A. Cebers

Deformation of intracellular endosomes under a magnetic field

We present a non-invasive method to monitor the membrane tension of intracellular organelles using a magnetic field as an external control parameter. By exploiting the spontaneous endocytosis of anionic colloidal ferromagnetic nanoparticles, we obtain endosomes possessing a superparamagnetic lumen in eukaryotic cells. Initially flaccid, the endosomal membrane undulates because of thermal fluctuations, restricted in zero field by the resting tension and the curvature energy of the membrane. When submitted to a uniform magnetic field, the magnetized endosomes elongate along the field, resulting in the flattening of the entropic membrane undulations. The quantification of the endosome deformation for different magnetic fields allows in situ measurement of the resting tension and the bending stiffness of the membrane enclosing the intracellular organelle.

Combined Electrostatics and Hydrogen Bonding Determine Intermolecular Interactions Between Polyphosphoinositides

Membrane lipids are active contributors to cell function as key mediators in signaling pathways controlling cell functions including inflammation, apoptosis, migration, and proliferation. Recent work on multimolecular lipid structures suggests a critical role for lipid organization in regulating the function of both lipids and proteins. Of particular interest in this context are the polyphosphoinositides (PPIs), especially phosphatidylinositol (4,5) bisphosphate (PIP 2). The cellular functions of PIP 2 are numerous but the organization of PIP 2 in the inner leaflet of the plasma membrane, as well as the factors controlling targeting of PIP 2 to specific proteins, remains poorly understood. To analyze the organization of PIP 2 in a simplified planar system, we used Langmuir monolayers to study the effects of subphase conditions on monolayers of purified naturally derived PIP 2 and other anionic or zwitterionic phospholipids. We report a significant molecular area expanding effect of subphase monovalent salts on PIP 2 at biologically relevant surface densities. This effect is shown to be specific to PIP 2 and independent of subphase pH. Chaotropic agents (e.g., salts, trehalose, urea, temperature) that disrupt water structure and the ability of water to mediate intermolecular hydrogen bonding also specifically expanded PIP 2 monolayers. These results suggest a combination of water-mediated hydrogen bonding and headgroup repulsion in determining the organization of PIP 2, and may contribute to an explanation for the unique functionality of PIP 2 compared to other anionic phospholipids.

Viscous fingering in a magnetic fluid. I. Radial Hele-Shaw flow

Viscous fingering phenomenon in a linear channel is studied for a magnetic fluid subjected to an external magnetic field. The competition between the hydrodynamic effects and the capillary effects leads to the formation of an interface between the air and the fluid which has a finger shape. It is the so-called Saffman–Taylor instability (STI). The influence of the magnetic effects depends on the direction of the applied field: it is possible either to enhance or to reduce the destabilizing phenomena. We study the onset of the STI and compare the experimental results with the linear analysis including the magnetic contribution. In the nonlinear regime, the measurement of the width of the finger as a function of the direction and the amplitude of the magnetic field is understood using a phenomenological approach.

Magnetic wire-based sensors for the microrheology of complex fluids.

We propose a simple microrheology technique to evaluate the viscoelastic properties of complex fluids. The method is based on the use of magnetic wires of a few microns in length submitted to a rotational magnetic field. In this work, the method is implemented on a surfactant wormlike micellar solution that behaves as an ideal Maxwell fluid. With increasing frequency, the wires undergo a transition between a steady and a hindered rotation regime. The study shows that the average rotational velocity and the amplitudes of the oscillations obey scaling laws with well-defined exponents. From a comparison between model predictions and experiments, the rheological parameters of the fluid are determined.

Labyrinthine instability of miscible magnetic fluids

The paper treats theoretically an inhomogeneous magnetic fluid (MF), modeling a miscible MF pair, in a Hele-Shaw cell subjected to a perpendicular magnetic field. As the existing experimental evidence indicates, a miscible form of the labyrinthine instability may occur in this system, with diffusion of magnetic particles playing the key role. Linear stability analysis is performed in the present paper: Analytically for a sharp interface and numerically for a diffused concentration distribution. For the sharp interface, assuming the Darcy law governs the flow, the neutral curves and the stability diagram are found along with the critical wavelength and the critical field intensity. Oscillatory and stationary instabilities are shown to substitute each other under certain conditions. For the diffused interface the viscous effects due to the flow nonuniformity in the plane of the cell are allowed for and found significant. Therefore, the conventional Darcy law that takes into account only the near-wall fricti...

Shape transitions of giant liposomes induced by an anisotropic spontaneous curvature.

We explore how a magnetic field breaks the symmetry of an initially spherical giant liposome filled with a magnetic colloid. The condition of rotational symmetry along the field axis leads either to a prolate or to an oblate ellipsoid. We demonstrate that an electrostatic interaction between the nanoparticles and the membrane triggers the shape transition.

Magnetic phospholipid tubes connected to magnetoliposomes: Pearling instability induced by a magnetic field

Abstract:We propose here a method to modify the membrane tension of phospholipid tubes with an applied magnetic field. The tubes are connected to giant liposomes capping the tubes at both ends. Tubes and liposomes are all filled with a magnetic fluid. The tension of the tube membrane is tuned by the deformation of the ending liposomes under the applied field. We modelize the magnetoliposome deformation and we are then able to describe the tube evolution. At low magnetic fields, the tube remains at equilibrium with a cylindrical shape and a uniform radius. It responds to an increase of membrane tension by a diameter reduction. Above a magnetic-field threshold, the cylindrical shape becomes unstable with respect to a pearling deformation. The tube shape then selected by the system is an unduloid, with a constant mean curvature equal to C0, the spontaneous curvature of the membrane.

Orientational dynamics of colloidal ribbons self-assembled from microscopic magnetic ellipsoids

We combine experiments and theory to investigate the orientational dynamics of dipolar ellipsoids, which self-assemble into elongated ribbon-like structures due to the presence of a permanent magnetic moment, perpendicular to the long axis in each particle. Monodisperse hematite ellipsoids are synthesized via the sol-gel technique and arrange into ribbons in the presence of static or time-dependent magnetic fields. We find that under an oscillating field, the ribbons reorient perpendicular to the field direction, in contrast with the behaviour observed under a static field. This observation is explained theoretically by treating a chain of interacting ellipsoids as a single particle with orientational and demagnetizing field energy. The model allows us to describe the orientational behaviour of the chain and captures well its dynamics at different strengths of the actuating field. The understanding of the complex dynamics and assembly of anisotropic magnetic colloids is a necessary step for controlling the structure formation, which has direct applications in different fluid-based microscale technologies.

Viscous fingering in magnetic fluids: numerical simulation of radial Hele–Shaw flow

Abstract In this paper, the viscous fingering phenomena of the magnetic liquids in the case when the magnetic field is applied normally to the Hele–Shaw cell is investigated by the numerical simulation technique. It is shown that the magnetic field causes additional destabilization of the free interface arising at the air injection in the magnetic liquid. Here the peculiarities of the interface dynamics – inward motion of the tips of the fjords, the gyration radius dependence on the perimeter found by the numerical simulations are in good accordance with the experiments.

Chaos: new trend of magnetic fluid research

Abstract Chaos phenomena in forced oscillations of magnetic fluids drops in an ac field are considered. It is shown that by period doubling, a bifurcation sequence regime of the oscillation with chaotic spontaneous rotation of the drops arises. At higher values of field strength direction of the drop rotation is also chaotic.