Active colloids in complex fluids

TL;DR

This review discusses recent advances in understanding how active colloids and swimmers behave in complex, non-Newtonian fluids, emphasizing the nonlinear interactions and their implications for collective behavior and transport.

Contribution

It highlights the emerging understanding of active colloids in complex fluids and calls for simple experiments to clarify their complex interactions.

Findings

Fluid rheology significantly affects swimmer gaits and speeds.

Swimmers induce time-dependent, 3D flows that alter fluid properties.

Nonlinear coupling influences collective behavior and passive tracer transport.

Abstract

We review recent work on active colloids or swimmers, such as self-propelled microorganisms, phoretic colloidal particles, and artificial micro-robotic systems, moving in fluid-like environments. These environments can be water-like and Newtonian but can frequently contain macromolecules, flexible polymers, soft cells, or hard particles, which impart complex, nonlinear rheological features to the fluid. While significant progress has been made on understanding how active colloids move and interact in Newtonian fluids, little is known on how active colloids behave in complex and non-Newtonian fluids. An emerging literature is starting to show how fluid rheology can dramatically change the gaits and speeds of individual swimmers. Simultaneously, a moving swimmer induces time dependent, three dimensional fluid flows, that can modify the medium (fluid) rheological properties. This two-way, non-linear coupling at microscopic scales has profound implications at meso- and macro-scales: steady state suspension properties, emergent collective behavior, and transport of passive tracer particles. Recent exciting theoretical results and current debate on quantifying these complex active fluids highlight the need for conceptually simple experiments to guide our understanding.