Swimming Microorganisms as Active Colloids

In the spirit of the main topics of active colloids in this book, microswimmers such as swimming bacteria are excellent examples of colloids that are able to perform self-propelled motion. A microorganism is, therefore, usually a living version of a colloidal particle, which generates thrust by consuming chemical energy or harnessing biological functions. These living objects exist on the microscale and have comparable kinetic energy to thermal fluctuations. Active colloids can be used as good models to capture the intrinsic features of these moving microorganism. The activity of bacteria is essentially generated by the flagellar motor, a tiny protein machine attached to the cell body and along with the long filament-like flagellum this swimming capability brings a new kind of diffusion, called active diffusion. This active diffusion combines net drift by swimming, random motion from thermal noise, and active noise due to run-and-tumble processes, which are discussed at the individual level with a model bacteria. These swimming dynamics are even more fascinating when the microswimmers are introduced into complex environments such as complex fluids from suspensions, confinements with various boundary walls. In the second part the cooperative behavior due the communication among individuals in a versatile way is discussed, the so called collective motion, which is a cutting-edge topics in swarming intelligence.