Session A17: Steerable Particles: New Ways to Manipulate Fluid-Mediated Forces

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In this talk I will describe a general strategy to assemble and transport polarizable microparticles in fluid media through combination of confinement and magnetic dipolar interactions. We use a homogeneous magnetic modulation to assemble dispersed particles into rotating dimeric state and frustrated binary lattices, and generate collective edge currents which arise from a novel, field-synchronized particle exchange process. The observed, net bidirectional current is composed of colloidal particles which periodically meet assembling into rotating dimers, and exchange their positions in a characteristic, “ceilidh"-like dance. We develop a theoretical model which explains the physics of the observed phenomena as dimer rupture and onset of current, showing agreement with Brownian dynamic simulations. Our results demonstrate an effective technique to drive microscale matter by using a combination of confinement and homogeneous field modulations, not based on any gradient of the applied field.   

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As we move towards development of more realistic and applications-driven active matter systems, we must understand how active particles interact with complex and crowded environments. We study this problem experimentally using a system which consists of weakly magnetic colloids suspended in water, and driven by a rotating magnetic field. These colloids are heavy and sedimented near the bottom of the sample chamber; hydrodynamic coupling between the particles and the floor induces propulsion. This results in a model system for driven colloids where the dominant force is hydrodynamics. Here we use microscopy and particle tracking to observe and characterize what happens when these rolling particles encounter various types of three-dimensional obstacles. We explore how variables such as roller angular velocity and obstacle geometry influence individual roller trajectories as well as collective interactions, measuring the effect of these parameters on roller movement. Additionally, we explore whether the hydrodynamic trapping effects seen in other active systems appear here, and how this trapping may be influenced by obstacle geometry.   

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Active colloids can generate autonomous motion in liquids, however due to their small size, their paths are rapidly randomised by Brownian diffusion. Consequently a long held aim is to develop methods to harness this motion more effectively in order to steer the colloids towards particular targets. A wide range of methods to achieve this goal for catalytic Janus colloids will be surveyed. These approaches exploit a range of phenomena including the colloids interaction with magnetic fields, gravity, and nearby surfaces; mediated by phoretic and hydrodynamic effects. The ability to directly observe these hydrodynamic interactions will also be demonstrated. A particular focus will be placed on recent advances which exploit the ability of catalytic active colloids in confined volumes to self-generate well defined fluid flows. We have recently found that the interaction of these self-induced flows with ellipsoidal Janus colloids provides a new method to access cross-flow migration behaviour, steering the colloids path in relation to the macroscopic containers geometry. Additionally we will report the potential for responsive active colloids to statistically migrate towards a chemically defined target based on their ability to change size and modulate their motion.   

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The gravitational settling of particles in a viscous fluid is a confounding problem in many-body physics, owing to the long-range flow fields set up by each falling particle. Here I will present experimental and theoretical results that show qualitatively new behaviour when the particles have non-trivial shape and orientational degrees of freedom, as do snowflakes, plankton, crystals, and other natural sediment. As examples of the richness that emerges from shape, I will discuss unusual phenomena in the sedimentation of individual polar and polygonal objects, of pairs of apolar and polar objects, and of a one-dimensional lattice of apolar discs. For instance, a pair of spheres falls with constant separation vector, whereas pairs of discs either fall into bound states with periodic twirling in the orientation, or fall into scattering states, in exact analogy with Keplerian orbits [1]. A line of spheres is unstable to small density perturbations, whereas a line of discs can alternatively display wave-like excitations of position and orientation[2]. Underlying these behaviours is a Hamiltonian description where orientation plays the role of an effective inertia. In the example of a sedimenting lattice, we also find a clean example of an unusual route to instability: while the system is linearly stable, there is transient growth in perturbations until nonlinear mechanisms can take over.

This work was done in collaboration with Rahul Chajwa, Alyssa Conway, Rama Govindarajan and Sriram Ramaswamy.

1. R. Chajwa et al. Kepler Orbits in Pairs of Disks Settling in a Viscous Fluid. Phys. Rev. Lett. 122, p.224501. 2019
2. R. Chajwa et al Waves, Algebraic Growth, and Clumping in Sedimenting Disk Arrays Phys. Rev. X 10, 041016 (2020)   

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Gravitational sedimentation may be the simplest way to steer a particle. In a suspension of many particles, a linear stability analysis shows particles with a non-uniform mass density will self-organize into hyperuniform structures via an effective repulsive force (Goldfriend et al., Phys. Rev. Lett., 2017). Here we experimentally test these predictions using non-uniform composite particles made from 2 mm individual spheres. We vary both the aspect ratio (κ) and center of mass offset (χ) by combining spheres of different materials. Particles sediment at low Reynolds number and are optically tracked in both 2D and 3D chambers. We quantified the mobility matrix for individual particles by fitting the data with simulated trajectories based on the Stokes equation. We find that two prolate (κ>1) particles with uniform density (χ=0) tend to constantly flip during sedimentation. However, when χ>0, we find that pairs of particles experience an effective repulsive force ~ (κ-1)/χ, so that particles with a small offset in their center of mass experience the largest repulsion (the small gravitational torque suppresses flipping). We also show that these repulsive effects can be readily seen in 3D suspensions of hundreds of particles with χ>0.   

