Guido Falk

Directed Self-Assembly of Colloidal Model Systems on Charge-Selective Surfaces in External Electric Fields: Theory and Numerical Analysis

Membrane electrophoretic deposition has a long-established reputation in delivering high quality nanoparticle compact and coating solutions in the field of high performance nanoparticle architectures made from aqueous nanoparticle suspensions. Although for a long time, it has been common practice in nanoparticle science and particle-based nanotechnology to use membrane electrophoretic shaping of nanoparticles, little is known about long-range electrohydrodynamic manipulation of the engineered assembly of colloidal particles at the nanoscale. Here, we analyze the interfacial field-induced flow of a strong electrolyte and its implications for the directed self-assembly of colloidal nanoparticles on nonuniform charge-selective ion-exchange membrane surfaces as well as on conducting microelectrodes imposed to electrophoretic deposition boundary conditions. Numerical calculations of the vortex streamlines are derived for the case of extreme diffusion limitation, concentration polarization near the limiting current, and induced electric forces acting upon the residual space charge. The system is modeled by coupled mass balances, Ohmic law, Navier-Stokes, and Nernst-Planck equations. Particularly, numerical calculations under bulk electroconvection conditions show that the latter provides an efficient intrinsic interfacial mechanism capable of accounting for the experimentally observed local electrophoretic deposition behavior of nanoparticles at ideal permselective membranes with nonplanar periodic charge discontinuity and metal microelectrodes.

Macro- and Microscale Fabrication by Field Assisted Nanoparticle Assembly - The Challenging Path from Science to Engineering

Local electrophoretic deposition of alumina nanoparticles under external DC electric field conditions with submerged impinging jet type capillaries arranged at ion exchange membrane substrates is presented. In order to evaluate particle deposition mechanisms a mathematical model is derived describing electroosmotic pumping of electrolyte through a micrometre scaled channel. The system is governed by surficial charge discontinuities and modeled by coupled mass balances, Ohmic law, Navier Stokes, and Nernst-Planck equations. Based on the boundary conditions of bulk convective electrodiffusion the effect of the imposed surface potential on the fluid flow behaviour and on particle tracing characteristics is studied by means of numerical analysis. The following findings have been obtained. At the corner edges of the charged surficial boundaries micro-vortices are generated to build up local stagnation points onto the modeled membrane surface. Particle tracing analysis reveal that the particle movement is caused by mass transport within the membrane directed velocity field to the stagnation point. The complex electrokinetics and electrohydro-dynamics suggest further investigations at membrane pore sizes in the range of the Debye-length to model the non-linear current-voltage characteristic that has already been experimentally proven for these kind of membrane EPD systems.

Sintering Behavior of Multimodal Ceria Powder Mixtures for Fuel Cell Applications

Sub-micrometer scaled CGO (Ce0,9Gd0,1O1,95) powder is used in order to manufacture Ni/CGO functionally graded green bodies with high green densities. Thereby the near-shape manufacturing of Ni/CGO cermets by gel casting of CGO and Ni powder mixtures and electrophoretic impregnation of sub-micro scaled ceria doped particles into a Ni/CGO mesoporous matrix is described. Solids content of dispersed suspensions is varied as well as the mixing ratio of sub-micro sized and micro sized particles. The influence on green density and sintering behavior is investigated in order to achieve maximum CGO densities in combination with maximum ionic conductivity for potential use in SOFC. Shrinkage rates from sintering experiments with constant rates of heating (CRH) are analyzed to determine most suitable co-sintering conditions of Ni-Ni/CGO composite anode materials.

