Particle-resolved instabilities in colloidal dispersions

Löwen, Hartmut

doi:10.1039/b923685f

Cited by 53 publications

(55 citation statements)

References 112 publications

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“…Colloidal mixtures are valuable model systems to explore gravity effects on the particle scale [13][14][15][16][17][18] both in equilibrium and nonequilibrium. Sediments of binary charged mixtures are commonly used to determine the phase behavior [19][20][21][22] .…”

Section: Introductionmentioning

confidence: 99%

Soft repulsive mixtures under gravity: Brazil-nut effect, depletion bubbles, boundary layering, nonequilibrium shaking

Kruppa

Neuhaus

Messina

et al. 2012

The Journal of Chemical Physics

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A binary mixture of particles interacting via long-ranged repulsive forces is studied in gravity by computer simulation and theory. The more repulsive A-particles create a depletion zone of less repulsive B-particles around them reminiscent to a bubble. Applying Archimedes' principle effectively to this bubble, an A-particle can be lifted in a fluid background of Bparticles. This "depletion bubble" mechanism explains and predicts a brazil nut effect where the heavier A-particles float on top of the lighter B-particles. It also implies an effective attraction of an A-particle towards a hard container bottom wall which leads to boundary layering of A-particles. Additionally, we have studied a periodic inversion of gravity causing perpetuous mutual penetration of the mixture in a slit geometry. In this nonequilibrium case of time-dependent gravity, the boundary layering persists. Our results are based on computer simulations and density functional theory of a two-dimensional binary mixture of colloidal repulsive dipoles. The predicted effects also occur for other long-ranged repulsive interactions and in three spatial dimensions. They are therefore verifiable in settling experiments on dipolar or charged colloidal mixtures as well as in charged granulates and dusty plasmas.

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Section: Introductionmentioning

confidence: 99%

Soft repulsive mixtures under gravity: Brazil-nut effect, depletion bubbles, boundary layering, nonequilibrium shaking

Kruppa

Neuhaus

Messina

et al. 2012

The Journal of Chemical Physics

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“…What is notable about the critters that emerge from this instability is that they are a persistent state which can be produced from hydrodynamic interactions. Other kinds of hydrodynamic clusters, such as those seen in sedimentation 30,31 , are always transient and not longlived structures. Almost all active matter systems exhibit some kind of clustering instability 3,4 , but it is usually a consequence of particle-particle interactions, either directly through an attractive potential, sensing, or via self-trapping, which is a consequence of a repulsive particle potential.…”

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confidence: 99%

Unstable fronts and motile structures formed by microrollers

et al. 2016

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1and driven systems [2][3][4][5] . It is commonly held that potential interactions 6 , depletion forces 7 , or sensing 8 are the only mechanisms which can create long-lived compact structures. Here we show that persistent motile structures can form spontaneously from hydrodynamic interactions alone, with no sensing or potential interactions. We study this structure formation in a system of colloidal rollers suspended and translating above a floor, using both experiments and large-scale three-dimensional simulations. In this system, clusters originate from a previously unreported fingering instability, where fingers pinch o from an unstable front to form autonomous 'critters', whose size is selected by the height of the particles above the floor. These critters are a stable state of the system, move much faster than individual particles, and quickly respond to a changing drive. With speed and direction set by a rotating magnetic field, these active structures o er interesting possibilities for guided transport, flow generation, and mixing at the microscale.We have identified a new instability in one of the most basic systems of low-Reynolds-number (steady Stokes or overdamped) flow, a collection of spheres rotating near a wall. This system has been well studied analytically and numerically 9,10 , since it is considered a base model for understanding many microbial and colloidal flows. The instability visually resembles wet paint dripping down a wall or individual droplets sliding down a windshield 11 -examples of Rayleigh-Taylor instabilities 12 . However, in those and other clustering phenomena, what holds things together is surface tension or other forces deriving from an interaction potential.Here we use a model system to explore whether hydrodynamic interactions alone, without particle collisions, attractions or sense/response redirection, can lead to stable finite clusters.The experimental system consists of polymer colloids with radius a = 0.66 µm which have a small permanent magnetic moment (|m| ∼ 5 × 10 −16 A m −2 ) from an embedded haematite cube 13 (see schematic in Fig. 1a). Inter-particle magnetic interactions are small compared to thermal energy (< 0.1k B T ). A rotating magnetic field (B = B 0 cos(ωt)x + sin(ωt)ẑ ) with magnitude B 0 and frequency f = ω/2π is applied, causing all the particles to rotate about the y-axis at the same rate ω. The particles rotate synchronously with the field for ω < ω c , where ω c is the critical frequency above which the applied magnetic torque is not enough to balance the viscous torque on the particle (see Supplementary Section I for details of the rotation mechanism). In all of our experiments, ω < ω c . In contrast with recent experiments on Quincke rollers 14 , the rotation direction is prescribed and does not arise from the system dynamics.Hydrodynamics is the dominant inter-particle interaction in this system, which is distinctly different from many other systems of rotating magnetic particles, where dynamics is found to be a strong function of inter-particle ...

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“…Extended numerical and theoretical investigations on lane formation in colloids were performed by one of us (HL) and collaborators [51]. Due to the (tunable) low damping in complex plasmas and the possibility to record the full kinetics of single dust grains, binary complex plasmas offer a unique research opportunity: namely to resolve the dynamical onset of lane formation, and investigate the temporal evolution of lanes and its dependence on complex plasma parameters.…”

Section: Lane Formation In Driven Binary Complex Plasmasmentioning

confidence: 99%

Non-equilibrium phase transitions in complex plasma

Sütterlin

Wysocki

Räth

et al. 2010

Plasma Phys. Control. Fusion

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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.

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Particle-resolved instabilities in colloidal dispersions

Cited by 53 publications

References 112 publications

Soft repulsive mixtures under gravity: Brazil-nut effect, depletion bubbles, boundary layering, nonequilibrium shaking

Soft repulsive mixtures under gravity: Brazil-nut effect, depletion bubbles, boundary layering, nonequilibrium shaking

Unstable fronts and motile structures formed by microrollers

Non-equilibrium phase transitions in complex plasma

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