Locomotion of a microorganism in weakly viscoelastic liquids

Enhanced rotational diffusion and spontaneous rotation of an active Janus disk in a complex fluid

Active colloids and self-propelled particles moving through microstructured fluids can display different behavior compared to what is observed in simple fluids. As they are driven out of equilibrium in complex fluids they can experience enhanced translational and rotational diffusion as well as instabilities. In this work, we study the deterministic and the Brownian rotational dynamics of an active Janus disk propelling at a constant speed through a complex fluid. The interactions between the Janus disk and the complex fluid are modeled using a fluctuating advection-diffusion equation, which we solve using the finite element method. Motivated by experiments, we focus on the case of a complex fluid comprising molecules that are much smaller than the size of the active disk but much bigger than the solvent. Using numerical simulations, we elucidate the interplay between active motion and fluid microstructure that leads to enhanced rotational diffusion and spontaneous rotation observed in experiments employing Janus colloids in polymer solutions. By increasing the propulsion speed of the Janus disk, the simulations predict the onset of a spontaneous rotation and an increase of the rotational diffusion coefficient by orders of magnitude compared to its equilibrium value. These phenomena depend strongly on the number density of the constituents of the complex fluid and their interactions with the two sides of the Janus disk. Given the simplicity of our model, we expect that our findings will apply to a wide range of active systems propelling through complex media.

Spontaneous emergence of motion of an isotropic active particle in a Carreau fluid

Active particles are self-propelling in nature due to the generation of a fore-aft asymmetry in the concentration of solutes around their surface. Both the surface activity and mobility play an important role in the particle dynamics. The solutes are the products of the chemical reaction between the active particle surface and suspending medium. Unlike Janus particles, isotropic active particles have been shown to undergo spontaneous self-propulsion beyond a critical particle size (or the Péclet number). Compared to Janus active particles, there is a third ingredient, namely, advection-induced instability that dictates the dynamics of such particles. The present study numerically investigates the role played by shear rate-dependent viscosity of a suspending medium in the self-phoretic dynamics of such isotropic active particles. Towards this, a non-Newtonian Carreau fluid is taken as the suspending medium. One of the important findings of this study is the presence of a second critical Péclet number beyond which the spontaneous motion of the particle ceases to exist. Even though this critical Péclet number had been previously investigated for Newtonian fluids, strong dependence of the former on the rheology of the suspending medium was not explored. The analysis also shows that a shear thinning fluid significantly reduces the maximum velocity of the particle. In addition, confinement is found to have a significant effect on the axial propulsive velocity of the particle suspended in a Carreau fluid.

Appetizer on soft matter physics concepts in mechanobiology

Mechanosensing, the active responses of cells to the mechanics on multiple scales, plays an indispensable role in regulating cell behaviors and determining the fate of biological entities such as tissues and organs. Here, I aim to give a pedagogical illustration of the fundamental concepts of soft matter physics that aid in understanding biomechanical phenomena from the scale of tissues to proteins. Examples of up-to-date research are introduced to elaborate these concepts. Challenges in applying physics models to biology have also been discussed for biologists and physicists to meet in the field of mechanobiology.

Dataset on rheological measurements of xanthan gum aqueous dispersions containing sodium chloride and settling dynamics of spheres and disks in these dispersions

This paper presents a dataset collected in laboratory experiments on the settling of solid spheres and disks in shear-thinning and viscoelastic aqueous solutions of xanthan gum with sodium chloride addition. Two types of spheres with density of 1.41 g/cm3 varying in diameter (3.00 mm and 1.59 mm) and four types of disks with density of 1.43 g/cm3 and thickness of 0.3 mm varying in diameter (1.5 mm, 2.0 mm, 2.5 mm, and 3.0 mm) were considered. A single particle was settling in a column filled with a test solution which varied in salt content (from 0 M to 0.9 M), while xanthan gum content was constant (1 g/L). The total of elven solutions were tested. For each experimental set, a sequence of images with a falling particle was captured using a camera with macro lenses. Dataset includes position of particle in time and enables the evaluation of settling velocity. Rheological measurements were carried out for each test solution to assess flow properties and viscoelasticity. The following measurements were performed: shear dependent viscosity, shear stress amplitude sweeps, frequency sweeps, the dependence of the first normal stresses difference on shear strain at constant frequency (1 Hz). Datasets may be useful in various areas on fluid mechanics and rheology, e.g., in research on the impact of salinity on rheological properties of exopolymer solutions, to develop numerical models on solid particles settling in non-Newtonian fluids, and in studies on the impact of exopolymers and electrolytes dissolved in water on settling dynamics of solid particles.

