Electrorotation of semiconducting microspheres

We study experimentally the electrorotation (ROT) of semiconducting microspheres. ZnO microspheres obtained by a hydrothermal synthesis method are dispersed in KCl aqueous solutions and subjected to rotating electric fields. Two ROT peaks are found in experiments: a counterfield peak and a cofield peak at somewhat higher frequencies. These observations are in accordance with recent theoretical predictions for semiconducting spheres. The counterfield rotation is originated by the charging of the electrical double layer at the particle-electrolyte interface, while the cofield rotation is due to the Maxwell-Wagner relaxation. Additionally, we also found that some microspheres in the sample behaved differently and only showed counterfield rotation. We show that the behavior of these particles can be described by the so-called shell model. The microstructure of the microspheres is analyzed with electron microscope techniques and related to the ROT measurements.Spanish Government Ministry MICINN under Contract No. PGC2018-099217-B-I00Junta de Andalucía Contract No. PEJUS-

Modeling the AC Electrokinetic Behavior of Semiconducting Spheres

We study theoretically the dielectrophoresis and electrorotation of a semiconducting microsphere immersed in an aqueous electrolyte. To this end, the particle polarizability is calculated from first principles for arbitrary thickness of the Debye layers in liquid and semiconductor. We show that the polarizability dispersion arises from the combination of two relaxation interfacial phenomena: charging of the electrical double layer and the Maxwell–Wagner relaxation. We also calculate the particle polarizability in the limit of thin electrical double layers, which greatly simplifies the analytical calculations. Finally, we show the model predictions for two relevant materials (ZnO and doped silicon) and discuss the limits of validity of the thin double layer approximation

Dipolophoresis and Travelling-Wave Dipolophoresis of Metal Microparticles

We study theoretically and numerically the electrokinetic behavior of metal microparticles immersed in aqueous electrolytes. We consider small particles subjected to non-homogeneous ac electric fields and we describe their motion as arising from the combination of electrical forces (dielectrophoresis) and the electroosmotic flows on the particle surface (induced-charge electrophoresis). The net particle motion is known as dipolophoresis. We also study the particle motion induced by travelling electric fields. We find analytical expressions for the dielectrophoresis and induced-charge electrophoresis of metal spheres and we compare them with numerical solutions. This validates our numerical method, which we also use to study the dipolophoresis of metal cylinders.Spanish Research Agency MCI under contract PGC2018-099217-B-I00

Proceso de corrección de errores técnicos en atletas en formación

The main purpose of this article is to analyze the process of correcting technical errors in athletes in training. It is descriptive research, with correlated study, using empirical methods observation and survey. The population investigated corresponds to 109 athletes of the categories (10-11; 12-13 and 14-15 years), of athletics sport who account for 100%. The technical tests were carried out in two moments, 6 months apart; in addition, the statistical method for processing the obtained data, with the SPSS version 23 program, was applied. The parents of families and managers of the Hermosillo Multi-Use Center, Sonora, provided us with their consents to access the research.El presente artículo tiene como principal propósito analizar el proceso de corrección de errores técnicos en atletas en formación. Es una investigación descriptiva, con estudio correlacional, empleando los métodos empíricos observación y encuesta. La población investigada corresponde a 109 atletas de las categorías (10-11; 12-13 y 14-15 años), del deporte atletismo que representan el 100%. Las pruebas técnicas, se realizaron en dos momentos, durante con un periodo de diferencia de 6 meses; además, se aplicó el método estadístico para procesar los datos obtenidos, con el programa SPSS, versión 23. Los padres de familias y directivos del Centro de Usos Múltiples de Hermosillo, Sonora, facilitaron sus consentimientos para acceder a la investigación

Avalanches in fine, cohesive powders

We have investigated the onset of avalanches in fine, cohesive granular materials. In our experiments shear stress is generated by tilting an initialized bed of powder and increasing the angle of tilt until the powder avalanches. We find that the angle a of the avalanche decreases with increasing bed width. The avalanche depth increases with the bed width and, in all cases, is of the order of several millimeters, which is much greater than the particle size. We carry out a macroscopic analysis of the avalanche process based on Coulomb’s method of wedges. This analysis shows the fundamental role played by powder cohesion and boundary conditions on avalanches in fine cohesive powders. This behavior contrasts with the behavior of noncohesive grains, such as dry sand, where avalanches consist of superficial layers of about ten grains. The reason behind this is that for our experimental powders (particle diameter ~10 mm) the van der Waals interparticle adhesive force exceeds several orders of magnitude particle weight. Adhesive forces oppose gravity, and as a result fine cohesive powders settle in very open structures as compared to noncohesive granular materials. Because of the dominance of adhesive forces over particle weight, our materials behave more like wet sand