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Microorganisms exhibit autonomous locomotion through complex media and can react to changes in their environment. This ability of bacteria and living cells to adapt their motion in response to external stimuli is called taxis [1]. Leptospira bacteria, for example, swim towards regions of high viscosity [2]. While different kinds of taxis have been studied, the mechanism behind viscotaxis was not investigated until recently. Liebchen et al. found that swimmers with nonuniaxial body shapes swim up viscosity gradients due to the generation of asymmetric viscous torques acting on different parts of the swimmer. We are testing these theoretical predictions using artificial microswimmers created by 3D printing based on two-photon lithography that gives us access to virtually any shape [4]. We have started with the fabrication of trimers on the micrometre scale, the simplest form of a nonuniaxial body shape.

1) D. B. Dusenbery, Living at Micro Scale, Harvard University Press, 2009.
2) M. G. Petrino et al., J. Gen. Microbiol., 1978, 109, 113-117.
3) B. Liebchen et al., Phys. Rev. Lett., 2018, 120, 208002.
4) R. P. Doherty et al., Soft Matter, 2020, DOI:10.1039/D0SM01320J.   

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Magnetic fields allow for the remote manipulation of bio-orthogonal microscale robots that have applications in drug delivery or microsurgery. Oscillations in triaxial magnetic fields induce non-reciprocal, wave-like motions on circular membrane patches. We show how the membrane actuation is controlled by dimensionless parameters that depend on the magnitude, angle and frequency of precessing magnetic fields to induce circumferential and radial membrane waves. Using the lattice Boltzmann method, we implement hydrodynamic interactions that reveal the flow field of the surrounding fluid and show how truncated circular patches swim at low Reynolds numbers.   

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Motile agents can condense even if they do not attract each other. This phenomenon, known as motility-induced phase separation (MIPS), has been seen to arise from central repulsive forces: self-propelled particles collide with one another, slow down, and self-trap in dense regions. In this talk, I will present a new mechanism for MIPS, which is based not on central forces but on torques. I will show that self-propelled Janus particles interact via non-reciprocal torques that reorient their motion toward high-density regions. Hence, particles self-propel up their own density gradient – a behavior dubbed autotaxis – which drives phase separation. Because this torque-based aggregation requires no self-trapping, particles in clusters retain substantial speed, and clusters remain dilute and fluid. Overall, our work establishes that orientational interactions (torques) can yield coexisting phases with no internal orientational order.   

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The advent of autonomous self-propulsion has directed research towards using active particles for a variety of functions such as sensing and environmental remediation. While much progress has been made in the last decade on various mechanisms to synthesize active particles, the ability to steer self-propelled colloidal devices has so far been limited. A critical barrier in increasing the impact of such particles is in directing their motion against the Brownian rotation, which randomizes particle orientations. Current strategies for directing self-propelled particles either lack autonomy or only constrain along one-dimension. Here we will discuss two strategies of steering catalytic particles namely their interactions with boundaries and the use of external fields. We will discuss how geometric constraints can be utilized to steer these active colloids along arbitrary trajectories. We will also demonstrate how the behavior of the particles can be programmed based on real-time image analysis and steering with external fields. These strategies can be used to render active particles ‘intelligent’ which will have a major impact on their application in robotics, engineering, biology, and medicine.   

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Steady streaming refers to the rectified flow patterns produced when solid boundaries interact with high-frequency oscillatory flows. This phenomenon is arguably the most efficient way to exploit inertia at the microscale, with several practical applications from mixing to particle manipulation. Here, we present numerical predictions and experimental verifications of steady streaming realized in a periodic lattice of cylinders with alternating curvatures. The interplay between multiple curvatures and oscillation frequency leads to a rich variety of flow topologies, beyond classically understood ones. We leverage this setup for tunable non-contact filtration and enhanced particle trapping.   

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Because bacterial adhesion on surfaces is the first step of infection and biofilm formation, surfaces have been developed to prevent cell adhesion. We demonstrate here that E cells can be immobilized on otherwise non-adhesive surfaces through the introduction of a non-adsorbing polyethylene oxide polymer. The depletion-driven cell capture can occur in gentle shear flow and is reversible. The free polymer additionally causes reversible aggregation of cells in free solution; however, cell concentrations are below those typically associated with depletion-driven fluid-solid phase transitions. Because cell aggregates experience stronger shearing forces than do individual cells, adhesion of individual cells is preferred over that of aggregates. While cells captured on physico-chemically adhesive surfaces tend to adsorb by their ends in a variety of tipped configurations relative to the surface, depletion-driven cell capture brings cells more nearly flat to the surface. The aggregated and captured cells are living and are observed to grow into microcolonies.