Anisotropic Diffusion in Driven Convection Arrays

We numerically investigate the transport of a Brownian colloidal particle in a square array of planar counter-rotating convection rolls at high Peclet numbers. We show that an external force produces huge excess peaks of the particle's diffusion constant with a height that depends on the force orientation and intensity. In sharp contrast, the particle's mobility is isotropic and force independent. We relate such a nonlinear response of the system to the advection properties of the laminar flow in the suspension fluid

Nonuniform self-organized dynamical states in superconductors with periodic pinning

We consider magnetic flux moving in superconductors with periodic pinning arrays. We show that sample heating by moving vortices produces negative differential resistivity (NDR) of both N and S type (i.e., N- and S-shaped) in the voltage-current characteristic (VI curve). The uniform flux flow state is unstable in the NDR region of the VI curve. Domain structures appear during the NDR part of the VI curve of an N type, while a filamentary instability is observed for the NDR of an S type. The simultaneous existence of the NDR of both types gives rise to the appearance of striking self-organized (both stationary and nonstationary) two-dimensional dynamical structures

Enhanced motility in a binary mixture of active nano/microswimmers

It is often desirable to enhance the motility of active nano- or microscale swimmers such as, e.g., self-propelled Janus particles as agents of chemical reactions or weak sperm cells for better chances of successful fertilization. Here we tackle this problem based on the idea that motility can be transferred from a more active guest species to a less active host species. We performed numerical simulations of motility transfer in two typical cases, namely for interacting particles with weak inertia effect, by analyzing their velocity distributions, and for interacting overdamped particles, by studying their effusion rate. In both cases we detected motility transfer with a motility enhancement of the host species of up to a factor of four. This technique of motility enhancement can find applications in chemistry, biology and medicine.Comment: 10 page

Visible Light Actuated Efficient Exclusion Between Plasmonic Ag/AgCl Micromotors and Passive Beads

Insight is provided into the collective behavior of visible‐light photochemically driven plasmonic Ag/AgCl Janus particles surrounded by passive polystyrene (PS) beads. The active diffusion of single Janus particles and their clusters (small: consisting of two or three Janus particles and large: consisting of more than ten Janus particles), and their interaction with passive PS beads, are analyzed experimentally and in simulations. The diffusivity of active Janus particles, and thus the exclusive effect to passive PS beads, can be regulated by the number of single Janus particles in the cluster. On the simulation side, the Langevin equations of motion for self‐propelled Janus particles and diffusing passive PS beads are numerically solved using Molecular‐Dynamics simulations. The complex interactions of both subsystems, including elastic core‐to‐core interactions, short‐range attraction, and effective repulsion due to light‐induced chemical reactions are considered. This complex mixed system not only provides insight to the interactive effect between active visible light‐driven self‐propelled micromotors and passive beads, but also offers promise for implications in light‐controlled propulsion transport and chemical sensing.Visible light actuated plasmonic Ag/AgCl‐based spherical Janus micromotors reveal efficient exclusion effects on surrounding passive beads in pure H2O. The exclusion efficiency is controlled by the number of single Janus particles composing micromotors. The system‐specific interaction parameter between Janus micromotors and passive beads is determined. It assures predictive power for further theoretical analysis of the complex dynamics of these heterogeneous active‐passive systems.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146656/1/smll201802537_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146656/2/smll201802537.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146656/3/smll201802537-sup-0001-S1.pd

Nonuniform Self-Organized Dynamical States in Superconductors with Periodic Pinning

We consider magnetic flux moving in superconductors with periodic pinning arrays. We show that sample heating by moving vortices produces negative differential resistivity (NDR) of both N and S type (i.e., N- and S-shaped) in the voltage-current characteristic (VI curve). The uniform flux flow state is unstable in the NDR region of the VI curve. Domain structures appear during the NDR part of the VI curve of an N type, while a filamentary instability is observed for the NDR of an S type. The simultaneous existence of the NDR of both types gives rise to the appearance of striking self-organized (both stationary and non-stationary) two-dimensional dynamical structures.Comment: 4 pages, 2 figure

