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ABSTRACT: Dense suspensions of self-propelled rod-like particles exhibit a fascinating variety of non-equilibrium phenomena. By means of computer simulations of a minimal model for rigid self-propelled colloidal rods with variable shape we explore the generic diagram of emerging states over a large range of rod densities and aspect ratios. The dynamics is studied using a simple numerical scheme for the overdamped noiseless frictional dynamics of a many-body system in which steric forces are dominant over hydrodynamic ones. The different emergent states are identified by various characteristic correlation functions and suitable order parameter fields. At low density and aspect ratio, a disordered phase with no coherent motion precedes a highly cooperative swarming state with giant number fluctuations at large aspect ratio. Conversely, at high densities weakly anisometric particles show a distinct jamming transition whereas slender particles form dynamic laning patterns. In between there is a large window corresponding to strongly vortical, turbulent flow. The different dynamical states should be verifiable in systems of swimming bacteria and artificial rod-like micro-swimmers.
Journal of Physics Condensed Matter 10/2012; 24(46):464130. · 2.55 Impact Factor
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ABSTRACT: We combine real-space experiments and lattice sum calculations to investigate the phase diagram of charged colloidal particles under soft confinement. In the experiments we explore the equilibrium phase diagram of charged colloidal spheres in aqueous suspensions confined between two parallel charged walls at low background salt concentrations. Motivated by the experiments, we perform lattice sum minimizations to predict the crystalline ground state of point-like Yukawa particles which are exposed to a soft confining wall potential. In the multilayered crystalline regime, we obtain good agreement between the experimental and numerical findings: upon increasing the density we recover the sequence 2□ → 2△ → 2hcp⊥ → 3□ → 3△ → 3hcp⊥ → 4□.
Journal of Physics Condensed Matter 10/2012; 24(46):464123. · 2.55 Impact Factor
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ABSTRACT: Both charged colloidal suspensions and complex (dusty) plasmas represent classical many-body strongly coupled Coulomb systems. Here we discuss their basic properties and focus on their heterogeneous crystallization from an undercooled melt. In particular, a model with different mobilities is proposed which is realizable in binary mixtures of charged particles. Within this binary-mobility model, the crystallization behaviour near a structured wall is explored by Brownian dynamics computer simulations. As a result, the propagation velocity of the crystal-fluid interface is a nonmonotonic function of the mobility ratio (if expressed in terms of an averaged mobility).
Journal of Physics Condensed Matter 07/2012; 24(28):284125. · 2.55 Impact Factor
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ABSTRACT: We investigated driven crystal formation events in thin layers of sedimented colloidal particles under low salt conditions. Using optical microscopy, we observe particles in a thermodynamically stable colloidal fluid to move radially converging towards cation exchange resin fragments acting as seed particles. When the local particle concentration has become sufficiently large, subsequently crystallization occurs. Brownian dynamics simulations of a 2D system of purely repulsive point-like particles exposed to an attractive potential, yield strikingly similar scenarios, and kinetics of accumulation and micro-structure formation. This offers the possibility of flexibly designing and manufacturing thin colloidal crystals at controlled positions and thus to obtain specific micro-structures not accessible by conventional approaches. We further demonstrate that particle motion is correlated with the existence of a gradient in electrolyte concentration due to the release of electrolyte by the seeds.
The Journal of chemical physics 04/2012; 136(16):164505. · 3.09 Impact Factor
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ABSTRACT: For many applications, it is important to catch collections of autonomously
navigating microbes and man-made microswimmers in a controlled way. Here we
propose an efficient trap to collectively capture self-propelled colloidal
rods. By means of computer simulation in two dimensions, we show that a static
chevron-shaped wall represents an optimal boundary for a trapping device. Its
catching efficiency can be tuned by varying the opening angle of the trap. For
increasing angles, there is a sequence of three emergent states corresponding
to partial, complete, and no trapping. A trapping `phase diagram' maps out the
trap conditions under which the capture of self-propelled particles at a given
density is rendered optimal.
02/2012;
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Advances in Chemical Physics 01/2012; 148:225. · 3.58 Impact Factor
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ABSTRACT: For sedimenting colloidal hard spheres, the propagation and broadening of the crystal-fluid interface is studied by Brownian dynamics computer simulations of an initially homogeneous sample. Two different types of interface broadenings are observed: the first occurs during growth and is correlated with the interface velocity, the second is concomitant with the splitting of the crystal-fluid interface into the crystal-amorphous and amorphous-liquid interfaces. The latter width is strongly peaked as a function of the gravitational driving strength with a huge amplitude relative to its equilibrium counterpart.