Combining DC and AC electric fields with deterministic lateral displacement for micro- And nano-particle separation

This paper describes the behavior of particles in a deterministic lateral displacement (DLD) separation device with DC and AC electric fields applied orthogonal to the fluid flow. As proof of principle, we demonstrate tunable microparticle and nanoparticle separation and fractionation depending on both particle size and zeta potential. DLD is a microfluidic technique that performs size-based binary separation of particles in a continuous flow. Here, we explore how the application of both DC and AC electric fields (separate or together) can be used to improve separation in a DLD device. We show that particles significantly smaller than the critical diameter of the device can be efficiently separated by applying orthogonal electric fields. Following the application of a DC voltage, Faradaic processes at the electrodes cause local changes in medium conductivity. This conductivity change creates an electric field gradient across the channel that results in a nonuniform electrophoretic velocity orthogonal to the primary flow direction. This phenomenon causes particles to focus on tight bands as they flow along the channel countering the effect of particle diffusion. It is shown that the final lateral displacement of particles depends on both particle size and zeta potential. Experiments with six different types of negatively charged particles and five different sizes (from 100 nm to 3 μm) and different zeta potential demonstrate how a DC electric field combined with AC electric fields (that causes negative-dielectrophoresis particle deviation) could be used for fractionation of particles on the nanoscale in microscale devices.Ministerio de Ciencia e Innovación PGC2018-099217-B-I0

The charged bouncing ball: An experimental model for period-doubling bifurcation

This paper presents an experimental and theoretical study of the dynamics of a conducting ball in a poorly conducting liquid subjected to an electric field. When the applied voltage is constant the ball bounces regularly on the lower electrode. If an AC voltage is superimposed, with a period equal to the unperturbed time between impacts, the ball undergoes a period-doubling bifurcation when increasing the amplitude of the AC signal. The non-linear map which describes the dynamics of the ball is closely related to the standard map and to the classical problem of a bouncing ball on a moving table.Dirección General de Investigación Científica y Técnica (DGICYT) PB93-118

Wall Repulsion during Electrophoresis: Testing the Theory of Concentration-Polarization Electroosmosis

We experimentally study the repulsion of charged microscopic particles with the channel walls during electrophoresis in microfluidic devices. For low frequencies of the electric fields (< 10 kHz), this repulsion is mainly due to the hydrodynamic interaction caused by the flow vortices that arise from the slip velocity induced by the electric field on the particle surface, as shown in a recent publication [Fernandez-Mateo et al., Physical Review Letters, 128, 074501, (2022)]. The maximum slip velocity on the particle surface is inferred from measurements of wall-particle separation. Importantly, this procedure allows us to infer very small slip velocities that otherwise are too weak to be measured directly. Data at small electric field amplitudes (E0) agree with theoretical predictions using the model of Concentration Polarization Electroosmosis (CPEO), which has recently been proposed as the mechanism behind the flow vortices on the surface of the particles. Data for higher electric fields show that the predictions of the CPEO theory for weak electric fields are not valid beyond E0 ∼ 60 kV/m. Additionally, we also show that, for sufficiently strong electric fields, the quadrupolar flow structures become disrupted, leading to a weaker wall repulsion.Ministerio de Ciencia e Innovación PGC2018-099217-B-I00, 10.13039/50110001103

Flow Regimes in Fine Cohesive Powders

Granular materials exhibit several regimes of behavior: plastic, inertial, fluidized, and entrained flow, but not all materials can pass through all of these states. Our concern is with the criteria that determine the transition from one regime to another and with the boundaries to the various flow regimes that these criteria define. Experimentally we have focused on fine, cohesive powders, where the interparticle cohesive force dominates over gravitational force and where entrained air can cause moving powder to become fluidized