Session S61: Steerable Colloids II

Role of shape and symmetry in externally driven micromotors

Motion in fluids at the micrometric scale is dominated by viscosity. One efficient propulsion method relies on a weak uniform rotating magnetic field that drives a chiral object. From bacterial flagella to artificial magnetic nanohelices, rotation of a corkscrew is considered as a universally efficient propulsion gait. Although approximate theories concerning dynamics of slender magnetic helices are available, actuation of geometrically achiral particles or random aggregates was not well understood. I will present a general theory of magnetized object of arbitrary shape in a rotating magnetic field. It appears that its propulsion velocity can be written in a compact form as Rayleigh quotient in terms of geometry-dependent chirality matrix Ch, where both the diagonal elements (owing to inherent handedness) and off-diagonal entries (that do not necessitate handedness) contribute in a similar way. The theory anticipates multiplicity of stable rotational states predicting that, e.g., two identical magnetic objects may propel with different speeds or even in opposite directions. However, for a class of simple planar objects, there is particular magnetization whereas the pair of symmetric rotational states degenerates into a unique propulsion gait closely resembling that of an ideal helix. In other words, geometrically achiral object can acquire effective chirality due to its interaction with the external magnetic field. The developed theory was further applied to optimize the geometry/magnetization and to obtain purely geometric constraint on propulsion speed of arbitrary shaped magnetic object. Finally, I will discuss general symmetries (such as parity and charge conjugation) and establish correspondence between propulsive states of geometrically achiral planar objects depending on orientation of the dipolar moment.

K. I. Morozov et al., Phys. Rev. Fluids 2, 044202 (2017)
Y. Mirzae et al., Science Robotics 3, eaas8713 (2018)
J. Sachs et al., Phys. Rev. E, to appear (2018)   

Programmable self-assembly of magnetic handshake materials

An outstanding intellectual problem in nanoscience is the programmable self-assembly of smart, digital, and mechanically functional structures [1]. We propose to combine magnetic patterning with the design principles of molecular biology for programmable self-assembly. To be specific, we harness magnetic forces from (i) panels with a 2 x2 pattern of magnetic domains so they bond together using specific, intelligent, interactions, analogous to Watson-Crick base pairs in DNA, and (ii) create programmed global structures, from assembly of magnetically patterned panels as well as strands that link these encoded panels in specific sequences. As a first step towards microscopic machines, we build macroscopic prototypes for proof-of-principle demonstration of information storage capability and programmable self-assembly of magnetic handshake materials.


References
[1] “Programmable self-assembly,” Ludovico Cademartiri and Kyle J. M. Bishop, Nature Materials 14, 2–9 (2015).   

Collapse dynamics of chains of paramagnetic particles: the role of the susceptibility.

We use computer simulations to study the dynamics of the collapse of chains of paramagnetic particles subject to rotating magnetic fields at various magnetic susceptibility. The system is initialized at constant magnetic field with particles forming chains along the field axis. The dynamics of the chains depends on the field rotation rate and, surprisingly, the particle susceptibility. At low susceptibility, and at sufficiently high field rotation rate, the particles undergo a period of chaotic motion and finally decay into a periodic orbit, consistent with previous simulations and experiments. Surprisingly, at high enough susceptibility we find a qualitatively different behavior. The initial chain state remains essentially intact for all time with the local moments strongly influenced by the orientation of the chain and less dependent on the applied field than in the low-susceptibility case. Our results should be important for applications of paramagnetic particles where rotating extended structures are desired such as micro mixing and should motivate new experiments on suspensions of paramagnetic particles in regimes of higher susceptibility.   

Hurricane dynamics in a membrane

We study driven microscopic rotors immersed in a membrane. We show that for small distances, interactions between rotors are identical to interactions between vortices in an ideal 2D Euler fluid, while the longer-ranged ones relate to classical models of atmospheric dynamics. Going beyond idealized interactions between rotors, we examine the more realistic setting where rotors also interact through local repulsion. We show that initially random distributions of rotors will rapidly self-organize into rotating uniform lattices or random hyperuniform structures, and exhibit activity-induced phase separation.   

Optically driven birefringent rotators

We use circularly polarized light to induce stable rotation in hundreds of colloidal micro-particles simultaneously. The optical angular momentum transferred to the particles creates a torque causing particle rotation. In the past, optical tweezers were used to rotate individual colloids while trapping them to the narrow waist of the focused beam. We use a defocused, circularly-polarized, beam to rotate multiple particles with minimal trapping. We developed a process for synthesizing stable birefringent vaterite particles that can rotate for long periods of times and characterized their optical and physical properties. Altering the handedness and intensity of the laser source allows us to control the frequency and switch between clockwise or counterclockwise rotation. We observe and discuss the interaction between the rotating particles.   

