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-

Enseñanza del árabe marroquí en la ciudad de Marrakech

Los datos que se presentan son fruto de una estancia de investigación en la ciudad de Marrakech a finales del pasado año. Se llevaron a cabo una serie de entrevistas a profesores, exprofesores y coordinadores de programas de intercambio, todos ellos relacionados de manera más o menos directa con la enseñanza del árabe marroquí. Para realizar estas entrevistas, en las que contamos finalmente con la desinteresada colaboración de una decena de personas, nos propusimos enfocar las cuestiones que quería tratar a través de un cuestionario previo. En dicho cuestionario, por una parte, considerábamos de gran importancia conocer de la mejor manera posible todo lo referente al comienzo de las clases de árabe marroquí en los centros donde éstas se impartían. Asimismo, conocer el perfil de los enseñantes se presentaba como un factor a tener en cuenta, sin obviar el enfoque metodológico del enseñante en el aula.Peer reviewe

Spin Analysis of the Process e+e- --> tau+tau- at LEP

Using the data collected by the four experiments at LEP during 1990-1994, a precise measurement of the tau longitudinal polarisation (P_tau) has been performed, as well as the measurement of the transverse-transverse and transverse-normal tau spin correlations. From the P_tau measurement, assuming lepton universality of the neutral currents, the effective weak mixing angle has been determined to be sin(theta_W) = 0.2325 +- 0.0006. The Standard Model predictions are consistent with the measured results.Comment: Invited talk to San Miniato 1997, 6 pages, 10 figures, Latex, macro espcrc2.sty include

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

Electro-orientation and electrorotation of metal nanowires

The physical mechanisms responsible for the electrical orientation and electrical rotation of metal nanowires suspended in an electrolyte as a function of frequency of the applied ac electric field are examined theoretically and experimentally. The alignment of a nanowire in an ac field with a fixed direction is called electro-orientation. The induced constant rotation of a nanowire in a rotating electric field is called electrorotation. In both situations, the applied electric field interacts with the induced charge in the electrical double layer at the metal-electrolyte interface, causing rotation due to the torque on the induced dipole, and also from induced-charge electro-osmotic flow around the particle. First, we describe the dipole theory that describes electro-orientation and electrorotation of perfectly polarizable metal rods. Second, based on a slender approximation, an analytical theory that describes induced-charge electro-orientation and electrorotation of metal nanowires is provided. Finally, experimental measurements of the electro-orientation and electrorotation of metal nanowires are presented and compared with theory, providing a comprehensive study of the relative importance between induced-dipole rotation and induced-charge electro-osmotic rotation