Inverse Solidification Induced by Active Janus Particles

Crystals melt when thermal excitations or the concentration of defects in the lattice is sufficiently high. Upon melting, the crystalline long‐range order vanishes, turning the solid to a fluid. In contrast to this classical scenario of solid melting, here a counter‐intuitive behavior of the occurrence of crystalline long‐range order in an initially disordered matrix is demonstrated. This unusual solidification is demonstrated in a system of passive colloidal particles accommodating chemically active defects—photocatalytic Janus particles. The observed crystallization occurs when the amount of active‐defect‐induced fluctuations (which is the measure of the effective temperature) reaches critical value. The driving mechanism behind this unusual behavior is purely internal and resembles a blast‐induced solidification. Here, the role of “internal micro‐blasts” is played by the photochemical activity of defects residing in the colloidal matrix. The defect‐induced solidification occurs under non‐equilibrium conditions: the resulting solid exists as long as a constant supply of energy in the form of ion flow is provided by the catalytic photochemical reaction at the surface of active Janus particle defects. The findings could be useful for the understanding of the phase transitions of matter under extreme conditions far from thermodynamic equilibrium.Inverse solidification driven by active colloids provides novel insight into the collective effects in mixed colloidal systems. It offers versatile possibilities to address the processes of solidification in various systems out of equilibrium, including the formation of bio‐molecular condensates or biomineralization, transitions from amorphous to polycrystalline state in condensed matter, or synthesis of materials under extreme conditions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/162773/3/adfm202003851.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/162773/2/adfm202003851-sup-0001-SuppMat.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/162773/1/adfm202003851_am.pd

High‐Motility Visible Light‐Driven Ag/AgCl Janus Micromotors

Visible light‐driven nano/micromotors are promising candidates for biomedical and environmental applications. This study demonstrates blue light‐driven Ag/AgCl‐based spherical Janus micromotors, which couple plasmonic light absorption with the photochemical decomposition of AgCl. These micromotors reveal high motility in pure water, i.e., mean squared displacements (MSD) reaching 800 µm2 within 8 s, which is 100× higher compared to previous visible light‐driven Janus micromotors and 7× higher than reported ultraviolet (UV) light‐driven AgCl micromotors. In addition to providing design rules to realize efficient Janus micromotors, the complex dynamics revealed by individual and assemblies of Janus motors is investigated experimentally and in simulations. The effect of suppressed rotational diffusion is focused on, compared to UV light‐driven AgCl micromotors, as a reason for this remarkable increase of the MSD. Moreover, this study demonstrates the potential of using visible light‐driven plasmonic Ag/AgCl‐based Janus micromotors in human saliva, phosphate‐buffered saline solution, the most common isotonic buffer that mimics the environment of human body fluids, and Rhodamine B solution, which is a typical polluted dye for demonstrations of photocatalytic environmental remediation. This new knowledge is useful for designing visible light driven nano/micromotors based on the surface plasmon resonance effect and their applications in assays relevant for biomedical and ecological sciences.Ag/AgCl‐based spherical Janus motors are demonstrated to reveal efficient propulsion when illuminated by visible blue light due to the surface plasmon resonance effect. The design rules to realize efficient visible‐light‐driven Janus micromotors are provided. In addition to the experimental and theoretical study of their complex dynamics, possible applications with visible light in physiological fluids and environmental remediation are highlighted.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146560/1/smll201803613-sup-0001-S1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146560/2/smll201803613.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146560/3/smll201803613_am.pd

Negative differential resistivity in superconductors with periodic arrays of pinning sites

We study theoretically the effects of heating on the magnetic flux moving in superconductors with a periodic array of pinning sites (PAPS). The voltage-current characteristic (VI-curve) of superconductors with a PAPS includes a region with negative differential resistivity (NDR) of S-type (i.e., S-shaped VI-curve), while the heating of the superconductor by moving flux lines produces NDR of N-type (i.e., with an N-shaped VI-curve). We analyze the instability of the uniform flux flow corresponding to different parts of the VI-curve with NDR. Especially, we focus on the appearance of the filamentary instability that corresponds to an S-type NDR, which is extremely unusual for superconductors. We argue that the simultaneous existence of NDR of both N- and S-type gives rise to the appearance of self-organized two-dimensional dynamical structures in the flux flow mode. We study the effect of the pinning site positional disorder on the NDR and show that moderate disorder does not change the predicted results, while strong disorder completely suppresses the S-type NDR.Comment: 10 pages, 1 table, 7 figure

Recent Advances in Superconductivity and Vortex Matter: Selected Topics

The more-than-a-century history of superconductivity is marked by a great deal of progress in the understanding of this fascinating phenomenon as well as discovery and design of new superconducting materials and systems with enhanced critical parameters. Despite the achievements in theory, experiment and technology, many questions still remain open related to the mechanisms of superconductivity in so-called unconventional materials and aspects related to applications of superconductors which stimulates continuous research interest in the field. In this short review paper, we discuss some important recent advances in superconductivity and vortex matter made over about the last decade. In particular, we focus on novel iron-based and multigap superconductors and vortex matter in multi-component systems including multiband superconductors and multi-component Bose–Einstein condensates. We also discuss recent advances in manipulating vortex matter in nano-engineered superconducting systems such as non-periodic pinning arrays which can be used as devices that enhance the critical superconducting parameters and thus open new possibilities for applications of superconductors