Transport Phenomena

Transport and migration of (bio-) macromolecules play an important role in biological- as well as technological processes. We focus on the dynamics of charged macromolecules under equilibrium conditions, filtration processes, and a new project in which we plan to model the post synapse signal transduction cascade.

Charged Colloids

Dispersions of charged colloids are ubiquitously occurring in bio-chemistry, medicine and pharmacy, and in many technological products. Using state-of-the art theoretical tools and simulations, and a variety of experimental methods, we study diffusion, rheology, electrokinetics, microstructure and phase behaviour of concentrated dispersions. The explored systems include suspensions of rigid synthetic particles [Banchio1999, Banchio2006, Holmqvist2010], ionic micro gels [Holmqvist2012], solutions of DNA fragments [McPhie2008], as well as protein- [Heinen2012, Riest2015] and and electrolyte solutions [Aburto2013]. Our aim is to understand these particulate systems on basis of their direct and hydrodynamic interactions [Nägele2013, Lisicki2015].

G. Nägele

Microgel Suspensions: Structure and Dynamics

Suspensions of ionic and non-ionic microgels are of fundamental technological interest, owing to the sensitivity of the microgel particle sizes on control parameters such as concentration, ionic strength and temperature. Microgels exhibit a profound environmental adaptability and capacity to partially interpenetrate and host small molecular species. This allows for their application as drug-delivery vehicles, functionalized colloids, switchable membrane filters, actuators and tunable microreactors. We have developed a versatile bottom-up approach for computing thermodynamic, structural and dynamic properties of crowded microgel suspensions based on the characteristics of single microgels [Schmid2014, Riest2015, Brito2019]. This has led, in particular, to new insights into concentration – induced microgel deswelling effects [Brito2019]. Our work is done in collaboration with the physical chemistry group headed by Prof. W. Richtering at the RWTH Aachen university (within SFB 985 on Functional Microgels) and Prof. A. Denton (North Dakota State University, USA).

G. Nägele

Filtration Modelling

Pressure-driven membrane filtration is commonly used for the concentration, purification and separation of particulate systems such as blood, protein solutions, microgel suspensions [Riest2015, Brito2019, Linkhorst2021], milk and beer. A frequently used technique is cross-flow filtration where the feed dispersion is continuously pumped through a parallel array of membrane pipes. We have developed a versatile, accurate method for calculating transport properties and the filtration efficiency in cross-flow filtration [Roa2015, Roa2016, Park2020, Park2021]. The method helps to optimize the filtration performance and to avoid unwarranted membrane fouling. Our filtration project bridges the gap from the theoretical exploration of concentrated dispersion transport properties to a realistic modelling of a technologically important filtration process. The project is done in collaboration with the process engineering group headed by Prof. M. Wessling at the RWTH Aachen University (project B6 in SFB 985 on Functional Microgels).

G. W. Park and G. Nägele

Dynamics of Quasi-two-dimensional Dispersions

Understanding the dynamics of monolayers of interacting proteins or colloidal particles confined to quasi-two-dimensional (Q2D) motion along a planar fluid interface (embedded in a three-dimensional bulk fluid) is a major challenge in biological soft matter science. Examples in case are Q2D dispersions of globular proteins with competing short-range attractive and long-range repulsive interactions [Riest2015, Riest2018]. The interplay of Q2D motion, direct interactions, and solvent-mediated hydrodynamic interactions (HIs) give rise to peculiar effects such as anomalously enhanced collective diffusion. Using an elaborate multiparticle collision dynamics (MPC) simulation method [Tan2021, Das2018], we explore hydrodynamic and direct interaction effects on Q2D translational and rotational particles diffusion. Our MPC study covers a broad range of correlation times, from very short times where the particle dynamics is non-instantaneously affected by sound propagation and transversal momentum diffusion, to long times where HIs are fully developed and quasi–instantaneous.

Z. Tan and G. Nägele

Post-synaptic Signal Transduction

Neuronal signal transmission is key to brain functioning. It involves molecular signalling cascades where hundreds of different biomacromolecules are diffusing and interacting. The cascades are initiated at the post-synaptic cell membrane and are responsible for information transmission and regulation of biological processes related to memory, learning, and mood. Using our newly developed multiparticle collision dynamics (MPC) mesoscale simulation method [Tan2021 and Das2018], we model postsynaptic membrane proteins as dumbbell – shaped Brownian particles migrating along a three-layer immiscible binary fluid [Tan2021]. Input parameters to the mesoscopic model such as protein – membrane effective interaction potentials are obtained from atomistic simulations [Cairano2021].  This has allowed us to bridge the gap between molecular and mesoscale time- and length scales. The project is done in collaboration with the Computational Biomedicine group at INM-9 (FZ – Jülich) headed by Prof. P. Carloni, in an effort towards gaining physical insight into postsynaptic signalling on a sub-cellular level.

Z. Tan, V. Calandrini, and G. Nägele

Self-Diffusion of Rods

Dispersions of colloidal viruses, in our case fd virus, display a cascade of phase transitions, from isotropic to the nematic [Lettinga2005], the smectic [Lettinga2007, Pouget2011] and the columnar [Naderi2013] phase with increasing concentration. Uncovering the dynamics underlying these phase transitions, which is fundamental to understand the process of self-assembly, as it is purely driven by entropy. We use fluorescence video microscopy in order to study the self-diffusion of rods in the different phases. This study gave and still gives a wealth of surprising features, such as discretised Brownian diffusion in the smectic and large rods that diffuse faster than short rods.

P. Lettinga