Quartz Crystal Microbalance Frequency Response to Discrete Adsorbates in Liquids.

Quartz crystal microbalance with dissipation monitoring (QCM-D) has become a major tool enabling accurate investigation of the adsorption kinetics of nanometric objects such as DNA fragments, polypeptides, proteins, viruses, liposomes, polymer, and metal nanoparticles. However, in liquids, a quantitative analysis of the experimental results is often intricate because of the complex interplay of hydrodynamic and adhesion forces varying with the physicochemical properties of adsorbates and functionalized QCM-D sensors. In the present paper, we dissect the role of hydrodynamics for the analytically tractable case of stiff contact, whereas the adsorbed rigid particles oscillate with the resonator without rotation.

Quantitative Prediction of Sling Events in Turbulence at High Reynolds Numbers.

Collisional growth of droplets, such as occurring in warm clouds, is known to be significantly enhanced by turbulence. Whether particles collide depends on their flow history, in particular on their encounters with highly intermittent small-scale turbulent structures, which despite their rarity can dominate the overall collision rate. Here, we develop a quantitative criterion for sling events based on the velocity gradient history along particle paths.

Fluid flow induced by helical microswimmers in bulk and near walls.

Magnetic nano- and microswimmers provide a powerful platform to study driven colloidal systems in fluidic media and are relevant to futuristic medical technologies requiring precise yet minimally invasive motion control at small scales. Upon the action of a rotating magnetic field, the helical microswimmers rotate and translate, generating flow in the surrounding fluid. In this paper, we study the fluid flow induced by the rotating helices using a combination of experiments, numerical simulations, and theory.

Planetary, inertia-gravity and Kelvin waves on the -plane and -plane in the presence of a uniform zonal flow.

A linear wave theory of the Rotating Shallow-Water Equations (RSWE) is developed in a channel on the midlatitude -plane or -plane in the presence of a uniform mean zonal flow that is balanced geostrophically by a meridional gradient of the fluid surface height. Here we show that this surface height gradient is a potential vorticity (PV) source that generates Rossby waves even on the -plane similar to the generation of these waves by PV sources such as the -effect, shear of the mean flow and bottom topography. Numerical solutions of the RSWE show that the resulting Rossby, Poincaré and "Kelvin-like" waves differ from their counterparts without mean flow in both their phase speeds and meridional structures.

Linear and nonlinear hydromagnetic stability in laminar and turbulent flows.

We consider the evolution of arbitrarily large perturbations of a prescribed pure hydrodynamical flow of an electrically conducting fluid. We study whether the flow perturbations as well as the generated magnetic fields decay or grow with time and constitute a dynamo process. For that purpose we derive a generalized Reynolds-Orr equation for the sum of the kinetic energy of the hydrodynamic perturbation and the magnetic energy.

Reynolds number dependence of Lyapunov exponents of turbulence and fluid particles.

The Navier-Stokes equations generate an infinite set of generalized Lyapunov exponents defined by different ways of measuring the distance between exponentially diverging perturbed and unperturbed solutions. This set is demonstrated to be similar, yet different, from the generalized Lyapunov exponent that provides moments of distance between two fluid particles below the Kolmogorov scale. We derive rigorous upper bounds on dimensionless Lyapunov exponent of the fluid particles that demonstrate the exponent's decay with Reynolds number Re in accord with previous studies.

Occupational structure of bearers of Jewish rabbinical, occupational and generic surnames.

We study choice of profession in three groups of Russian-speaking Jewish families with different occupational distributions of the ancestors. This study continues exploration of the persistence of social status of families over centuries that was initiated in recent years. It was found previously that in some cases professions remain associated with the same surnames for many generations.

Efficient mate finding in planktonic copepods swimming in turbulence.

Zooplankton live in dynamic environments where turbulence may challenge their limited swimming abilities. How this interferes with fundamental behavioral processes remains elusive. We reconstruct simultaneously the trajectories of flow tracers and calanoid copepods and we quantify their ability to find mates when ambient flow imposes physical constrains on their motion and impairs their olfactory orientation.

