Research

There are three active lines of research in the Hermans lab (from left to right): 1) Dissipative nonequilibrium self-assembly in supramolecular systems, 2) Chiral separation using Taylor-Couette flow, and 3) Microfluidics without walls.
topics

1) Dissipative nonequilibrium self-assembly

Supramolecular polymerization has been traditionally focused on the thermodynamic equilibrium state, where one-dimensional assemblies reside at the global minimum of the Gibbs free energy. The pathway and rate to reach the equilibrium state are irrelevant, and the resulting assemblies remain unchanged over time. In the past decade, the focus has shifted to kinetically trapped (non-dissipative non-equilibrium) structures that heavily depend on the method of preparation (i.e., pathway complexity), and where the assembly rates are of key importance. Kinetic models have greatly improved our understanding of competing pathways, and shown how to steer supramolecular polymerization in the desired direction (i.e., pathway selection). The most recent innovation in the field relies on energy or mass input that is dissipated to keep the system away from the thermodynamic equilibrium (or from other non-dissipative states). We demonstrated supramolecular pathway selection of a perylenediimide derivative in aqueous solution using chemically fueled redox reactions to control assembly/disassembly cycles. The number and frequency of cycles affect the nucleation and growth process, providing control over the size and internal order of the resulting self-assembled structures. Controlling the shape, molecular organization, chirality, and dispersity of self-assembled structures is crucial for applications e.g. in supramolecular materials, energy conversion and biomedicine.

2) Chiral separation using Taylor-Couette flow

Most bioactive molecules and pharmaceutical targets are chiral and need careful chiral separation to isolate only the active enantiomer. Asymmetric synthesis yielding pure enantiomers is very appealing, but remains hard to implement for most drugs on the market. Therefore, chiral resolution of enantiomers remains one of the most important challenges in the development of drugs, food, and cosmetics, and is currently predominantly  done  using chromatographic methods. In recent years, however, we and others have demonstrated resolution methods based on mechanical separation, and in particular the interaction of shear flows to separate objects from the millimeter down to the micrometer scales. Our recent studies are focused on chiral resolution in Couette cells, consisting of two concentric cylinders, where the inner cylinder is stationary and the outer one rotating in either clock-wise or counter-clockwise direction. We have first demonstrated this technology at the millimeter scale, where complete resolution of 3D-printed helices was achieved within minutes. Now our aim is to downscale our chiral resolution principle to the molecular scale, by using increasingly strong shear flows. Molecular scale chiral resolution using shear flows would be of great benefit in industry and academia. Two patents on this project have been filed so far (2014, 2015) together with SATT Conectus.

3) Microfluidics without walls

Microfluidics has been an active research field aiming at the realization of “Lab-on-a-chip”. As the surface-to-volume ratio increases by miniaturizing the fluidic channel, the importance of surface increases and causes various problems such as fouling, clogging, and large pressure drops. We are developing a new wall-less technology to overcome these problems. By controlling magnetic field configuration, we successfully confined fluidic channels inside ferrofluids and replaced solid walls by stable liquid-liquid interfaces. Such a wall-less tube is self-healing, uncloggable and frictionless as demonstrated by various experiments. As the tube is defined by a magnetic field, various shapes of tubes can be achieved by proper designing of the magnetic field. Moreover, in theory, the fluidic channels can be reconfigured in real-time if the field can be controlled in-situ. As a proof-of-concept, we demonstrated that the valving of a tube can be performed by inserting extra magnets to reconfigure the magnetic field inside the wall-less tube. The same concept was used to pump liquids through temporal modulation of magnetic fields. Another interesting property of the wall-less tube is the efficient mixing of liquid without any sophisticated physical structure. Various properties of wall-less tubes are under development.