27 research outputs found
Controlling stability and transport of magnetic microswimmers by an external field
We investigate the hydrodynamic stability and transport of magnetic
microswimmers in an external field using a kinetic theory framework. Combining
linear stability analysis and nonlinear 3D continuum simulations, we show that
for sufficiently large activity and magnetic field strengths, a homogeneous
polar steady state is unstable for both puller and pusher swimmers. This
instability is caused by the amplification of anisotropic hydrodynamic
interactions due to the external alignment and leads to a partial
depolarization and a reduction of the average transport speed of the swimmers
in the field direction. Notably, at higher field strengths a reentrant
hydrodynamic stability emerges where the homogeneous polar state becomes stable
and a transport efficiency identical to that of active particles without
hydrodynamic interactions is restored
Effects of inertia on conformation and dynamics of active filaments
Many macroscopic active systems such as snakes, birds and fishes have
flexible shapes and inertial effects on their motion, in contrast to their
microscopic counterparts, cannot be ignored. Nonetheless, the consequences of
interplay between inertia and flexibility on their shapes and dynamics remain
unexplored. Here, we examine inertial effects on the most studied active
flexible system, {\it i.e.} linear active filaments pertinent to worms, snakes
and filamentous robots. Performing Langevin dynamics simulations of active
polymers with underdamped and overdamped dynamics for a wide range of contour
lengths and activities, we uncover striking inertial effects on their
conformation and dynamics. Inertial collisions increase the persistence length
of active polymers and remarkably alter their scaling behavior. In stark
contrast to passive polymers, inertia leaves its fingerprint at long times by
an enhanced diffusion of the center of mass. We rationalize inertia-induced
enhanced dynamics by analytical calculations of center of mass velocity
correlations, revealing significant contributions from active force
fluctuations convoluted by inertial relaxation.Comment: 5 pages, 4 figure
Dynamic Monte Carlo Simulations of Anisotropic Colloids
We put forward a simple procedure for extracting dynamical information from
Monte Carlo simulations, by appropriate matching of the short-time diffusion
tensor with its infinite-dilution limit counterpart, which is supposed to be
known. This approach --discarding hydrodynamics interactions-- first allows us
to improve the efficiency of previous Dynamic Monte Carlo algorithms for
spherical Brownian particles. In a second step, we address the case of
anisotropic colloids with orientational degrees of freedom. As an illustration,
we present a detailed study of the dynamics of thin platelets, with emphasis on
long-time diffusion and orientational correlations.Comment: 12 pages, 9 figure
Active motion of tangentially driven polymers in periodic array of obstacles
One key question about transport of active polymers within crowded environments is how spatial order of obstacles influences their conformation and dynamics when compared to disordered media. To this end, we computationally investigate the active transport of tangentially driven polymers with varying degrees of flexibility and activity in two-dimensional square lattices of obstacles. Tight periodic confinement induces notable conformational changes and distinct modes of transport for flexible and stiff active filaments. It leads to caging of low activity flexible polymers inside the inter-obstacle pores while promoting more elongated conformations and enhanced diffusion for stiff polymers at low to moderate activity levels. The migration of flexible active polymers occurs via hopping events, where they unfold to move from one cage to another, similar to their transport in disordered media. However, in ordered media, polymers are more compact and their long-time dynamics is significantly slower. In contrast, stiff chains travel mainly in straight paths within periodic inter-obstacle channels while occasionally changing their direction of motion. This mode of transport is unique to periodic environment and leads to more extended conformation and substantially enhanced long-time dynamics of stiff filaments with low to moderate activity levels compared to disordered media. At high active forces, polymers overcome confinement effects and move through inter-obstacle pores just as swiftly as in open spaces, regardless of the spatial arrangement of obstacles. We explain the center of mass dynamics of semiflexible polymers in terms of active force and obstacle packing fraction by developing an approximate analytical theory
Experimental observation of the intricate free-energy landscape for a soft glassy system
In the free energy landscape picture of glassy systems, the slow dynamics
characteristic of these systems is believed to be due to the existence of a
complicated free-energy landscape with many local minima. We show here that for
a colloidal glassy material multiple paths can be taken through the free energy
landscape, that can even lead to different 'final' non-ergodic states at the
late stages of aging. We provide clear experimental evidence for the
distinction of gel and glassy states in the system and show that for a range of
colloid concentrations, the transition to non-ergodicity can occur in either
direction (gel or glass), and may be accompanied by 'hesitations' between the
two directions. This shows that colloidal gels and glasses are merely global
free-energy minima in the same free energy landscape, and that the paths
leading to these minima can indeed be complicated.Comment: 5 pages, 5 figure
Interplay of anisotropy in shape and interactions in charged platelet suspensions
Motivated by the intriguing phase behavior of charged colloidal platelets, we
investigate the structure and dynamics of charged repulsive disks by means of
Monte-Carlo simulations. The electrostatic interactions are taken into account
through an effective two-body potential, obtained within the non-linear
Poisson-Boltzmann formalism, which has the form of anisotropic screened Coulomb
potential. Recently, we showed that the original intrinsic anisotropy of the
electrostatic potential in competition with excluded volume effects leads to a
rich phase behavior that not only includes various liquid-crsytalline phases
but also predicts the existence of novel structures composed of alternating
nematic-antinematic sheets. Here, we examine the structural and dynamical
signatures of each of the observed structures for both translational and
rotational degrees of freedom. Finally, we discuss the influence of effective
charge value and our results in relation to experimental findings on charged
platelet suspensions.Comment: 22 pages, 17 figure
The interplay between chemo-phoretic interactions and crowding in active colloids
Many motile microorganisms communicate with each other and their environments via chemical signaling which leads to long-range interactions mediated by self-generated chemical gradients. However, consequences of the interplay between crowding and chemotactic interactions on their collective behavior remain poorly understood. In this work, we use Brownian dynamics simulations to investigate the effect of packing fraction on the formation of non-equilibrium structures in a monolayer of diffusiophoretic self-propelled colloids as a model for chemically active particles. Focusing on the case when a chemical field induces attractive positional and repulsive orientational interactions, we explore dynamical steady-states of active colloids of varying packing fractions and degrees of motility. In addition to collapsed, active gas, and dynamical clustering steady-states reported earlier for low packing fractions, a new phase-separated state emerges. The phase separation results from a competition between long-range diffusiophoretic interactions and motility and is observed at moderate activities and a wide range of packing fractions. Our analysis suggests that the fraction of particles in the largest cluster is a suitable order parameter for capturing the transition from an active gas and dynamical clustering states to a phase-separated state