Alexey Snezhko

Magnetic manipulation of self-assembled colloidal asters.

Self-assembled materials must actively consume energy and remain out of equilibrium to support structural complexity and functional diversity. Here we show that a magnetic colloidal suspension confined at the interface between two immiscible liquids and energized by an alternating magnetic field dynamically self-assembles into localized asters and arrays of asters, which exhibit locomotion and shape change. By controlling a small external magnetic field applied parallel to the interface, we show that asters can capture, transport, and position target microparticles. The ability to manipulate colloidal structures is crucial for the further development of self-assembled microrobots.

Driving self-assembly and emergent dynamics in colloidal suspensions by time-dependent magnetic fields

In this review we discuss recent research on driving self-assembly of magnetic particle suspensions subjected to alternating magnetic fields. The variety of structures and effects that can be induced in such systems is remarkably broad due to the large number of variables involved. The alternating field can be uniaxial, biaxial or triaxial, the particles can be spherical or anisometric, and the suspension can be dispersed throughout a volume or confined to a soft interface. In the simplest case the field drives the static or quasistatic assembly of unusual particle structures, such as sheets, networks and open-cell foams. More complex, emergent collective behaviors evolve in systems that can follow the time-dependent field vector. In these cases energy is continuously injected into the system and striking flow patterns and structures can arise. In fluid volumes these include the formation of advection and vortex lattices. At air-liquid and liquid-liquid interfaces striking dynamic particle assemblies emerge due to the particle-mediated coupling of the applied field to surface excitations. These out-of-equilibrium interface assemblies exhibit a number of remarkable phenomena, including self-propulsion and surface mixing. In addition to discussing various methods of driven self-assembly in magnetic suspensions, some of the remarkable properties of these novel materials are described.

Magnetic nanoparticles as efficient bulk pinning centers in type-II superconductors

Enhancement of vortex pinning by magnetic nanoparticles embedded into the bulk of type–II superconductor is studied both theoretically and experimentally. Magnetic part of the pinning force associated with the interaction between a finite-size spherical magnetic inclusion and an Abrikosov vortex was calculated in London approximation. Calculations are supported by the experimental results obtained on sonochemically modified MgB2 superconductor with embedded magnetic Fe2O3 nanoparticles and compared to MgB2 with nonmagnetic Mo2O5 pinning centers of similar concentration and particle size distribution. It is shown that ferromagnetic nanoparticles result in a considerable enhancement of vortex pinning in large-κ type-II superconductors.

Emergent colloidal dynamics in electromagnetic fields

We present a current review of the collective dynamics that can arise in colloidal systems subjected to electromagnetic fields. The focus is on phenomena that are not simply understandable purely from a dipolar model, but instead emerge from the collective behavior of many discrete interacting components driven out of equilibrium by external forces. We examine in particular the fascinating diversity of large-scale dynamical structures that arise due to the interplay between the induced interactions, time-dependent energy injection, and coupling with the fluid flow.

Evidence for unconventional superconductivity in the nonoxide perovskite MgCNi 3 from penetration depth measurements

The London penetration depth

Non-equilibrium magnetic colloidal dispersions at liquid-air interfaces: dynamic patterns, magnetic order and self-assembled swimmers.

\ensuremath{\lambda}(T)

Rewritable artificial magnetic charge ice

was measured in polycrystalline powders of the nonoxide perovskite superconductor

Sonochemical modification of the superconducting properties of MgB2

{\mathrm{MgCNi}}_{3}

Pulsating–gliding transition in the dynamics of levitating liquid nitrogen droplets

by using a sensitive tunnel-diode resonator technique. The penetration depth exhibits distinctly non

Emergence of reconfigurable wires and spinners via dynamic self-assembly

s