Filip Strubbe

Transport of charged Aerosol OT inverse micelles in nonpolar liquids.

Surfactants such as Aerosol OT (AOT) are commonly used to stabilize and electrically charge nonpolar colloids in devices such as electronic ink displays. The electrical behavior of such devices is strongly influenced by the presence of charged inverse micelles, formed by excess surfactant that does not cover the particles. The presence of charged inverse micelles results in increased conductivity of the solution, affecting both the energy consumption of the device and its switching characteristics. In this work, we use transient current measurements to investigate the electrical properties of suspensions of the surfactant Aerosol OT in dodecane. No particles are added, to isolate the effect of excess surfactant. The measured currents upon application of a voltage step are found to be exponentially decaying, and can be described by an analytical model based on an equivalent electric circuit. This behavior is physically interpreted, first by the high generation rate of charged inverse micelles giving the suspension resistor like properties, and second by the buildup of layers of charged inverse micelles at both electrodes, acting as capacitors. The model explains the measurements over a large range of surfactant concentrations, applied voltages, and device thicknesses.

Screening and separation of charges in microscale devices: complete planar solution of the Poisson-Boltzmann equation

The Poisson-Boltzmann (PB) equation is widely used to calculate the interaction between electric potential and the distribution of charged species. In the case of a symmetrical electrolyte in planar geometry, the Gouy-Chapman (GC) solution is generally presented as the analytical solution of the PB equation. However, we demonstrate here that this GC solution assumes the presence of a bulk region with zero electric field, which is not justified in microdevices. In order to extend the range of validity, we obtain here the complete numerical solution of the planar PB equation, supported with analytical approximations. For low applied voltages, it agrees with the GC solution. Here, the electric double layers fully absorb the applied voltage such that a region appears where the electric field is screened. For higher voltages (of order 1 V in microdevices), the solution of the PB equation shows a dramatically different behavior, in that the double layers can no longer absorb the complete applied voltage. Instead, a finite field remains throughout the device that leads to complete separation of the charged species. In this higher voltage regime, the double layer characteristics are no longer described by the usual Debye parameter kappa, and the ion concentration at the electrodes is intrinsically bound (even without assuming steric interactions). In addition, we have performed measurements of the electrode polarization current on a nonaqueous model electrolyte inside a microdevice. The experimental results are fully consistent with our calculations, for the complete concentration and voltage range of interest.

Diffuse double layer charging in nonpolar liquids

The presence of charges in nonpolar liquids is usually neglected. However, in some applications, they have important effects. In this work, an analytical description is given for the dynamics of diffuse double layers when a voltage step of the order of kT∕e is applied. Part of the result is an expression for the current, which is consistent with current measurements on nonpolar liquids and with other works. In measurements for higher voltages, we observe and briefly discuss nonlinear effects. From measurements both at high and at low voltages, we can obtain the same properties of the liquid.

Power-law transient charge transport in a nonpolar liquid

Transient current measurements in response to a voltage step can reveal a lot of information about the nature, the origin, and the properties of electrically charged species in nonpolar media. In situations where the influence of the charges on the electric field can be neglected, interpretation is straightforward. However, this condition is not fulfilled in many applications. This work provides an approximate analytical description, illustrated by measurements and simulations, for cases in which the contribution of the charge density to the electric field is comparable in magnitude to the contribution of the applied voltage.

Fast and versatile deposition of aligned semiconductor nanorods by dip-coating on a substrate with interdigitated electrodes

Semiconductor nanorods mainly absorb and emit light with the electric field along the axis of the rods, it is therefore important to align the nanorods along a preferred direction. The homogeneous deposition of aligned nanorods on large substrates is interesting for large size applications such as solar cells and OLEDs. In this work, we present a fast and versatile method for the homogeneous deposition and alignment of nanorods from a colloidal suspension. The method is based on a low-cost dip-coating procedure during which an alternating electric field is applied. The accumulation, orientation, and polarized fluorescence of the nanorods is verified by AFM and polarized fluorescence microscopy. An alignment with order parameter of 0.67 has been obtained with this method.

