Differently from passive Brownian particles, active particles, also known as self-propelled Brownian particles or microswimmers and nanoswimmers, are capable of taking up energy from their environment and converting it into directed motion. Because of this constant flow of energy, their behavior can be explained and understood only within the framework of nonequilibrium physics. In the biological realm, many cells perform directed motion, for example, as a way to browse for nutrients or to avoid toxins. Inspired by these motile microorganisms, researchers have been developing artificial particles that feature similar swimming behaviors based on different mechanisms. These man-made micromachines and nanomachines hold a great potential as autonomous agents for health care, sustainability, and security applications. With a focus on the basic physical features of the interactions of self-propelled Brownian particles with a crowded and complex environment, this comprehensive review will provide a guided tour through its basic principles, the development of artificial self-propelling microparticles and nanoparticles, and their application to the study of nonequilibrium phenomena, as well as the open challenges that the field is currently facing.

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Active Particles in Complex and Crowded Environments

Active Brownian particles, also referred to as microswimmers and nanoswimmers, are biological or manmade microscopic and nanoscopic particles that can self-propel. Because of their activity, their behavior can only be explained and understood within the framework of nonequilibrium physics. In the biological realm, many cells perform active Brownian motion, for example, when moving away from toxins or towards nutrients. Inspired by these motile microorganisms, researchers have been developing artificial active particles that feature similar swimming behaviors based on different mechanisms; these manmade micro- and nanomachines hold a great potential as autonomous agents for healthcare, sustainability, and security applications. With a focus on the basic physical features of the interactions of active Brownian particles with a crowded and complex environment, this comprehensive review will put the reader at the very forefront of the field of active Brownian motion, providing a guided tour through its basic principles, the development of artificial self-propelling micro- and nanoparticles, and their application to the study of nonequilibrium phenomena, as well as the open challenges that the field is currently facing.

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Active Brownian Particles in Complex and Crowded Environments

Biological microorganisms swim with flagella and cilia that execute nonreciprocal motions for low Reynolds number (Re) propulsion in viscous fluids. This symmetry requirement is a consequence of Purcell's scallop theorem, which complicates the actuation scheme needed by microswimmers. However, most biomedically important fluids are non-Newtonian where the scallop theorem no longer holds. It should therefore be possible to realize a microswimmer that moves with reciprocal periodic body-shape changes in non-Newtonian fluids. Here we report a symmetric 'micro-scallop', a single-hinge microswimmer that can propel in shear thickening and shear thinning (non-Newtonian) fluids by reciprocal motion at low Re. Excellent agreement between our measurements and both numerical and analytical theoretical predictions indicates that the net propulsion is caused by modulation of the fluid viscosity upon varying the shear rate. This reciprocal swimming mechanism opens new possibilities in designing biomedical microdevices that can propel by a simple actuation scheme in non-Newtonian biological fluids.

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Swimming by reciprocal motion at low Reynolds number.

Life at Low Reynolds Number

Microfluidic devices have been widely used since 1990s for diverse manipulations of particles (a general term of beads, cells, vesicles, drops, etc.) in a variety of applications. Compared to the active manipulation via an externally imposed force field, the passive manipulation of particles exploits the flow-induced intrinsic lift and/or drag to control particle motion with several advantages. Along this direction, inertial microfluidics has received tremendous interest in the past decade due to its capability to handle a large volume of samples at a high throughput. This inertial lift-based approach in Newtonian fluids, however, becomes ineffective and even fails for small particles and/or at low flow rates. Recent studies have demonstrated the potential of elastic lift in non-Newtonian fluids for manipulating particles with a much smaller size and over a much wider range of flow rates. The aim of this article is to provide an overview of the various passive manipulations, including focusing, separation, washing and stretching, of particles that have thus far been demonstrated in non-Newtonian microfluidics.

Particle manipulations in non-Newtonian microfluidics: A review.

