Thomas Bickel

Cryo-electron tomography of nanoparticle transmigration into liposome

Nanoparticle transport across cell membrane plays a crucial role in the development of drug delivery systems as well as in the toxicity response induced by nanoparticles. As hydrophilic nanoparticles interact with lipid membranes and are able to induce membrane perturbations, hypothetic mechanisms based on membrane curvature or hole formation have been proposed for activating their transmigration. We report on the transport of hydrophilic silica nanoparticles into large unilamellar neutral DOPC liposomes via an internalization process. The strong adhesive interactions of lipid membrane onto the silica nanoparticle triggered liposome deformation until the formation of a curved neck. Then the rupture of this membrane neck led to the complete engulfment of the nanoparticle. Using cryo-electron tomography we determined 3D architectures of intermediate steps of this process unveiling internalized silica nanoparticles surrounded by a supported lipid bilayer. This engulfing process was achieved for a large range of particle size (from 30 to 200 nm in diameter). These original data provide interesting highlights for nanoparticle transmigration and could be applied to biotechnology development.

Flow pattern in the vicinity of self-propelling hot Janus particles.

We study the temperature field and the resulting flow pattern in the vicinity of a heated metal-capped Janus particle. If its thickness exceeds about 10 nm, the cap forms an isotherm and the flow pattern comprises a quadrupolar term that decays with the square of the inverse distance ~r(-2). For much thinner caps the velocity varies as ~r(-3). These findings could be relevant for collective effects in dense suspensions and for the circular tracer motion observed recently in the vicinity of a tethered Janus particle.

Local entropic effects of polymers grafted to soft interfaces

Abstract:In this paper, we study the equilibrium properties of polymer chains end-tethered to a fluid membrane. The loss of conformational entropy of the polymer results in an inhomogeneous pressure field that we calculate for Gaussian chains. We estimate the effects of excluded volume through a relation between pressure and concentration. Under the polymer pressure, a soft surface will deform. We calculate the deformation profile for a fluid membrane and show that close to the grafting point, this profile assumes a cone-like shape, independently of the boundary conditions. Interactions between different polymers are also mediated by the membrane deformation. This pair-additive potential is attractive for chains grafted on the same side of the membrane and repulsive otherwise.

Polarization of active Janus particles.

We theoretically study the motion of surface-active Janus particles, driven by an effective slip velocity due to a nonuniform temperature or concentration field ψ. With parameters realized in thermal traps, we find that the torque exerted by the gradient ∇ψ inhibits rotational diffusion and favors alignment of the particle axes. In a swarm of active particles, this polarization adds a novel term to the drift velocity and modifies the collective behavior. Self-polarization in a nonuniform laser beam could be used for guiding hot particles along a given trajectory.

A note on confined diffusion

The random motion of a Brownian particle confined in some finite domain is considered. Quite generally, the relevant statistical properties involve infinite series, whose coefficients are related to the eigenvalues of the diffusion operator. Because the latter depend on space dimensionality and on the particular shape of the domain, an analytical expression is in most circumstances not available. In this article, it is shown that the series may in some circumstances sum up exactly. Explicit calculations are performed for 2D diffusion restricted to a circular domain and 3D diffusion inside a sphere. In both cases, the short-time behaviour of the mean square displacement is obtained.

Brownian motion near a liquid-like membrane

Abstract.The dynamics of a tracer molecule near a fluid membrane is investigated, with particular emphasis given to the interplay between the instantaneous position of the particle and membrane fluctuations. It is found that hydrodynamic interactions creates memory effects in the diffusion process. The random motion of the particle is then shown to cross over from a “bulk” to a “surface” diffusive mode, in a way that crucially depends on the elastic properties of the interface.

Hindered mobility of a particle near a soft interface.

The translational motion of a solid sphere near a deformable fluid interface is studied in the low Reynolds number regime. In this problem, the fluid flow driven by the sphere is dynamically coupled to the instantaneous conformation of the interface. Using a two-dimensional Fourier transform technique, we are able to account for the multiple backflows scattered from the interface. The correction to the mobility tensor is then obtained from the matrix elements of the relevant Greens function. Our perturbative analysis allows us to express the explicit position and frequency dependence of the mobility for small particles. We recover in the steady limit the result for a sphere near a perfectly flat interface. At intermediate time scales, the mobility exhibits an imaginary part which is a signature of the elastic response of the interface. In the short time limit, we find that the perpendicular mobility may, under some circumstances, become lower than the bulk value. All the results can be explained using the definition of the relaxation time of the soft interface.

Nonequilibrium fluctuations of an interface under shear

The steady-state properties of an interface in a stationary Couette flow are addressed within the framework of fluctuating hydrodynamics. Our study reveals that thermal fluctuations are driven out of equilibrium by an effective shear rate that differs from the applied one. In agreement with experiments, we find that the mean-square displacement of the interface is reduced by the flow. We also show that nonequilibrium fluctuations present a certain degree of universality in the sense that all features of the fluids can be factorized into a single control parameter. Finally, the results are discussed in the light of recent experimental and numerical studies.

Probing nanoscale deformations of a fluctuating interface

We consider the contribution of thermal capillary waves to the interaction between a fluid-fluid interface and a nearby nanoparticle. Fluctuations are described thanks to an effective interaction potential which is derived using the renormalization group. The general theory is then applied to a spherical particle interacting with the interface through van der Waals forces. Surprisingly enough, we find that fluctuations contribute significantly to the deformation profile. Our study therefore reveals that thermal fluctuations cannot be ignored when probing nanoscale deformations of a soft interface.

Scale-dependent rigidity of polymer-ornamented membranes

Abstract:We study the fluctuation spectrum of fluid membranes carrying grafted polymers. Contrary to usual descriptions, we find that the modifications induced by the polymers cannot be reduced to the renormalization of the membrane bending rigidity. Instead we show that the ornamented membrane exhibits a scale-dependent elastic modulus that we evaluate. In ornamented lamellar stacks, we further compute the polymer contribution to the Caillé parameter characterizing the power law singularities of the Bragg peaks.