On-chip light sheet illumination enables diagnostic size and concentration measurements of membrane vesicles in biofluids

Cell-derived membrane vesicles that are released in biofluids, like blood or saliva, are emerging as potential non-invasive biomarkers for diseases, such as cancer. Techniques capable of measuring the size and concentration of membrane vesicles directly in biofluids are urgently needed. Fluorescence single particle tracking microscopy has the potential of doing exactly that by labelling the membrane vesicles with a fluorescent label and analysing their Brownian motion in the biofluid. However, an unbound dye in the biofluid can cause high background intensity that strongly biases the fluorescence single particle tracking size and concentration measurements. While such background intensity can be avoided with light sheet illumination, current set-ups require specialty sample holders that are not compatible with high-throughput diagnostics. Here, a microfluidic chip with integrated light sheet illumination is reported, and accurate fluorescence single particle tracking size and concentration measurements of membrane vesicles in cell culture medium and in interstitial fluid collected from primary human breast tumours are demonstrated.

Electrokinetics of colloidal particles in nonpolar media containing charged inverse micelles

We have compared optical tracking and electric current measurements to study the electrokinetics of colloidal particles in nonpolar media containing charged inverse micelles. Particle trajectories are measured in response to a voltage step, revealing spatial and temporal variations of the electric field when space-charge layers are created by charged inverse micelles. The particle trajectories and current measurements are in good agreement with simulations and analytical approximations based on drift and diffusion of charges. Electrohydrodynamic effects observed at high concentrations of charged inverse micelles are explained by injection of charged inverse micelles from the bulk into the space-charge layers.

Thermal Charging of Colloidal Quantum Dots in Apolar Solvents: A Current Transient Analysis

We analyze thermal charging in additive-free colloidal CdSe quantum dot (QD) dispersions by means of the transient electric current resulting from a voltage step applied across the QD dispersion. On the basis of the initial current and the total charge separated, we find that the CdSe dispersion behaves as a 1:1 electrolyte where equal fractions of the QDs carry a single positive or a single negative charge. This conclusion is confirmed by a more detailed fitting of the current transient using the Nernst-Planck-Poisson equations. Using equilibrium thermodynamics, we relate the fraction of charged QDs to the QD charging energy. The magnitude of the charging energy corresponds to values found using known models for the charging energy of either a spherical surface in a dielectric or a charge within a dielectric sphere. However, the experimental dependence of the charging energy on the dielectric constant of the solvent is far less pronounced than predicted by these models. A better correspondence is found based on the charging energy of a spherical surface embedded in a compound medium consisting of the ligand shell and the solvent.

Micellization and adsorption of surfactant in a nonpolar liquid in micrometer scale geometries

Mixtures of nonpolar liquid and surfactant are used increasingly in applications with microscopic dimensions. However, most methods to characterize them are performed on bulk solutions. We measure electrical transient currents in thin layers of nonpolar liquid with surfactant and derive several properties from these measurements. This paper reports the results for different liquid layer thicknesses and surfactant concentrations. We observe a dependence on the layer thickness of the inverse micelle concentration, which cannot be explained by bulk micellization alone. A model including surface adsorption is proposed that describes surfactant behavior in microscale geometries.

Characterizing Generated Charged Inverse Micelles with Transient Current Measurements

We investigate the generation of charged inverse micelles in nonpolar surfactant solutions relevant for applications such as electronic ink displays and liquid toners. When a voltage is applied across a thin layer of a nonpolar surfactant solution between planar electrodes, the generation of charged inverse micelles leads to a generation current. From current measurements it appears that such charged inverse micelles generated in the presence of an electric field behave differently compared to those present in equilibrium in the absence of a field. To examine the origin of this difference, transient current measurements in which the applied voltage is suddenly increased are used to measure the mobility and the amount of generated charged inverse micelles. The mobility and the corresponding hydrodynamic size are found to be similar to those of charged inverse micelles present in equilibrium, which indicates that other properties determine their different behavior. The amplitude and shape of the transient currents measured as a function of the surfactant concentration confirm that the charged inverse micelles are generated by bulk disproportionation. A theoretical model based on bulk disproportionation with simulations and analytical approximations is developed to analyze the experimental transient currents.