Nucleation of Chemically Active Droplets

by Noah Ziethen
Doctoral thesis
Date of Examination:2024-08-08
Date of issue:2024-09-10
Advisor:Dr. David Zwicker
Referee:Dr. David Zwicker
Referee:Prof. Dr. Peter Sollich
Persistent Address: http://dx.doi.org/10.53846/goediss-10731

 

 

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Abstract

English

The interior of a cell is a complicated mixture of many components, and dynamics need to be orchestrated in time and space to ensure the proper functioning of the cell. One key strategy for achieving this tight control is the hierarchical structuring of its components. Liquid-liquid phase separation emerged as a crucial organizing principle inside biological cells, giving rise to many intracellular compartments. Unique to the cellular context, these condensates can consist of only a few hundred molecules and are affected by nonequilibrium processes. In particular, active chemical conversion between condensate material and proteins in the surrounding cytoplasm can control multiple aspects of the condensates. In addition, inside cells, various organelles coexist and interact with the complex cytoplasm. However, it remains unclear how intracellular compartments can form from a mixed cytoplasm under such conditions. Here, we investigate the homogeneous and heterogeneous nucleation of chemically active droplets from a metastable well-mixed phase. Our numerical simulations reveal that reactions generally suppress nucleation, which is valid for the homogeneous case and in the presence of a boundary. Using an equilibrium surrogate model, we find an effective increase in the energy barrier and, thus, decelerated transitions between the well-mixed and the droplet states. Using classical nucleation theory, we approximate the full dynamics by diffusion in a free energy potential described by an analytical expression that only depends on the droplet size and the reaction rate. This surrogate model allows us to construct a phase diagram capturing how driven chemical reactions affect the stability of the homogeneous state. In the presence of a system boundary, we uncover a coupled effect of wall interaction and chemical reactions, leading to shapes that deviate from spherical caps. We establish that these distortions result from anisotropic fluxes responding to the boundary conditions dictated by the Young Dupré equation. In cells, all these effects might be crucial for controlling the nucleation of droplets and their morphology.
Keywords: Nucleation; Biomolecular Condensates; Phase Separation; Chemically Active Droplets; Reaction-Diffusion System
 
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