Fluid-Mediated Force on a Particle Due to an Oscillating Plate and Its Effect on Deposition Measurements by a Quartz Crystal Microbalance.

Interaction of particles with boundaries is a fundamental problem in many fields of physics. In this Letter, we theoretically examine the fluid-mediated interaction between a horizontally oscillating plate and a spherical particle, revealing emergence of the novel nonlinear vertical force exerted on the particle. Although we demonstrate that the phenomenon only slightly alters deposition of colloidal (sub-)μm-sized particles measured by quartz crystal microbalance, it can result in levitation of larger particles above the plate, considerably hindering their deposition.

Large deviations, singularity, and lognormality of energy dissipation in turbulence.

We study implications of the assumption of power-law dependence of moments of energy dissipation in turbulence on the Reynolds number Re, holding due to intermittency. We demonstrate that at Re→∞ the dissipation's logarithm divided by lnRe converges with probability one to a negative constant. This implies that the dissipation is singular in the limit, as is known phenomenologically.

Density and tracer statistics in compressible turbulence: Phase transition to multifractality.

We study the statistics of fluid (gas) density and concentration of passive tracer particles (dust) in compressible turbulence. As Ma increases from small or moderate values, the density and the concentration in the inertial range go through a phase transition from a finite continuous smooth distribution to a singular multifractal spatial distribution. Multifractality is associated with scaling, which would not hold if the solenoidal and the potential components of the flow scaled differently, producing transport which is not self-similar.

The mixed Rossby-gravity wave on the spherical Earth.

This work revisits the theory of the mixed Rossby-gravity (MRG) wave on a sphere. Three analytic methods are employed in this study: (a) derivation of a simple ad hoc solution corresponding to the MRG wave that reproduces the solutions of Longuet-Higgins and Matsuno in the limits of zero and infinite Lamb's parameter, respectively, while remaining accurate for moderate values of Lamb's parameter, (b) demonstration that westward-propagating waves with phase speed equalling the negative of the gravity-wave speed exist, unlike the equatorial β-plane, where the zonal velocity associated with such waves is infinite, and (c) approximation of the governing second-order system by Schrödinger eigenvalue equations, which show that the MRG wave corresponds to the branch of the ground-state solutions that connects Rossby waves with zonally symmetric waves. The analytic conclusions are confirmed by comparing them with numerical solutions of the associated second-order equation for zonally propagating waves of the shallow-water equations.

Dispersion of particles in an infinite-horizon Lorentz gas.

We consider a two-dimensional Lorentz gas with infinite horizon. This paradigmatic model consists of pointlike particles undergoing elastic collisions with fixed scatterers arranged on a periodic lattice. It was rigorously shown that when t→∞, the distribution of particles is Gaussian.

Integral representation of channel flow with interacting particles.

We construct a boundary integral representation for the low-Reynolds-number flow in a channel in the presence of freely suspended particles (or droplets) of arbitrary size and shape. We demonstrate that lubrication theory holds away from the particles at horizontal distances exceeding the channel height and derive a multipole expansion of the flow which is dipolar to the leading approximation. We show that the dipole moment of an arbitrary particle is a weighted integral of the stress and the flow at the particle surface, which can be determined numerically.

Zooplankton can actively adjust their motility to turbulent flow.

Calanoid copepods are among the most abundant metazoans in the ocean and constitute a vital trophic link within marine food webs. They possess relatively narrow swimming capabilities, yet are capable of significant self-locomotion under strong hydrodynamic conditions. Here we provide evidence for an active adaptation that allows these small organisms to adjust their motility in response to background flow.

Classification of eastward propagating waves on the spherical Earth.

