Deterministic ratchets for larger-scale separation of suspensions

Solid-liquid separation is a very common process operation in the chemical and food industry. Current technologies, such as membrane separation, consume large amounts of energy and water and often suffer from fouling issues. A novel, promising separation principle was identified for possible large scale application. This principle has been studied in microfluidic systems and employs so-called deterministic ratchets. Ratchet separationrelies on particle interactions with a series of obstacle arrays positioned in a flow field. Particles above a critical size are forced from their streamlines and migrate into another direction than the main flow direction. The objective of this thesis was to understand the mechanisms relevant for suspension separation with deterministic ratchets and to develop guidelines for the design of this technology at a larger scale. An up-scaled device was developed to investigate separation of model suspensions with larger particles (~101 - ~102 µm). Experiments at increasing volume particle fractions yielded final particle concentrations up to 12 v/v% without particle accumulation. The separation efficiency was discovered strongly influenced by the hydrodynamic conditions. High speed camera images and fluid flow simulations provided insight that a vortex pair developed behind obstacles and that inertial forces improved displacement behavior of particles. Different designs suitable for larger-scale application were evaluated. A mirrored (axisymmetric) obstacle array was found more effective in displacement of particles. Different designs were identified for cleaning as well as concentration applications. Finally, a simple, but effective sparse ratchet design is proposed by replacing full obstacle arrays by selected single lines of obstacles. The degree of sparseness is found a design parameter for accommodating differences in concentrations. Although the application of the principle is still challenging for smaller particle diameters (~100 - ~101 µm), this study shows that the principle of deterministic ratchet separation holds potential for larger-scale separation of suspensions.