I Ivo Roghair

Optofluidic lens with tunable focal length and asphericity

Adaptive micro-lenses enable the design of very compact optical systems with tunable imaging properties. Conventional adaptive micro-lenses suffer from substantial spherical aberration that compromises the optical performance of the system. Here, we introduce a novel concept of liquid micro-lenses with superior imaging performance that allows for simultaneous and independent tuning of both focal length and asphericity. This is achieved by varying both hydrostatic pressures and electric fields to control the shape of the refracting interface between an electrically conductive lens fluid and a non-conductive ambient fluid. Continuous variation from spherical interfaces at zero electric field to hyperbolic ones with variable ellipticity for finite fields gives access to lenses with positive, zero, and negative spherical aberration (while the focal length can be tuned via the hydrostatic pressure).

The clustering morphology of freely rising deformable bubbles

We investigate the clustering morphology of a swarm of freely rising deformable bubbles. A three-dimensional Voronoi analysis enables us to distinguish quantitatively between two typical preferential clustering configurations: a regular lattice arrangement and irregular clustering. The bubble data are obtained from direct numerical simulations using the front-tracking method. It is found that the bubble deformation, represented by the aspect ratio χ , plays a significant role in determining which type of clustering is realized: nearly spherical bubbles form a regular lattice arrangement, while more deformed bubbles show irregular clustering. Remarkably, this criterion for the clustering morphology holds for different diameters of the bubbles, surface tensions and viscosities of the liquid in the studied parameter regime. The mechanism of this clustering behaviour is most likely connected to the amount of vorticity generated at the bubble surfaces.

Novel developments in fluidized bed membrane reactor technology

Abstract This chapter outlines recent and ongoing investigations on the effect of using membranes in a fluidized bed reactor (FBR), using numerical simulations and experiments. Fluidized bed membrane reactors are a novel, integrated type of reactors where heterogeneously catalyzed reactions can be performed with simultaneous reactant feeding or product extraction in a single unit operation. While this operating technique is beneficial for various reasons (e.g., shift of chemical equilibrium, very good mass and heat transfer), addition or extraction of components can significantly change the behavior of the fluidized bed compared to traditional FBRs, hydrodynamics (bubble and emulsion phase behavior), and mass and heat transfer may be severely affected by the presence of the membranes. A number of experimental measurement techniques are discussed, with a focus on noninvasive optical techniques such as particle image velocimetry and digital image analysis, as well as a number of academic numerical modeling tools such as discrete particle model and two-fluid model. Not only hydrodynamic aspects, such as the emergence of defluidized zones and solids circulation profile inversion, but also the effect on the bubble size distributions are discussed for wall-mounted membranes and horizontally immersed membranes. The development of two novel experimental techniques, which may be used for studying concentration profiles in the gas phase, and for studying the fluidized bed at reaction conditions, are outlined in Section 5 .