We investigate the rheological characteristics of human blood plasma in shear and elongational flows. While we can confirm a Newtonian behavior in shear flow within experimental resolution, we find a viscoelastic behavior of blood plasma in the pure extensional flow of a capillary breakup rheometer. The influence of the viscoelasticity of blood plasma on capillary blood flow is tested in a microfluidic device with a contraction-expansion geometry. Differential pressure measurements revealed that the plasma has a pronounced flow resistance compared to that of pure water. Supplementary measurements indicate that the viscoelasticity of the plasma might even lead to viscoelastic instabilities under certain conditions. Our findings show that the viscoelastic properties of plasma should not be ignored in future studies on blood flow.

Rheology of human blood plasma: viscoelastic versus Newtonian behavior.

Conventionally, a microscopic particle that performs a reciprocal stroke cannot move through its environment. This is because at small scales, the response of simple Newtonian fluids is purely viscous and flows are time-reversible. We show that by contrast, fluid elasticity enables propulsion by reciprocal forcing that is otherwise impossible. We present experiments on rigid objects actuated reciprocally in viscous fluids, demonstrating for the first time a purely elastic propulsion set by the object's shape and boundary conditions. We describe two different artificial “swimmers” that experimentally realize this principle.

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Fluid elasticity can enable propulsion at low Reynolds number

The recent popularity of inertial microfluidic devices has driven attention towards the behavior of particles in flows within curvilinear microchannels for particle separation and concentration applications. The robust technique of inertial focusing is particularly advantageous in such applications, not only because curved geometries can greatly reduce the footprint of a lab-on-chip devices, but also because the coupling of secondary Dean flows to inertial forces allows for exquisite particle manipulations. However, the ability to accurately predict the behavior of inertial particles in a curvilinear channel is often based on empirical results, as the commonly used theoretical models treat the effects of Dean flow on an inertial particle to be a consequence of the undisturbed velocity field. In particular, this simplification is problematic when the size of the particle is comparable to that of the confining channel. Here we present the first complete direct numerical model that directly simulates a particle within a confined curvilinear flow. This numerical model allows us to not only investigate the three-dimensional focusing behavior of inertial particles but also determine the applicability of the point particle assumptions previous researchers have proposed. Finally, we propose a more computationally efficient second-order model that takes into account the full physics by relying on a perturbation expansion of the lateral forces, where the perturbation parameter is the curvature ratio of the channel. This simpler model can be used to efficiently and accurately predict the behavior of particles in complex channel geometries where the curvature may not be constant.

A model for inertial particles in curvilinear flows

An experimental investigation in a novel tangential flow filtration geometry at a moderate Reynolds number reveals how the presence of a secondary flow can can modify inertial migration and focusing of microparticles.

Inertial particle dynamics in the presence of a secondary flow

The recent advent of advanced microfabrication capabilities of microfluidic devices has driven attention towards the behavior of particles in inertial flows within microchannels for applications related to the separation and concentration of bio-particles. The phenomena of inertial focusing has been demonstrated to be a robust technique in such applications, where the flow of particles in a curvilinear geometry has proven to be particularly advantageous, not only because the geometry can reduce the foot-print of a lab-on-chip device, but also because the coupling of secondary Dean flows to inertial forces allows for exquisite particle manipulations. However, the ability to design a curvilinear channel for a specific application is often based on empirical results, as theoretical models to date typically do not include the effects of a finite sized particle within the flow. Here we present a complete numerical model that directly simulates a particle within a confide curvilinear flow and using this model we investigate the three dimensional focusing behavior of inertial particles as well as the applicability of the point particle assumptions previous researchers have proposed. Finally, we propose a new model that takes into account the full physics, but relies on a perturbation expansion of the lateral forces, where the perturbation parameter is the curvature ratio of the channel. This simple model can be used to predict the behavior of particles in complex channel geometries where the curvature may not be constant.

Mike Garcia

Papers

Rheology of human blood plasma: viscoelastic versus Newtonian behavior.

Fluid elasticity can enable propulsion at low Reynolds number

A model for inertial particles in curvilinear flows

Inertial particle dynamics in the presence of a secondary flow

A model for inertial particles in curvilinear flows.