Lattice Boltzmann Simulations of Two Linear Microswimmers Using the Immersed Boundary Method

The performance of a single or the collection of microswimmers strongly depends on the hydrodynamic coupling among their constituents and themselves.We present a numerical study for a single and a pair of microswimmers based on lattice Boltzmann method(LBM)simulations.Our numerical algorithm consists of two separable parts.Lagrange polynomials provide a discretization of the microswimmers and the lattice Boltzmann method captures the dynamics of the surrounding fluid.The two components couple via an immersed boundary method.We present data for a single swimmer system and our data also show the onset of collective effects and,in particular,an overall velocity increment of clusters of swimmers.

Self-assembled vesicle–colloid hybrid swimmers:Non-reciprocal strokes with reciprocal actuation

We consider a self-assembled hybrid system,composed of a bilayer vesicle to which a number of colloids are adhered.Based on known results of membrane curvature elasticity,we predict that,for sufficiently deflated prolate vesicles,the colloids can self-assemble into a ring at a finite distance away from the vesicle equator,thus breaking the up–down symmetry in the system.Because the relative variation of the position of the colloidal ring along the vesicle endows the system with an effective elasticity,periodic cycles of inflation and deflation can lead to non-reciprocal shape changes of the vesicle–colloid hybrid,allowing it to swim in a low Reynolds number environment under reciprocal actuation.We design several actuation protocols that allow control over the swimming direction.

A composite micromotor driven by self-thermophoresis and Brownian rectification

Brownian motors and self-phoretic microswimmers are two typical micromotors,for which thermal fluctuations play different roles.Brownian motors utilize thermal noise to acquire unidirectional motion,while thermal fluctuations randomize the self-propulsion of self-phoretic microswimmers.Here we perform mesoscale simulations to study a composite micromotor composed of a self-thermophoretic Janus particle under a time-modulated external ratchet potential.The composite motor exhibits a unidirectional transport,whose direction can be reversed by tuning the modulation frequency of the external potential.The maximum transport capability is close to the superposition of the drift speed of the pure Brownian motor and the self-propelling speed of the pure self-thermophoretic particle.Moreover,the hydrodynamic effect influences the orientation of the Janus particle in the ratched potential,hence also the performance of the composite motor.Our work thus provides an enlightening attempt to actively exploit inevitable thermal fluctuations in the implementation of the self-phoretic microswimmers.

Simulation of microswimmer hydrodynamics with multiparticle collision dynamics

In this review we discuss the recent progress in the simulation of soft active matter systems and in particular the hydrodynamics of microswimmers using the method of multiparticle collision dynamics,which solves the hydrodynamic flows around active objects on a coarse-grained level.We first present a brief overview of the basic simulation method and the coupling between microswimmers and fluid.We then review the current achievements in simulating flexible and rigid microswimmers using multiparticle collision dynamics,and briefly conclude and discuss possible future directions.

Speedup of self-propelled helical swimmers in a long cylindrical pipe

Pipe-like confinements are ubiquitously encountered by microswimmers.Here we systematically study the ratio of the speeds of a force-and torque-free microswimmer swimming in the center of a cylindrical pipe to its speed in an unbounded fluid(speed ratio).Inspired by E.coli,the model swimmer consists of a cylindrical head and a double-helical tail connected to the head by a rotating virtual motor.The numerical simulation shows that depending on swimmer geometry,confinements can enhance or hinder the swimming speed,which is verified by Reynolds number matched experiments.We further developed a reduced model.The model shows that the swimmer with a moderately long,slender head and a moderately long tail experiences the greatest speed enhancement,whereas the theoretical speed ratio has no upper limit.The properties of the virtual motor also affect the speed ratio,namely,the constant-frequency motor generates a greater speed ratio compared to the constant-torque motor.

Design and Characterization of Magnetically Actuated Helical Swimmers at Submillimeter-scale

Bacteria with helical flagella show an ideal mechanism to swim at low Reynolds number. For application of artificial mi- croswimmers, it is desirable to identify effects of structural and geometrical parameters on the swimming performance. In this study, a double-end helical swimmer is proposed based on the usual single-end helical one to improve the forward-backward motion symmetry, The propulsion model of the artificial helical microswimmer is described. Influences of each helix parameter on the swimming velocity and propulsion efficiency are further analyzed. The optimal design for achieving a maximum propulsion velocity of submillimeter scale swimmers is performed based on some constraints. An experimental setup consisting of three-pair of Helmholtz coils is built for the helical microswimmers. Experiments of microswimmers with several groups of parameters were performed, and the results show the validity of the analysis and design.

Closed-loop Control Using High Power Hexapole Magnetic Tweezers for 3D Micromanipulation

This paper presents the design,modeling,integration,and application of 3D printed high power hexapole magnetic tweezers for 3D micromanipulation applications.Six sharp-tipped magnetic poles were configured with electromagnetic coils and mounted on 3D printed magnetic yokes to form a tilted Cartesian coordinate system for actuation.A closed loop control algorithm was developed to automatically manipulate external power supplies connected to the magnetic tweezers,by using 3D positional information obtained from real-time image processing techniques.When compared against other designs of magnetic tweezers,our system has a larger working space and can generate higher magnetic field strengths.This allows for more diverse applications regarding small scale manipulation,including cell manipulation and cell therapy.Experiments and analytics explained in this paper demonstrate the closed-loop manipulation of microswimmers can provide a magnetic force as high as 800 pN while maintaining a positional error below 4μm in 3D and 1.6μm in 2D.Using the desired location as the control input,the microswimmers investigated were able to achieve arbitrary 2D and 3D trajectories.We also show that the implemented hexapole magnetic tweezers have adequate power to control microswimmers in Newtonian fluid environments.The system will later be optimized and deployed to control microswimmers in non-Newtonian fluid environments.