Self-Propelled Colloidal Particles: From Isotropic to Anisotropic Microswimmers

The problem of locomotion at the microscale has to be faced by living microorganisms as well as in many artificial systems. Various intriguing strategies have been developed in order to achieve a directed swimming motion in spite of the lack of inertia. While biological microswimmers often rely on mechanical self-propulsion mechanisms, such as beating flagella or shape deformations, many realizations of artificial self-propelled colloidal particles are based on phoretic driving mechanisms. The main focus of this book is on the influence of the body shape on the dynamics. Different systems ranging from spherical particles, via deformable and rodlike microswimmers, to completely asymmetric objects are investigated. In particular, a theoretical framework for the dynamics of asymmetric self-propelled particles based on coupled Langevin equations for the translational and rotational motion is provided. In the last part of this book, the introduced theory is used in order to contribute to the understanding of the gravitactic behavior which is observed in many microorganisms. Although several explanatory approaches exist, the detailed origin of gravitaxis is still an unsolved question in microbiology. Here, by studying synthetic microswimmers, a possible explanation based on an asymmetric body shape is given. Regarding artificial self-propelled particles, an especially promising field is the development of novel micromachines. Such devices have great potential for medical purposes as well as for many other technological applications. By appropriately designing the particle shape based on the findings of this book the motional behavior can be systematically tuned.