A Review on the Some Issues of Multiphase Flow with Self-Driven Particles

Multiphase flow with self-driven particles is ubiquitous and complex. Exploring the flow properties has both important academic meaning and engineering value. This review emphasizes some recent studies on multiphase flow with self-driven particles: the hydrodynamic interactions between self-propelled/self-rotary particles and passive particles; the aggregation, phase separation and sedimentation of squirmers; the influence of rheological properties on its motion; and the kinematic characteristics of axisymmetric squirmers. Finally, some open problems, challenges, and future directions are highlighted.

Squirming in a viscous fluid enclosed by a Brinkman medium

Cell motility plays important roles in a range of biological processes, such as reproduction and infections. Studies have hypothesized that the ulcer-causing bacterium Helicobacter pylori invades the gastric mucus layer lining the stomach by locally turning nearby gel into sol, thereby enhancing its locomotion through the biological barrier. In this work, we present a minimal theoretical model to investigate how heterogeneity created by a swimmer affects its own locomotion. As a generic locomotion model, we consider the swimming of a spherical squirmer in a purely viscous fluid pocket (representing the liquified or degelled region) surrounded by a Brinkman porous medium (representing the mucus gel). The use of the squirmer model enables an exact, analytical solution to this hydrodynamic problem. We obtain analytical expressions for the swimming speed, flow field, and power dissipation of the swimmer. Depending on the details of surface velocities and fluid properties, our results reveal the existence of a minimum threshold size of mucus gel that a swimmer needs to liquify in order to gain any enhancement in swimming speed. The threshold size can be as much as approximately 30% of the swimmer size. We contrast these predictions with results from previous models and highlight the significant role played by the details of surface actuations. In addition to their biological implications, these results could also inform the design of artificial microswimmers that can penetrate into biological gels for more effective drug delivery.

Medical micro/nanorobots in complex media

Medical micro/nanorobots have received tremendous attention over the past decades owing to their potential to be navigated into hard-to-reach tissues for a number of biomedical applications ranging from targeted drug/gene delivery, bio-isolation, detoxification, to nanosurgery. Despite the great promise, the majority of the past demonstrations are primarily under benchtop or in vitro conditions. Many developed micro/nanoscale propulsion mechanisms are based on the assumption of a homogeneous, Newtonian environment, while realistic biological environments are substantially more complex. Moving toward practical medical use, the field of micro/nanorobotics must overcome several major challenges including propulsion through complex media (such as blood, mucus, and vitreous) as well as deep tissue imaging and control in vivo. In this review article, we summarize the recent research efforts on investigating how various complexities in biological environments impact the propulsion of micro/nanoswimmers. We also highlight the emerging technological approaches to enhance the locomotion of micro/nanorobots in complex environments. The recent demonstrations of in vivo imaging, control and therapeutic medical applications of such micro/nanorobots are introduced. We envision that continuing materials and technological innovations through interdisciplinary collaborative efforts can bring us steps closer to the fantasy of "swallowing a surgeon".

Simulating squirmers with multiparticle collision dynamics

Multiparticle collision dynamics is a modern coarse-grained simulation technique to treat the hydrodynamics of Newtonian fluids by solving the Navier-Stokes equations. Naturally, it also includes thermal noise. Initially it has been applied extensively to spherical colloids or bead-spring polymers immersed in a fluid. Here, we review and discuss the use of multiparticle collision dynamics for studying the motion of spherical model microswimmers called squirmers moving in viscous fluids.

