Hydrodynamic irreversibility of non-Brownian suspensions in highly confined duct flow

A.H. acknowledges support from the US Department of Energy, Advanced Scientific Computing Research programme, under the Scalable, Efficient and Accelerated Causal Reasoning Operators, Graphs and Spikes for Earth and Embedded Systems (SEA-CROGS) project, FWP 80278. Pacific Northwest National Laboratory (PNNL) is a multi-programme national laboratory operated for the US Department of Energy by the Battelle Memorial Institute under contract no. DE-AC05-76RL01830. F.V. acknowledges funding from the University of Granada through the Brown/CASA-UGR Research Collaboration Fund and MICINN PID2019-104883GB-I00 project (Spain).Supplementary movies are available at https://doi.org/10.1017/jfm.2023.793The irreversible behaviour of a highly confined non-Brownian suspension of spherical particles at low Reynolds number in a Newtonian fluid is studied experimentally and numerically. In the experiment, the suspension is confined in a thin rectangular channel that prevents complete particle overlap in the narrow dimension and is subjected to an oscillatory pressure-driven flow. In the small cross-sectional dimension, particles rapidly separate to the walls, whereas in the large dimension, features reminiscent of shear-induced migration in bulk suspensions are recovered. Furthermore, as a consequence of the channel geometry and the development and application of a single-camera particle tracking method, three-dimensional particle trajectories are obtained that allow us to directly associate relative particle proximity with the observed migration. Companion simulations of a steadily flowing suspension highly confined between parallel plates are conducted using the force coupling method, which also show rapid migration to the walls as well as other salient features observed in the experiment. While we consider relatively low volume fractions compared to most prior work in the area, we nevertheless observe significant and rapid migration, which we attribute to the high degree of confinement.US Department of Energy: FWP 80278, DE-AC05-76RL01830University of GranadaMICINN PID2019-104883GB-I00 project (Spain

Oscillating Magnetic Drop: How to Grade Water-Repellent Surfaces

Evaluation of superhydrophobic (SH) surfaces based on contact angle measurements is challenging due to the high mobility of drops and the resolution limits of optical goniometry. For this reason, some alternatives to drop-shape methods have been proposed such as the damped-oscillatory motion of ferrofluid sessile drops produced by an external magnetic field. This approach provides information on surface friction (lateral/shear adhesion) from the kinetic energy dissipation of the drop. In this work, we used this method to compare the low adhesion of four commercial SH coatings (Neverwet, WX2100, Ultraever dry, Hydrobead) formed on glass substrates. As ferrofluid, we used a maghemite aqueous suspension (2% v/v) synthesized ad hoc. The rolling magnetic drop is used as a probe to explore shear solid–liquid adhesion. Additionally, drop energy dissipates due to velocity-dependent viscous stresses developed close to the solid–liquid interface. By fitting the damped harmonic oscillations, we estimated the decay time on each coating. The SH coatings were statistically different by using the mean damping time. The differences found between SH coatings could be ascribed to surface–drop adhesion (contact angle hysteresis and apparent contact area). By using this methodology, we were able to grade meaningfully the liquid-repelling properties of superhydrophobic surfaces.This research was financed by the State Research Agency (SRA) and European Regional Development Fund (ERDF) through the project MAT2017-82182-R. Fernando Vereda acknowledges financial support from MAT 2016-78778-R and PCIN-2015-051 projects (Spain)

Ternary solid-ferrofluid-liquid magnetorheological fluids

A new kind of magnetorheological fluid is proposed that exhibits both enhanced magnetorheological effect and kinetic stability against sedimentation. It includes the presence of small amounts of an emulsified aqueous ferrofluid as a third phase in a conventional oil-based magnetorheological fluid prepared by the dispersion of carbonyl iron microparticles.This work was supported by MINECO MAT 2016-78778-R and PCIN-2015-051 projects (Spain), the European Regional Development Fund (ERDF) and by the Junta de Andalucía P11-FQM-7074 project (Spain). J J Yang acknowledges the Chinese State Scholarship Fund. J R Morillas acknowledges the FPU14/01576 fellowship

Specific ion effects on the electrokinetic properties of iron oxide nanoparticles: Experiments and simulations