Experimental and numerical analysis of electrophoretic deposition on charge selective surfaces in external electric fields

Abstract In this paper, we present chronopotentiometric results of direct current (DC) membrane electrophoretic deposition (DC-M-EPD) experiments and analyse the influence of the parameters solid concentration, particle size and electrical conductivity of colloidal fumed silica model suspensions on voltage–current curves. The expected streamline patterns are numerically modelled based on coupled mass balances, Ohmic law, Navier–Stokes and Nernst-Planck equations. The results confirm that overlimiting current condition is an obligatory condition for the formation of EPD green deposits under suitable DC-M-EPD conditions. Furthermore, micro-EPD experiments in external alternating current (AC) fields (micro-AC-EPD) using video microscopy come to the conclusion that the observed formations of microvortices are in accordance to the numerically modelled streamline patterns. Finally, the formation of AC-EPD microdeposits exhibiting a torus-like microstructure is substantially explained based on microfluidic streamline pattern analysis.

Electrophoresis in Membrane Separation Processes: From Lab to Field Scale Experiments

In this paper results of electrophoretically activated processes for domestic wastewater treatment in lab and field scale experiments are presented. The principal mechanisms of non-membrane and membrane based electrokinetic solid liquid separation by electrophoresis are described. In the case of non-membrane based electrokinetic wastewater treatment a modular processing scheme is suggested in order to achieve economically and ecologically suitable processing conditions based on colloidal wastewater characteristics. In the case of membrane based electrokinetic waste water treatment an effective anti-membrane fouling process is designed controlled by colloidal characteristics of the wastewater, especially zetapotential, as well as external field parameters and microfiltration module geometries. The specific energy input of the membrane based and non-membrane based electrophoretic waste water treatment methodologies are compared and future perspectives of electrokinetic activated waste water purification processes are proposed.

Advances in Microscale and Nanoscale Mechanisms of Electrophoretic Deposition in Aqueous Media

The processing of ceramic thick and thin films, nano- and micro-scaled ceramic structures as well as bulk ceramics of high quality and precise dimensions under electrophoretic boundary conditions requires a full understanding of the dynamics of relevant interfacial mechanisms and interactions of colloidal phases at the nano- and micro-scale. Recent findings and latest insights on the importance of electrokinetic and electrohydrodynamic interfacial processes for membrane electrophoretic depositon in aqueous media are summarised. In this context, the paper addresses the fundamental importance of surficial charge heterogeneities, electric double layer instabilities, electrokinetically induced micro-vortex dynamics, as well as lateral and medial effective electrical field gradients. These phenomena are evaluated in terms of reasonable correlations and mechanistic coincidences of general EPD deposition principles. The experimental results are based on potentiometry, in-situ videomicroscopy, high-resolution as well as secondary electron microscopy. A numerical method for the simulation of the electrophoretic deposition process is suggested based on a multiphysical Finite Element approach given by Nernst-Planck, Poisson- and Navier-Stokes equations. The results of the simulations provide adequate agreement with experimental findings.

Electrohydrodynamics and Electrokinetics of Colloidal Ceramic Gels: Fundamentals and Applications

Processing of colloidal gels with electrified interfaces to form ceramic bulk and composite materials as well as coatings of complex shapes with well definded structures in the nanoscale requires the understanding of fundamental role of the coupling between hydrodynamic and electric interactions in colloidal deposition and colloidal arrangement. In this article, fundamental equations of electrohydrodynamics of membrane supported electrophoretic deposition (EPD) as well as exemplary numerical FEM-simulations of electrophoretically deposited colloidal gels are presented. Based on microfluidic physics a model is then used to study electrohydrodynamic EPD phenomena, especially electroosmotic flow incorporated with modulated surface potentials, particle interactions and electrostatic potentials. Some practical applications of EPD as well as visions of general purpose of the computational modelling results were given.

Non-thermal effects of mm-wave sintering on the microstructure of zirconia ceramics

The potential of high frequency (30 GHz) microwave sintering has been investigated in field of sintering Y/sub 2/O/sub 3/ stabilized, tetragonal and cubic ZrO/sub 2/ in comparison to conventional sintering techniques. Therefore by electrophoretic deposition improved green bodies were produced with high green densities and homogeneous pore size distribution. After systematic variation of sintering parameters, the subsequent characterization of the sintered samples has been focused on the influence of microwave sintering to the microstructure evolution.