Microswimmers in dynamic environments: active colloidal particles in evaporating liquid droplets
Final Report Abstract
Inspired by biological microswimmers, various types of artificial self-propelled particles have been developed and thoroughly characterized in recent years. While in most studies quiescent solvents or stationary flow fields were considered, much less is known about the behavior of synthetic microswimmers in more complicated flow environments as they often occur in biological systems under non-laboratory conditions. In this research project, we investigated the dynamics of self-propelling Janus particles in an evaporating liquid droplet. Here, the competition between the flows due to the evaporation and the active motion of the particles leads to a complex dynamical behavior. However, since on average the evaporative flow tends to carry the particles towards the outer region of the droplet, after a while more and more particles accumulate close to the contact line. Near the end of the lifetime of the droplet, the flows become faster and faster since they still have to replenish the liquid that has evaporated from the edges, in spite of the shrinking vertical cross section of the droplet. When the liquid has vanished completely, only the colloidal particles remain on the substrate, forming a ring-like deposit, which is also known as coffee ring effect. Altering the structure of this deposit by adding active particles to the evaporating droplet could possibly enable new approaches for self-assembly and for the creation of novel materials with specifically designed properties. Further scientific output obtained during this research project includes a detailed analysis of the dynamics of mass-anisotropic self-propelling Janus particles under gravity, new insights into the navigation of microswimmers in viscosity gradients, a systematic study of the segregation dynamics in a binary mixture of active colloidal particles moving on a motility contrast landscape, and the design and characterization of a flashing motility ratchet which might be useful for drug delivery applications since it opens a route to create pulsating transport by using laser light, allowing one to ‘bombard’ a distant target with short and intense pulses of active particles.
Publications
- Helical paths, gravitaxis, and separation phenomena for mass-anisotropic selfpropelling colloids: experiment versus theory. J. Chem. Phys. 147, 084905 (2017)
A. I. Campbell, R. Wittkowski, B. ten Hagen, H. Löwen, and S. J. Ebbens
(See online at https://doi.org/10.1063/1.4998605) - Viscotaxis: microswimmer navigation in viscosity gradients. Phys. Rev. Lett. 120, 208002 (2018)
B. Liebchen, P. Monderkamp, B. ten Hagen, and H. Löwen
(See online at https://doi.org/10.1103/PhysRevLett.120.208002) - Colloidal Brazil nut effect in microswimmer mixtures induced by motility contrast. J. Chem. Phys. 150, 114902 (2019)
S. Jahanshahi, C. Lozano, B. ten Hagen, C. Bechinger, and H. Löwen
(See online at https://doi.org/10.1063/1.5083098) - Propagating density spikes in light-powered motility-ratchets. Soft Matter 15, 5185 (2019)
C. Lozano, B. Liebchen, B. ten Hagen, C. Bechinger, and H. Löwen
(See online at https://doi.org/10.1039/c9sm00727j)