Light-driven capillary assembly and motion of hydrogel nanocomposite disks at the air/water interface

Modulation of capillary forces by active changes in shape is an approach used by a number of insects to achieve propulsion at air-water interfaces. Learning from nature, we have designed light-responsive sub-millimeter hydrogel disks that exhibit: (1) attraction, leading to formation of well-defined assemblies; (2) repulsion; or (3) sustained rotation, depending on the pattern of illumination. Fabrication of light-responsive hydrogel disks containing patterned gold nanoparticles (Au NPs) was achieved by photo-chemical reduction of Au³⁺ ions to metallic Au NPs within micro-patterned temperature-responsive hydrogels. Photothermal deswelling of illuminated Au NP-containing hydrogel regions induced out-of-plane buckling and deformation of the air/water interface, with a characteristic response time of seconds. This provided a versatile means to control capillary interactions in both time and space, thereby leading to a diverse range of behaviors at the air/water interface.   

Controlling Colloidal Assembly through Optical Binding

Directed assembly of colloidal particles has attracted considerable interest from both a fundamental and applied perspective. Most approaches to colloidal self-assembly use short range forces, e.g. via patchy or shaped particles. As an alternative, I will discuss the possibility of using 'optical binding' - a long range inter-particle force generated by multiple light scattering - to control the assembly of particles via a tunable external field. Although this approach has many advantages in principle; significant experimental and modelling work is required to make this a practical approach. I will discuss our efforts in both directions.   

Observations of Optically Bound Colloidal Assemblies

In recent years there has been growing interest in assembling complex structures from colloidal suspensions for use in both fundamental and applied studies. We are exploring a relatively new and highly tunable inter-particle colloidal force known as the optical binding force. The optical binding force is mediated by light scattering between two or more particles in an intense optical field which can be externally modified. We use experimental methods to impose this optical binding force onto clusters of dielectric colloidal particles. We will discuss unique behaviors that arise from this force, including the formation of multiple extended particle chains and formation of planar arrays.   

Harnessing Complex Fluid Interfaces to Control Colloidal Assembly and Deposition

Using lattice Boltzmann-Brownian dynamics (LB-BD) simulations, we model large-scale assembly of nanoparticles on liquid-vapor interfaces with complex geometries and investigate subsequent deposition upon complete evaporation. Particles aggregate into hexagonally close-packed monolayers on flat and spherical interfaces given appropriate value of the interaction parameter that couples fluid hydrodynamics and discrete particle dynamics. Detailed force analysis reveals a long-range attraction between particles, mimicking capillary interactions due to interface disturbance. On curved fluid interfaces with complex curvature fields, the particle dynamics is governed by pair capillary interactions and curvature-induced capillary migration. We develop a minimal theoretical model to predict equilibrium particle distribution on non-evaporating curved interfaces, which agrees well with simulation observation. Finally, we demonstrate that the interplay between evaporation-induced convective flow, particle pair interaction, and curvature-particle interaction results in distinct deposition patterns, which were obtained by using curved fluid interfaces as templates.   

Interstitial Particle Design for Active Colloidal Microstructures

Defect microstructures in colloidal crystals can be viewed as complex localized motifs, distinct from the environments present throughout the bulk of the crystalline phase. These motifs are persistent sites which interstitial particles can adsorb onto and be trapped by. The degree to which interstitial particles are bound to the vicinity of defects can be explored as a function of particle geometry. We present here a method to maximize the strength of the preferential interaction that a rod-like interstitial experiences with defect microstructures that include edge dislocations. We show that for sufficiently strongly bound interstitials, microstructural migration can be induced by applying forces to the designed particle, making it active. This approach opens up many possibilities for dynamically manipulating the microstructure of colloidal crystals, with applications in shape changing colloidal assemblies. Furthermore, the line-like nature of dislocations permits widely separated interstitials to be connected, correlating their transport properties in a manner not typically possible with local particle interactions.   

Maze-solving via diffusiophoresis

Mazes provide a simple model for many frequently-occurring structures and phenomena (eg. human circulatory system, traffic in a city). It is often of interest to find the most efficient path in a maze (eg. in order to transport materials). The problem of maze-solving poses a mathematical challenge: most analytic solutions can become computationally demanding as mazes become more complex, often relying on brute-force methods that involve exploring multiple paths before arriving at the solution. In this work, we present a novel method of solving the problem by exploiting the diffusiophoretic motion of colloids in a maze.We demonstrate our method experimentally via a microfluidic maze.