Active Biological Matter

Dynamical Self-organization inside Evaporating Droplet
 

Life has evolved in the liquid phase, which allows many interactions in the micrometer and submicrometer range. In addition to large water deposits, liquid droplets are very common in nature, such as aerosols, raindrops, etc. A liquid droplet can contain various components such as particles and macromolecules. Any droplet on a surface exposed to air is subject to evaporation. Therefore, it is important to understand how evaporation can lead to a certain self-organization of the droplet's components. As studied extensively, the evaporating droplet proves to be a robust experimental setup for studying the dynamics of non-active or active biological materials. Therefore, we apply the setup of the evaporating droplet for two types of systems, namely, non-active and active components:

• non-active spherical particles in micro- and nano-meter sizes
• Active cytoskletal filaments such as mixture of microtubules filaments and associated
  motor proteins.

In the case of non-active spherical particles, we study the pattern formation of the particles with different chemical composition of the droplet on altered substrates. The surface interaction with the particles and the particle-particle interaction are coupled with flow fields inside the droplet, leading to the formation of regular patterns. We show how the chemistry of the droplet affects the particle-particle interaction at the interface of the evaporating droplet. Two unique patterns are seen: a flower-like lattice and a regular lattice formed by micro- and nano-sized particles, respectively. We wonder how the coupling between the internal flow of the droplet and the particle-particle interaction can lead to the formation of regular patterns. Broadly, this research aims at a comprehensive understanding of pattern formation in non-active droplets, which might be associated with some favorable self-assembly, similar to biologically active substances.

In the active mixture of cytoskeletal filaments, we observed that the coupling of flow fields inside the evaporating droplet with the active mixture led to the formation of a symmetrical and ring-like arrangement. The observed dynamics was due to the activity of kinesin motors, and the absence of motor activity resulted in phase separation of the filaments from the aqueous phase of the droplet. We attempt to characterize these experimental observations and reveal that the addition of depleting agents is critical for promoting activity in the active droplet. These experiments suggest that a membraneless droplet of cell size can be used to study the mechanical forces induced by cytoskeletal filaments in a minimal system, offering applicability for artificial cell studies.

Pattern formation in the evaporation droplet with active and non-active components. a) Schematic representation of the evaporating droplet with active cytoskeletal filaments as a mixture of microtubule filaments and kinesin motor proteins. b) Time-lapse images of the experiment with the active evaporating droplet show a symmetrical ring-like arrangement of the active network. c) Formation of a lattice pattern in a non-active evaporating droplet containing quantum dots (7 nm). d) Formation of a flower-like pattern of 1 μm sized particles by the Marangoni effect. e) Radial aggregation of the 1 μm sized particles in the evaporating droplet.

1. arXiv:2305.07099 (2023). https://doi.org/10.48550/arXiv.2305.07099
2. Nature 588, 210-213 (2020).   https://doi.org/10.1038/d41586-020-03461-4
3. Nat Commun 11, 5167 (2020).   https://doi.org/10.1038/s41467-020-18815-9