Swimming efficiency in a shear-thinning fluid

Micro-organisms expend energy moving through complex media. While propulsion speed is an important property of locomotion, efficiency is another factor that may determine the swimming gait adopted by a micro-organism in order to locomote in an energetically favorable manner. The efficiency of swimming in a Newtonian fluid is well characterized for different biological and artificial swimmers. However, these swimmers often encounter biological fluids displaying shear-thinning viscosities. Little is known about how this nonlinear rheology influences the efficiency of locomotion. Does the shear-thinning rheology render swimming more efficient or less? How does the swimming efficiency depend on the propulsion mechanism of a swimmer and rheological properties of the surrounding shear-thinning fluid? In this work, we address these fundamental questions on the efficiency of locomotion in a shear-thinning fluid by considering the squirmer model as a general locomotion model to represent different types of swimmers. Our analysis reveals how the choice of surface velocity distribution on a squirmer may reduce or enhance the swimming efficiency. We determine optimal shear rates at which the swimming efficiency can be substantially enhanced compared with the Newtonian case. The nontrivial variations of swimming efficiency prompt questions on how micro-organisms may tune their swimming gaits to exploit the shear-thinning rheology. The findings also provide insights into how artificial swimmers should be designed to move through complex media efficiently.

Dynamics of a microorganism in a sheared viscoelastic liquid

In this paper, we investigate the dynamics of a model spherical microorganism, called squirmer, suspended in a viscoelastic fluid undergoing unconfined shear flow. The effect of the interplay of shear flow, fluid viscoelasticity, and self-propulsion on the orientational dynamics is addressed. In the limit of weak viscoelasticity, quantified by the Deborah number, an analytical expression for the squirmer angular velocity is derived by means of the generalized reciprocity theorem. Direct finite element simulations are carried out to study the squirmer dynamics at larger Deborah numbers. Our results show that the orientational dynamics of active microorganisms in a sheared viscoelastic fluid greatly differs from that observed in Newtonian suspensions. Fluid viscoelasticity leads to a drift of the particle orientation vector towards the vorticity axis or the flow-gradient plane depending on the Deborah number, the relative weight between the self-propulsion velocity and the flow characteristic velocity, and the type of swimming. Generally, pullers and pushers show an opposite equilibrium orientation. The results reported in the present paper could be helpful in designing devices where separation of microorganisms, based on their self-propulsion mechanism, is obtained.

Reply to "Comment on 'Locomotion of a microorganism in weakly viscoelastic liquids' "

In the present reply we show that the comments casting doubts on the results of our recent paper [M. De Corato et al., Phys. Rev. E 92, 053008 (2015)PLEEE81539-375510.1103/PhysRevE.92.053008] are based on a misinterpretation of the second-order fluid constitutive equation. Nevertheless, we show that, by considering alternative constitutive equations for the viscoelastic stress, we recover, to first-order in the Deborah number, the same results already obtained by De Corato et al. [Phys. Rev. E 92, 053008 (2015)PLEEE81539-375510.1103/PhysRevE.92.053008], thus dissipating any possible doubt about their validity.

Comment on "Locomotion of a microorganism in weakly viscoelastic liquids"

We point out, and show the implications of resolving, an apparent conceptual difficulty in a recent article by De Corato et al. [Phys. Rev. E 92, 053008 (2015)PLEEE81539-375510.1103/PhysRevE.92.053008] on the locomotion of certain microorganisms in a second-grade fluid. The difficulty arises due to the assumption that α_{1}<0, where α_{1} is the first normal stress modulus of the (non-Newtonian) liquid, was chosen for this study. In particular, this choice of sign for α_{1} is inconsistent with thermodynamics, and as such casts considerable doubt on De Corato et al.'s assumption regarding the existence of a steady-state solution of the equations of motion of the fluid.

Dynamics of Self-Propelled Janus Particles in Viscoelastic Fluids

We experimentally investigate active motion of spherical Janus colloidal particles in a viscoelastic fluid. Self-propulsion is achieved by a local concentration gradient of a critical polymer mixture which is imposed by laser illumination. Even in the regime where the fluid's viscosity is independent of the deformation rate induced by the particle, we find a remarkable increase of up to 2 orders of magnitude of the rotational diffusion with increasing particle velocity, which can be phenomenologically described by an effective rotational diffusion coefficient dependent on the Weissenberg number. We show that this effect gives rise to a highly anisotropic response of microswimmers in viscoelastic media to external forces, depending on its orientation.