EPL (Europhysics Letters) 07/2011; 95(3):38004. · 2.17 Impact Factor
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ABSTRACT: Overdamped Brownian motion of a self-propelled particle is studied by solving the Langevin equation analytically. On top of translational and rotational diffusion, in the context of the presented model, the 'active' particle is driven along its internal orientation axis. We calculate the first four moments of the probability distribution function for displacements as a function of time for a spherical particle with isotropic translational diffusion, as well as for an anisotropic ellipsoidal particle. In both cases the translational and rotational motion is either unconfined or confined to one or two dimensions. A significant non-Gaussian behaviour at finite times t is signalled by a non-vanishing kurtosis γ(t). To delimit the super-diffusive regime, which occurs at intermediate times, two timescales are identified. For certain model situations a characteristic t(3) behaviour of the mean-square displacement is observed. Comparing the dynamics of real and artificial microswimmers, like bacteria or catalytically driven Janus particles, to our analytical expressions reveals whether their motion is Brownian or not.
Journal of Physics Condensed Matter 05/2011; 23(19):194119. · 2.55 Impact Factor
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ABSTRACT: By lattice sum minimization, we predict the ground state of particles interacting via a Yukawa potential which are confined in a quasi–one-dimensional cylindrical tube. As a function of screening strength and particle density, the zero-temperature phase diagram exhibits a cascade of stable crystals with both helical and non-helical structures. These quasi–one-dimensional crystals can be confirmed in experiments on confined charged colloidal suspensions, trapped dusty plasmas or ions in nanotubes.
EPL (Europhysics Letters) 04/2011; 94(2):28005. · 2.17 Impact Factor
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ABSTRACT: The dynamics of an adsorbed polymer chain along the substrate is studied using Brownian dynamics computer simulations of a bead-spring model. The polymer is end-functionalized and exposed both to a linear shear flow and an external field oscillatory in time which couples oppositely to the end beads and acts normal to the substrate. Different propagation modes are identified including a waddling-like and somersault-like polymer motion.
EPL (Europhysics Letters) 01/2011; 93(1):18002. · 2.17 Impact Factor
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K R Sütterlin,
A Wysocki,
C Räth,
A V Ivlev,
H M Thomas,
S Khrapak,
S Zhdanov,
M Rubin-Zuzic,
W J Goedheer,
V E Fortov,
A M Lipaev,
V I Molotkov,
O F Petrov,
G E Morfill, H Löwen
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ABSTRACT: Complex plasma being the 'plasma state of soft matter' is especially suitable for investigations of non-equilibrium phase transitions. Non-equilibrium phase transitions can manifest in dissipative structures or self-organization. Two specific examples are lane formation and phase separation. Using the permanent microgravity laboratory PK-3 Plus, operating onboard the International Space Station, we performed unique experiments with binary mixtures of complex plasmas that showed both lane formation and phase separation. These observations have been augmented by comprehensive numerical and theoretical studies. In this paper we present an overview of our most important results. In addition we put our results in context with research of complex plasmas, binary systems and non-equilibrium phase transitions. Necessary and promising future complex plasma experiments on phase separation and lane formation are briefly discussed.
Plasma Physics and Controlled Fusion 11/2010; 52(12):124042. · 2.42 Impact Factor
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A Wysocki,
C Räth,
A V Ivlev,
K R Sütterlin,
H M Thomas,
S Khrapak,
S Zhdanov,
V E Fortov,
A M Lipaev,
V I Molotkov,
O F Petrov, H Löwen,
G E Morfill
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ABSTRACT: Using experiments and combining theory and computer simulations, we show that binary complex plasmas are particularly good model systems to study the kinetics of fluid-fluid demixing at the "atomistic" (individual particle) level. The essential parameters of interparticle interactions in complex plasmas, such as the interaction range(s) and degree of nonadditivity, can be varied significantly, which allows systematic investigations of different demixing regimes. The critical role of competition between long-range and short-range interactions at the initial stage of the spinodal decomposition is discussed.
Physical Review Letters 07/2010; 105(4):045001. · 7.37 Impact Factor
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ABSTRACT: The nonequilibrium response of a colloidal liquid-crystalline nematic phase to an external aligning field, which rotates in a plane, is explored by dynamical fundamental measure density-functional theory. Depending on the drive frequency, different dynamical states are found, which are characterized by towing and overtaking of the nematic director by the field as well as by breathing and dynamical splitting of the orientational distribution peak. This complex response can be exploited for smart optical switching and mixing devices.
Physical Review E 05/2010; 81(5 Pt 1):051703. · 2.26 Impact Factor
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ABSTRACT: The recently developed fundamental measure density functional theory (Hansen-Goos and Mecke 2009 Phys. Rev. Lett. 102 018302) for an inhomogeneous anisotropic hard body fluid is used as a basic ingredient in treating the Brownian dynamics of hard spherocylinders. After discussing the relevance of a free parameter in the fundamental measure density functional for the isotropic-nematic transition in equilibrium, we discuss the equilibrium phase behaviour of hard spherocylinders in a static external potential which couples only to the orientations. For external potentials favouring rod orientations along the poles of the unit sphere, there is a well-known paranematic-nematic transition which ceases to exist above a threshold of the strength V(0) of the external potential. However, when orientations along the equator are more favoured, in the plane of the potential energy V(0) and density, there is a phase transition from paranematic to nematic for any strength, which becomes second order above a critical threshold of V(0). The full equilibrium phase diagram in the V(0)-density plane is computed for a fixed rod aspect ratio of 5. For the equatorial cases, strength V(0) is then oscillating in time and dynamical density functional theory is used to compute the evolution of the orientational distribution. A subtle resonance for increasing oscillation frequencies is detected if the oscillating V(0) crosses the paranematic-nematic phase transition.