Observational evidence for an equatorial non-dispersive mode propagating at the speed of gravity waves is strong, and while the structure and dispersion relation of such a mode can be accurately described by a wave theory on the equatorial β-plane, prior theories on the sphere were unable to find such a mode except for particular asymptotic limits of gravity wave phase speeds and/or certain zonal wave numbers. Here, an ad hoc solution of the linearized rotating shallow-water equations (LRSWE) on a sphere is developed, which propagates eastward with phase speed that nearly equals the speed of gravity waves at all zonal wave numbers. The physical interpretation of this mode in the context of other modes that solve the LRSWE is clarified through numerical calculations and through eigenvalue analysis of a Schrödinger eigenvalue equation that approximates the LRSWE.

Solvable continuous-time random walk model of the motion of tracer particles through porous media.

We consider the continuous-time random walk (CTRW) model of tracer motion in porous medium flows based on the experimentally determined distributions of pore velocity and pore size reported by Holzner et al. [M. Holzner et al.

Clustering of particles in turbulence due to phoresis.

We demonstrate that diffusiophoretic, thermophoretic, and chemotactic phenomena in turbulence lead to clustering of particles on multifractal sets that can be described using one single framework, valid when the particle size is much smaller than the smallest length scale of turbulence l_{0}. To quantify the clustering, we derive positive pair correlations and fractal dimensions that hold for scales smaller than l_{0}. For scales larger than l_{0} the pair-correlation function is predicted to show a stretched exponential decay towards 1.

Inhomogeneous distribution of water droplets in cloud turbulence.

We consider sedimentation of small particles in the turbulent flow where fluid accelerations are much smaller than acceleration of gravity g. The particles are dragged by the flow by linear friction force. We demonstrate that the pair-correlation function of particles' concentration diverges with decreasing separation as a power law with negative exponent.

Phytoplankton's motion in turbulent ocean.

We study the influence of turbulence on upward motion of phytoplankton. Interaction with the flow is described by the Pedley-Kessler model considering spherical microorganisms. We find a range of parameters when the upward drift is only weakly perturbed or when turbulence completely randomizes the drift direction.

Quasispecies in population of compositional assemblies.

Background: The quasispecies model refers to information carriers that undergo self-replication with errors. A quasispecies is a steady-state population of biopolymer sequence variants generated by mutations from a master sequence. A quasispecies error threshold is a minimal replication accuracy below which the population structure breaks down.

Convective stability of turbulent Boussinesq flow in the dissipative range and flow around small particles.

We consider arbitrary, possibly turbulent, Boussinesq flow which is smooth below a dissipative scale l_{d}. It is demonstrated that the stability of the flow with respect to growth of fluctuations with scale smaller than l_{d} leads to a nontrivial constraint. That involves the dimensionless strength of fluctuations of the gradients of the scalar in the direction of gravity Fl and the Rayleigh scale L depending on the Rayleigh number Ra, the Nusselt number Nu, and l_{d}.

Inhomogeneous quasistationary state of dense fluids of inelastic hard spheres.

We study closed dense collections of freely cooling hard spheres that collide inelastically with constant coefficient of normal restitution. We find inhomogeneous states (ISs) where the density profile is spatially nonuniform but constant in time. The states are exact solutions of nonlinear partial differential equations that describe the coupled distributions of density and temperature valid when inelastic losses of energy per collision are small.

Distribution of particles and bubbles in turbulence at a small Stokes number.

The inertia of particles driven by the turbulent flow of the surrounding fluid makes them prefer certain regions of the flow. The heavy particles lag behind the flow and tend to accumulate in the regions with less vorticity, while the light particles do the opposite. As a result of the long-time evolution, the particles distribute over a multifractal attractor in space.

Physical aspects of precision in genetic regulation.

The process by which transcription factors (TFs) locate specific DNA binding sites is stochastic and as such, is subject to a considerable level of noise. TFs diffuse in the three-dimensional nuclear space, but can also slide along the DNA. It was proposed that this sliding facilitates the TF molecules arriving to their binding site, by effectively reducing the dimensionality of diffusion.