Electrification in granular gases leads to constrained fractal growth

The empirical observation of aggregation of dielectric particles under the influence of electrostatic forces lies at the origin of the theory of electricity. The growth of clusters formed of small grains underpins a range of phenomena from the early stages of planetesimal formation to aerosols. However, the collective effects of Coulomb forces on the nonequilibrium dynamics and aggregation process in a granular gas -- a model representative of the above physical processes -- have so far evaded theoretical scrutiny. Here, we establish a hydrodynamic description of aggregating granular gases that exchange charges upon collisions and interact via the long-ranged Coulomb forces. We analytically derive the governing equations for the evolution of granular temperature, charge variance, and number density for homogeneous and quasi-monodisperse aggregation. We find that, once the aggregates are formed, the system obeys a physical constraint of nearly constant dimensionless ratio of characteristic electrostatic to kinetic energy $\mathcal{B}(t)\le 1$. This constraint on the collective evolution of charged clusters is confirmed both by the theory and the detailed molecular dynamics simulations. The inhomogeneous aggregation of monomers and clusters in their mutual electrostatic field proceeds in a fractal manner. Our theoretical framework is extendable to more precise charge exchange mechanism, a current focus of extensive experimentation. Furthermore, it illustrates the collective role of long-ranged interactions in dissipative gases and can lead to novel designing principles in particulate systems

Dynamics of a Passive Droplet in Active Turbulence

We numerically study the effect of an active turbulent environment on a passive deformable droplet. The system is simulated using coupled hydrodynamic and nematodynamic equations for nematic liquid crystals with an active stress which is non-zero outside the droplet, and is zero inside. The droplet undergoes deformation fluctuations and its movement shows periods of ``runs" and ``stays". The mean square displacement of the geometric center of the droplet shows an extended ballistic regime and a transition to normal diffusive regime which depends on the size of the droplet. We relate this transition with a temporal scale associated with velocity autocorrelation function of the droplet trajectories, and with a spatial scale associated with one-time two-point velocity correlation function of the surrounding active medium. As the radius is decreased below the integral length scale, the velocity autocorrelation time of trajectories increases and the transition to normal diffusion is delayed

Levitation of non-magnetizable droplet inside ferrofluid

The central theme of this work is that a stable levitation of a denser non-magnetizable liquid droplet, against gravity, inside a relatively lighter ferrofluid -- a system barely considered in ferrohydrodynamics -- is possible, and exhibits unique interfacial features; the stability of the levitation trajectory, however, is subject to an appropriate magnetic field modulation. We explore the shapes and the temporal dynamics of a plane non-magnetizable droplet levitating inside ferrofluid against gravity due to a spatially complex, but systematically generated, magnetic field in two dimensions. The effect of the viscosity ratio, the stability of the levitation path and the possibility of existence of multiple-stable equilibrium states is investigated. We find, for certain conditions on the viscosity ratio, that there can be developments of cusps and singularities at the droplet surface; this phenomenon we also observe experimentally and compared with the simulations. Our simulations closely replicate the singular projection on the surface of the levitating droplet. Finally, we present an dynamical model for the vertical trajectory of the droplet. This model reveals a condition for the onset of levitation and the relation for the equilibrium levitation height. The linearization of the model around the steady state captures that the nature of the equilibrium point goes under a transition from being a spiral to a node depending upon the control parameters, which essentially means that the temporal route to the equilibrium can be either monotonic or undulating. The analytical model for the droplet trajectory is in close agreement with the detailed simulations. (See draft for full abstract).Comment: This article has been published in a revised form in Journal of Fluid Mechanics http://dx.doi.org/10.1017/jfm.2018.733. Copyright: copyright holde

Anisotropic short-range attractions precisely model branched erythrocyte aggregates

Homogeneous suspensions of red blood cells (RBCs or erythrocytes) in blood plasma are unstable in the absence of driving forces and form elongated stacks, called rouleau. These erythrocyte aggregates are often branched porous networks -- a feature that existing red blood cell aggregation models and simulations fail to predict exactly. Here we establish that alignment-dependent attractive forces in a system of dimers can precisely generate branched structures similar to RBC aggregates observed under a microscope. Our simulations consistently predict that the growth rate of typical mean rouleau size remains sub-linear -- a hallmark from past studies -- which we also confirm by deriving a reaction kernel taking into account appropriate collision cross-section, approach velocities, and an area-dependent sticking probability. The system exhibits unique features such as the existence of percolated and/or single giant cluster states, multiple coexisting mass-size scalings, and transition to a branched phase upon fine-tuning of model parameters. Upon decreasing the depletion thickness we find that the percolation threshold increases and the morphology of the structures opens up towards an increased degree of branching. Remarkably the system self-organizes to produce a universal power-law size distribution scaling irrespective of the model parameters

Barn og straff : Anvendelsen av samfunnsstraff ovenfor unge lovbrytere og dens rettstutvikling

Temaet for oppgaven er barn og straff. Problemstillingen er anvendelsen av samfunnsstraff ovenfor unge lovbrytere og dens rettsutvikling. §28a straffeloven og rettsutviklingen i Høyesterett for anvendelse av samfunnsstraff som straffereaksjon ovenfor lovbrytere mellom 15-18 år

Electrification in granular gases leads to constrained fractal growth

The empirical observation of aggregation of dielectric particles under the influence of electrostatic forces lies at the origin of the theory of electricity. The growth of clusters formed of small grains underpins a range of phenomena from the early stages of planetesimal formation to aerosols. However, the collective effects of Coulomb forces on the nonequilibrium dynamics and aggregation process in a granular gas – a model representative of the above physical processes – have so far evaded theoretical scrutiny. Here, we establish a hydrodynamic description of aggregating granular gases that exchange charges upon collisions and interact via the long-ranged Coulomb forces. We analytically derive the governing equations for the evolution of granular temperature, charge variance, and number density for homogeneous and quasi-monodisperse aggregation. We find that, once the aggregates are formed, the granular temperature of the cluster population, the charge variance of the cluster population and the number density of the cluster population evolve in such a way that their non-dimensional combination obeys a physical constraint of nearly constant dimensionless ratio of characteristic electrostatic to kinetic energy. This constraint on the collective evolution of charged clusters is confirmed both by our theory and our detailed molecular dynamics simulations. The inhomogeneous aggregation of monomers and clusters in their mutual electrostatic field proceeds in a fractal manner. Our theoretical framework is extendable to more precise charge exchange mechanisms, a current focus of extensive experimentation. Furthermore, it illustrates the collective role of long-ranged interactions in dissipative gases and can lead to novel designing principles in particulate systems

Guided run-and-tumble active particles: wall accumulation and preferential deposition

Asymmetric rotational drift induced by guiding fields, and variability in bacterial running strategies, are important physical mechanisms to understand the organization and early biofilm formation in collections of confined active particles.</jats:p

Correction: Guided run-and-tumble active particles: wall accumulation and preferential deposition

Correction for ‘Guided run-and-tumble active particles: wall accumulation and preferential deposition’ by Chamkor Singh, Soft Matter, 2021, 17, 8858–8866, DOI: 10.1039/D1SM00775K.</jats:p