of the desired on-field shape and exposing this to a homogeneous magnetic field until the gel point is reached. At this point the material may be removed from the mold and fully gelled in the desired off-field shape. The resultant magnetic shape-memory material then transitions between these two shapes when subjected to successive cycles of a homogeneous magnetic field, acting as a large deformation actuator. For actuators that are planar in the off-field state, this can result in significant bending to return to the on-field state. In addition, it is possible to make shape-memory materials that twist under the application of a magnetic field. For these torsional actuators, both experimental and theoretical results are given.Departamento de Física AplicadaGrupo FQM144Ministerio de Ciencia, Innovación y UniversidadesAgencia Estatal de InvestigaciónDeutsche Forschungsgemeinschaft (DFG