Markus Feist

Investigation of the nonlinear effects during the sedimentation process of a charged colloidal particle by direct numerical simulation

In this article we study the settling process of a colloidal particle under the influence of a gravitational or centrifugal field in an unbounded electrolyte solution. Since particles in aqueous solutions normally carry a non-zero surface charge, a microscopic electric field develops which alters the sedimentation process compared to an uncharged particle. This process can be mathematically modelled via the Stokes-Poisson-Nernst-Planck system, a system of coupled partial differential equations that have to be solved in an exterior domain. After a dimensional analysis we investigate the influence of the various characteristic dimensionless numbers on the sedimentation velocity. Thereby the linear-response (weak-field) approximation that underpins almost all existing theoretical work on classical electrokinetic phenomena is relaxed, such that no additional assumption on the thickness of the double layer as well as on its displacement is needed. We show that there exists a strong influence of the fluid Reynolds number and the ionic strength on the sedimentation velocity. Further we have developed an asymptotic expansion to describe the limit of small values of the surface potential of a single particle. This expansion incorporates all nonlinear effects and extends the well-known results of Booth (1954) [1] and Ohshima et al. (1984) [2] to higher fluid Reynolds numbers.

Experimental Results for the Settling Behaviour of Particle-Fiber Mixtures

Sedimentation of organic fibres and inorganic particles can be observed in several industrial applications. Fibres are involved not only in wastewater treatment but also in other separation applications. In the paper industry, the separation of inorganic filler and coating particles from short cellulose fibres is still a challenge in the recycling process. During that process, particles have to be removed to obtain a purified fibre suspension. These fibres can be used again to produce new paper. With the currently applied techniques, like screening and flotation, the efficiency of short fibre separation is very poor. Moreover, also separation by sedimentation fails due to similar settling velocities of heavy-small particles and the light and larger fibres. This paper concentrates on the sedimentation of organic fibres and inorganic particles in water. The investigated suspensions are made by resolving two different and specially produced papers a coated and an uncoated one, as well as the single components used for its production. We observe a different sedimentation behaviour according to the concentration of fibres and particles in the suspension and the pH-value of it. The main result is that, according to the fibre rate, the sedimentation behaviour is particle dominant or fibre dominant.

Direct Numerical Simulation of the Settling of a Charged Colloidal Particle

In many industrial processes it is necessary to control the interactions between particles and the ambient electrolyte in which they are dispersed. In this paper we consider the sedimentation of a single charged colloidal particle in a Newtonian electrolyte to study the basic physical effects of this process. We investigate the settling of the particle by direct numerical simulation. This approach finally leads to the Stokes‐Poisson‐Nernst‐Planck system, a system of coupled partial differential equations. For the numerical approximation of the solution we use the finite element method. We show that the settling velocity does not solely depend on the ion concentration of the electrolyte, but also has a significant dependence on the particle Reynolds number. In the case of small particle Reynolds numbers our results are in good agreement with the existing literature. We have further developed a new zeta potential expansion that is capable of describing the whole range of particle Reynolds numbers in the Sto...

An Integral Approach to Master Physicochemical Properties in the Solid Chain of a Colloidal Pharmaceutical Product

In a wide range of industrial applications particle design is governed by the process precipitation followed by isolation like filtration or washing and drying. Particle characteristics play a crucial role on the behavior of the suspension during all production steps. This especially counts for pigments or dye stuff as well as for life science products. Particularly in the pharmaceutical industry bioavailability of a product is of utmost importance, which is often improved by designing colloidal particle collectives with a high surface area, however, often compromising the solid-liquid separation efficiency. This study is concerned with the interaction between crystallization, sedimentation, and washing efficiency all impacted by physical chemical properties of the particles itself, but also on the liquid phases used. The effect of dosage time of the reactants, stirrer speed, ionic strength, and pH-value are investigated in the precipitation process and on their impact on the solid-liquid separation behavior.