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Abstract

Active matter taps local energy sources to generate forces and motion. A key example is the locomotion of microorganisms, which can be modeled as active Brownian particles. A particularly intriguing case involves chiral active particles that follow a preferred sense of rotation. Working across relevant scales, I show theoretically that malaria parasites, owing to their high speeds and curved shape, provide an excellent model system for this class.

First, I built an automated image-processing pipeline to analyze experimentally measured trajectories in a three-dimensional hydrogel. This established proof of uniformly right-handed chirality, which also controls transitions between two- and three-dimensional environments.

I then formulated a stochastic theory for chiral active particles based on an Ornstein–Uhlenbeck process for rotational dynamics, demonstrating that helical motion is more robust to fluctuations and can, statistically, yield larger net displacements—so that a helix can be ``straighter than a straight line”.

Finally, I developed a theory for the self-organized surface flow of adhesins, driving the motion. This suggested that the parasites’ curved shape is an evolutionary adaptation to avoid on-the-spot rotations. An extension of the theory that incorporates mechanical deformations attributes the observed right-handedness to an asymmetric release of adhesion molecules; this prediction was corroborated experimentally.