Journal of Physics Condensed Matter 03/2010; 22(10):104112. · 2.55 Impact Factor
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H Löwen
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ABSTRACT: The physics of colloidal suspensions confined in slits and cavities has significantly advanced during the last twenty years. In particular, freezing transitions in confinement have been addressed by theory and simulations and experimental realizations were proposed to confine colloidal particles to two dimensions. After reviewing this progress, we discuss the generalization to time-dependent confinement which leads to nonequilibrium situations. This is elaborated further for unstable situations where the particles can leave the confinement. In particular, the completely overdamped Brownian motion of a colloidal particle in a time-dependent harmonic trap is considered. The analytically soluble model of a time-dependent quadratic potential is used to extract the dynamical properties of the potential if the potential undergoes periodic switching from a confining harmonic potential to an unstable one. The amplitudes of the oscillating particle response can strongly grow in time, which we refer to as 'giant breathing'. This giant breathing process occurs also in anharmonic potentials and is verifiable in real-space experiments of colloids in laser-optical fields.
Journal of Physics Condensed Matter 11/2009; 21(47):474203. · 2.55 Impact Factor
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ABSTRACT: We examine binary mixtures of superparamagnetic colloidal particles confined to a two-dimensional water–air interface both by real-space experiments and Monte Carlo computer simulations at high coupling strength. In the simulations, the interaction is modelled as a pairwise dipole–dipole repulsion. While the ratio of magnetic dipole moments is fixed, the interaction strength governed by the external magnetic field and the relative composition is varied. Excellent agreement between simulation and experiment is found for the partial pair distribution functions including the fine structure of the neighbour shells at high coupling. Furthermore local crystal nuclei in the melt are identified by bond-orientational order parameters and their contribution to the pair structure is discussed.
Journal of Physics Condensed Matter 10/2009; 21(46):464114. · 2.55 Impact Factor
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ABSTRACT: Based on primitive model computer simulations with explicit microions, we calculate the effective interactions in a binary mixture of charged colloids with species A and B for different size and charge ratios. An optimal pairwise interaction is obtained by fitting the many-body effective forces. This interaction is close to a Yukawa (or Derjaguin-Landau-Verwey-Overbeek (DLVO)) pair potential but the AB cross-interaction is different from the geometric mean of the two direct AA and BB interactions. As a function of charge asymmetry, the corresponding nonadditivity parameter is first positive, then significantly negative and is then positive again. We finally show that an inclusion of nonadditivity within an optimal effective Yukawa model gives better predictions for the fluid pair structure than DLVO theory.
Journal of Physics Condensed Matter 10/2009; 21(42):424117. · 2.55 Impact Factor
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ABSTRACT: The crystalline ground state of macroions confined between two neutral parallel plates in the presence of their homogeneously spread counterions is calculated by lattice sum minimization of candidate phases involving up to six layers. For increasing macroion density, a cascade of solid-solid transitions is found involving various multilayered crystals. The cascade includes triangular monolayer and buckled bilayer as well as rhombic, squared and triangular phase structures.
Journal of Physics Condensed Matter 10/2009; 21(42):424110. · 2.55 Impact Factor
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ABSTRACT: The distance-resolved effective interaction between two colloidal particles in a subcritical solvent is explored both by an explicit and implicit modeling. An implicit solvent approach based on a simple thermodynamic interface model is tested against grand-canonical Monte Carlo computer simulations using explicit Lennard-Jones solvent molecules. Close to liquid-gas coexistence, a joint gas bubble surrounding the colloidal particle pair yields an effective attraction between the colloidal particles, the strength of which can be vastly tuned by the solvophobicity of the colloids. The implicit model is in good agreement with our explicit computer simulations, thus enabling an efficient modeling and evaluation of colloidal interactions and self-assembly in subcritical solvent environments.
The Journal of chemical physics 08/2009; 131(4):044513. · 3.09 Impact Factor
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ABSTRACT: The phase diagram of Yukawa particles confined between two parallel hard walls is calculated at zero temperature beyond the bilayer regime by lattice-sum-minimization. Tuning the screening, a rich phase behavior is found in the regime bounded by stable two triangular layers and three square layers. In this regime, alternating prism phases with square and triangular basis, structures derived from a hcp bulk lattice, and a structure with two outer layers and two inner staggered rectangular layers, reminiscent of a Belgian waffle iron, are stable. These structures are verifiable in experiments on charged colloidal suspensions and dusty plasma sheets.
EPL (Europhysics Letters) 04/2009; 86(2):28002. · 2.17 Impact Factor