We report experimental and simulation studies on ion specificity in aqueous colloidal suspensions of positively charged, bare magnetite nanoparticles. Magnetite has the largest saturation magnetization among iron oxides and relatively low toxicity, which explain why it has been used in multiple biomedical applications. Bare magnetite is hydrophilic and the sign of the surface charge can be changed by adjusting the pH, its isoelectric point being in the vicinity of pH = 7. Electrophoretic mobility of our nanoparticles in the presence of increasing concentrations of different anions showed that anions regarded as kosmotropic are more efficient in decreasing, and even reversing, the mobility of the particles. If the anions were ordered according to the extent to which they reduced the particle mobility, a classical Hofmeister series was obtained with the exception of thiocyanate, whose position was altered. Monte Carlo simulations were used to predict the diffuse potential of magnetite in the presence of the same anions. The simulations took into account the ion volume, and the electrostatic and dispersion forces among the ions and between the ions and the solid surface. Even though no fitting parameters were introduced and all input data were estimated using Lifshitz theory of van der Waals forces or obtained from the literature, the predicted diffusion potentials of different anions followed the same order as the mobility curves. The results suggest that ionic polarizabilities and ion sizes are to a great extent responsible for the specific ion effects on the electrokinetic potential of iron oxide particles.The authors thank the financial support from the following institutions: (i) ‘Ministerio de Economía y Competitividad, Plan Nacional de Investigación, Desarrollo e Innovación Tecnológica (I + D + i)’, Projects MAT2013-44429-R, MAT2012-36270-C04-04 and -02. (ii) ‘Consejería de Innovación, Ciencia y Empresa de la Junta de Andalucía’, Projects P09-FQM-4698, P10-FQM-5977, and P11-FQM-7074. (iii) European Regional Development Fund (ERDF)

Control of surface morphology and internal structure in magnetite microparticles: from smooth single crystals to rough polycrystals

Magnetite particles in the micrometer range have been obtained by the oxidative aging of ferrous hydroxide with KNO3. The surface morphology and the number of crystallites that constitute each particle can be controlled by adjusting the Fe2+ excess in the reaction media. Thus, for a relatively low [Fe2+]Exc we obtained smooth polyhedral single-crystal particles, whereas for larger [Fe2+] Exc the particles had rough surfaces and a raspberry-like appearance due to their polycrystalline nature. The differences in the surface morphology of the particles are intimately related to the differences in the internal structure, which are the outcome of particular growth mechanisms. These mechanisms of particle formation can therefore also be controlled and can be qualitatively explained in terms of the interparticle electrostatic interactions after the initial nucleation. Magnetic properties were also connected to the internal structure of the particles. Because of the relatively large size of the crystalline domains, magnetization reversal took place by magnetic domain wall motion and all the particles we obtained were magnetically soft at room temperature. At 5 K the more complex structure of the rough particles resulted in a larger coercivity. © 2013 The Royal Society of Chemistry.F. Vereda is especially grateful to the ‘Programa de reincorporación de doctores de la Universidad de Granada’. This work was supported by the MICINN MAT 2010-15101 and MAT2011-23641 projects (Spain), the European Regional Development Fund (ERDF) and the Junta de Andalucía P10-FQM-5977, P10-RNM-6630 and P11-FQM-7074 projects (Spain).Peer Reviewe

Hydrodynamic irreversibility of non-Brownian suspensions in highly confined duct flow

The irreversible behavior of a highly confined non-Brownian suspension of spherical particles at low Reynolds number in a Newtonian fluid is studied experimentally and numerically. In experiment, the suspension is confined in a thin rectangular channel that prevents complete particle overlap in the narrow dimension and subjected to an oscillatory pressure-driven flow. In the small cross-sectional dimension particles rapidly separate to the walls, whereas in the large dimension features reminiscent of shear-induced migration in bulk suspensions are recovered. Furthermore, as a consequence of the channel geometry and the development and application of a single-camera particle tracking method, three-dimensional particle trajectories are obtained that allow us to directly associate relative particle proximity with the observed migration. Companion simulations of a steadily flowing suspension highly confined between parallel plates are conducted using the Force Coupling Method and recover many of the salient features observed in the experiment.Comment: 17 pages, 12 figure

Rough and Hollow Spherical Magnetite Microparticles: Revealing the Morphology, Internal Structure, and Growth Mechanism

We report the fabrication, characterization, and a tentative growth mechanism of spherical microparticles with a rough surface fabricated by oxidative aging of ferrous hydroxide. The aging involves the transformation of ferrous hydroxide into Fe₃O₄ and the growth of the magnetite particles. Scanning electron microscopy, focused ion beam, and transmission electron microscopy studies of the spherical microparticles show that they have a small void in the center and that they are polycrystalline with a typical grain size of 150 nm. The crystallites are oriented along the radial direction of the spheres, stretching themselves from the central cavity to the particle surface, and their crystalline orientations do not keep any obvious relationship. The collected data and the structure suggest that the microparticles’ growth mechanism has four main stages: (1) initial nucleation of small magnetite nanoparticles, (2) aggregation to form spherical polycrystalline clusters, (3) direct crystal growth from species in solution, and (4) development of the outer facets. Magnetization measurements are in agreement with the observed crystalline structure. Both X-ray powder diffraction and magnetization measurements indicate that the stoichiometry of these particles is slightly oxidized with respect to Fe₃O₄