In this thesis, we explore how the shape of anisotropic microswimmers influences their swimming modes, clustering behavior, and interactions with other active particles. Inspired by the Plasmodium falciparum malaria parasite, which enhances its transmission efficiency through its distinctive curved, crescent-shaped morphology, we focused on the fabrica tion and analysis of catalytically active bent rods.
In Chapter 2, we demonstrate how the bent rod geometry dramatically enhances out-of equilibrium self-organization. We experimentally and numerically investigate a model class of microswimmers with a shape that can be continuously tuned from spherical to bent and straight rod configurations. We find that bent rods promote interlocking assembly even at extremely low particle densities, with semicircular shapes (crescents) showing the highest clustering efficiency. Our analysis reveals that clustering dynamics are governed by a balance between the probability of...
Show moreIn this thesis, we explore how the shape of anisotropic microswimmers influences their swimming modes, clustering behavior, and interactions with other active particles. Inspired by the Plasmodium falciparum malaria parasite, which enhances its transmission efficiency through its distinctive curved, crescent-shaped morphology, we focused on the fabrica tion and analysis of catalytically active bent rods.
In Chapter 2, we demonstrate how the bent rod geometry dramatically enhances out-of equilibrium self-organization. We experimentally and numerically investigate a model class of microswimmers with a shape that can be continuously tuned from spherical to bent and straight rod configurations. We find that bent rods promote interlocking assembly even at extremely low particle densities, with semicircular shapes (crescents) showing the highest clustering efficiency. Our analysis reveals that clustering dynamics are governed by a balance between the probability of interlocking and the stability of the resulting clusters. We also report for the first time that crescent-shaped particles reverse their swimming direction at higher fuel concentrations — a phenomenon that significantly reduces their clustering efficiency.
Building on this observation, we investigate the direction reversal of crescent-shaped particles in greater detail in Chapter 3. We experimentally show that the swimming direc tion can change with increasing fuel concentration, not only for crescent-shaped particles but also for other anisotropic swimmers such as disks and tori. At low hydrogen peroxide concentrations, these swimmers propel with their inert side forward, whereas at high concentrations, they move catalytic side forward. We attribute this reversal to combined effects of pH-dependent surface properties, particle geometry, and substrate-induced solute confinement. For anisotropic prolate-like geometries and relative zeta-potential ratios of the two Janus sides close to unity, small variations in surface mobility can reverse the propulsion direction, whereas symmetric or oblate shapes require mobilities of opposite sign. The nearby substrate further confines reaction products and reshapes the concentration gradients in a shape-dependent manner, thereby amplifying this effect.
After having studied in Chapter 2 the particle interactions in a pure crescent sample, Chapter 4 focuses on lock-and-key type interactions of crescent-shaped swimmers with dif ferent ”key” particles. We study mixtures of different types of active shape-complementary colloids and analyze the resulting pair formation. We find that the self-assembly process can be described with a chemical equilibrium model with equilibrium constant K. We further demonstrate that K can be tuned by the shape of the key-partner and that, surprisingly, the most efficient clustering remains the crescent–crescent interaction.
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