Showing papers by "Pablo García-Sánchez published in 2019"
TL;DR: A novel particle separation technique that combines deterministic lateral displacement (DLD) with orthogonal electrokinetic forces leading to enhanced sorting of particles below the critical diameter of the device is described.
Abstract: We describe a novel particle separation technique that combines deterministic lateral displacement (DLD) with orthogonal electrokinetic forces. DLD is a microfluidic technique for continuous flow particle separation based on size. We describe new tunable devices that use a combination of AC electric fields with DLD to separate particles below the critical diameter. Planar electrodes were integrated into a classical DLD device to produce a force orthogonal to the fluid flow direction. Experiments with 3.0 μm, 1.0 μm and 500 nm diameter microspheres show that at low frequencies (up to 500 Hz) particles oscillate in the direction of the field due to electrophoretic (EP)/electroosmotic (EO) forces. As the frequency of the field increases, the amplitude of these oscillations vanishes and, eventually dielectrophoresis (DEP) becomes the dominant electrokinetic force on the particles (DEP arises from electric field inhomogeneities caused by the presence of the DLD posts). Both mechanisms alter the paths of the particles inside the DLD devices leading to enhanced sorting of particles below the critical diameter of the device.
47 citations
TL;DR: It is shown that particles significantly smaller than the critical diameter of the device can be efficiently separated by applying orthogonal electric fields, and that the final lateral displacement of particles depends on both particle size and zeta potential.
Abstract: 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.
24 citations
TL;DR: The particle polarIZability is calculated from first principles for arbitrary thickness of the Debye layers in liquid and semiconductor and it is shown that the polarizability dispersion arises from the combination of two relaxation interfacial phenomena: charging of the electrical double layer and the Maxwell–Wagner relaxation.
Abstract: 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.
14 citations
TL;DR: It is found that some microspheres in the sample behaved differently and only showed counterfield rotation, and it is shown that the behavior of these particles can be described by the so-called shell model.
Abstract: 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.
11 citations
TL;DR: The cylinder rotation induced by the ICEO mechanism can be calculated by using the Lorentz reciprocal theorem, while the rotation due to the induced dipole is calculated from the cylinder polarizability using the thin-double-layer approximation.
Abstract: We study theoretically the rotation induced on an uncharged metal nanocylinder immersed in an electrolyte by AC electric fields. We consider the rotation of the cylinder when subjected to a rotating electric field (electrorotation) and the orientation of the cylinder in an AC field with constant direction (electro-orientation). The cylinder rotation is due to two mechanisms: the electric field interaction with the induced dipole on the particle and the hydrodynamic stress on the particle originated by the induced-charge electro-osmotic (ICEO) flow around the particle. The cylinder rotation induced by the ICEO mechanism can be calculated by using the Lorentz reciprocal theorem, while the rotation due to the induced dipole is calculated from the cylinder polarizability. We employ 3D numerical computations using finite elements for the general case as well as analytical methods for slender cylinders. Both calculations use the thin-double-layer approximation. We compare the results for slender cylinders of both methods showing good agreement. The electro-orientation (EOr) due to dipole torque aligns the axis of slender cylinders with the applied field, but aligns the axis of short cylinders perpendicularly to the field. The EOr due to ICEO torque always aligns the axis of cylinders with the field. The rotation induced by ICEO torque tends to disappear for frequencies of the applied field much greater than the characteristic frequency for charging the double-layer capacitance of the metal-electrolyte interface.
5 citations
TL;DR: A group of top high school Spanish students attended a summer scientific camp at the University of Seville as discussed by the authors, where the topic of the project was magnetic levitation, and during the one-week-long stay, the students investigated several possible ways of achieving stable levitation.
Abstract: During July 2018, a group of top high school Spanish students attended a summer scientific camp at the University of Seville. The topic of the project was magnetic levitation. During the one-week-long stay, the students investigated several possible ways of achieving stable levitation. These included diamagnetic levitation, use of superconductors, induced current levitation, and the toy Levitron®. The experiments were accompanied by their corresponding conceptual and theoretical explanations.
1 citations
TL;DR: The use of AC electric fields for manipulating and/or characterizing liquids and small particles in suspension is well-known and has applications in medicine, physics, and electronics.
Abstract: The use of AC electric fields for manipulating and/or characterizing liquids and small particles in suspension is well-known [...]
1 citations
Posted Content•
TL;DR: In this paper, the authors present supplementary material for the article with the same title that appeared in American Journal of Physics in 2019, which is referred to as the supplementary material of this article.
Abstract: This document contains supplementary material for the article with the same title that appeared in American Journal of Physics in 2019
TL;DR: In this paper, the authors report the production of bicomponent Janus filaments of miscible aqueous fluids in a microfluidic electro-flow focusing device under the action of an AC electric field.
Abstract: We report the production of bicomponent Janus filaments of miscible aqueous fluids in a microfluidic electro-flow-focusing device under the action of an AC electric field. The production of liquid filaments can lead to the generation of microfibers by adding a subsequent process of polymerization. Janus microfibers are of paramount importance in biomedical applications such as tissue production on crimped scaffolds. We show that the filament length is a function of the frequency signal, voltage amplitude and of the viscosity and conductivity of the dispersed phase. In particular, Janus filaments with diameters $$\sim 10\,\upmu$$
m and longer than 1 mm are produced by AC voltages with frequencies below 150 kHz and a viscosity of the dispersed phase $$\sim 10\,$$
cP.