On the theoretical side, correlations for the dependency of the dispersion coefficient and the local Sherwood number as a function of the Peclet number have been established. These calculations define a new yardstick for the chromatographic performance of perfectly ordered structures. This work has been published in the Journal of Chromatography (Matheuse et al., 2020). Two more papers in the Journal of Chromatography (Desmet et al., 2020a-b) were devoted to the discovery of a new mode hydrodynamic dispersion regime in ordered flow-through media. The unique feature of this regime is that it represents a local decrease of the dispersion, whereas in all other regimes the dispersion increases monotonically with the velocity.
Next, a radically new computational approach allowing to study dispersion problems using steady-state computations instead of the conventional time-dependent simulations has been developed. This work, based on the age field theory, has already led to two fundamentally new insights (logarithmic dependence on the velocity as a characteristic trait of randomly fluctuating velocity fields; logarithmic growth of plate heights with length and width in systems with radial velocity bias) which are in the course of being submitted to the Journal of Fluid Mechanics and Analytical Chemistry.
Concerning the core goal of the project, i.e. developing a rapid large-scale method to produce ordered 2D sphere arrays, an important breakthrough was realized by generating stable particles clouds using an electrostatic cell as a means to present the particles to the vacuum-driven assembly membranes. This work has been published as a cover article in Materials & Design (Van Geite et al., 2022) and in Powder Technology (Jimidar et al., 2021) and offers a new method to produce for the rapid (<1s) assembly of high quality monolayers (errors <0.1% measured over large repeat series). We also demonstrated these can subsequently be deposited onto a second surface. Unfortunately, this surface is currently limited to soft surfaces and not to the rigid surface that are needed to 3D print particle assemblies. The electrostatic particle cloud generation method produced more repeatable and flawless layers than those obtained with the wet assembly processes and the pressure-shock method (both published in Powder Technology: Berneman et al., 2020 and Van Geite et al., 2022) we initially mainly focused on.
Another major finding in the project has been the development of a method to uniformly fill an array of circular pockets with single particles. This technique has subsequently been applied to fill networks of interconnected pockets (so-called structured micro-grooves) to produce perfectly ordered particle beds for miniature-scale chromatography. This work has been reported as a cover story in Langmuir (Verloy et al., 2022) and protected by means of a European patent application.
As an unexpected result, we discovered a new method for the directed particle segregation & self-assembly of silica particles by rubbing-induced tribo-electrification on patterned surfaces (pattern of fluorocarbon coated patches). Our key finding is the fact that silica particles are tribo-charged due to the rubbing process and therefore stick on the fluorocarbon coating, while they are repelled from the silicon regions in between the fluorocarbon patches because of the opposite charge. During the rubbing, a monolayer of particles is on the fluorocarbon surfaces, thus creating an ordered array of self-assembled particle collections. Using Kelvin Probe Force Microscopy (KPFM), we could show that the rubbing process induces electric charge on the FC coated surface and particles. This work was published in Langmuir (Jimidar et al., 2020) and in Soft Matter (Jimidar et al., 2020).
