The removal of ultrathin amorphous polymer films in contact with nonsolvent/solvent binary mixtures is addressed by means of neutron reflectometry and atomic force microscopy. The high resolution of neutron scattering makes it possible to resolve the distribution profiles of heavy water and benzyl alcohol inside Laropal (R) A81, often employed as a protective varnish layer for Culture Heritage in restoration of easel paintings. The swelling kinetics and distribution profiles were recorded as a function of time and increasing benzyl alcohol concentration in water. The varnish film swells by penetration of the good solvent. At higher concentrations water-filled cavities appear inside the varnish and grow with time. Contrary to homogeneous dissolution dewetting is observed at late stages of exposure to the liquid which leads to the Breakup of the film. The high resolution measurements are compared to bulk behaviour characterized by the ternary phase diagram and the Flory-Huggins interaction parameters are calculated and used to predict the swelling and solvent partition in the films. Distinct differences of the thin film to bulk behaviour are found. The expectations made previously for the behaviour of solvent/non-solvent mixtures on the removal of thin layers in the restoration of easel paintings should be revised in view of surface interactions.

The actin cytoskeleton has the unique capability of producing pushing forces at the leading edge of motile cells without the implication of molecular motors. This phenomenon has been ex- tensively studied theoretically, and molecular models, including the widely known Brownian ratchet, have been proposed. However, supporting experimental work is lacking, due in part to hardly accessible molecular length scales. We designed an experiment to directly probe the mechanism of force generation in a setup where a population of actin filaments grows against a load applied by magnetic microparticles. The filaments, arranged in stiff bundles by fascin, are constrained to point toward the applied load. In this protrusion-like geometry, we are able to directly measure the velocity of filament elongation and its dependence on force. Using numerical simulations, we provide evidence that our experimental data are consistent with a Brownian ratchet-based model. We further demonstrate the existence of a force regime far below stalling where the mechanical power transduced by the ratcheting filaments to the load is maximal. The actin machinery in migrating cells may tune the number of filaments at the leading edge to work in this force regime.

Superparamagnetic colloids are used in many studies to apply a force or a torque on biological systems such as cells or DNA molecules. The physical process by which magnetization appears in those beads, allowing them to exert forces, is still poorly understood. In a first part, with an original experimental setup based on optical anisotropy measurements, we show that the apparently magnetically isotropic
beads can orient themselves in a magnetic field. Equilibrium and out of equilibrium measurements allowed ,to develop a model explaining how a few big grains in the beads are responsible for the whole coupling. The robustness of this model lies in its generalization to several sizes of beads and its consistency with the properties of the ferrofluid used to make them. In a second part, the reaction of a few actin filaments to the pressure applied by superparamagnetic colloids is studied. This setup represents a direct way to probe the physical mechanism underlying the production of force by actin filaments polymerization in cellular motility. Controlling the filaments’ organization allows to study two ideal ideal situations. We experimentally show that in the former situation the polymerization velocity is independent of the applied force. This is no longer true in the latter situation where the filaments are verys ti. Thus, we experimentally show for the first time the eect of force on actin filaments polymerization. A Brownian ratchet based numerical model with no load sharing can describe the production of force in that case.

The polymerization of actin in filaments generates forces that play a pivotal role in many cellular processes. We introduce a novel technique to determine the force-velocity relation when a few independent anchored filaments grow between magnetic colloidal particles. When a magnetic field is applied, the colloidal particles assemble into chains under controlled loading or spacing. As the filaments elongate, the beads separate, allowing the force-velocity curve to be precisely measured. In the widely accepted Brownian ratchet model, the transduced force is associated with the slowing down of the on-rate polymerization. Unexpectedly, in our experiments, filaments are shown to grow at the same rate as when they are free in solution. However, as they elongate, filaments are more confined in the interspace between beads. Higher repulsive forces result from this higher confinement, which is associated with a lower entropy. In this mechanism, the production of force is not controlled by the polymerization rate, but is a consequence of the restriction of filaments' orientational fluctuations at their attachment point.

We study the thermal expansion of chains formed by self-assembly of magnetic colloidal particles in a magnetic field. Using video microscopy, complete positional data of all the particles of the chains is obtained. By changing the ionic strength of the solution and the applied magnetic field, the interaction potential can be tuned. We analyze the thermal expansion of the chain using a simple model of a one-dimensional anharmonic crystal of finite size.

In 1994 Leal Calderon et al. (Phys. Rev. Lett. 72, 2959 (1994)) introduced the magnetic chaining technique to directly probe the force-distance profile between colloidal particles. In this paper, we revisit this approach in two ways. First, we describe a new experimental design which allows us to utilize sample volumes as low as a few microliters, involving femtomoles of surface active macromolecules. Secondly, we extensively describe the characterization and preparation of the magnetic colloids, and we give a quantitative evaluation of performance and resolution of the technique in terms of force and interparticle separation.

Several biological mechanisms use polymerization of actin filaments as mechanical motor. The chemical energy released with the addition of a monomer in the filament is converted into mechanical work thereby generating a force. Filaments are regulated by actin binding proteins, and form structures that possess different mechanical and elastical properties, and also different functions in the biological processes. Our experimental setup permits to study the link between the mechanical properties and the mechanisms that produce a force. The polymerization of actin filaments is directly initiated on the surface of magnetic particles. Within a magnetic field, particles organize themselves in a chain due to dipole-dipole interactions, and a compressive force is induced on the polymerizing filaments. Polymerization pushes the particles apart over time and as a function of the applied force, the velocity of the particles gap slows down. While following the evolution of the distance between particles, we obtain information on the force-velocity profile and the mechanical properties of the filaments.